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The interband GaSb-based diode lasers emitting simultaneously in two narrow bands separated by either ~1.6 or ~3.3 THz were designed, fabricated and characterized. The device active region contained one asymmetric tunnel-coupled double quantum well with separation between two lowest electron subbands controlled by thickness of the tunnel barrier. The Y-branch 6th order distributed Bragg reflector devices have been fabricated with either deep or shallow etched ridge waveguides. The increase of the deeply etched ridge waveguide width from 10 to $20~\mu \text{m}$ improved laser threshold and efficiency thanks to reduction of the relative role of the sidewall defect recombination. Further improvement of the device performance parameter was achieved by shallow etching. The shallow etched lasers with stable dual-wavelength emission spectrum generated 100 mW of continuous wave output power at 20 °C.

The passively mode-locked type-I quantum well cascade diode lasers operating near 2.7 and 3.2 μm generated trains of the ∼10 ps long pulses with average power up to 10 mW. The devices based on laser heterostructures with reinforced carrier confinement requires increased reverse bias voltages applied to absorber sections to operate in mode-locked regime. The autocorrelation measurements showed that lasers generated strongly chirped pulses with temporal width an order of magnitude above transform limit. The application of the external feedback led to narrowing of the laser emission spectra accompanied by an order of magnitude reduction of the intermodal beat note linewidth. The multiheterodyne beat notes have been observed for devices stabilized by external feedback.

In this talk I will discuss how resonant light-matter interaction in the gain medium of quantum cascade lasers gives rise to a rich nonlinear multimode dynamics and a variety of phase-locked multimode regimes, most notably optical frequency combs with separation between the comb lines changing from one to many dozen round-trip frequencies. I will review recent progress in understanding why frequency comb formation seems to be so ubiquitous in QCLs.

Interference of laser beams in a suitable medium creates dynamic optical gratings, which can serve for a wide variety of applications, ranging from real-time holography to ultrasound generation. Typically, the interference occurs in a sample material that is separated from the laser sources. Here, we explore a new aspect of laser-induced dynamic gratings: microwave generation occurring inside the cavity of semiconductor laser frequency comb generators, such as quantum cascade lasers and interband cascade lasers. The interplay between laser light and microwaves in these devices reveals intracavity spatiotemporal phenomena that are of great importance for the understanding of their physics and for their operation to be efficient. Grating effects related to laser locking dynamics, structured cavities, and microwave propagation are demonstrated. Applications in science and technology based on these phenomena, including the realization of novel hybrid electronic–photonic devices, will also be presented.

A numerical model is developed for a phase-locked array of quantum cascade lasers. The population density is derived from rate equations. The temperature distribution for stationary generation is produced by ohmic heating. It is shown that the results of above-threshold operation modeling by the semi-vectorial beam propagation method are in good agreement with the modal analysis provided by the vectorial-COMSOL solver supplemented with the Rigrod’s model estimations. The wall-plug efficiency and the limits of the single-mode lasing are found. Thermal lensing is shown to be the main reason limiting the single-spatial-mode power under CW-operating conditions.

Quantum Cascade Lasers (QCLs) are compact and powerful sources of frequency combs in the mid-IR and THz region [1] , [2] . They naturally tend to operate on the fundamental comb state i.e. the mode spacing is given by the round-trip time of the cavity. Lately it was observed that free running mid-IR QCLs operate, under optical feedback reduction, in a so-called harmonic comb state where the mode spacing is a multiple of the fundamental one [3] . The origin arises from a single mode instability close to threshold which evolves to the harmonic state and normally falls into the fundamental comb state. Harmonic combs in the THz were so far only observed in mode-locked THz QCLs [4] .

New experiments with THz QCLs reveal the harmonic frequency comb regime, in which isolated modes are separated by several free spectral ranges and the optical spectrum does not have a reflection symmetry. Here we study the mechanism of the harmonic frequency combs. Our calculations show that the harmonic comb is favored if the QCL contains two lower laser states, with different but comparable strength of the optical transitions. The asymmetry between the two optical transitions can be linked to a linewidth enhancement factor. Our work can be extended to mid-IR QCLs with an asymmetric gain spectrum.

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.

THz quantum cascade lasers (QCL) frequency combs are recently attracting attention both as valuable sources for THz spectroscopy as well as a model system to study non linear generation and laser dynamics. Harmonic comb state has proven to be emerging in quantum cascade lasers and promoted by an interplay between parametric gain and spatial hole burning. We report here on robust, self-starting harmonic mode locking in double-metal THz quantum cascade lasers. Different harmonic orders can be excited in the same laser cavity depending on the pumping conditions. The harmonic state can be RF injected and frequency pulled. We study the dependence of harmonic state on the laser cavity and we also report results on harmonic state in external cavities under RF injection.

We report on high temperature, self-starting THz Quantum Cascade Laser harmonic frequency combs. Their coherence is assessed by electrical beatnote measurements and a self-mixing technique and their beatnote can be injection-locked to a RF source.

The recent study of ring quantum cascade lasers [1] , [2] (QCLs, Fig. 1a ) revealed a new laser instability. It is triggered by phase turbulence akin to the wave instabilities that occur in other nonlinear systems such as fluids, superconductors and Bose-Einstein condensates. The choice of the ring geometry took inspiration from Kerr combs [3] , that are commonly generated in passive ring microresonators and have attracted great attention within the photonics community in the last years thanks to their rich physics.

Popular models of the phenomenological relaxation operators that are widely used in the master equation formalism for open condensed-matter systems have significant flaws ranging from limited applicability to violation of fundamental physical principles. We propose a relatively simple universal model of the relaxation operator which is free from these flaws, has a correct static limit, correct direct-current limit in a uniform electric field, includes both interband and intraband transitions, and is valid for an arbitrary dispersion of quasiparticles in a solid. We use the proposed operator to generalize the Lindhard formula and derive explicit expressions for the relaxation operator for Dirac materials with an unconventional energy spectrum of quasiparticles, such as graphene and Weyl semimetals. We compare the linear susceptibility spectra for graphene obtained with different relaxation models and show that the proposed relaxation operator leads to physically meaningful behavior of the susceptibility at low frequencies whereas the existing models become completely invalid., 37 pages, 3 figures

We report systematic theoretical studies of the inverse Faraday effect in materials with massless Dirac fermions, both in two dimensions such as graphene and surface states in topological insulators, and in three dimensions such as Dirac and Weyl semimetals. Both semiclassical and quantum theories are presented, with dissipation and finite size effects included. We find that the magnitude of the effect can be much stronger in Dirac materials as compared to conventional semiconductors. Analytic expressions for the optically induced magnetization in the low temperature limit are obtained. Strong inverse Faraday effect in Dirac materials can be used for the optical control of magnetization, all-optical modulation, and optical isolation., 28 pages, 5 figures

We report on development of the mid-infrared antimonide based laser technology targeting dual wavelength operation for intra-cavity difference frequency generation. The devices utilize Y-branch architecture with high order DBR reflectors controlling the laser emission spectrum. The device active region contain asymmetric tunnel coupled quantum well with built in resonant second order nonlinearity. The epitaxially regrown photonic crystal surface emitting GaSb-based lasers will be demonstrated.

Passively mode-locked type-I quantum well cascade diode lasers emitting near 3.25 μm were designed, fabricated, and characterized. The deep etched ∼5.5-μm-wide single spatial mode ridge waveguide design utilizing split-contact architecture was implemented. The epi-side-up mounted 3-mm-long ridge lasers with both 2.7-mm-long gain and 300-μm-long absorber sections forward biased operated in the continuous wave regime with above 12 mW of output power at 20 °C. The devices with a reverse-biased absorber section generated ∼10 ps pulses at the repetition rate of ∼13.2 GHz with average power exceeding 1 mW. The laser emission spectrum in the mode-locked regime comprised a smooth bell-shape-like frequency comb covering about a 20 nm range at the full-width at half-maximum level. Second-order interferometric autocorrelation studies revealed a strong spectral chirp in the pulse. The intermodal beat note of the Lorentzian shape with a 180 kHz line width was observed, corresponding to pulse-to-pulse timing jitter of ...

A monochromatic wave that circulates in a nonlinear and dispersive optical cavity can become unstable and form a structured waveform. This phenomenon, known as modulation instability, was encountered in fiber lasers, optically pumped Kerr microresonators and, most recently, in monolithic ring quantum cascade lasers (QCLs). In ring QCLs, the instability led to generation of fundamental frequency combs—optical fields that repeat themselves once per cavity round trip. Here we show that the same instability may also result in self-starting harmonic frequency combs—waveforms that repeat themselves multiple times per round trip, akin to perfect soliton crystals in ring Kerr microresonators. We can tailor the intermode spacing of harmonic frequency combs by placing two minute defects with a well-defined separation between them along the ring waveguide. On-demand excitation of frequency comb states with few powerful modes spaced by hundreds of gigahertz may find their use in future sub-terahertz generators.

Optical frequency combs 1,2 establish a rigid phase-coherent link between microwave and optical domains and are emerging as high-precision tools in an increasing number of applications 3 . Frequency combs with large intermodal spacing are employed in the field of microwave photonics for radiofrequency arbitrary waveform synthesis 4,5 and for the generation of terahertz tones of high spectral purity in future wireless communication networks 6,7 . Here, we demonstrate self-starting harmonic frequency comb generation with a terahertz repetition rate in a quantum cascade laser. The large intermodal spacing caused by the suppression of tens of adjacent cavity modes originates from a parametric contribution to the gain due to temporal modulations of population inversion in the laser 8,9 . Using multiheterodyne self-detection, the mode spacing of the harmonic comb is shown to be uniform to within 5 × 10−12 parts of the central frequency. This new harmonic comb state extends the range of applications of quantum cascade laser frequency combs 10–13 . Self-starting harmonic frequency comb generation with a THz repetition rate in a q uantum cascade l aser is demonstrated . T he mode spacing uniformity is verified to within 5 × 10−12 parts of the central frequency. The findings extend the range of applications of quantum cascade laser frequency combs.

We demonstrate that ring lasers can attain multimode operation at a very low pumping level, contrary to common belief. The instability is explained via the Ginzburg-Landau theory, establishing a connection between laser and microresonator combs.

Using plasmonic nanofocusing coupled with ultrafast femtosecond pulses, we perform spatio-temporal nonlinear optical imaging of graphene and MoSe2. We resolve for the first time the nanoscale heterogeneity in the few-femtosecond coherent dynamics in two-dimensional materials.

Semiconductor ring lasers are miniaturized devices that operate on clockwise and counterclockwise modes. These modes are not coupled in the absence of intracavity reflectors, which prevents the formation of a standing wave in the cavity and, consequently, of a population inversion grating. This should inhibit the onset of multimode emission driven by spatial hole burning. Here we show that, despite this notion, ring quantum cascade lasers inherently operate in phase-locked multimode states, that switch on and off as the pumping level is progressively increased. By rewriting the master equation of lasers with fast gain media in the form of the complex Ginzburg-Landau equation, we show that ring frequency combs stem from a phase instability---a phenomenon also known in superconductors and Bose-Einstein condensates. The instability is due to coupling of the amplitude and phase modulation of the optical field in a semiconductor laser, which plays the role of a Kerr nonlinearity, highlighting a connection between laser and microresonator frequency combs.

Phase-locked array of diode lasers is an attractive way to achieve multi-Watt power single-mode operation. Theoretical analysis of the optical modes of planarized phase-locked quantum cascade laser arrays (QCLA) emitting at 4.8 pm was performed in [1], [2]. These high-index-contract photonic crystal (HC-PC) structures allow global coupling between all array elements. Our numerical modelling predicts that even in the presence of strong thermal lensing under CW operation, 3-element QCLA can achieve more than 1W power.

Since the days of Hertz, radio transmitters have evolved from rudimentary circuits emitting around 50 MHz to modern ubiquitous Wi-Fi devices operating at gigahertz radio bands. As wireless data traffic continues to increase, there is a need for new communication technologies capable of high-frequency operation for high-speed data transfer. Here, we give a proof of concept of a compact radio frequency transmitter based on a semiconductor laser frequency comb. In this laser, the beating among the coherent modes oscillating inside the cavity generates a radio frequency current, which couples to the electrodes of the device. We show that redesigning the top contact of the laser allows one to exploit the internal oscillatory current to drive a dipole antenna, which radiates into free space. In addition, direct modulation of the laser current permits encoding a signal in the radiated radio frequency carrier. Working in the opposite direction, the antenna can receive an external radio frequency signal, couple it to the active region, and injection lock the laser. These results pave the way for applications and functionality in optical frequency combs, such as wireless radio communication and wireless synchronization to a reference source.

Despite successful modeling of graphene as a 0.34-nm thick optical film synthesized by exfoliation or chemical vapor deposition (CVD), graphene induced shift of surface plasmon resonance (SPR) of gold films has remained controversial. Here we report the resolution of this controversy by developing a clean CVD graphene transfer method and extending Maxwell-Garnet effective medium theory (EMT) to 2D materials. A SPR shift of 0.24 is obtained and it agrees well with 2D EMT in which wrinkled graphene is treated as a 3-nm graphene/air layered composite, in agreement with the average roughness measured by atomic force microscope. Because the anisotropic built-in boundary condition of 2D EMT is compatible with graphene's optical anisotropy, graphene can be modelled as a film thicker than 0.34-nm without changing its optical property; however, its actual roughness, i.e., effective thickness will significantly alter its response to strong out-of-plane fields, leading to a larger SPR shift., 18 pages, 5 figures

Cascade pumping of type-I quantum well gain sections led to increase of the output power and efficiency of GaSb-based diode lasers operating in spectral region from 1.9 to 3.3 µm. The wide stripe multimode lasers based on cascade lasers heterostructures generate watt class output power levels up to 3 µm. The corresponding narrow ridge single spatial mode and single frequency mode distributed feedback devices generate tens of mW. The external cavity lasers utilizing gain chips based on cascade diode laser heterostructures demonstrate extra wide tuning range. The short pulse passively mode-locked lasers generate optical frequency combs.

Stable high-power narrow-linewidth operation of the 2.05–2.1 µm GaSb-based diode lasers was achieved by utilizing the sixth-order surface-etched distributed Bragg reflector (DBR) mirrors. The DBR multimode devices with 100 µm wide ridge waveguides generated ∼ 850 m W in the continuous wave (CW) regime at 20°C. The device CW output power was limited by thermal rollover. The laser emission spectrum was defined by Bragg reflector reflectivity at all operating currents in a wide temperature range. The devices operated at DBR line with detuning from gain peak exceeding 10 meV.

Harmonic comb state has proven to be emerging in quantum cascade lasers and promoted by an interplay between parametric gain and spatial hole burning. We report here on robust, pure, self-starting harmonic mode locking in Copper-based double-metal THz quantum cascade lasers. Different harmonic orders can be excited in the same laser cavity depending on the pumping condition and stable harmonic combs spanning more than 600 GHz bandwidth at 80 K are reported. Such devices can be RF injected and the free running coherence is assessed by means of self-mixing technique performed at 50 GHz. A theoretical model based on Maxwell-Bloch equations including an asymmetry in the gain profile is used to interpret the data., Comment: Submitted to APL, 6 pages, 5 figures

Light–matter interaction in two-dimensional photonic or phononic materials allows for the confinement and manipulation of free-space radiation at sub-wavelength scales. Most notably, the van der Waals heterostructure composed of graphene (G) and hexagonal boron nitride (hBN) provides for gate-tunable hybrid hyperbolic plasmon phonon-polaritons (HP3). Here, we present the anisotropic flow control and gate-voltage modulation of HP3 modes in G-hBN on an air–Au microstructured substrate. Using broadband infrared synchrotron radiation coupled to a scattering-type near-field optical microscope, we launch HP3 waves in both hBN Reststrahlen bands and observe directional propagation across in-plane heterointerfaces created at the air–Au junction. The HP3 hybridization is modulated by varying the gate voltage between graphene and Au. This modifies the coupling of continuum graphene plasmons with the discrete hBN hyperbolic phonon polaritons, which is described by an extended Fano model. This work represents the fir...

With its linear energy dispersion and large transition dipole matrix element, graphene is an attractive material for nonlinear optoelectronic applications. However, the mechanistic origin of its strong nonlinear response, the ultrafast coherent dynamics and the associated nanoscale phenomena have remained elusive due to a lack of suitable experimental techniques. Here, using adiabatic nanofocusing and imaging, we study the broadband four-wave mixing (FWM) response of graphene with nanometre and femtosecond spatio-temporal resolution. We detect a nonlinear signal enhancement at the edges and dependence on the number of layers from excitation areas as small as 104 carbon atoms. Femtosecond FWM nanoimaging and concomitant frequency-domain measurements reveal dephasing on T2 ≈ 6 ± 1 fs timescales, which we attribute to a strong electron–electron interaction. We also identify an unusual non-local FWM response on ~100–400 nm length scales, which we assign to a Doppler effect controlling the nonlinear interaction between the tip near-field momenta and the graphene electrons with high Fermi velocity. These results illustrate the distinct nonlinear nanooptical properties of graphene, expected also in related classes of two-dimensional materials, that could form the basis for improved nonlinear and ultrafast nanophotonic devices. Ultrafast nanoimaging reveals the origin of the nonlinear optical properties of graphene.

Laser dynamics encompasses universal phenomena that can be encountered in many areas of physics, such as bifurcation and chaos, mode competition, resonant nonlinearities, and synchronization---or locking---of oscillators. When a locking process occurs in a multimode laser, an optical frequency comb is produced, which is an optical spectrum consisting of equidistant modes with a fixed phase relationship. Describing the formation of self-starting frequency combs in terms of fundamental laser equations governing the field inside the cavity does not allow one, in general, to grasp how the laser synchronizes its modes. Our finding is that, in a particular class of lasers where the output is frequency modulated with small or negligible intensity modulation, a greatly simplified description of self-locking exists. We show that in quantum cascade lasers---solid-state representatives of these lasers characterized by an ultrashort carrier relaxation time---the frequency comb formation obeys a simple variational principle, which was postulated over 50 years ago and relies on the maximization of the laser output power. The conditions for the breakdown of this principle are also experimentally identified, shedding light on the behavior of many different types of lasers, such as dye, diode, and other cascade lasers. This discovery reveals that the formation of frequency-modulated combs is an elegant example of an optimization problem solved by a physical system.

We develop a rigorous quantum-mechanical theory of the nonlinear optical process of difference frequency generation of surface plasmon-polaritons in Landau-quantized graphene. Although forbidden in the electric-dipole approximation, the second-order susceptibility is surprisingly high, equivalent to the bulk magnitude above ${10}^{\ensuremath{-}3}$ m/V. We consider the graphene monolayer as a nonlinear optical component of a monolithic photonic chip with integrated pump fields. The nonlinear power conversion efficiency of the order of tens of $\ensuremath{\mu}\mathrm{W}/{\mathrm{W}}^{2}$ is predicted from structures of $10--100\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$ size. We investigate a variety of waveguide configurations to identify the optimal geometry for maximum efficiency.

The recently discovered ability of the quantum cascade laser to produce a harmonic frequency comb has attracted new interest in these devices for both applications and fundamental laser physics. In this review we present an extensive experimental phenomenology of the harmonic state, including its appearance in mid-infrared and terahertz quantum cascade lasers, studies of its destabilization induced by delayed optical feedback, and the assessment of its frequency comb nature. A theoretical model explaining its origin as due to the mutual interaction of population gratings and population pulsations inside the laser cavity will be described. We explore different approaches to control the spacing of the harmonic state, such as optical injection seeding and variation of the device temperature. Prospective applications of the harmonic state include microwave and terahertz generation, picosecond pulse generation in the mid-infrared, and broadband spectroscopy.

We consider a quantum-electrodynamic problem of the spontaneous emission from a two-dimensional (2D) emitter, such as a quantum well or a 2D semiconductor, placed in a quasi-2D waveguide or cavity with subwavelength confinement in one direction. We apply the Heisenberg-Langevin approach which includes dissipation and fluctuations in the electron ensemble and in the electromagnetic field of a cavity on equal footing. The Langevin noise operators that we introduce do not depend on any particular model of dissipative reservoir and can be applied to any dissipation mechanism. Moreover, our approach is applicable to nonequilibrium electron systems, e.g. in the presence of pumping, beyond the applicability of the standard fluctuation-dissipation theorem. We derive analytic results for simple but practically important geometries: strip lines and rectangular cavities. Our results show that a significant enhancement of the spontaneous emission, by a factor of order 100 or higher, is possible for quantum wells and other 2D emitters in a subwavelength cavity., Comment: 22 pages including Appendix and 2 figures

We observe enhanced second-harmonic generation in monolayer graphene in the presence of an ultra-strong terahertz field pulse with a peak amplitude of 250 kV/cm. This is a strongly nonperturbative regime of light-matter interaction in which particles get accelerated to energies exceeding the initial Fermi energy of 0.2 eV over a timescale of a few femtoseconds. The second-harmonic current is generated as electrons drift through the region of momenta corresponding to interband transition resonance at an optical frequency. The resulting strongly asymmetric distortion of carrier distribution in momentum space gives rise to an enhanced electric-dipole nonlinear response at the second harmonic. We develop an approximate analytic theory of this effect which accurately predicts observed intensity and polarization of the second-harmonic signal.

With the external injection of an optical seed, we demonstrate how to control the spectrum of multimode Fabry-Perot quantum cascade lasers by reaching a watt-level single-mode emission and tunable harmonic comb regime.

Controlling the spacing of self-starting harmonic frequency combs in QCLs by design of fundamental laser parameters is arduous. New ways to shape such combs by means of original electrical, optical and radiofrequency techniques are presented.

Free-running Fabry-Perot lasers normally operate in a single-mode regime until the pumping current is increased beyond the single-mode instability threshold, above which they evolve into a multimode state. As a result of this instability, the single-mode operation of these lasers is typically constrained to few percents of their output power range, this being an undesired limitation in spectroscopy applications. In order to expand the span of single-mode operation, we use an optical injection seed generated by an external-cavity single-mode laser source to force the Fabry-Perot quantum cascade laser into a single-mode state in the high current range, where it would otherwise operate in a multimode regime. Utilizing this approach we achieve single-mode emission at room temperature with a tuning range of $36 \, \mathrm{cm}^-1$ and stable continuous-wave output power exceeding 1 W. Far-field measurements show that a single transverse mode is emitted up to the highest optical power indicating that the beam properties of the seeded Fabry-Perot laser remain unchanged as compared to free-running operation., 4 figures, 10 pages

High-quality thin films of highly aligned semiconducting single-wall carbon nanotubes have been recently demonstrated. They have excellent absorption and photoluminescence properties; however, fast nonradiative recombination of carriers prevents their use as a gain medium in lasers. Here we predict that such films can operate as efficient sources of ultrashort radiation pulses under the conditions of superfluorescence, i.e. cooperative interband recombination of injected electrons and holes. Superfluorescence develops much faster than nonradiative recombination and leads to high-intensity, coherent pulses of near/mid-infrared radiation.

The third-order nonlinear susceptibility of Landau-quantized graphene is studied by degenerate time-integrated four-wave mixing in the THz regime. The revealed resonance behavior and the observed field dependencies are in agreement with our theoretical calculations.

Using degenerate four-wave mixing (DFWM), we have investigated the coherent polarization between the lowest Landau levels in graphene under resonant excitation with narrowband THz pulses. A pronounced DFWM signal is observed and its dependence on THz field strength and magnetic field detuning is explored and compared with theoretical expectations.

The single-mode to multimode transition in standing-wave quantum cascade lasers exhibits a harmonically mode-locked regime skipping cavity modes. The large sideband separation unravels a new mechanism for frequency comb generation seeded by the RNGH instability.

We show that a strong infrared laser beam obliquely incident on graphene can experience a parametric instability with respect to decay into lower-frequency (idler) photons and THz surface plasmons. The instability is due to a strong in-plane second-order nonlinear response of graphene which originates from its spatial dispersion. The parametric decay leads to efficient generation of THz plasmons and gives rise to quantum entanglement of idler photons and surface plasmon states.

We overview the results of recent experimental and theoretical studies of nonlinear dynamics of mid-infrared quantum cascade lasers (QCLs) associated with nonlinear interactions of laser modes. Particular attention is paid to phase-sensitive nonlinear mode mixing which turns out to be quite prominent in QCLs of different kinds and which gives rise to frequency and phase locking of laser modes. Nonlinear phase coupling of laser modes in QCLs leads to a variety of ultrafast and coherent phenomena: synchronization of transverse modes, beam steering, the RNGH multimode instability, and generation of mode-locked ultrashort pulses.

Self-starting harmonic frequency combs in quantum cascade lasers exhibit skipping of several tens of longitudinal modes of the cavity, producing widely spaced frequency combs which may be used for a number of applications, such as the generation of high-spectral-purity microwave and terahertz tones. Under pure electrical injection, the spacing of such combs is fixed by fundamental laser parameters and can hardly be controlled. Here, we demonstrate that harmonic frequency combs in quantum cascade lasers can be induced by optical injection of an external seed provided by a tunable source. This scheme enables wide tunability of the harmonic comb spacing, allowing the skipping between 44 and 171 longitudinal modes in a single device.Self-starting harmonic frequency combs in quantum cascade lasers exhibit skipping of several tens of longitudinal modes of the cavity, producing widely spaced frequency combs which may be used for a number of applications, such as the generation of high-spectral-purity microwave and terahertz tones. Under pure electrical injection, the spacing of such combs is fixed by fundamental laser parameters and can hardly be controlled. Here, we demonstrate that harmonic frequency combs in quantum cascade lasers can be induced by optical injection of an external seed provided by a tunable source. This scheme enables wide tunability of the harmonic comb spacing, allowing the skipping between 44 and 171 longitudinal modes in a single device.

In standing-wave lasers, spatial hole burning induces a static grating of the population inversion, enabling multimode operation with several independent lasing modes. In the presence of a mode-locking mechanism, these modes may become correlated, giving origin to a frequency comb. Quantum cascade lasers, owing to their ultrafast gain dynamics, are ideally suited to achieve comb operation. Here we experimentally demonstrate that the modes of a quantum cascade laser frequency comb coherently beat to produce time-dependent population inversion gratings, which spatially modulate the current in the device at frequencies equal to the mode separation and its higher harmonics. This phenomenon allows the laser to serve as a phased collection of microwave local oscillators and is utilized to demonstrate quadrature amplitude modulation, a staple of modern communications. These findings may provide for a new class of integrated transmitters, potentially extending from the microwave to the low terahertz band.

Terahertz (THz) quantum cascade lasers (QCLs) are currently the most advanced electrically pumped semiconductor lasers in the spectral range 1-5 THz. However, their operation at room temperature is still an unresolved challenge. In this paper, we discuss our efforts to improve the temperature performance of these devices. In particular, we present THz QCLs that approach thermoelectric cooled operation and discuss factors that limit their high-temperature performance. We also discuss a different type of THz QCL source that produces coherent THz radiation without population inversion across the THz transition. These devices are based on intracavity difference-frequency generation in dual-wavelength mid-IR QCLs, and can now provide microwatt levels of coherent THz radiation up to room temperature. We discuss how the output power of these devices can be further improved to produce milliwatts of THz radiation at room temperature.

We propose mid/far-infrared photodetectors based on coherent sum-frequency generation in a near-resonant cascade of interband and intersubband transitions in high optical nonlinearity asymmetric quantum well structures. Such structures can yield high detectivity and responsivity in the bandwidth of the order of 30% of a central frequency in the mid-infrared range. Resonant up-conversion detectors can be designed for both collinear and perpendicular pump and signal beams. They can be integrated with semiconductor pump lasers to yield compact devices. We present specific device designs based on GaAs/AlGaAs and InGaAs/AlInAs heterostructures and calculate their expected figures of merit.

We investigated the laser pulse-width dependence of dense plasmas confined within the magnetic length of $In_{0.2}Ga_{0.8}As$ /GaAs multiple quantum wells under high magnetic fields up to 31 T. To fully fill the Landau levels of effectively zero-dimensional system, we used intense femtosecond (fs) laser pulses to create carrier densities near $10^{13}/cm^2$ . The observed photoluminescence showed a characteristic of superfluorescence, above critical magnetic field when being excited by pulses shorter than coherence buildup time.