Seungyong Jung

Broadly tunable monolithic room-temperature terahertz quantum cascade laser sources

Electrically pumped room-temperature semiconductor sources of tunable terahertz radiation in 1-5 THz spectral range are highly desired to enable compact instrumentation for THz sensing and spectroscopy. Quantum cascade lasers with intra-cavity difference-frequency generation are currently the only room-temperature electrically pumped semiconductor sources that can operate in the entire 1-5 THz spectral range. Here we demonstrate that this technology is suitable to implementing monolithic room-temperature terahertz tuners with broadband electrical control of the emission frequency. Experimentally, we demonstrate ridge waveguide devices electrically tunable between 3.44 and 4.02 THz.

External cavity terahertz quantum cascade laser sources based on intra-cavity frequency mixing with 1.2–5.9 THz tuning range

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 generation in mid-infrared quantum cascade lasers with a dual-upper-state active region

We report the performance of room temperature terahertz sources based on intracavity difference-frequency generation in mid-infrared quantum cascade lasers with a dual-upper-state (DAU) active region. DAU active region design is theoretically expected to produce larger optical nonlinearity for terahertz difference-frequency generation, compared to the active region designs of the bound-to-continuum type used previously. Fabricated buried heterostructure devices with a two-section buried distributed feedback grating and the waveguide designed for Cherenkov difference-frequency phase-matching scheme operate in two single-mode mid-infrared wavelengths at 10.7 μm and 9.7 μm and produce terahertz output at 2.9 THz with mid-infrared to terahertz conversion efficiency of 0.8 mW/W2 at room temperature.

Spectroscopic Study of Terahertz Generation in Mid-Infrared Quantum Cascade Lasers

Terahertz quantum cascade laser sources based on intra-cavity difference-frequency generation are currently the only room-temperature mass-producible diode-laser-like emitters of coherent 1–6 THz radiation. Device performance has improved dramatically over the past few years to reach milliwatt-level power output and broad tuning from 1.2 to 5.9 THz, all at room-temperature. Terahertz output in these sources originates from intersubband optical nonlinearity in the laser active region. Here we report the first comprehensive spectroscopic study of the optical nonlinearity and investigate its dependence on the mid-infrared pump frequencies. Our work shows that the terahertz generation efficiency can vary by a factor of 2 or greater depending on the spectral position of the mid-infrared pumps for a fixed THz difference-frequency. We have also measured for the first time the linewidth for transitions between the lower quantum cascade laser states, which is critical for determining terahertz nonlinearity and predicting optical loss in quantum cascade laser waveguides.

GaSb based light emitting diodes with strained InGaAsSb type I quantum well active regions

Mid-IR (λ≈3–3.5 μm) light emitting diodes with quinternary AlInGaAsSb barriers and InGaAsSb strained quantum wells grown on GaSb substrates have been demonstrated. The devices produced a quasi-cw emission power of 0.7 mW at room temperature and 2.5 mW at T=80 K.

High dimensional addressable LED arrays based on type I GaInAsSb quantum wells with quinternary AlGaInAsSb barriers

GaSb-based type I InGaAsSb quantum well mid-infrared (mid-IR) light-emitting diodes (LEDs) operated at wavelengths up to 3.66 µm are demonstrated. The application of quinternary AlGaInAsSb barriers improved hole confinement in the quantum wells and enabled an LED radiant excitance of 1.3 W cm−2 (λ = 3.66 µm) at 100 K which corresponds to the emittance of a blackbody at 1350 K. High-contrast individually addressed 512 × 512 LED arrays were designed and fabricated using wet etching. An accurate characterization technique for mid-IR LEDs has been developed.

Dual wavelength GaSb based type I quantum well mid-infrared light emitting diodes

We have designed and developed dual wavelength type I quantum well light emitting diodes (LEDs) operating at 2 μm and 3–3.4 μm wavelengths with independently controlled intensities. The room temperature quasicontinuous wave output power was 2.8 mW at 2 μm and 0.14 mW at 3 μm. The design of the dual wavelength structure allows for monolithically integrating LED pixels with different wavelengths opening the way for the fabrication of multiwavelength LED arrays for multispectral and hyperspectral imaging applications.

Widely tunable terahertz source based on intra-cavity frequency mixing in quantum cascade laser arrays

We demonstrate a compact monolithic terahertz source continuously tunable from 1.9 THz to 3.9 THz with the maximum peak power output of 106 μW at 3.46 THz at room temperature. The source consists of an array of 10 electrically tunable quantum cascade lasers with intra-cavity terahertz difference-frequency generation. To increase fabrication yield and achieve high THz peak power output in our devices, a dual-section current pumping scheme is implemented using two electrically isolated grating sections to independently control gain for the two mid-IR pumps.

Recent Progress in Widely Tunable Single-Mode Room Temperature Terahertz Quantum Cascade Laser Sources

We present the operating principle, design, and performance of external-cavity (EC) and monolithic terahertz (THz) tuners based on intracavity difference-frequency generation (DFG) in midinfrared (mid-IR) quantum cascade lasers (QCLs). A DFG-QCL gain chip employed in a Littrow-type THz EC system was optimized for wide tunability in 1-6 THz range using broad mid-IR gain bandwidth active region with integrated optical nonlinearity and Cherenkov THz phase-matching scheme. The EC system demonstrated ultrabroadband single-mode tuning from 1.2 to 5.9 THz. Beam steering in THz far field due to refractive index dispersion of InP substrate has been successfully suppressed by bonding the QCL chip on a high-resistivity silicon substrate. We also discuss the design of compact widely tunable monolithic THz DFG-QCL sources. Devices use two electrically isolated grating sections for independent electronic tuning of the two mid-IR pump frequencies. THz DFG frequency tuning from 3.44 to 4.02 THz, corresponding to 580 GHz or 15% of center frequency, is obtained.

Spectral purity and tunability of terahertz quantum cascade laser sources based on intracavity difference-frequency generation

Difference frequency generation quantum cascade lasers are well-suited for applications requiring narrow-linewidth emitters. 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.