Terahertz Quantum Cascade Laser Source Based on Intra-Cavity Difference-Frequency Generation

A monolithic tunable single mode terahertz quantum cascade laser source based on intra-cavity difference frequency generation with a tuning range of ~580 GHz is demonstrated at room temperature.

Terahertz sources based on intracavity difference-frequency generation in mid-infrared quantum cascade lasers (THz DFG-QCLs) have recently emerged as the first monolithic electrically pumped semiconductor sources capable of operating at room temperature across the 1- to 6-THz range. Despite tremendous progress in power output, which now exceeds 1 mW in pulsed and 10 μW in continuous-wave regimes at room temperature, knowledge of the major figure of merits of these devices for high-precision spectroscopy, such as spectral purity and absolute frequency tunability, is still lacking. By exploiting a metrological grade system comprising a terahertz frequency comb synthesizer, we measure, for the first time, the free-running emission linewidth (LW), the tuning characteristics, and the absolute center frequency of individual emission lines of these sources with an uncertainty of 4 × 10-10. The unveiled emission LW (400 kHz at 1-ms integration time) indicates that DFG-QCLs are well suited to operate as local oscillators and to be used for a variety of metrological, spectroscopic, communication, and imaging applications that require narrow-LW THz sources.

Room-temperature terahertz (THz) quantum cascade laser sources with intracavity difference-frequency nonlinear mixing are electrically pumped monolithic semiconductor laser sources operating in the 0.6–6 THz spectral range. We report widely tunable, low-frequency THz quantum cascade laser sources using a lens-coupled Cherenkov waveguide scheme. Based on a watt-class high-power, λ ∼ 13.7 μm quantum cascade laser, the monolithic THz source is strongly coupled with a high-resistivity silicon lens, which causes a major increase in the THz coupling efficiency and demonstrates significant performance improvements. A room-temperature 1.5 THz device produces a 0.2 mW peak output power with a high-quality beam pattern. Improved THz outcoupling efficiency using the lens-coupled scheme enabled the demonstration of a high-performance external-cavity semiconductor THz source that is tunable from 420 GHz to 2 THz. The external-cavity, lens-coupled device configuration can technically be assembled into a butterfly-style package for a thumb-sized, widely frequency tunable THz semiconductor source.

Room-temperature, narrow-linewidth terahertz quantum cascade laser sources based on intracavity Cherenkov difference-frequency generation with emission at 3.5 THz, nearly 60µW power output, and conversion efficiency of approximately 0.55 mW/W2 are reported.

We discuss the design and operation of widely-tunable terahertz sources based on Cherenkov intra-cavity difference-frequency generation in mid-infrared quantum cascade lasers. Laser chips are integrated into a Littrow-type external cavity system. Devices demonstrate continuous terahertz emission tuning at room temperature with a record tuning range from 1.2 THz to 5.9 THz and peak power output varying between 5 and 90 μW, depending on the operating frequency. Beam steering of terahertz Cherenkov emission with frequency is suppressed and mid-infrared-to-terahertz conversion efficiency is improved by bonding devices onto high-resistivity silicon substrates that have virtually no refractive index dispersion and vanishingly-small optical loss in 1–6 THz range.

We analyze the influence of conduction-band nonparabolicity on bound electronic states in the active region of a quantum cascade laser (QCL). Our model assumes expansion of the conduction-band dispersion relation up to a fourth order in wavevector and use of a suitable second boundary condition at the interface of two III-V semiconductor layers. Numerical results, obtained by the transfer matrix method, are presented for two mid-infrared GaAs/Al0.33Ga0.67As QCL active regions, and they are in very good agreement with experimental data found in the literature. Comparison with a different nonparabolicity model is presented for the example of a GaAs/Al0.38Ga0.62As-based mid-IR QCL. Calculations have also been carried out for one THz QCL structure to illustrate the possible application of the model in the terahertz part of the spectrum.

Intrinsic photoconductivity of CdS in the band edge region

The single lobe far-field patterns produced from terahertz quantum cascade lasers (QCLs) are greatly demanded for various applications, such as imaging, data transmission, etc. However, for a ridge waveguide terahertz QCL, the far-field beam divergence is large due to the fact that the waveguide aperture is far smaller than the terahertz wavelength. This is the case typically for double-metal waveguide terahertz QCL which emits terahertz photons in almost every direction in the space. Even for a single plasmon waveguide terahertz QCL, the divergence angle is as large as 30 in both horizontal and vertical direction. Here, in this work we design and fabricate a double metal third-order distributed feedback terahertz QCL emitting around 4.3 THz, and investigate the characteristics of the longitudinal and transverse modes. This work aims to achieve high beam quality for terahertz QCL by exploiting the third-order distributed feedback geometry, and in the meantime to achieve single longitudinal mode operation. The electromagnetic field distribution in the waveguide is modelled by employing a finite element method. The mode selection mechanism is studied by using the eigen frequency analysis, and the far-field beam is simulated by applying the near-field to far-field Fourier transform technique. The QCL active region used in this work is based on the resonant-phonon design, which is grown by a molecular beam epitaxy (MBE) system on a semi-insulating GaAs (100) substrate. The wafer bonding and traditional semiconductor device fabrication technology, i.e., optical lithography, electron beam evaporation, lift-off, wet and dry etching, are used to process the MBE-growth wafer into the third-order distributed feedback geometry with double-metal waveguides. By carefully designing the grating structures and optimizing the fabrication process, we achieve third-order distributed feedback terahertz QCL with quasi-single-longitudinal mode operation and single lobe far-field beam pattern with low beam divergence in both vertical and horizontal directions. The effect of grating duty cycle on the far-field beam divergence is systematically studied theoretically and experimentally. By the simulation, we finally achieve the divergence angle of 1213 for a third-order distributed feedback laser with a grating duty cycle of 12% that results in an effective refractive index close to 3. The experimental results show good agreement with the simulation. There is still room to further reduce the beam divergence of third-order distributed feedback terahertz QCL by improve the accuracy of the simulation and the fabrication.

In this paper, we consider terahertz sources based on Čerenkov difference frequency generation in a dual-wavelength mid-infrared quantum cascade laser and calculate analytically the optical external efficiency of the out-going terahertz wave. The mid-infrared pumps operate simultaneously and have a common upper level. For short cavity lengths and low terahertz losses in the substrate, we find that implementation of the Čerenkov emission scheme in terahertz difference frequency generation quantum cascade lasers improves the optical external efficiency dramatically. The maximum value of the latter is close to 19%, but is only attainable if the laser exhibits negligible mid-infrared transmission losses.

Regrowth of quantum cascade laser active regions on metamorphic buffer layers

We present a detailed experimental and theoretical study of a $\mathrm{GaAs}∕\mathrm{AlGaAs}$ mid-infrared quantum cascade laser (QCL) under a strong magnetic field. The emitted power and the magnetoresistance show strong oscillations as a function of $B$, due to a magnetic field modulation of the lifetime of the upper state of the laser transition. The analysis of these oscillations shows a modulation of the electron lifetime due to inelastic (LO-phonon emission) and elastic scattering mechanisms. We show that at low temperature, the interface roughness scattering is the most efficient elastic relaxation mechanism; it becomes dominant whenever LO-phonon emission is inhibited by the magnetic field. By comparing experimental and theoretical results, we are able to estimate the relative weight of elastic and inelastic scattering. The magnetic field is thus a powerful spectroscopic tool to study the scattering mechanisms in QCL active regions.

We report terahertz quantum cascade laser sources based on intra-cavity difference-frequency generation processed into double-metal waveguides with surface-grating outcouplers. Over 112 μW of peak power output is produced at room temperature at 1.9 THz.

We present a novel device concept for a low threshold terahertz source. Our device is based on difference frequency generation in a dual-wavelength quantum cascade laser for terahertz emission and utilizes a Cerenkov phase matching scheme. The active region is an injectorless design to reach lower current threshold densities than injectorbased devices.

The transmission coefficients and wave function of electron in single period of resonant phonon (RP) Terahertz Quantum Cascade Laser active region were calculated with bias by Airy function substitution and transmission matrix methods, distribution of the electron wave function and relationship curve of quasi-bound level (Subband) and bias were obtained. Based on simulation results, a RP Terahertz Quantum Cascade Laser active region structure was designed. The simulation results showed that THz lasing appeared when bias of SP structure was 51.67mV. The wave function corresponding with E2 and E1 energy level could overlap at lasing region and related lasing wave which wavelength was 90.0μm and frequency was 3.2 THz was within THz frequency. At population inversion region, the transition energy from E1 to E0 was 38.62meV, which was approximate to the energy of longitudinal optical (LO) phonon. It was good for achieving large population inversions.

Recently we investigated cascaded schemes for down-conversion to the mid-IR spectral range above 5 μm by implementing the second stage based on a non-oxide nonlinear crystal inside the cavity of a ns optical parametric oscillator (OPO) based on Rb:PPKTP, pumped by a Q-switched Nd:YAG laser [1-3]. Intracavity difference-frequency generation (DFG) [2, 3] turned out to be more efficient compared to intracavity pumped OPO [1], however wavelength tuning is restricted and a broadband dichroic half-wave plate is needed for either type-I or type-II phase-matching in the studied crystals of AgGaSe2 and BaGa4Se7. Low-symmetry cubic semiconductors, such as orientation-patterned GaAs offer greater variety of phase-matching configurations in which intracavity polarization rotation becomes redundant. With a similar spectral coverage of up to ∼18 μm and 2 to 3 times higher effective nonlinearity, OPGaAs exhibits 50 to 100 times higher thermal conductivity, better damage resistivity, and offers the unique possibility of convenient temperature tuning in both stages in non-critical configurations. These advantages point at possible extensions to much higher repetition rates (1–100 kHz). On one hand such low energy regimes will utilize perfectly the limited aperture of the presently available OPGaAs samples but on the other hand will enable the generation of unprecedented average powers in the mid-IR spectral range. In a first demonstration of such intracavity DFG we implement OPGaAs in the same Rb:PPKTP OPO operated at 100 Hz [1-3], using tighter focusing.