Alexey Belyanin

Room temperature terahertz quantum cascade laser source based on intracavity difference-frequency generation

We report on our progress in the development of a terahertz quantum cascade laser source based on intracavity terahertz difference-frequency mixing in a dual-wavelength mid-infrared quantum cascade laser with the active region engineered to possess giant second-order nonlinear susceptibility. In this letter, we demonstrate devices that operate in mid-infrared at λ1=8.9μm and λ2=10.5μm and produce terahertz output at λ≈60μm via difference-frequency generation with 7μW output power at 80K, 1μW output at 250K, and still approximately 300nW output at 300K.

Raman injection laser.

Stimulated Raman scattering is a nonlinear optical process that, in a broad variety of materials, enables the generation of optical gain at a frequency that is shifted from that of the incident radiation by an amount corresponding to the frequency of an internal oscillation of the material. This effect is the basis for a broad class of tunable sources known as Raman lasers. In general, these sources have only small gain (∼ 10-9 cm W-1) and therefore require external pumping with powerful lasers, which limits their applications. Here we report the realization of a semiconductor injection Raman laser designed to circumvent these limitations. The physics underlying our device differs in a fundamental way from existing Raman lasers: it is based on triply resonant stimulated Raman scattering between quantum-confined states within the active region of a quantum cascade laser that serves as an internal optical pump—the device is driven electrically and no external laser pump is required. This leads to an enhancement of orders of magnitude in the Raman gain, high conversion efficiency and low threshold. Our lasers combine the advantages of nonlinear optical devices and of semiconductor injection lasers, and could lead to a new class of compact and wavelength-agile mid-and far-infrared light sources.

Mode-Locked Pulses from Mid-Infrared Quantum Cascade Lasers

In this study, we report the unequivocal demonstration of midinfrared mode-locked pulses from quantum cascade lasers. The train of short pulses was generated by actively modulating the current and hence the gain of an edge-emitting quantum cascade laser (QCL). Pulses with duration of about 3 ps at full-width-at-half-maxima and energy of 0.5 pJ were characterized using a second-order interferometric autocorrelation technique based on a nonlinear quantum well infrared photodetector. The mode-locking dynamics in the QCLs was modeled based on the Maxwell-Bloch equations in an open two-level system. Our model reproduces the overall shape of the measured autocorrelation traces and predicts that the short pulses are accompanied by substantial wings as a result of strong spatial hole burning. The range of parameters where short mode-locked pulses can be formed is found.

Coherent instabilities in a semiconductor laser with fast gain recovery

We report the observation of a coherent multimode instability in quantum cascade lasers QCLs, which is driven by the same fundamental mechanism of Rabi oscillations as the elusive Risken-Nummedal-Graham- Haken RNGH instability predicted 40 years ago for ring lasers. The threshold of the observed instability is significantly lower than in the original RNGH instability, which we attribute to saturable-absorption nonlinearity in the laser. Coherent effects, which cannot be reproduced by standard laser rate equations, can play therefore a key role in the multimode dynamics of QCLs, and in lasers with fast gain recovery in general.

High-Temperature Operation of Terahertz Quantum Cascade Laser Sources

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.

Optimized second-harmonic generation in quantum cascade lasers

Optimized second-harmonic generation (SHG) in quantum cascade (QC) lasers with specially designed active regions is reported. Nonlinear optical cascades of resonantly coupled intersubband transitions with giant second-order nonlinearities were integrated with each QC-laser active region. QC lasers with three-coupled quantum-well (QW) active regions showed up to 2 /spl mu/W of SHG light at 3.75 /spl mu/m wavelength at a fundamental peak power and wavelength of 1 W and 7.5 /spl mu/m, respectively. These lasers resulted in an external linear-to-nonlinear conversion efficiency of up to 1 /spl mu/W/W/sup 2/. An improved 2-QW active region design at fundamental and SHG wavelengths of 9.1 and 4.55 /spl mu/m, respectively, resulted in a 100-fold improved external linear-to-nonlinear power conversion efficiency, i.e. up to 100 /spl mu/W/W/sup 2/. Full theoretical treatment of nonlinear light generation in QC lasers is given, and excellent agreement with the experimental results is obtained. For the best structure, a second-order nonlinear susceptibility of 4.7/spl times/10/sup -5/ esu (2/spl times/10/sup 4/pm/V) is calculated, about two orders of magnitude above conventional nonlinear optical materials and bulk III-V semiconductors.

Efficient nonlinear generation of THz plasmons in graphene and topological insulators.

Surface plasmons in graphene may provide an attractive alternative to noble-metal plasmons due to their tighter confinement, peculiar dispersion, and longer propagation distance. We present theoretical studies of the nonlinear difference frequency generation (DFG) of terahertz surface plasmon modes supported by two-dimensional layers of massless Dirac electrons, which includes graphene and surface states in topological insulators. Our results demonstrate strong enhancement of the DFG efficiency near the plasmon resonance and the feasibility of phase-matched nonlinear generation of plasmons over a broad range of frequencies.

Improvement of second-harmonic generation in quantum-cascade lasers with true phase matching

About a 100-fold improvement of the second-harmonic generation in a quantum-cascade laser with integrated optical nonlinearity was obtained by including phase-matching considerations in the design of the deep-etched ridge waveguide. The waveguide layer structure was optimized to minimize the phase mismatch of the zero-order mode of the fundamental light with the second-order transverse mode of the second-harmonic light. Exact phase matching is made possible by the faster decrease of the modal refractive index of the fundamental light with decreasing ridge width relative to the refractive index of the second-harmonic light. Up to 240 μW of the second-harmonic power and a nonlinear power conversion efficiency of up to 36 mW/W2 were achieved.

Giant optical nonlinearity of graphene in a strong magnetic field

We show that graphene in a magnetic field possesses by far the highest third-order optical nonlinearity among all known materials. The giant nonlinearity originates from unique electronic properties and selection rules for transitions between Landau levels near the Dirac point. As a result, even one monolayer of graphene gives rise to appreciable nonlinear frequency conversion efficiency for incident infrared radiation. We present quantum-mechanical density-matrix formalism for calculating the nonlinear optical response of graphene in the magnetic field, valid for arbitrarily strong magnetic and optical fields.

Constraints on the extremely high-energy cosmic ray accelerators from classical electrodynamics

We formulate the general requirements, set by classical electrodynamics, on the sources of extremely high-energy cosmic rays (EHECRs). It is shown that the parameters of EHECR accelerators are strongly limited not only by the particle confinement in large-scale magnetic fields or by the difference in electric potentials (generalized Hillas criterion) but also by the synchrotron radiation, the electro-bremsstrahlung, or the curvature radiation of accelerated particles. Optimization of these requirements in terms of an accelerators size and magnetic field strength results in the ultimate lower limit to the overall source energy budget, which scales as the fifth power of attainable particle energy. Hard \ensuremath{\gamma} rays accompanying generation of EHECRs can be used to probe potential acceleration sites. We apply the results to several populations of astrophysical objects\char22{}potential EHECR sources\char22{}and discuss their ability to accelerate protons to