We investigate the nonlinear optical properties of transparent optical materials using ultrashort midwave infrared laser pulses between 3 and 4 microns. Random quasi-phase matching in polycrystalline materials generates multiple frequency harmonics of both odd and even orders throughout the transmission window of the target. We also investigate single crystal and amorphous materials and demonstrate a range of frequency conversion and pulse broadening. Simulations using a nonlinear polarization model enhanced with ionization and experimentally measured n2 values provide good qualitative agreement with experimental data.
We investigate the nonlinear optical properties of ZnSe and ZnS using ultrashort (pulse duration approximately 200 fs) midwave infrared laser pulses between 3 and 4 μm. Multiple harmonic generation in both materials was observed, as well as significant spectral modification of the fundamental pulse. Simulations using a nonlinear polarization model enhanced with ionization compared favorably with experimental data. Random quasi phase matching in the materials is the likely generator of the observed harmonics.
We have identified major paradigm shifts relative to near-IR filamentation when high power multiple terawatt laser pulses are propagated at mid-IR and long-IR wavelengths within key atmospheric transmission windows. Individual filaments at near-IR (800 nm) wavelengths typically persist only over tens of centimeters, despite the whole beam supporting them being sustained over about a Rayleigh range. In the important mid-IR atmospheric window (3.2 - 4 μm) optical carrier wave self-steepening (carrier shocks) tend to dominate and modify the onset of long range filaments. These shocks generate bursts of higher harmonic dispersive waves that constrain the intensity growth of the filament to well below the traditional ionization limit, making long range low loss propagation possible. For long wavelength pulses in the 8-12 μm atmospheric transmission window, many-electron dephasing collisions from separate gas species act to dynamically suppress the traditional Kerr self-focusing lens and leads to a new type of whole beam self-trapping over multiple Rayleigh ranges. This prediction is key, since strong linear diffraction at these wavelengths are the major limitation and normally requires large launch beam apertures. We will present simulation results that predict multiple Rayleigh range propagation paths for whole beam self-trapping and will also discuss some recent efforts to extend the HITRAN linear atmospheric transmission/refractive index database to include nonlinear responses of important atmospheric molecular constituents.
There is a strong push worldwide to develop multi-Joule femtosecond duration laser pulses at wavelengths around 3.5-4 and 9-11μm within important atmospheric transmission windows. We have shown that pulses with a 4 μm central wavelength are capable of delivering multi-TW powers at km range. This is in stark contrast to pulses at near-IR wavelengths which break up into hundreds of filaments with each carrying around 5 GW of power per filament over meter distances. We will show that nonlinear envelope propagators fail to capture the true physics. Instead a new optical carrier shock singularity emerges that can act to limit peak intensities below the ionization threshold leading to low loss long range propagation. At LWIR wavelengths many-body correlations of weakly-ionized electrons further suppress the Kerr focusing nonlinearity around 10μm and enable whole beam self-trapping without filaments.
In the absence of external excitation, light trapped within a dielectric medium generally decays by leaking out—and also by getting absorbed within the medium. We analyze the leaky modes of a parallel-plate slab, a solid glass sphere, and a solid glass cylinder, by examining those solutions of Maxwell’s equations (for dispersive as well as non-dispersive media) which admit of a complex-valued oscillation frequency. Under certain circumstances, these leaky modes constitute a complete set into which an arbitrary distribution of the electromagnetic field residing inside a dielectric body can be expanded. We provide completeness proofs, and also present results of numerical calculations that illustrate the relationship between the leaky modes and the resonances of dielectric cavities formed by a simple parallel-plate slab, a glass sphere, and a glass cylinder.
Computer simulations of ultrafast optical pulses face multiple challenges. This requires one to construct a propagation model to reduce the Maxwell system so that it can be efficiently simulated at the temporal and spatial scales relevant to experiments. The second problem concerns the light-matter interactions, demanding novel approaches for gaseous and condensed media alike. As the nonlinear optics pushes into new regimes, the need to honor the first principles is ever greater, and requires striking a balance between computational complexity and physical fidelity of the model. With the emphasis on the dynamics in intense optical pulses, this paper discusses some recent developments and promising directions in the field of ultrashort pulse modeling.
After a brief historical review, we describe recent research in the study of tera-Watt class femtosecond lasers propagating in air and condensed media. Here critical self-focusing of the light field reflects the presence of a famous singularity (blow-up in finite time) in the governing Nonlinear Schrö dinger equation (NLS) — this contribution deals with moving into a regime where NLSE fails and more exact optical carrier resolved pulse propagators need to be developed and secondly, addresses the failure of well-established phenomenological nonlinear optical susceptibilities and their replacement by more fundamental quantum models.
Ultrafast intense femtosecond laser pulses spontaneously undergo critical collapse in air and
condensed media above some critical power. In normally dispersive media, such pulses can
spontaneously generate dynamical X-waves where distinct X-features appear in the spectrally-resolved
far-field. These nonlinear self-trapped pulses resemble linear Bessel beams - the latter
exhibit extended line rather than point foci and are robust to strong perturbations. Nonlinear X-waves
can be directly generated by using an axicon lens and have the potential to generate
highly nonlinear, extended interaction zones relative to pulses with Gaussian profiles. Potential
applications of these pulsed sources to controlling and extending white light supercontinuum and
plasma channel generation will be discussed. X-wave generation in normally dispersive media is
associated witha cascade of pulse splittings where individual split pulses have been identified
with different arms of the spectrally observed X-feature. This allows for novel pump-probe
experiments where a seed pulse can selectively generate Raman Stokes shifted waves by
scattering off of different arms of the X-feature. We will discuss a 3-wave interaction picture that
allows for a transparent physical interpretation of these complex spatio-temporal events.
Rapid progress in recent years in the development of high power ultrashort pulse laser systems has opened up a whole new vista of applications and computational challenges. Amongst those applications that are most challenging from a computational point of view are those involving explosive critical self-focusing with concomitant explosive growth in the generated light spectrum. Moreover, new experimental developments in the field of extreme nonlinear optics will require more rigorous propagation models beyond those existing in the current literature. Specific applications areas chosen for illustration in this paper include atmospheric light string propagation and nonlinear self-trapping in condensed media. These examples exhibit rather different aspects of intense femtosecond pulse propagation and demonstrate the robustness and flexibility of the unidirectional Maxwell propagator.
A novel aspect of our approach is that the pulse propagator is designed to faithfully capture the light-material interaction over the broad spectral landscape of relevance to the interaction. Moreover the model provides a seamless and physically self-consistent means of deriving the many ultrashort pulse propagation equations presented in the literature.
Vertical external cavity surface emitting lasers (VECSELs) have been considered the “ultimate disk-laser” due
to their extremely thin active regions and because they take advantage of the high gain found in semiconductor
material. This paper discusses power scaling limitations, including heating effects, surface roughness losses, and
laterally guided amplified spontaneous emission (ASE).
Modeling of high-power diodes poses several numerical problems. They require algorithms capable of capturing accurately the fast temporal and spatial dynamics in a broad spectral range. Another problem is how to reconcile vastly different time scales of various physical processes involved. We present an outline of a semiconductor laser simulation engine that incorporates both the first-principles many body gain calculations, and the carrier and heat transport simulation into an interactive computer laser model.
Semiconductor and fiber amplifiers and lasers are amongst the most complex and critically important components in most modern optical telecommunications systems. The ever increasing demand for bandwidth places severe constraints on component design. Active materials need to be accurately characterized in terms of their optical properties. In addition, realistic simulation tools must be capable of resolving the multi-THz bandwidths while providing a rapid turn around to the system designer. We will report on the implementation of an extremely efficient algorithm running within an object-oriented simulation environment. As an illustration, we will present results showing how a WDM-based semiconductor optical amplifier and a TDM Mach-Zehnder interferometer gate can be optimized using rigorously computed and experimentally validated semiconductor optical material properties.
Synchronization of chaotic semiconductor lasers has now been demonstrated experimentally using a variety of coupling schemes. Coupling methods include situations where the transmitter laser system is itself chaotic and drives a passive receiver system, both lasers are individually chaotic and, both lasers induce the chaos through mutual self-coupling. The qualitative dynamics for each of these scenarios is adequately captured by an appropriate set of coupled Lang-Kobayashi lumped rate equation models. Such lumped models however cannot distinguish between the possible coupling geometries realizable in real experimental systems and ignore multiple feedback from external reflecting surfaces. For example, real lasers may have AR/HR coated facets and there are several choices of placement of external reflectors and coupling paths relative to these facets. Moreover, nominally single mode FP lasers may exhibit pronounced multi-longitudinal mode dynamics in the presence of weak external reflection and DFB lasers may exhibit dual-wavelength operation or strongly asymmetric spatial hole-burning due to the presence of finite facet reflectivity.
High-power, femtosecond light filaments, also termed light strings, are experimentally observed to propagate over distances which substantially exceed the diffraction lengths that would correspond to their transverse dimensions. Thus, they provide a way to deliver high powers of focused light over long distance, and may potentially serve as light probes in remote sensing. We concentrate on a theoretical understanding of the underlying physics. In this talk, we review the results of our computer simulations providing insight into the rich spatio-temporal dynamics of this interesting phenomenon.
We are currently developing 2 semiconductor laser simulators built on a first-principles microscopic physics basis. The first is a PC-based, plane-wave simulator for both component and system-level design of low-power optoelectronic devices. The second is a supercomputer-based simulator that models the fully time-dependent and spatially-resolved optical, carrier, and temperature fields for arbitrary geometry, high-power semiconductor lasers. Both simulators are based on a comprehensive gain model that includes the relevant bandstructure of the quantum wells and confining barrier regions together with a fully quantum mechanical many-body calculation that takes all occupied bands into account.
Large-scale computer simulations of wide-beam, high-power femtosecond laser pulse propagation in air are presented. Our model, based on the nonlinear Schrodinger equation for the vector field, incorporates the main effects present in air, including diffraction, group-velocity dispersion, absorption and defocusing due to plasma, multiphoton absorption, nonlinear self-focusing and rotational stimulated Raman scattering. The field equation is coupled to a model that describes the plasma density evolution. Intense femtosecond pulses with powers significantly exceeding the critical power for self-focusing in air are simulated to study turbulence-induced filament formation, their mutual interaction via a low-intensity background, dynamics of the field polarization, and evolution of the polarization patterns along the propagation direction.
A robust, modular and comprehensive simulation model, built on a first-principles microscopic physics basis, includes the fully time-dependent and spatially resolved internal optical, carrier and temperature fields within an arbitrary geometry edge-emitting high-power semiconductor laser device. The simulator is designed to run interactively on a multi- processor shared memory graphical supercomputer by utilizing a highly efficient algorithm running in parallel over multiple CPUs. The experimentally validated semiconductor optical response is computed using a microscopic approach that includes the relevant bandstructure of the Quantum Well and confining barrier regions together with a fully quantum mechanical many-body calculation that takes all occupied bands into account. The latter quantity is introduced into the simulator via a multidimensional look-up table that captures the local dependence of the gain and refractive index of the structure over a broad range of frequencies and carrier densities. The simulator is designed in a modular form so as to be able to include differing device geometries (broad area, flared, multiple contacts, arrays, ..), filters (DBR or DFB grating sections), index/gain-guiding, temperature and current profiles and so on. Results will be presented for both broad area and MOPA devices.
Large-scale computer simulations of wide-beam, high-power femtosecond laser pulse propagation in air are presented. Our model, based on the nonlinear Schroedinger equation for the vector field, incorporates the main effects present in air, including diffraction, group-velocity dispersion, absorption and defocusing due to plasma, multiphoton absorption, nonlinear self-focusing and rotational stimulated Raman scattering. The field evolution is coupled to a model that describes the plasma density evolution. Intense femtosecond pulses with powers significantly exceeding the critical power for self-focusing in air are simulated to study turbulence-induced filament formation, their mutual interaction via a low-intensity background, dynamics of the field polarization, and evolution of the polarization patterns along the propagation direction.