The quantum cascade laser pumped molecular laser (QPML) is a polyvalent source spanning the THz gap, with demonstrated operation from 200 GHz up to more than 5 THz. Using various models for molecular relaxation permits the derivation of a laser model that describes the pressure dependent behavior of the QPML. Here, we will discuss new designs that enable high performance operation.
The quest for mastering high-power lasers has advanced from initial power scaling to sophisticated beam control. With chirped-pulse amplification catalyzing terawatt and petawatt-scale advancements, the current frontier is refining beam shaping at these extreme powers. We introduce robust monolithic meta-optics, capable of enduring and manipulating high-peak-power laser beams across a wide spectral range, from near-UV to near-IR. These all-glass metasurfaces exhibit a damage threshold at the material's limit, vastly outperforming traditional heterogeneous metasurfaces. We demonstrate the meta-optics robustness with high-peak-power femtosecond pulses. Our nanofabrication protocol allows for precise vertical nanopillars, overcoming previous challenges in tapered sidewalls. We exploit the form birefringence of these nanopillars to induce optical anisotropy in inherently isotropic glass. A geometric perspective underpins our approach to overcoming scaling challenges associated with long-wavelength lasers. Our metasurfaces pave the way for unprecedented control in high-power laser applications.
Active resonators based on semiconductor gain media encompass a large optical nonlinearity that arises from gain saturation and enables bright soliton generation. The ability to operate these resonators below the lasing threshold as tunable passive devices –– filters, modulators, phase shifters –– opens up an untapped potential of seamlessly integrated reconfigurable devices for both generation of multimode mid-infrared (4 – 12 μm) light and its manipulation.
Bright pulses of light are unstable states in free-running semiconductor lasers. Stable bright solitons require an optical bistability---as predicted by mean-field theories such as the Complex Ginzburg Landau Equation (CGLE) or the Lugiato-Lefever Equation (LLE). However, this restriction is relaxed when two lasers are coupled to one another. Here, we identify a new state of light in a pair of semiconductor ring lasers with fast gain dynamics. Two racetrack (RT) quantum cascade lasers (QCLs) when coupled along their straight sections spontaneously produce a frequency comb over the hybridized modes of the coupled cavity. Waveform reconstruction measurements reveal the hybridized comb manifests itself as a pair of bright and dark pulses circulating the coupled cavity simultaneously. In addition, split-step integration of a pair of mutually forced CGLEs faithfully reproduces our experimental measurements, providing some insight on the formation of such states.
Optical frequency combs (OFCs) stand as the cornerstone of modern optics, with
applications ranging from fundamental science to sensing and spectroscopy. Generation of
short optical soliton pulses in passive media such as optical fibers and microresonators has
been an established technique for stable OFC formation with a broad optical spectrum –
however these platforms are driven by an external optical signal and often rely on
additional bulky elements that increase the complexity of the system.
Here, we aim to overcome these difficulties by direct OFC generation in mid-infrared
semiconductor lasers, such as quantum and interband cascade lasers. After a general
introduction to such combs and their nonlinear dynamics, the soliton concept from
microresonator Kerr combs will be generalized to active media that are electrically-driven
and a new type of solitons in free-running semiconductor laser integrated on a chip will be
demonstrated.
Understanding the structure of forests is essential to gain insights into the diversity of ecosystems, which is key to determining their resilience to abiotic and biotic hazards. Forests play a critical role in the storage of biomass and are therefore affecting the global carbon cycle and future trajectories of atmospheric CO2 concentration. Earth observations and satellite remote sensing data have become valuable resources for observing forested areas on a global scale. From this viewpoint the Global Ecosystem Dynamics Investigation (GEDI) LIDAR (Light Detection and Ranging) mission, developed by NASA (National Aeronautics and Space Administration), measures an unprecedented range of parameters that describe forest structural complexity. In this paper, we investigate the ability of machine learning tools to discern variations in canopy height and evaluate their performance with a view to determine the most accurate one for the Iberian Peninsula. A novel approach is proposed for integrating Sentinel-1, Alos-Palsar, Sentinel-2 and spectral indices data to create an input dataset for machine learning models. Random Forest (RF), XGBoost. and Multi-Layer Perceptron (MLP) are trained to leverage canopy height data obtained from GEDI metrics. Our results show that the three algorithms perform similarly in terms of accuracy and errors, presenting MLP with the highest correlation with canopy height. Additionally, we evaluated the feature importance of each model, providing insights into the variables that influence the model's decision-making. Our analysis revealed that MLP gave greater importance to Sentinel-2 bands, whereas Random Forest relied more heavily on vegetation indices. In contrast, XGBoost exhibited a balanced approach and utilized information from both features evenly. It is noteworthy that the red-edge channels and their derived indices have demonstrated a significant level of importance across all models employed in this study.
Quantum cascade lasers and other semiconductor laser types constitute an attractive integrated platform for spectroscopic applications, as they emit self-starting Frequency Combs (FCs), unlike traditionally-used mode-locked lasers. Here, we explain self-starting FCs due to nonlinear effects arising from the laser gain itself, with particular attention on the coupling of the amplitude and phase of light, quantified by the Linewidth Enhancement Factor (LEF). We study both cavity geometries, Fabry-Perot and ring, reporting the conditions for stable comb formation and different methods of optimizing their performance. In analogy with Kerr microresonators, ring lasers show the formation of temporal localized soliton-like structures, indicating towards an untapped potential for discovering new states of light.
Optical nonlinearities are known to coherently couple the amplitude and the phase of light, which can lead to the formation of perfectly periodic waveforms – known as frequency combs. Recently, self-starting frequency combs that do not rely on the emission of short pulses are appearing in numerous semiconductor laser types, among which is the quantum cascade laser. Here we discuss the role of a Bloch gain induced giant Kerr nonlinearity in Fabry-Pérot and ring cavity QCLs, paving the way towards electrically pumped Kerr combs.
We use a prototypical model based on the Complex Ginzburg Landau Equation to study the dynamics of a multimode ring quantum cascade laser. We predict the existence and stability of different classes of localized structures in the system. In presence of coherent injection, we report the formation dissipative Temporal Solitons (TSs) which manifest multi-stability and coexistence with a stable CW solution. We show how these features allow for external manipulation of the spectral content of the optical frequency combs associated with TSs with a big impact on applications in the field of e.g. high precision spectroscopy and wireless communications.
Monolithic ring Quantum Cascade Lasers (QCLs) have recently emerged as a new platform for frequency comb generation in the mid-infrared with immediate applications in molecular gas spectroscopy and photonic generation of stable coherent sub-THz tones. In this talk I will show that depending on the way they are driven, ring QCLs can act as carrier generators, integrated intensity modulators, tunable filters, and on-chip optical amplifiers. The natural predisposition of these components to photonic integration opens a route to compact mid-infrared WDM transceivers for free space optical links and miniaturized 2D IR spectrometers.
We generalized the well-known Lugiato-Lefever Equation to unify the description of combs and localized structures formation in nonlinear optical systems such as Kerr micro-resonators (passive systems) and Quantum Cascade Lasers (QCL) (active systems). In particular this model was applied to the study of pattern formation in a unidirectional ring QCL driven by a coherent injected field. We showed the existence of Dissipative Solitons (DS) and Turing rolls associated to standard and harmonic Optical Frequency Combs (OFC) in the system. We also provided a proof of principle demonstration of the possibility to deterministically control the spectral properties of these OFC by switching-on one or more DS with suitable addressing pulses.
These results considerably increase the theoretical insight in chip-scale combs sources in the mid-infrared region of the electromagnetic spectrum for timely applications in the field of e.g. high resolution and/or time resolved molecular spectroscopy, long range and high bit rate wireless communications.
We demonstrate a non-Hermitian system by introducing topological dynamics in coherent laser arrays. It consists of a large set of mutually interacting optical vortices subject to arbitrary topological transformations realized by metasurfaces in a closed environment. Like atoms in a crystal, the vortices are arranged in a regular lattice producing Bragg diffraction peaks. Prior art focussed on demonstrations of single vortex sources employing concepts from non-Hermitian and topological photonics to add charge reconfigurability and to mitigate manufacturing fault effects. We extrapolate these concepts to vortex arrays that demonstrate tunability and robustness akin to those of topical single vortex sources.
Optical nonlinearities are known to coherently couple the amplitude and the phase of light, which can lead to the formation of perfectly periodic waveforms – known as frequency combs. Recently, self-starting frequency combs that do not rely on the emission of short pulses are appearing in numerous semiconductor laser types, among which is the quantum cascade laser. This novel type of combs is gaining vast attention from researchers due to their self-starting nature and compactness, making them an ideal platform for further development of spectroscopic applications. Their spontaneous formation was explained through an interplay of phenomenological nonlinearity and dispersion in the laser active region, although the actual physical processes remained unclear until now. Here we show that Bloch gain – a phenomenon described by Bloch and Zener in the 1930s – plays an essential role in their formation. We demonstrate that a Bloch gain contribution is present in any quantum cascade laser and becomes particularly dominant under saturation.
Bloch gain in QCLs with ultrafast gain recovery induces a giant Kerr nonlinearity, which is two orders of magnitude larger than the bulk values. The resonant Kerr nonlinearity provides coherent coupling between the amplitude and the phase of the laser field, which serves as a locking mechanism for frequency comb operation. We show that in Fabry-Pérot QCLs this results in frequency-modulated combs with a linear frequency chirp. In ring cavity QCLs, the Bloch gain is able to induce a single-mode instability by tuning the laser in the phase turbulence regime. This can lead to the formation of locked spatial patterns that are related to dissipative Kerr solitons, paving the way towards electrically pumped Kerr combs.
We generate a large coherent laser array imbued with topological charge by addressing the phase retrieval problem to reconstruct the desired phase distribution from its corresponding Fourier intensity pattern. By employing the many modes in a degenerate laser cavity as a parallelised solver and by limiting its finite domain to lift the degeneracy between the competing phase distributions, an optimum solution with tailored multi-singularities is found. We implement the required constraints within the cavity using binary amplitude masks as opposed to sophisticated phase devices, paving the way as a simple technique to generate large structured vortex arrays at the source.
The recent generalised theory of frequency comb generation in externally pumped cavities with and without population inversion suggested an intimate link between quantum cascade lasers (QCLs) and Kerr resonators. In this talk we overview recent experimental developments in chip-scale ring cavity QCLs with and without output coupling ports, that allow operation in self-pumped and externally pumped configurations, and their ability to support cavity solitons.
We demonstrate a non-Hermitian system by introducing topological dynamics in coherent laser arrays. It consists of a large set of mutually interacting optical vortices subject to arbitrary topological transformations realized by metasurfaces in a closed environment. Like atoms in a crystal, the vortices are arranged in a regular lattice producing Bragg diffraction peaks. Prior art focussed on demonstrations of single vortex sources employing concepts from non-Hermitian and topological photonics to add charge reconfigurability and to mitigate manufacturing fault effects. We extrapolate these concepts to vortex arrays that demonstrate tunability and robustness akin to those of topical single vortex sources.
Metasurfaces are a promising platform to exceed their traditional counterparts not only in compactness but also for functionality. However, current designs are limited when trying to implement multiple, non-paraxial functions with a single metasurface as they are bound to either a small angular range or to low efficiencies.
Here, we present a new non-local metasurface design that enables the implementation of multiple, independent functions with a large difference in deflection angle. We further demonstrate the capabilities of this approach for advanced control of light emission systems by creating a wavelength-tunable external cavity laser with holographic output based on such metasurface.
Frequency combs are ideal candidates to realize miniaturized spectrometers without moving parts and hence are of great interest for integrated photonics. Here, an overview on the generation electrically pumped optical frequency combs on integrated platforms using semiconductor lasers.
This includes self-starting generation of frequency modulated combs in quantum cascade laser in the 8um and interband cascade lasers in the 3-4um wavelength region, respectively. Furthermore, we will discuss how to integrate efficient high-speed modulators in these devices in order to facilitate the generation of picosecond pulses.
We propose a generalization of the well-known Lugiato-Lefever Equation to unify the description of combs- and structures- forming nonlinear optical systems. This approach rigorously connects for the first time passive systems such as Kerr micro-resonators and active systems such as Quantum Cascade Lasers (QCLs) which were so far treated separately. The model effectively describes a unidirectional ring QCL driven by a coherent signal where we show the existence of temporal solitons and Turing rolls, previously identified only in Kerr micro-resonators, considerably increasing the theoretical insight and the technological potential of chip-scale comb sources.
It is a well-established truth that spatial hole burning (SHB) in a standing-wave cavity is an essential single-mode instability mechanism for multimode operation of quantum cascade lasers (QCLs). We discovered recently that another instability mechanism–phase turbulence–is capable of triggering an onset of previously unseen types of frequency combs in traveling-wave ring cavity QCLs in absence of SHB. This new regime of laser operation reveals a connection with Kerr combs and paves the way to manipulation and engineering of comb states in QCLs.
The generation of short pulses with quantum cascade lasers (QCLs) remains challenging to date due to their ultrafast gain dynamics. Here, we report on active mode-locking of mid-infrared QCLs. For the first time we show, that picosecond pulses can be generated also at room-temperature using high-performance QCL material. Mounted epi-up, the QCLs emit a train of pulses as short as a 7ps with an average power of 100mW. The nonlinear autocorrelation shows reveals the famous 8:1 ratio, which proves unambiguously that the QCL operates in the mode-locked regime. This result is further verified using the beatnote spectroscopy technique SWITS.
Water is always at risk of accidental or intentional pollution that would consist of introducing a harmful chemical into a drinking water reservoir. Fiber-optics evanescent wave sensing has been shown to be an efficient sensor scheme for direct in-water sensing. Here we demonstrate a system for the detection of chemicals dissolved in water by using quantum cascade lasers (QCLs) coupled into a silver halide fiber. The study was performed over two frequency ranges: short wavelength (i.e. 3µm and 5µm) and long wavelength (between 8µm and 10µm) and using two different types of QCL source: pulsed and continuous wave.
Quantum cascade lasers are often operated in pulsed regime for low-power applications due to the large thermal dissipation required for continuous wave operation. The typical pulse length is of the order of 100 ns with a duty cycle below 1%. Fourier transform infrared spectrometers, commonly used in the mid-infrared, typically have a spectral resolution of the order of 3 GHz and rely on the acquisition of a path-difference interferogram. As a consequence, when measuring devices operated in pulsed regime such spectrometers can only measure the spectrum averaged over several pulses.
We propose a method to determine the absolute instantaneous frequency of a pulsed laser with a precision of 10 MHz. First, the light from the laser is sent through a 30 cm long Fabry-Perot resonator under vacuum. The temporal waveform of the transmitted signal, which is measured using an HgCdTe detector, contains fringes corresponding to constructive and destructive interference occurring as function of time. This experiment allows to determine the chirp rate. The Fabry-Perot cavity is then filled with a known gas exhibiting an absorption line lying within the laser emission range, which can be used as an absolute frequency reference. By combining this measurement with the chirp rate, we obtain the instantaneous frequency of the laser as a function of time. Complex spectral behavior of pulsed DFB lasers, such as mode-hopping and dual-wavelength lasing, can also be properly identified using this technique.
Thanks to its high Kerr non-linearity and its low linear absorption, silicon is a material of choice for optical devices in the mid-infrared (from 3 to 5 microns) such as microresonators. In this wavelength range, the available optical sources such as quantum cascade lasers have a limited tunability. Tuning the refractive index of silicon can be achieved by a temperature change of the chip and has been previously demonstrated on ring resonators using integrated heaters or thermo-electric elements. We present a new method for thermo-optical tuning of silicon devices by directly using the light from a laser diode operating at 450 nm. The blue light focused on the silicon induces a local elevation of temperature and thus the refractive index locally increases. When applying this method on silicon ring resonator, the elevation of temperature leads to a decreasing free-spectral range and thus shift the resonances to lower frequencies. At 4.5 µm we measured a tuning efficiency of 200 MHz per mW of incident light. Numerical simulations of the thermo-optical effect show the locality of this tuning method, and confirm the experimental results. Finally a frequency study of the response of this method is performed and a time constant of the order of the micro-second is measured. In conclusion, we propose a fast, local, and non-invasive method for tuning silicon resonators operating in the mid-infrared that can be extended to any silicon-based device.
Following the goals of single-chip integrated dual comb spectrometers, we report on recent results on mid-infrared frequency combs. We demonstrate frequency comb operation with a bi-functional quantum cascade material, which allows the integration of lasers and detectors on one chip. With this device, we hold the power and efficiency record of QCL frequency combs. In the second part, we will present first evidence of frequency comb generation using mode-locked interband cascade lasers. With the demonstration of picosecond pulse generation in the mid-infrared, we open a new path towards battery driven sensitive high-resolution spectrometers miniaturized to chip-scale dimensions.
We present a model of carrier distribution and transport accounting for quantum localization effects in disordered semiconductor alloys. It is based on a recent mathematical theory of quantum localization which introduces a spatial function called localization landscape for carriers. These landscapes allow us to predict the localization of electron and hole quantum states, their energies, and the local densities of states. The various outputs of these landscapes can be directly implemented into a drift-diffusion model of carrier transport and into the calculation of absorption/emission transitions. This model captures the two major effects of quantum mechanics of disordered systems: the reduction of barrier height (tunneling) and lifting of energy ground states (quantum confinement), without having to solve the Schrödinger equation. Comparison with exact Schrödinger calculations in several one-dimensional structures demonstrates the excellent accuracy of the approximation provided by the landscape theory [1]. This approach is then used to describe the absorption Urbach tail in InGaN alloy quantum wells of solar cells and LEDs. The broadening of the absorption edge for quantum wells emitting from violet to green (indium content ranging from 0% to 28%) corresponds to a typical Urbach energy of 20 meV and is closely reproduced by the 3D sub-bandgap absorption based on the localization landscape theory [2]. This agreement demonstrates the applicability of the localization theory to compositional disorder effects in semiconductors.
[1] M. Filoche et al., Phys. Rev. B 95, 144204 (2017)
[2] M. Piccardo et al., Phys. Rev. B 95, 144205 (2017)
We discuss the unambiguous detection of Auger electrons by electron emission (EE) spectroscopy from a cesiated InGaN/GaN light-emitting diode (LED) under electrical injection. Electron emission spectra were measured as a function of the current injected in the device. The appearance of high-energy electron peaks simultaneously with the droop in LED efficiency shows that hot carriers are being generated in the active region (InGaN quantum wells) by an Auger process. A linear correlation was measured between the high energy emitted electron current and the “droop current” - the missing component of the injected current for light emission. We conclude that the droop originates from the onset of Auger processes. We compare such a direct identification of the droop mechanism with other identifications, most of them indirect and based on the many-parameter modeling of the dependence of the external quantum efficiency on the carrier injection.
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