This PDF file contains the front matter associated with SPIE Proceedings Volume 11770, including the Title Page, Copyright information, and Table of Contents.

The nascent field of semiconductor core fibres is attracting increased interest as a means to exploit the excellent optical and optoelectronic functionality of the semiconductor material directly within the fibre geometry. Compared to their planar counterparts, this new class of waveguide retains many advantageous properties of the fibre platforms such as flexibility, cylindrical symmetry, and long waveguide lengths. Furthermore, owing to the robust glass cladding it is also possible to employ standard fibre post-processing procedures to tailor the waveguide dimensions and reduce the optical losses over a broad wavelength range, of particular use for nonlinear applications. This presentation will review progress in the development of nonlinear devices from the semiconductor core fibre platform and outline exciting future prospects for the field.

This talk will focus on three absorbing lines of research into nanophotonics. It will begin with the first experimental observation of a chiroptical effect that was predicted 40 years ago.[1,2] Our team recently demonstrated that in chiral metal nanoparticles (those that lack mirror symmetry) the intensity of light, scattered at the secondharmonic frequency, is proportional to the chirality. This effect was predicted 40 years ago, it is >10,000 more sensitive than corresponding linear optical effects and it could enable safer pharmaceuticals. Next, it will consider the smallest backjets (‘nanojets’) ever created and will discuss how they can serve to assemble novel metamaterials based on nano metalworking.[3,4] Nano metalworking is an exciting, emerging field that is largely unexplored. Finally, it will illustrate how a vapour stabilization technique can greatly enhance quantum sensors.[5] REFERENCES [1] Phys Rev. X 9, 011024 (2019). [2] J. Chem. Phys. 70, 1027 (1979). [3] Adv. Mater. 24, OP29-OP35 (2012). [4] Nat. Commun. 5, 4568 (2014). [5] Nat. Commun. 10, 2328 (2019).

The ability of plasmonic nanosurfaces to produce strong electromagnetic fields in their vicinity upon illumination can be used to enhance effects, such as those originating from chirality (lack of mirror symmetry) of molecules. We numerically investigate chiral nanosurfaces composed of plasmonic nanobars with varying packing densities. We identify the optimum illumination conditions for maximal field enhancement. Under these illumination conditions, the optical chirality near the surface exceeds the optical chirality of the incident light by almost an order of magnitude in a large area (200 nm × 200 nm) near the surface. Our simulations prove the nanosurfaces to be promising candidates for enhancement of chiral-optical effects.

Graphene nanoribbons provide the means to excite surface plasmon upon direct excitation, avoiding then the momentum mismatch in extended graphene. We describe those collective modes employing a rigorous quantum-mechanical simulation, which accounts for nonlocal, quantum finite-size, and edge-termination effects manifested in the optical response. Our simulations reveal a strong dependence on such phenomena for excitation with a high optical momentum component along the direction of transverse symmetry in both the linear and nonlinear optical response, wherein particular second-order nonlinear phenomena are found to manifest with high efficiency due to the breaking of inversion symmetry. These results can be employed to describe emitters, as two-level atoms, close by used to excite and determine the nonlinear dynamics originated by those plasmons in the graphene nanoribbons.

Atomically thin films possess appealing intrinsic properties for nonlinear optics that we explore employing a rigorous quantum-mechanical description. The main optical response is driven by plasmons, the collective oscillation of conduction electrons, which are modeled through the proper consideration of the dominant features of their electronic band structure, including surface and quantum well states along with the bandgap arising for each crystallographic facet. We report a beneficial use of films decreasing in thickness down to few-atom-thick sizes, influencing both the linear and the nonlinear optical response. Such a degree of tunability makes them unique together with lower losses compared to their amorphous counterparts. These results facilitate the development of atomically-thin nonlinear nanophotonic devices based on well-defined crystalline metal films.

Our studies are focused on high intensity spatiotemporal parametric processes in a microdispersive non-centrosymmetric medium. We solve numerically the system of coupled parabolic equations for second harmonic generation that are supplemented with terms responsible for self- and cross- modulations. Joint influence of quadratic and cubic nonlinearities is studied for the case of anomalous dispersion at both frequencies. Recently it was demonstrated that provided small phase mismatches a stable two-color light bullet may be generated and then it propagates in a medium with pure quadratic nonlinearity. At the same time, pure Kerr nonlinearity is responsible for bullet instability. In our investigations we reveal a threshold condition of the two-color bullet stability in the medium with combined quadraticcubic nonlinearity.

Recently a possibility of the formation of “dancing” light bullets was analytically shown in graded-index waveguides with both Kerr and quadratic nonlinearities. At that, a two-color “dancing” light bullet in a quadratic nonlinear waveguide may form at both anomalous and normal group velocity dispersion at the fundamental frequency. Here and in what follows we refer as “dancing” light (optical) bullets spatiotemporal solitons whose trajectories can be spatial Lissajous figures with anisotropic spatial distribution of the refractive index in the cross section of the waveguide. Stability of a two-color “dancing” light bullet was theoretically demonstrated without additional conditions for its temporal duration, aperture, and power. These results were established with the help of the averaged Lagrangian method in the framework of the quasi-optical approach under the conditions of phase and group matching. The mentioned conditions are known to be hardly achievable in experiment. In the present work we study two- color “dancing” light bullet generation by means of numerical simulation including the cases when only one of the conditions of synchronism is fulfilled. We demonstrate that a pulse at the doubled frequency is generated in the waveguide at second harmonic generation (SHG), followed by the formation of the bound state of both harmonics representing a two-color “dancing” light bullet

The effect of the self-injection locking (SIL) is well-known for many years in the theory of oscillations, radiophysics and optics and is actively used for the stabilization and spectral purification of the corresponding generators. Last years it has attracted even more attention due to the possibility of using such stabilized lasers as pump sources for the realization of the nonlinear processes in the same microresonators, simultaneously used for laser linewidth reduction. However, existing linear theories of the self-injection locking unable to predict soliton generation because enough value of the pump frequency detuning can not be obtained in the linear regime. The development of full nonlinear theory becomes even more important, since recently generation of the solitonic pulses at normal group velocity dispersion has been demonstrated in the self-injection locking regime We developed an original model describing the process of the frequency comb generation in the self-injection locking regime and performed numerical simulation of this process. Generation of the dissipative Kerr solitons in the self-injection locking regime at anomalous group velocity dispersion was studied numerically. Different regimes of the soliton excitation depending on the locking phase, backscattering parameter and pump power were identified. It was also proposed and confirmed numerically that self-injection locking may provide an easy way for the generation of the frequency combs at normal group velocity dispersion. Generation of platicons was demonstrated and studied in detail. Parameter range providing platicon excitation was found.

Bound states in the continuum (BICs) represent dark modes trapped in the radiation continuum. BICs received significant attention in optics and photonics as a simple tool to achieve giant quality factors by transforming them into quasi-BICs. Here, we report the observation of high-harmonic generation in dielectric metasurfaces hosting BICs. The metasurface is composed of a square lattice with parallel Si bars of a slightly different width placed on a transparent substrate. The structure is engineered to support a quasi-BIC in the mid-IR with a high quality factor. We tune the metasurface asymmetry to enable the optimal coupling condition that provide the highest high-harmonic generation efficiency. In the experiment, we demonstrate the generation of odd optical harmonics from the 3rd to the 11th order in the BIC regime and study their polarization dependence. We measure the dependence of the high-harmonic signal on the input intensity. The concept of metasurfaces with highly localized light boosted by BIC resonances provides a new degree of freedom to control experimentally strong nonlinear optical response.

Bose-Einstein condensates of exciton-polaritons in inorganic semiconductor microcavities are known to possess strong interparticle interactions attributed to their excitonic component. The interactions play a crucial role in the nonlinear dynamics of such systems and can be witnessed as the high energy shift of polariton states. However, the localised nature of Frenkel excitons in strongly coupled organic microcavities precludes interparticle Coulomb exchange-interactions that affect nonlinear dynamics and change mechanisms of polariton energy shifts accordingly. We have scrutinized the origins of energy shifts in connection with nonlinear dynamics in Frenkel exciton-polariton condensates and examined the possible contributions: intracavity optical Kerr-effect, gain-induced frequency pulling, polariton interactions and effects related to saturation of optical transitions for "dark"- and "bright" molecules [1]. Unlike the conventional strongly coupled semiconductor microcavities, we have shown that nonlinear interactions within the condensate do not rely on polariton interactions but instead originated from Pauli-blocking principle forbidding double excitation of the same molecular states. We have developed a theoretical model explaining the omnipresent energy shift of the condensate wavefunction together with its spectral and polarization features at the BEC transition in a consistent way. The crucial role of intermolecular energy transfer and “dark” exciton reservoir has been demonstrated for the first time. We believe the principles explored in this work are relevant to other systems exhibiting strong coupling of Frenkel excitons with a cavity mode, regardless of a cavity type, whether one dealing with Fabry-Perot cavities or plasmonic nanocavities etc and; therefore, provide general insight on nonlinear phenomena in composite light-matter condensates. [1] T. Yagafarov, D. Sannikov, A. Zasedatelev, K. Georgiou, A. Baranikov, O. Kyriienko, I. Shelykh, L. Gai, Z. Shen, D. G. Lidzey, P. Lagoudakis, Mechanisms of blueshifts in organic polariton condensates, Commun Phys 3, 18 (2020). https://doi.org/10.1038/s42005-019-0278-6

Spectral analysis of high-order harmonics generated by ultrashort mid-infrared pulses in molecular nitrogen reveals well-resolved signatures of inverse Raman scattering, showing up near the frequencies of prominent vibrational transitions of nitrogen molecules. When tuned on a resonance with the vʹ = 0 → vʺ = 0 pathway within the B3Πg → C3Πu second positive system of molecular nitrogen, the eleventh harmonic of a 3.9-μm sub-80-fs driver is shown to acquire a distinctive antisymmetric spectral profile with red-shifted bright and blue-shifted dark features as indicators of stimulated Raman gain and loss. This high-harmonic setting extends the inverse Raman effect to a vast class of strong-field light–matter interaction scenarios.

With the wavelength of a short-pulse laser driver shifted to the midinfrared range, the low-frequency output of laser-induced plasmas can be drastically enhanced, as our experiments show, providing a source of ultrabroadband radiation with a spectrum spanning across the entire terahertz, millimeter-wave, and microwave bands. At low gas pressures, such ultrabroadband field waveforms are shown to rapidly build up their coherence, developing a well-resolved emission cone, dominated by a radial radiation energy flux. As counterintuitive as it may seem, this behavior of the intensity, coherence, and polarization of the low-frequency plasma output is shown to be consistent with a physical scenario of Cherenkov-type radiation emission by ponderomotively driven plasma currents.

We investigate the photomixing Kelly sidebands of soliton mode-locking fiber laser to generate the Terahertz (THz) signal. A simulation model about soliton fiber laser with obvious ±1 order Kelly sidebands is built up to design the sidebands with strong intensity, narrow bandwidth and adjustable spacing. It is found that the parameters of mode-locking fiber laser such as the total dispersion, filtering bandwidths, pump power of the laser play significant roles in determining the features of the ±1 order Kelly sidebands and the relationships between them are investigated. By photomixing the ±1 order sidebands, the experimental result shows that the desired THz signals with high optical-to-THz conversion are generated at frequencies of 0.187THz from the lab-made Yb-doped mode-locking fiber lasers. Well-designed Er-doped ring-cavity soliton fiber laser is used for obtaining THz signal of narrow linewidth with 1.35THz frequency tunable range.

In this report, FBG inscription by the point-by-point method using a femtosecond laser is described. The results concerning the dependence of the spectral characteristics of the FBG on the power of the inscribing radiation are demonstrated. A new approach to FBG inscription is reported, based on the modernization of the point-by-point method, which makes it possible to control FBG inscription with high accuracy. The dynamics of inscription by a similar method is presented.

Possibility of femtosecond laser pulses to affect the materials properties arises the interest in ultrafast processes based research and technology. In the case of graphene surface modification and functionalization using femtosecond laser, there are several effects appear, such as ablation, covalent bonding of different chemical groups, re-crystallization in three-dimensional shapes. CVD grown graphene was transferred on Si/SiO₂. Through several lithography steps, graphene-based field-effect transistors were formed with Cr/Au source-drain electrodes and Si back gate electrode. For graphene modification we used 100 fs 80 MHz laser with 780 nm wavelength with different irradiation doses. Exposure of graphene to a femtosecond laser pulse is determined by the prevalence of physical or chemical effects during exposure to a laser pulse. The range of laser exposure was narrowed down to values causing the formation of atomic defects in the carbon lattice, which makes it possible to form nanopores in graphene and these doses are below the graphene ablation. The main tool for studying the effect of femtosecond laser irradiation was Raman spectroscopy. By evaluating the intensity ratio of certain peaks, namely the G-band (~1600 cm^-1) and D-band (~ 1350 cm^-1), the degree of functionalization, or amorphization of graphene, was estimated. It was found that the ablation threshold starts from 18 mW at the beam speed in the range of 400-500 μm/s. Just below this range, both graphene functionalization and a change in the graphene surface roughness were observed. Despite the change in the morphology of graphene, the graphene resistance fell by only ~4 times, and the transfer current-voltage curves of the graphene transistor did not change much, showing a shift towards higher voltages. With a decrease in the slope of the transfer current-voltage characteristics, the resistance of the structure also decreases with an increase in the dose of laser exposure, since the number of defects and functional groups in graphene increases. In addition, we found the effect of the laser polarization on the modification of graphene. The difference in parameters between the samples modified with different polarization directions along the direction of the beam motion can be explained as the interference interaction of the electron density in graphene. A beam passing over the graphene region excites hot electrons, which partially cause the graphene modification. After passing by the laser, the electron density does not have time to relax, and the next beam of photons affects the already excited electrons, increasing the total dose of laser radiation.

Nonlinear and electro-optic devices are present in our daily life with many applications: light sources for microsurgery, green laser pointers, or modulators for telecommunication. They mainly use bulk materials such as glass fibers or high-quality crystals, hardly integrable or scalable due to low signal and difficult fabrication. Here I will show several strategies to enhance optical signals by engineering metal-oxides. First, I will explain some fundamental aspects of quadratic materials. Then, I will show several photonic systems relying on bottom-up assemblies of barium titanate nanoparticles either to obtain electro-optic metasurfaces or broadband Mie driven microspheres.

This work presents an approach for continuous-wave laser emission in the far-UVC using second-harmonic generation in barium borate (BBO) thin-film waveguides. The crystal orientation is analytically considered for waveguides, and two designs with perfect modal phase matching (PhM) are simulated. The first design includes a BBO ridge-waveguide defined by dry etching. This allows for good confinement and spatial overlap of the relevant optical modes. The second design includes a strip-loaded waveguide, which aims at suppressing the side-wall scattering by confining the optical modes in the BBO thin-film. Showing that the conversion efficiency can be greatly enhanced.

Self-injection locking is an effective tool of laser stabilization, known from the origins of radiophysics. Recently it was shown to be useful for perspective compact sources of optical frequency combs. This, however, implies the nonlinearity and high power inside the microresonator. Recently, we showed, that the account for the nonlinearity in the self-injection locking model substantially changes the system behaviour. However, thermal effects, inevitably arising from the high intracavity power, have not been considered yet. This point is also of great importance as these effects are known to be a serious obstacle for stable dissipative Kerr soliton generation. In this work we develop further the self-injection locking theory and show that thermal nonlinearity also introduce novel and important features. We analyse in detail several possible regimes arising due to thermal and Kerr nonlinearity competition and analyse tuning curves at different signs of thermorefractive coefficient. We confirm our predictions that the locking should help to overcome the temperature drift problem and show that nonlinear frequency shift helps to reach the desired detuning for the soliton comb state inside the locking band.

In this paper, we have studied ML features in a sub-200-fs erbium-doped all-fiber laser based on a saturable absorber obtained by high-pressure-high-temperature treatment of commercially available single-walled carbon nanotubes. We have shown that there is a significant effect on the ultrafast optical properties of saturable absorber due to the high-density well-aligned structure of newly-developed single-walled carbon nanotubes and related it to the mode-locked laser performance. We have obtained a low-intensity-noise ultra-short stretched pulse generation with a repetition rate of 42.22 MHz, a spectral pulse width of 30 nm, and average output power of 11.3 mW with long-term stability of ~ 0.06% during 3 h measurement time

The use of semiconductors, metals and conductive oxides in the process of fabrication of actual nano devices is at the front edge of nowadays technology, exploiting the properties of light propagation and localization at nanometric scale in new and surprising ways. At these scales the usual theory describing the nonlinear (NL) effects of electromagnetic fields should be revisited and analyzed. We report a collection of experimental results of nonlinear harmonic generation in different nanolayers: semiconductors, conductive oxides and metals. The comparison of these experimental results with numerical predictions of our theoretical model identifies, distinguishes and explains the different nonlinear contributions to the harmonics generated by these materials at nanoscale. Our model accounts for surface, magnetic and bulk nonlinearities arising from free and bound charges, preserving linear and nonlinear dispersion, nonlocal effects due to pressure and viscosity.

Currently, nanostructures are routinely fabricated and integrated in different photonic devices for a variety of purposes and applications. For instance, in order to engineer properly nano-antennas or filters, it is important to understand accurately how light interacts with metals, semiconductors, or ordinary dielectrics at the nanoscale. When the nanoscale is reached, light-matter interactions displays new phenomena and conventional approximations may not always be applicable. Thus, new strategies must be sought in order to study and understand light-matter interactions at the nanoscale. In this work, we present experimental results of second and third harmonic generation from gold nanolayers, raveling novel behavior at nanoscale. These measurements are compared with numerical simulations based on a microscopic hydrodynamic model which accounts for surface, magnetic and bulk nonlinearities arising from both free and bound charges, preserving linear and nonlinear dispersion, nonlocal effects due to pressure and viscosity, and an intensity dependent free electron density, to which we refer as hot electrons contribution.

We report efficient second-harmonic emission by single multilayer metal–dielectric nanocavities. Engineering the intrinsic interface-induced symmetry breaking by resonant optical absorption design, allows to achieve almost two orders of magnitude higher second-harmonic generation efficiency compared to gold nanostructures with the same geometry. We estimate a second-order nonlinear susceptibility of the order of 1 pm/V, which is comparable to widely used nonlinear crystals. We envision that our system, which combines the advantages of both plasmonic and dielectric materials, might enable the realization of composite nano-systems for an efficient multi-purpose manipulation of nonlinear optical processes at the nanoscale.

In this work, the second harmonic generation was investigated in colloidal solution of CdSe nanoplatelets with two and three monolayers of CdS shell in the case of the two-photon laser excitation of the light-hole – electron transition (1lh-e). In addition to the two-photon photoluminescence spectrum a narrow spectral peak was observed at the wavelength of the laser’s second harmonic radiation, which intensity increases quadratically with incident laser intensity growth taking into account linear absorption of colloidal solution. Also the cause of the second harmonic generation in CdSe/CdS nanoplatelets was considered and explained. It was suggested that the second harmonic in CdSe/CdS colloidal solution is mostly generated by surface atoms for the part of the nanoplatelets satisfying to phase matching condition of the fundamental and second harmonic waves. The concentration of CdSe/CdS nanoplatelets in colloidal solution at which the second harmonic generation can be observed was determined.

Semiconductor nanocrystals have been actively studied due to their unique physical and chemical properties and are actively being implemented in nanophotonics devices. Nanostructures created by the colloidal synthesis with design shape, size and crystal structure are widely used. Recently, colloidal semiconductor quantum wells (nanoplatelets) have been created. Colloidal semiconductor nanoplatelets (NPLs) are atomically flat nanocrystals which demonstrate a zinc blend crystal structure with a [001] axis. Strong quantum confinement of NPLs and high exciton binding energy are provided by anisotropic of nanocrystals with several nanometers thick and tens of nanometers in lateral dimensions which can be used to tune the optical absorption and photoluminescence spectra. In this paper we present the peculiarities of excitation, interaction and relaxation of excitons in colloidal CdSe nanoplates depend on type and thickness of shell in the case of one-photon exciton excitation by laser pulses (λ=540 nm, τ=10 ns). The linear and nonlinear absorption spectra of colloidal CdSe NPLs were studied. The linear absorption spectrum of the NPLs demonstrate three well-resolved absorption bands that correspond to heavy hole, light hole and spin-orbital exciton transitions at room temperatures due to the almost complete absence size dispersion of nanocrystals. The differential transmission spectra allowed us to reveal experimentally the lowest four band structure of CdSe/CdS nanoplatelets at the Γ point of Brillouin zone and its modification with CdS shell thickness changing for the first time. In addition, the features of exciton-exciton interaction, exciton-phonon interaction, as well as the process of energy transfer between light and heavy excitons to exciton relaxation were investigated. The rate equations describing the exciton-exciton and exciton-electron interactions was applied for analyzing the recombination and interaction of excitons in the colloidal NPLs under high excitation densities. This work was partially supported by the Russian Foundation for Basic Research №20-32-70001

Strongly coupled organic microcavities are up-and-coming material systems for ambient polaritonics. A broad range of suitable materials made possible the experimental observation of polariton lasing across the whole visible range, as well as device-concepts ranging from ultra-fast transistors and all-optical logic gates to single-photon switching, all at room temperature under ambient conditions. Unlike the case of inorganic semiconductor microcavities, where continuous-wave excitation allows for the replenishment of particle losses, leading to the realization of steady-state polariton condensates, in organic semiconductors photobleaching and polaron formation prevent CW operation. BODIPY dyes have been the subject of thorough studies for their applications in the strong coupling regime. Strongly coupled BODIPY microcavities and polariton lasing in these structures allow for highly monochromatic tunable coherent emission of duration up to ~two picoseconds. Here, we use a single-mode lambda/2 strongly coupled microcavity of a BODIPY dye molecule, employ a single-shot dispersion imaging technique to study polariton lasing in a planar organic microcavity, and achieve a quasi-steady state exciton-polariton condensation under single-shot excitation in such systems. Temporal dynamics of a single-shot exciton-polariton lasing is of particular interest and importance for understanding rates of depletion and replenishment of the exciton reservoir and polariton states, respectively, under pulsed excitation. Moreover, the direct measurement of the condensate lifetime provides valuable insight into the transient processes of nonequilibrium polariton condensation. Long-lasting condensates exceeding polariton lifetime for several orders of magnitude push the system one step closer towards the regime of dynamic equilibrium and could be a missing puzzle towards polariton applications such as connected polariton devices and condensate lattices implemented at ambient conditions, opening the possibility for all-optical polariton circuitry on a chip.

The need for faster and energy-efficient computing technologies has recently pushed for major developments on alternative computing paradigms to the common von Neumann architecture. Amongst those, reservoir computing framework is an emerging concept that leverages a simple training process and eases transference to hardware implementations, allowing any given nonlinear physical system to act as a computing platform. In this work, we explore how we can make use of a discrete chain of solitons to create an effective reservoir computing framework, investigating not only the ability to learn data but also to predict models depending on the strength of the nonlinear interaction of the media. Probing the role of the nonlinear separation for tasks involving nonlinear separable data, these results open new possibilities for a multitude of physical implementations in the context of optical sciences, from optical fibers to nonlinear crystals.

Benefitting from recent innovations in the smartphone sector, liquid optics in very compact designs have been cost-effectively introduced to the market. Without mechanical actuation, a focus variation can be adjusted within fractions of a second by curving a boundary layer between two liquids by applying a pulse width or amplitude modulated potential. Especially in the field of endoscopy, these innovative optical components open up many application possibilities. Conventional, mechanical zoom lenses are not very common in endoscopy and can only be miniaturized at considerable effort due to the necessary actuation and the complex design. In addition, the mechanical response is slow, which is a particular disadvantage in hand-held operation. A calibrated camera provides a two-dimensional camera pixel translated into a three-dimensional beam and, together with distortion correction enables the extraction of metric information. This approach is widely used in endoscopy, for example, to measure objects in the scene or to estimate the camera position and derive a trajectory accordingly. This is particularly important for triangulation-based 3D reconstruction such as photogrammetry. The use of liquid lenses requires a new data set with an adapted camera calibration for each focus adjustment. In practice, this is not feasible and would result in an extensive calibration effort. This paper therefore examines, on the basis of an experimental setup for automated endoscopic camera calibration, the extent to which certain calibration parameters can be modelled and approximated for each possible focal adjustment, and also investigates the influence of a liquid lens on the quality of the actual calibration.

Despite the ubiquity of Raman spectroscopy, fluorescence, poor signal strength and photobleaching pose a significant challenge to researchers in the biomedical field. Here, we demonstrate a 17-fold signal enhancement in Raman spectra of crystal violet via surface-enhanced Raman scattering (SERS). The SERS substrate was fabricated by electron beam lithography (EBL); the nanostructured surface was an array of G-shaped elements made of Au on SiO2/Si. In addition to the SERS spectra, finite-difference time-domain simulations were performed to illustrate the distribution of electric-field hot-spots on the SERS substrate. The electric-field hot-spots were prominent at the vertices and edges of the nanostructured G-shaped motifs. The results presented here demonstrate that EBL is a high-end choice for SERS substrate fabrication that opens the way for more complex Raman spectroscopies, for instance involving nonlinear optics or chiral analytes.

The nonlinear Fourier transform (NFT) based signal processing has attracted considerable attention as a promising tool for fibre nonlinearity mitigation in optical transmission. However, the mathematical complexity of NFT algorithms and the noticeable distinction of the latter from the “conventional” (Fourier-based) methods make it difficult to adapt this approach for practical applications. In our work, we demonstrate a hardware implementation of the fast direct NFT operation: it is used to map the optical signal onto its nonlinear Fourier spectrum, i.e. to demodulate the data. The main component of the algorithm is the matrix-multiplier unit, implemented on field-programmable gate arrays (FPGA) and used in our study for the estimation of required hardware resources. To design the best performing implementation in limited resources, we carry out the processing accuracy analysis to estimate the optimal bit width. The fast NFT algorithm that we analyse, is based on the FFT, which leads to the O(N log²₂ N) method’s complexity for the signal consisting of N samples. Our analysis revealed the significant demand in DSP blocks on the used board, which is caused by the complex-valued matrix operations and FFTs. Nevertheless, it seems to be possible to utilise further the parallelisation of our NFT-processing implementation for the more efficient NFT hardware realisation.

In this study, we report on experiments of spatio-temporal nonlinear frequency conversion in a periodically poled Lithium Niobate (PPLN) crystal designed for second-harmonic generation (SHG). We demonstrated a novel supercontinuum source based on the mixing of second and third-order nonlinearities. We could adjust the (X(2), X(3)) nonlinearities by controlling the input laser polarization orientation, the pulse duration and the PPLN crystal temperature. We obtained an ultra-broadband spectrum, ranging from visible to infrared domains, by pumping a 20-mm-long PPLN crystal with a 3 ps pulse at 1030 nm. This broadband pulse was used to achieve direct multiplex Coherent Anti-Stokes Raman Scattering (M-CARS) imaging, without the need for any optical delay line to temporally synchronize the pump and the Stokes waves. Simultaneous vibrational signatures ranging from -3200 cm-1 to -500 cm-1 were obtained. Several filters were placed on the broadband supercontinuum path, to shape the output spectrum between 1030 nm and 1650 nm, before sending it into the microscope. The output spectral analysis allows for the demonstration of multimodal imaging, by using SHG, M-CARS and multiphoton fluorescence processes.

The Theory of General Relativity is currently the most accepted model to describe gravity, and although many experiments and observations continue to validate it, recent astrophysical and cosmological observations require to include new forms of matter and energy (dark matter and dark energy), to be consistent. Modified theories of gravity with non-minimal coupling between curvature and matter are extensions of the Theory of General Relativity and have been proposed to address these shortcomings. Interestingly, matter at large scales behaves as a fluid and under certain approximations, the field equations can be approximated to a generalized Schr¨odingerNewton system of equations. This model is largely found in the nonlinear optical systems, in particular to describe light propagating in nonlinear and nonlocal optical materials and also as a base model for the development of many optical analogues. Due to this, there are a wide variety of numerical methods developed to tackle this type of mathematical models, and that can be used to study these alternative gravity models. In this work, we explore the application of these numerical techniques based on GPGPU supercomputing, initially developed to study light propagating in nonlinear optical systems, to explore a particular non-minimal coupled gravity model. This model, in the nonrelativistic limit, modifies the hydrodynamic equations with the introduction of an attractive Yukawa potential and a repulsive one proportional to the matter density. We used the Schr¨odinger-Newton formalism to numerically study this model and, through the imaginary-time propagation method, we found stationary solutions that were sustained by the repulsive potential introduced by the non-minimal coupled model, even in the absence of a pressure term. We developed an analytical study in the Thomas-Fermi approximation and compared the predictions with numerical solutions. Finally, we explored how this gravity model may be emulated in the laboratory as an optical analogue.

One of the most widespread and reliable techniques for spectral phase characterization of optical elements is spectrally resolved interferometry (SRI), which utilizes a two-arm interferometer illuminated by a broadband light source and a spectrometer to record the generated spectral fringes, which are usually evaluated by using Fourier-transform-based methods. One detriment to these methods, that they require high spectral resolution. Considering that most of the spectrometers in the mid-infrared (MIR) region have typically a few nm spectral resolution, we propose two alternative methods for measurements in this spectral region, second harmonic assisted SRI (SH-SRI) and dual-band SRI. The SH-SRI method utilizes second harmonic generation, which allows for high-precision phase retrieval by shifting the detection range from the MIR regime to the near-infrared, where high-resolution spectrometers are commercially available. The dual-band SRI method has the capabilities of the SH-SRI, while simultaneously probing the optical element at the second harmonic frequency band as well. Using the Fourier transform method, the acquired phase can be obtained from a single interferogram at both frequency bands simultaneously, which effectively doubles the measurement bandwidth compared to the original SRI and the SH-SRI without increasing the requirements on the bandwidth and the resolution of the spectrometer. To determine the performance of these new schemes, we have measured the spectral phase shift of several optical windows with both SH-SRI and dual-band SRI. Their accuracy was significantly higher than that of the conventional SRI technique that relied on low-resolution spectrometers operating in the MIR region. We used the SH-SRI technique also to verify the phase performance of custom-made MIR dispersive mirrors, designed with specific dispersion characteristics. With the dispersive mirrors, we were able to compress our spectrally broadened pulses to 19.6 fs (<2 cycles) at 3170 nm central wavelength with 8.2 W average power.

We demonstrate a random distributed feedback Raman fiber laser that operates in a self-gain-switched regime with pulse operation. We find that the repetition rate of the output pulse train depends on pump power, and introduce a simple formula for oscillation frequencies.

In this paper, the ultraviolet output of frequency-doubled OPO laser at 266 nm was employed to obtain highly-resolved (2+1) resonance-enhanced multiphoton ionization (REMPI) and mass spectra of supersonically-expanded and jet-cooled molecular pulses of iodomethane. The molecular sample consisted of a 3% neat iodomethane gas dissolved in helium. The REMPI and mass spectra of iodomethane were obtained using a two meter time-of-flight (TOF) mass spectrometer coupled with a microchannel plate detector that was kept at 4K volts. The laser power density effect on the (2+1) REMPI process was also investigated by focusing and defocusing the laser pulses using 30 cm and 75 cm biconvex lenses, respectively. By monitoring I⁺ and CH³⁺ fragments, it was evident that 3 photons were involved in the REMPI process.

Linewidth narrowing and stabilization of semiconductor laser light generation is of great research interest governed by the huge demand of compact cost-effective narrow-band laser sources for many potential applications. In 2012 we have demonstrated a simple kHz-linewidth laser just splicing a standard distributed feedback (DFB) laser diode and a few passive telecommunication components. The principle of operation employs the mechanism of self-injection locking that significantly improves DFB laser performance. While a typical linewidth of free-running DFB semiconductor lasers ranges from a few to tens MHz, self-injection locking of a DFB laser through an external fiber ring cavity causes a drastic reduction of its laser linewidth down to a few kHz. The advantage of the proposed configuration is that the same external fiber ring cavity could be used for self-injection locking of a DFB laser and as Brillouin scattering media to generate Stokes shifted optical wave. However, a continuous laser operation at two frequencies has not been reported yet preventing it from many prosperous photonic applications. Here, we introduce a simple dual-frequency laser configuration. In our approach, the implementation of self-injection locking into the Brillouin ring fiber laser helps to maintain coupling between the DFB laser and an external high-Q fiber cavity enabling dual-frequency laser operation. Specifically, the same ring fiber cavity is used to generate narrow-band light at the pump frequency (through self-injection locking mechanism) and narrow-band laser light at Stokes frequency (through stimulated Brillouin scattering). The system is supplied by a low-bandwidth active optoelectronic feedback circuit controlled by a low-cost USB-DAQ card that helps the laser to maintain the desired operation mode. The fiber configuration reduces the natural Lorentzian linewidth of light emitted by the laser at pump and Stokes frequencies down to 270 Hz and 220 Hz, respectively, and features a stable 300-Hz-width RF spectrum recorded with the beating of two laser outputs. We have explored key features of the laser performance, revealing its stability and applicability to RF harmonic generation of high spectral purity as an additional benefit of the proposed technique.

Surface plasmon–polaritons (SPPs), propagating bound oscillations of electrons and light at a metal surface, exhibit the capacity for subwavelength field confinement and enhancement that make them a promising photonic platform at the nanometer scale. In this work we report the results of theoretical investigations of SPP propagation in metal film with varying thickness lying on a dielectric substrate. It is shown that the SPP propagation can be described by an equation of the type of the nonlinear Schrödinger equation. The main parameters of the nonlinear Schrödinger equation for the SPP − Kerr nonlinearity and dispersion coefficients− are derived from the basic SPP dispersion relation. The dependences of these coefficients on the film thickness are obtained. It is shown that the high-frequency SPP dispersive branch satisfies the conditions of the development of modulation instability. According to estimates, the magnitude of the modulation gain is sufficient to generate ultrashort pulse train from the initial modulated SPP wave on the propagation length of about 100 nm. The peculiar features of modulated SPP propagation in the metal film with varying thickness are studied. Analogies between the light wave propagation in a nonlinear dispersion decreasing fiber and SPP propagation in the metal film with increasing thickness are shown. Numerical simulations performed confirm the theoretically predicted generation of ultrashort pulse trains with THz an sub-THz repetition rates from an initially modulated SPP wave in a film with a longitudinally increasing thickness on propagation length of about 100-200 nm.

Low-noise lasers are a powerful tool in precision spectroscopy, displacement measurements, and the development of advanced optical atomic clocks. All applications benefit from lower frequency noise and robust design, however, the generation of microwave signals additionally requires narrowband lasing at two frequencies. Here, we introduce a simple optoelectronic oscillator enabling the generation of a stable ultra-narrow microwave carrier signal with low phase noise based on stimulated Brillouin scattering. A cost-effective sub-kilohertz Brillouin fiber ring laser with stabilized selfinjection locked pump DFB (Distributed Feedback Laser) laser is used for this purpose. The system is supplied by a low-bandwidth active optoelectronic feedback controlled by a low-cost USB-DAQ card. The full-width of generated microwave signal at -3 dB level is approximately equal to 300 Hz with a peak maximum at ~10.946 GHz. The strongest parasitic harmonics shifted from carrier signal peak by ±50 kHz, ±450 kHz, and ±900 kHz are below the main peak by 45-50 dB. A phase noise below −90 dBc/Hz for a frequency offset above 10 kHz from the carrier after passing the 20 km length test fiber has been achieved.

Soliton fiber lasers with passive harmonic mode-locking (HML) have been known for several decades as reliable sources of pulse trains with a high repetition rate. They are commonly employed as frequency comb generators in a wide list of applications in spectroscopy, telecommunications, microwave photonics. Besides that exhibiting advantageous consumer properties, such as compactness, reliability, low cost and convenience of beam delivery approach these sources are among the most attractive alternatives for material processing, medicine lasers and other areas. In the first group of these lasers the HML occurs due to special intra-cavity periodic filter with a free spectral range (FSR) which is a multiple of the main cavity FSR and equals to the pulse repetition rate. The second group of HML lasers in this classification is attractive for the scientific community due to the automatic arrangement of strongly periodic pulse pattern in the laser cavity through pulse repulsion. However, it is difficult to specify the mechanism of pulse repulsion for each case. It can be based on interaction through saturating and relaxing dissipative parameters, continuous-wave (CW) radiation component, dispersion waves, or acoustic waves induced by electrostriction, etc. A common feature of all interaction induced effects is low intensity, in many cases only slightly exceeding the noise level. The noise-induced fluctuations in the time interval of the HML pulse train are known as HML timing jitter, and its value is significantly higher than the timing jitter in lasers operating at fundamental frequency. It is a major drawback of the HML laser technology. We report the stabilizing frequency shift effect in harmonic mode-locking ring soliton fiber laser that is studied theoretically and numerically. We demonstrate that the frequency shift contributes to an increase in the hardness of interpulse interactions and can led to stabilization of the periodic arrangement of pulses. The experiment carried out confirms the theoretical predictions and the results of numerical simulation.

Ionization of polarized states of Ar and Xe atoms by femtosecond probe pulse in a supersonic beam in the presence of magnetic field was studied theoretically and experimentally. The revealed oscillation structure in the photoionization signals occurs due to the Larmor precession of exited atomic states in a magnetic field. We derived analytical formulas for the photoelectron current and explained the detected oscillations in terms of photon and atomic polarization moments. Our results indicate the possibility of implementing Doppler-free spectroscopy involving bound-free transitions.

2D organic–inorganic hybrid perovskites are promising materials for next-generation optoelectronic devices. However, 2D perovskites still suffer from performance-limiting trap states and possess fundamental photophysical effects that remain poorly understood. Herein, a photoinduced trap passivation for enhanced photoluminescence (PL) in 2D hybrid perovskites is reported. Under two-photon excitation, the PL emission of the 2D hybrid perovskites exhibits a gradual increase in intensity and lifetime, due to the photoinduced trap passivation by interacting with oxygen in ambient environment. Interestingly, the PL increase shows distinct features at the surface and in the bulk of the 2D perovskites, which can be well revealed by a photoinduced oxygen-diffused trap-passivation model. Moreover, the PL increase and relaxation can recycle for several times, which suggests an excellent repeatability of the photoinduced trap passivation. The photoinduced trap passivation is critical for fundamental study of light–matter interaction in 2D perovskites, and shows great promise for improving the optoelectronic responses for functional devices.

Bound states in the continuum are non-radiating solutions of the wave equation which are completely decoupled from the radiation continuum and therefore it is impossible to excite them from the far-field in linear systems. In this work, we develop a general theory of parametric excitation of BICs in nonlinear systems via spontaneous symmetry breaking, which results in a coupling between BIC and bright modes of the system. Using the temporal coupled-mode theory and perturbation analysis, we found the threshold intensity for excitation of BIC and study the possible stable solutions depending on the pump intensity and frequency detuning between the pump and BIC. We revealed that at some parameters of the pump beam, both BIC and the bright mode can have non-zero amplitudes forming a stable hybrid state. Also, at some parameters, there are no stable solutions and BIC can be used for frequency comb generation. Our findings can be very promising for use in nonlinear photonic devices and all-optical networks.

We propose in this paper to come back to the starting conditions of optoelectronic oscillators and in particular with regard to the Barkhausen conditions. The context is first of all the anniversary of the hundred years since the discovery of We propose in this paper to come back to the starting conditions of optoelectronic oscillators and in particular with regard to the Barkhausen conditions. The context is first of all the anniversary of the hundred years since the discovery of oscillations conditions and their optimization by Heinrich Barkhausen, whose principles we recall [1]. We then give through the study of an example, how these conditions are relevant to optimize the start of the oscillation. The type of optoelectronic oscillator chosen for this study is an optoelectronic oscillator based on optical delay lines. The optical signal carrier wavelength is 1.55 μm, the typical oscillation frequency is 10 GHz [2 – 5]. It is all the more interesting to approach this subject in the context of the centenary of this discovery, which we can see how the advances made possible by H. Barkhausen are essential and still relevant today

Through this paper, we strive to describe the method that allows us to give an estimate of the uncertainty. This paper aims to give an estimation of the uncertainty on the frequency peak by Brillouin Scattering Stimulation. It corresponds to the speed of phonons inside a material excited by a 532 nm wavelength laser. The guideline follows the Guide to the Expression of Uncertainty in Measurement and first estimation indicates speed of the phonons at 5% at 2 sigma.

In the present work, we show the results of theoretical calculations of nonlinear optical properties of choline chloride – urea deep eutectic solvent mixture. Deep eutectic solvents are a novel kind of solvents with similar properties to classic ionic liquids but with advantages like been easy to prepare and biodegradable. We had obtained the most stable structure using DFT optimization with M06-2X/6-311G(d,p). Then their static and dynamic (at 477 nm) first and second order hyperpolarizabilities were obtained. These results were used to estimate the third order nonlinear susceptibility showing that this solvent is promising for nonlinear optics-related application.

The last years saw the emergence of nonlinear optical materials, with local and nonlocal nonlinearities, as experimentally accessible systems to implement optical analogues of quantum fluids. In these systems, a light beam propagating in the nonlinear medium can be interpreted as a fluid, where the diffraction in the transverse plane to the propagation gives the effective mass of the fluid and the medium nonlinearity mediates the required interactions between the photons. This fluid interpretation and its application have been extensively studied, from the creation of superfluid-like flows and the study of phenomena associated with this effect to the implementation of gravity analogues. Furthermore, many optical materials have been considered, with a special interest in the ones that offer tunable mechanisms that allow to easily control the system properties to better explore and emulate the different phenomena. Recently, nematic liquid crystals have been proposed as an interesting tunable material capable of supporting superfluids of light. These systems have a nonlocal character and offer external mechanisms that can be used to tailor the nonlinearity to better emulate the desired analogue system. Indeed, through the application of an external electric field perpendicular to the direction of propagation, is it possible to control the nonlocal length of the nonlinearity. This mechanism offers interesting opportunities in the present context. In this work, through numerical methods based on GPGPU supercomputing, we explore the possibility of observing superfluid effects in defocusing nematic liquid crystals. In particular, we explore the possibility of observing the drag force cancelation and the emission of quantized vortices, which are two manifestations of a superfluid flow. Furthermore, we also discuss the possibility of using these systems for creating an analogue of quantum turbulence with these materials. These studies constitute a stepping-stone towards the implementation of gravity analogues with nematic liquid crystals.

In this work we use the concept of paraxial fluids of light to explore quantum turbulence, probing a turbulent regime induced on an optical beam propagating inside a defocusing nonlinear media. For that purpose, we establish a physical analogue of a two-component quantum fluid by making use of orthogonal polarizations and incoherent beam interaction, obtaining a system for which the perturbative excitations follow a modified Bogoliubov-like dispersion relation. This dispersion relation features regions of instability that define an effective range of energy injection and that are easily tuned by manipulating the relative angle of incidence between the two components. Our numerical results support the predictions and show evidence of direct and inverse turbulent cascades expected from weak wave turbulence theories, which may inspire new ways to explore to quantum turbulence with optical analogues.

In this study, J-type dimeric copper phthalocyanine in dimethylformamide (DMF) was investigated to show strong nonlinear absorption due to the reverse saturable absorption (RSA) under single nanosecond pulses (16 ns at a wavelength of 532 nm), which significantly exceeds the lifetimes of excited states. In the case of a femtosecond laser, the effect of spatial self-phase modulation was observed, with no contribution from nonlinear absorption being found. The only linear absorption was detected under pulses of 140 fs at a wavelength of 790 nm. The femtosecond pulse repetition rate was 80 MHz which corresponds to a delay between single pulses of about 12.5 ns. Both analytical wavelengths used were outside the intense absorption region (Q- and B- bands) of J-type dimeric copper phthalocyanine in the UV/Vis/NIR spectrum. In the case of femtosecond laser radiation, a sufficient population of excited states was not achieved due to the low peak fluence (~ 0.03 mJ/cm²). For the single nanosecond pulses, the threshold fluence density was ca. 30 mJ/cm². In the switch circuit, such material should be placed horizontally to exclude asymmetric thermal convection resulting from the gravity effect.

We demonstrated a new approach to measuring third-order nonlinear optical effects using an experimental setup incorporating both Z-scan and Mach-Zehnder interferometer methods. This method could be especially purposive for thermo-optical effect studies as it can simultaneously probe thermal gradient profile as well as absolute temperature changes induced by an optical beam. The experimental setup was tested using chloroform. Experimental measurements were carried out using 1064 nm Nd:YAG laser with 8 ns pulse width and 40 kHz pulse repetition rate. The measured nonlinear refractive index of chloroform was mainly induced by the thermo-optical effect. As thermo-optical response depends on beam size at focal length, measurements were carried out with 35 mm and 125 mm focal length lenses. Measured nonlinear refractive index values of Z-scan and interferometric measurements gave the same value indicating that this method can be used for thermo-optical effect studies.