In this conference, we will discuss the advancements in Terahertz (THz) spectroscopy and imaging regarding biophotonics applications. In recent years, THz technology helped various fields, from fundamental physics to industrial process control. THz spectrometers offer a wide spectrum range and high dynamic range, enabling the analysis of semi-transparent materials and living tissues. THz imaging has shown potential in applications such as cancer detection, diabetes foot syndrome, burn wound analysis, and plant hydration assessment. Spectroscopy in the THz range is particularly promising for studying biological systems, including proteins and their compounds. However, challenges such as the strong absorption of water, temperature stability, and small sample sizes need to be overcome. Techniques like scanning microscopy, integrated photonics, and metallic broadband approaches have been explored to enhance THz analysis. Despite the challenges, THz technology has found industrial applications and efforts are being made to improve data processing and error evaluation. Additionally, a proposed method aims to enhance the resolution of THz systems, making them suitable for gas spectroscopy and the sensing of volatile organic compounds relevant to biology.
We explored two ways to enhance light matter interaction in the THz range through spatial confinement of the electric field. Firstly, a broadband metallic waveguide with low losses and low dispersion used in a TDS setup to measure samples with volume as low as 200pL. In this proceeding, we explore a resonant structure allowing for tighter confinement at the price of narrower bandwidth. Split ring resonators are resonant structures analogous to LC circuit, where the electric field is confined in the capacitive part of the device. We fabricated SRRs with capacitive gaps as small as 30nm for measurements on extremely low volume sample such as macromolecules or viruses.
Terahertz spectroscopy provides information on the motion of the charges in a sample at a picosecond scale. To recover this information from Terahertz time-domain spectroscopy (THz-TDS), one usually extracts the experimental refractive index then fits these curves. This approach suffers from several limitations, among them the difficulty to compare models of motions, provide the error bar associated with the extracted magnitude and a resolution limitation coming from the Fourier criteria of the fast Fourier transform. By adopting a Bayesian framework taking into account the experimental uncertainties and directly fitting the time-domain trace, we overcame these limitations. When correlated and epistemic uncertainties/noise are present, the algorithm considers its distribution as part of the data to fit and can mistake it for real physical features. Hence, it offers poor discrimination between good models and bad ones. After a thorough analysis of the experimental noise, we developed a preprocessing software removing epistemic noise on the time traces and providing an estimate of the noise correlation matrix (generalization of the standard deviation). It allows the proper weighting of the error function of the fit using these uncertainties and therefore the derivation of the Akaike information criteria, a metric enabling to calculate the most probable model from a set of models one wants to compare. In addition, by being in the time domain we avoid the Fourier criteria for the resolution and thus could get information on experimental lines down to 30 MHz with a commercial THz-TDS system.
The Terahertz (THz) technology has now reached a level of maturation, which allows its uses beyond its core domains of application (telecom and imaging for security or healthcare). Vibrational spectroscopy in the THz range is employed in various fields and is specifically promising in (μ)biology. Indeed, the probed vibrational states extend over several nanometers and give a signature of the sample 3D structure at the nanoscale. This is particularly salient for macromolecules (proteins, DNA and RNA strands etc.) since, on one hand, their 3D structure is very difficult to probe in physiological condition with other techniques, and on the other hand, this structure determines their function and is consequently of utmost importance for the living. A major hurdle still arises when applying THz spectroscopy on biological or macromolecular samples. The samples are generally smaller than the THz wavelength, which requires concentrating the THz field in the sample. Solutions aimed at tackling this challenge by using μ/nano technology of THz field concentration and a proper data analysis will be presented.
In this proceeding, we present a software Fit@TDS that enables to retrieve the refractive index of a sample from a timedomain spectroscopy experiment. The software include commonly used methods where the refractive index is extracted from frequency domain data. This method has limitations when the signal is too noisy or when the absorption peak saturates the absorption. Thus, the software includes as well a new method where the refractive index are directly fitted using a model (the Drude-Lorentz for example) in the time domain. This method uses an optimization algorithm that retrieves the parameters of the model corresponding to the studied material. In this proceeding, we explain the methods and test them on fictitious samples to probe the feasibility and reliability of the proposed model.
In this work we show the potential of the ZnO/ZnMgO material system for intersubband (ISB)-based devices. This family of alloys presents a unique set of properties that makes it highly attractive for THz emission as well as strong coupling regimes: it has a very large longitudinal optical phonon energy of 72 meV, it can be doped up to ~1021 cm-3, it is very ionic with a large difference between the static and high frequency dielectric constants, and it can be grown homoepitaxially on native substrates with low defect densities. The films analyzed here are grown by molecular beam epitaxy (MBE) on a non-polar orientation, the m-plane, with varying QW thicknesses and 30% Mg concentrations in the barrier, and are examined with polarization-dependent IR absorption spectroscopy. The QW band structure and the intersubband transitions energies are modeled considering many body effects, which are key to predict correctly the measured values.
The development of Zinc Oxide (ZnO)-based applications have been strongly limited due to the lack of reproducible p-type doping. Here we present novel opportunities in the field of unipolar oxide wide band gap semiconductors. First we have developed the growth of nonpolar ZnO/ZnMgO multiple quantum wells (MQWs) and then we demonstrate that the structural and optical properties of the MQWs are reaching the required level for intersubband devices in terms of defects, surface and interface roughness and doping. We will show and discuss the most recent results as, for instance, intersubband transitions which have been observed in such structures.
This "Zoterac" project has received funding from the European Union’s Horizon 2020 research and
innovation programme under grant agreement No 665107
ZnO has great potential for devices in the mid IR and the THz range through the use of intersubband (ISB) transitions in multiple quantum wells (MQWs), although exploiting these transitions requires great control of the epitaxial layers as well as of the physics involved. In this work we present an analysis of non-polar ZnO grown homoepitaxially by molecular beam epitaxy on m-plane ZnO substrates as an ISB optical absorber. The MQWs were characterized under a 45°-bevelled multi-pass waveguide configuration allowing the observation at room temperature of an ISB transition in the 4-6 μm region for p-polarized incident light.
Buried heterostructure (BH) lasers are routinely fabricated for telecom applications. Development of quantum cascade
lasers (QCL) for sensing applications has largely benefited from the technological achievements established for telecom
lasers. However, new demands are to be met with when fabricating BH-QCLs. For example, hetero-cascade and multistack
QCLs, with several different active regions stacked on top of each other, are used to obtain a broad composite gain
or increased peak output power. Such structures have thick etch ridges which puts severe demand in carrying out
regrowth of semi-insulating layer around very deeply etched (< 10 μm) ridges in short time to realize BH-QCL. For
comparison, telecom laser ridges are normally only <5 μm deep. We demonstrate here that hydride vapour phase epitaxy
(HVPE) is capable of meeting this new demand adequately through the fabrication of BH-QCLs in less than 45 minutes
for burying ridges etched down to 10-15 μm deep. This has to be compared with the normally used regrowth time of
several hours, e.g., in a metal organic vapour phase epitaxy (MOVPE) reactor. This includes also micro-stripe lasers
resembling grating-like ridges for enhanced thermal dissipation in the lateral direction. In addition, we also demonstrate
HVPE capability to realize buried heterostructure photonic crystal QCLs for the first time. These buried lasers offer
flexibility in collecting light from the surface and relatively facile device characterization feasibility of QCLs in general;
but the more important benefits of such lasers are enhanced light matter interaction leading to ultra-high cavity Q-factors,
tight optical confinement, possibility to control the emitted mode pattern and beam shape and substantial reduction in
laser threshold.
In this communication, we present the potentialities offered by 2D photonic crystals to trap and absorb photons in thin silicon layers. We will specifically focus on the impact of the photonic crystal unit cells symmetry, and the possibility to increase light absorption and generated photocurrent using multi-periodic and pseudo-disordered photonic nanostructures.
Recently, we developed a new family of 3D photonic hollow resonators which theoretically allow tight confinement
of light in a fluid (gaz or liquid): the photon cages. These new resonators could be ideal for sensing applications
since they not only localize the electromagnetic energy in a small mode volume but also enforce maximal overlap
between this localized field and the environment (i.e. a potential volume of nano-particles). In this work, we will
present numerical and experimental studies of the interaction of a photon cage optical mode with nano-emitters. For
this, PbS quantum dot emitters in a PDMS host matrix have been introduced in photon cages designed to have
optimal confinement properties when containing a PDMS-based active medium. Photoluminescence measurements
have been performed and the presence of quantum dot emitters in the photon cages has been demonstrated.
High index contrast periodic structures can be exploited to perform an arbitrarily adjustable spatio-temporal molding of
light at the wavelength scale. This tight control of photons is obtained through a shaping of resonant modes and a
suitable adjustment of their coupling with propagative modes. In this talk, surface addressable (or above the light line)
Bloch modes in photonic crystal membranes (PCMs) will be investigated. The concepts governing the properties of these
modes will be first presented. It will be demonstrated that simple models can be considered to understand their behavior
and, more specifically, their ability to capture photons during the desired lifetime. Bloch modes with a very large band
width or a very high Q factor can be easily designed this way. The wide application range of these PCMs will be
illustrated by different devices. Low Q Bloch modes will be first used as efficient and broadband reflectors and exploited
to realize compact and efficient vertical cavity lasers with unprecedented functionalities. In addition, it will be shown
that this kind of mirrors can be bent in order to confine photons in the 3 directions even in low index materials, opening
the way to a new class of emitters and sensors. Finally, the exploitation of high Q Bloch modes to design highly
directional thermal emitters with wavelength selectivity will be presented.
In silicon-based solar cells, a substantial part of the energy losses is related to the charge carriers thermalization in the UV-blue range and the week carriers collection at these wavelenghts. To avoid this issue, we introduce a new concept which combines a rare-earths doped thin layer with a photonic crystal (PC) layer, allowing an efficient conversion from UV-blue photons to near-IR photons. We report on the feasibility of such a nanostructured down-converter module using an active rare-earth doped CaYAlO4 thin layer and a silicon nitride PC on top. By means of optical numerical simulations, the promising potentialities of the concept are demonstrated.
In order to achieve high efficiency photovoltaic devices and sensors, we propose to implement photonic crystals on thin
absorbing layers in such a way to generate two Bloch mode resonances with opposite symmetries. Through FDTD and
RCWA simulations, we track and adjust the characteristics of these modes so as to reach their degeneracy. Design and
simulations were carried out considering a hydrogenated amorphous silicon layer. We demonstrate that up to 92%
absorption can be achieved, far above the 50% limit corresponding to the critical coupling condition between an incident
wave and an optical resonance. Moreover, the robustness of the absorption peak was tested by varying both the
topographical parameters of the PhC membrane and the angle of incidence. Finally, some guidelines are provided to
generalize our approach for the design of broadband absorbers.
Yb3+-doped fiber is one of the most promising hosts for high-power fiber lasers [1]. However, in the late 1990s,
photodarkening effect, i.e., the creation of color centers induced by light, was observed and reported [2]. Like in other
rare-earth-doped materials, a broad visible and near infrared absorption band appears during laser operation, which
strongly lowers the laser efficiency. We have shown how thulium impurities, present at the parts-per-billion weights
(ppb) level in the raw doping material, can induce UV emission which in turn creates defects responsible for
photodarkening in ytterbium-doped continuous-wave fiber lasers [3]. This new vision is not in contradiction with others
but it gives for the first time a possible explanation for UV defect creation by an infrared beam.
KEYWORDS: Photonic crystals, Near field scanning optical microscopy, Light scattering, Laser crystals, Scattering, Finite-difference time-domain method, Multiple scattering, Near field optics, Photons, Near field
Anderson localization (AL), the localization phenomenon of waves in random media, was theoretically predicted for
electrons in a random potential in 1958 and still has been a recondite puzzle today. Stemming from interferences of
multiply scattered waves, the principle is applicable to whole quantum as well as classical waves. Although experimental
attempts toward AL of light had been performed in fully random structures such as aggregates of fine grains, it had been
difficult to achieve because it demands materials with both extremely high scattering strength and low absorption losses.
It was predicted in 1987 that localization may be more achievable in a randomized photonic crystal which supports a
wide photonic band gap. However, AL of light is not yet experimentally exhibited except by far-field indirect
observations in one- and two-dimensional structures. Here we show the first direct near-field observation of two-dimensional
AL in random photonic crystal lasers by use of SNOM (Scanning Near-field Optical Microscope). We
fabricated two-dimensional random photonic crystal lasers to which structural randomness is introduced by dislocating
the positions of air holes to random directions. We show that only slight amount of randomness induces the extended
Slow Bloch Modes to be Anderson localized, but too much randomness releases the light confinement. In addition, by
performing FDTD computational method we confirm the experimental results to be consistent with theoretical prospects.
Our results directly expose the detailed appearance of two-dimensional Anderson localized light first time ever.
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