Karun Vijayraghavan

Broadly tunable terahertz generation in mid-infrared quantum cascade lasers

Room temperature, broadly tunable, electrically pumped semiconductor sources in the terahertz spectral range, similar in operation simplicity to diode lasers, are highly desired for applications. An emerging technology in this area are sources based on intracavity difference-frequency generation in dual-wavelength mid-infrared quantum cascade lasers. Here we report terahertz quantum cascade laser sources based on an optimized non-collinear Cherenkov difference-frequency generation scheme that demonstrates dramatic improvements in performance. Devices emitting at 4 THz display a mid-infrared-to-terahertz conversion efficiency in excess of 0.6 mW W(-2) and provide nearly 0.12 mW of peak power output. Devices emitting at 2 and 3 THz fabricated on the same chip display 0.09 and 0.4 mW W(-2) conversion efficiencies at room temperature, respectively. High terahertz-generation efficiency and relaxed phase-matching conditions offered by the Cherenkov scheme allowed us to demonstrate, for the first time, an external-cavity terahertz quantum cascade laser source tunable between 1.70 and 5.25 THz.

Terahertz sources based on Čerenkov difference-frequency generation in quantum cascade lasers

We report room-temperature terahertz sources based on Cerenkov difference-frequency generation in dual-wavelength mid-infrared quantum cascade lasers with giant resonant optical nonlinearities originating from intersubband transitions. A Cerenkov difference-frequency generation scheme allows for extraction of THz radiation along the whole length of the laser waveguide and provides directional terahertz emission. Experimentally, our sources demonstrate a conversion efficiency of up to 70 μW/W2 approximately an order of magnitude improvement over the previous reports.

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.

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.

GaAs/Al0.15Ga0.85As terahertz quantum cascade lasers with double-phonon resonant depopulation operating up to 172 K

We report the design and performance of GaAs/Al0.15Ga0.85As terahertz quantum cascade lasers with double-phonon resonant depopulation and a vertical lasing transition. Lasing at 3 THz was observed up to a heat-sink temperature of 172 K.

Improved terahertz quantum cascade laser with variable height barriers

Using an ensemble Monte-Carlo analysis, it is found that relaxing the constraint of identical barrier heights can result in an improved temperature performance. Exploiting this additional design degree of freedom, modified structures with non-uniform barrier heights are developed based on the current record temperature design. For an optimized structure with reduced diagonality, we predict an increase of 31 K for the maximum operating temperature. Furthermore, we develop improved designs with the same oscillator strength as for the reference design. Using a genetic algorithm for optimization, an improvement of the maximum operating temperature by 38 K is obtained. These results aim to show the potential of varying the barrier heigths for the design of high temperature performance terahertz quantum cascade lasers.

THz Difference-Frequency Generation in MOVPE-Grown Quantum Cascade Lasers

We report mass-producible room-temperature electrically-pumped THz sources based on intra-cavity difference-frequency generation in mid-infrared InGaAs/AlInAs/InP quantum cascade lasers. Devices are grown by a commercial foundry using metal organic vapor phase epitaxy. A dual-stack active region possessing giant optical nonlinearity for 3.5 THz generation and a non-collinear Cherenkov waveguide THz outcoupling scheme is employed. Fabry-Pérot devices provided broad emission in the 3-4 THz range with a peak power of 5 μW. Single color THz sources were processed using surface distributed feedback gratings to produce narrowband emission at 3.5 THz with nearly 40 μW of peak power and a mid-infrared-to-THz conversion efficiency of 0.36 mW/W2. To better understand the dynamics of the DFG process, time gated spectral measurements of the mid-infrared pumps were performed and simultaneous lasing over the duration of the applied bias pulse was observed, thereby resulting in efficient difference frequency generation.

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.

Experimental investigation of terahertz quantum cascade laser with variable barrier heights

We report an experimental study of terahertz quantum cascade lasers with variable barrier heights based on the Al xGa1–xAs/GaAs material system. Two new designs are developed based on semiclassical ensemble Monte Carlo simulations using state-of-the-art Al 0.15Ga0.85As/GaAs three-quantum-well resonant phonon depopulation active region design as a reference. The new designs achieved maximum lasing temperatures of 188 K and 172 K, as compared to the maximum lasing temperature of 191 K for the reference structure. These results demonstrate that terahertz quantum cascade laser designs with variable barrier heights provide a viable alternative to the traditional active region designs with fixed barrier composition. Additional design space offered by using variable barriers may lead to future improvements in the terahertz quantum cascade laser performance.