Difference-Frequency Generation Terahertz Quantum Cascade Lasers with Surface Grating Outcouplers

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.

: This report results from a contract tasking University of Cambridge as follows: Description of Work: The first phase of the project aims to reproduce the performance figures from the reference THz QCL design. To achieve high temperature operation the device utilises a double-metal (DM) waveguide. This utilises a novel gold-to-gold thermal compression wafer bonding technique that has been developed at the Cavendish Laboratory which incorporates a self-aligned dry etch process. Standard electrical (I-V) and lasing emission (L-I, L-f, L-T, electroluminescence) characterisation will be taken and compared with the simulation data. Following validation of the USAF modelling code the first LO phonon injection design will be fabricated. The design aims to directly inject electrons into the upper state of the lasing transition through an LO phonon transition; with the aim of preserving the upper state lifetime at elevated temperatures. Again standard characterisation of this device in a DM waveguide will be performed. Depending on the promise of this design philosophy, further iterative improvements will be scheduled. If the above device shows significantly degraded performance an alternative design approach will be adopted.

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.

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.

Terahertz quantum cascade laser sources based on intra-cavity nonlinear frequency mixing are currently the only electrically pumped monolithic semiconductor light source operating in THz frequency range at room temperature. We demonstrated the power enhancement of the devices in 0.4-3 THz range by using a lens-coupled Cherenkov radiation scheme. Adopting this scheme, the device produces ∼1 milli-watt peak power at a frequency of 2.38 THz at room temperature, which was increased by a factor of approximately four compared to that of a conventional Cherenkov extraction scheme.

This paper aims to integrate AlInAs/InGaAs quantum well structures with high second-order nonlinear susceptibility into a dual-wavelength quantum cascade laser (QCL) for difference frequency generation (DFG) of THz radiation. The QCL active region contains two sections: a section with 30 cascades of bound-to-continuum design with integrated optical nonlinearity emitting at 8.7 mum and a section with 20 cascades of two-phonon design emitting at 7.6 mum. The nonlinear susceptibility for the DFG process is found to have a value of chi <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(2)</sup> ~4times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> pm/V.

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.

Terahertz (THz) quantum cascade laser sources based on optical nonlinearity are the only electrically pumped monolithic semiconductor sources operable at room temperature in the 0.6–6 THz range. We investigated the temperature dependence of the imaging characteristics of a broadband THz nonlinear quantum cascade laser and evaluated several important properties: the spectrum, far-field pattern and THz imaging results. Consequently, we found that the far-field patterns were single-lobed Gaussian-like, and THz images were well-resolved despite the lower operating temperature of the device. The stable temperature-performance indicates that this broadband THz source is promising for THz imaging applications.

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.

Terahertz (THz) Quantum Cascade Lasers (QCLs) have been of large interest in the context of very broadband and compact frequency combs in the THz spectral range [1]. For THz QCLs, the double metal waveguide proved to be advantageous in terms of bandwidth, compactness and low dispersion. However, as it confines the optical mode to deeply subwavelength dimensions, the output beam is highly divergent, as displayed in Fig. 1(a). Previous solutions addressing this issue were either impractical or not so broadband [2,3]. To improve the far-field properties of the double metal waveguide across a very large frequency range, we designed, fabricated and characterized a broadband, narrow beam Vivaldi Antenna [4].

The active region of a THz (terahertz) quantum-cascade laser based on three tunnel-coupled GaAs/Al0.15Ga0.85As quantum wells with a resonance-phonon depopulation scheme is designed. Energy levels, matrix elements of dipole transitions, and gain spectra are calculated as functions of the applied electric-field strength F and temperature. It is shown that the maximum gain is implemented at a frequency of 3.37 THz and F = 12.3 kV/cm. Based on the proposed design, a quantum-cascade laser emitting at ~3.3 THz with a double metal waveguide and Tmax ~ 84 K is fabricated. The activation energy Ea = 23 meV for longitudinal-optical (LO) phonon emission upon the stimulated recombination of hot electrons from the upper laser level to the lower one is determined from the Arrhenius temperature dependence of the output power.

We present a terahertz quantum cascade laser based on InGaAs/InAlAs/InP material system with double metal waveguide that reaches operating temperature of 123K with continuous wave power output 4.5mW at 10K and slope efficiency comparable to GaAs/AlGaAs counterparts.

Previous research has demonstrated a reduction in peak power via isokinetic dynamometry and power testing. The purpose of this study was to determine the effects of static stretching (SS) on the Wingate test (WAnT). Thirteen recreationally active males (mean age: 22.5±1.9 years, height: 172.5±8.4 cm, body mass: 83.4±24.6 kg) were tested for peak (PP), average (AP), and minimum (MP) power output using the WAnT. Two WAnT trials were completed by each subject on a Monark 894-E ergometer that was interfaced with a desktop computer. Participants completed a 5-minute warm-up on the cycle ergometer and then performed the pre-WAnT. Upon completion, the stretching protocol four static stretches were performed: one was participant self-administered and the remaining three by the investigator. Each stretch was held for 30 seconds and was repeated 4 times on each leg and repeated after 4 minutes of rest. Significant differences (P<0.05) were found for PP only for the pre- versus post-WAnT (802.6±207.6 vs. 768.8±199.7 W). Static stretching before performance of the WAnT demonstrated a significant decrease in peak power output. Based upon these results, it appears that SS decreases peak power output when performing an anaerobic capacity test on a cycle ergometer.

In this study, AlGaN-based ultraviolet-B band laser diodes with 150-mW peak output power in pulsed operation were demonstrated at room temperature. The oscillation wavelength, differential quantum efficiency, and slope efficiency of a laser diode were 300 nm, 3.6%, and 0.15 W/A, respectively. These results were obtained by increasing the injection efficiency and decreasing the positive fixed polarization charge formed at the interface between a p-side waveguide layer and an electron blocking layer when polarization doping is formed in a p-AlGaN cladding layer.