Alan Wei Min Lee

High-power and high-temperature THz quantum-cascade lasers based on lens-coupled metal-metal waveguides

A metal-metal waveguide quantum cascade laser with an abutted silicon hyperhemispherical lens is demonstrated at ~4.1 THz. The device produced 145 mW of peak pulsed power at 5 K with a wall-plug power efficiency of 0.7%, lasing up to a maximum operating temperature of 160 K. The far-field beam pattern has a full width at half-maximum value of ~4.8 degrees in the H plane. The same device produced ~26 mW of peak power using a Winston cone instead of a lens, lasing up to 165 K. The large increase in output power is mainly attributed to an increase in collection efficiency.

Tunable terahertz quantum cascade lasers with external gratings

We demonstrate a frequency tunable external cavity terahertz quantum cascade laser using an abutted antireflection-coated silicon lens to reduce facet reflection and as a beam-forming element, with an external grating providing frequency selective optical feedback. Angle tuning of the grating allows a single longitudinal mode of the laser ridge to be selected, resulting in discontinuous tuning over a 165 GHz range around a center frequency of 4.4 THz. Another device exhibited 145 GHz of total tuning with 9 GHz of continuous tuning near the longitudinal modes of the laser.

A terahertz pulse emitter monolithically integrated with a quantum cascade laser

A terahertz pulse emitter monolithically integrated with a quantum cascade laser (QCL) is demonstrated. The emitter facet is excited by near-infrared pulses from a mode-locked Ti:sapphire laser, and the resulting current transients generate terahertz pulses that are coupled into an electrically isolated QCL in proximity. These pulses are used to measure the gain of the laser transition at ∼2.2 THz, which clamps above threshold at ∼18 cm−1 and has a full width at half-maximum linewidth of ∼0.67 THz. The measurement also shows the existence of absorption features at different biases that correspond to misalignment of the band structure and to absorption within the two injector states. The simplicity of this scheme allows it to be implemented alongside standard QCL ridge processing and to be used as a versatile tool for characterizing QCL gain media.

Terahertz tomography using quantum-cascade lasers

The interfaces of a dielectric sample are resolved in reflection geometry using light from a frequency agile array of terahertz quantum-cascade lasers. The terahertz source is a 10-element linear array of third-order distributed-feedback QCLs emitting at discrete frequencies from 2.08 to 2.4 THz. Emission from the array is collimated and sent through a Michelson interferometer, with the sample placed in one of the arms. Interference signals collected at each frequency are used to reconstruct an interferogram and detect the interfaces in the sample. Because of the long coherence length of the source, the interferometer arms need not be adjusted to the zero-path delay. A depth resolution of 360 µm in the dielectric is achieved with further potential improvement through improved frequency coverage of the array. The entire experiment footprint is <1 m × 1 m with the source operated in a compact, closed-cycle cryocooler.

Real-time imaging using a 4.3-THz quantum cascade laser and a 240×320 element focal-plane array

A real-time reflection/transmission imaging system is demonstrated using a 50-mW, 4.3-THz quantum cascade laser source in a closed-cycle cryorefrigerator, with a 240times320 pixel microbolometer detector. A signal-to-noise ratio of 340 is achieved with a 20-frame/sec acquisition rate.

Real-Time, Transmission-Mode, Terahertz Imaging Over a 25-meter Distance

We demonstrate transmission-mode imaging over a 25 meter distance using a ~4.9- THz quantum-cascade laser, frequency-optimized for a low-loss (-0.5 dB/m) atmospheric window. The ~17-mW peak power allows real-time imaging with a 320x240 element microbolometer camera.

Terahertz quantum cascade lasers

We provide an overview of terahertz quantum cascade lasers based on resonant-phonon depopulation and metal-metal waveguides, including two-phonon resonant-phonon depopulation schemes, long wavelength operation, and real time terahertz imaging.

Terahertz quantum cascade laser based optical coherence tomography

The interfaces of a dielectric sample are resolved in reflection geometry using light from a frequency agile array of terahertz quantum-cascade lasers. The terahertz source is a 10-element linear array of third-order distributed feedback QCLs emitting at discrete frequencies from 2.08 to 2.4 THz. Emission from the array is collimated and sent through a Michelson interferometer, with the sample placed in one of the arms. Interference signals collected at each frequency are used to reconstruct an interferogram and detect the interfaces in the sample. Due to the long coherence length of the source, the interferometer arms need not be adjusted to the zero-path delay. A depth resolution of 360 μm in the dielectric is achieved with further potential improvement through improved frequency coverage of the array. The entire experiment footprint is <1 m x 1 m with the source operated in a compact, closed-cycle cryocooler.

High-Power Metal-Metal Waveguide Terahertz Quantum-Cascade Laser with a Hyperhemispherical Lens

We demonstrate an ~85 mW (pulsed, 5 K), metal-metal waveguide, terahertz quantum- cascade laser using an optically coupled lens. The device has a FWHM of ~6 Deg. and a maximum pulsed operating temperature of 155 K.