Benjamin S. Williams

Operation of terahertz quantum-cascade lasers at 164 K in pulsed mode and at 117 K in continuous-wave mode.

We report the demonstration of a terahertz quantum-cascade laser that operates up to 164 K in pulsed mode and 117 K in continuous-wave mode at approximately 3.0 THz. The active region was based on a resonant-phonon depopulation scheme and a metal-metal waveguide was used for modal confinement. Copper to copper thermocompression wafer bonding was used to fabricate the waveguide, which displayed improved thermal properties compared to a previous indium-gold bonding method.

3.4-THz quantum cascade laser based on longitudinal-optical-phonon scattering for depopulation

We report the development of a quantum cascade laser, at λ=87.2 μm, corresponding to 3.44 THz or 14.2 meV photon energy. The GaAs/Al0.15Ga0.85As laser structure utilizes longitudinal-optical (LO) phonon scattering for electron depopulation. Laser action is obtained in pulsed mode at temperatures up to 65 K, and at 50% duty cycle up to 29 K. Operating at 5 K in pulsed mode, the threshold current density is 840 A/cm2, and the peak power is approximately 2.5 mW. Based on the relatively high operating temperatures and duty cycles, we propose that direct LO-phonon-based depopulation is a robust method for achieving quantum cascade lasers at long-wavelength THz frequencies.

Terahertz quantum-cascade laser at λ≈100 μm using metal waveguide for mode confinement

We report lasing at ∼3.0 THz (λ≈98–102 μm) in a quantum-cascade structure in which mode confinement is provided by a double-sided metal waveguide. The depopulation mechanism is based on resonant phonon scattering, as in our previous work. Lasing takes place in pulsed mode up to a heat-sink temperature of 77 K. The waveguide consists of metallic films placed above and below the 10-μm-thick multiple-quantum-well gain region, which gives low losses and a modal confinement factor of nearly unity. Fabrication takes place via low-temperature metallic wafer bonding and subsequent substrate removal using selective etching. This type of waveguide is expected to be increasingly advantageous at even longer wavelengths.

Real-time terahertz imaging over a standoff distance (>25meters)

The authors demonstrate the use of a terahertz quantum cascade laser (QCL) for real-time imaging in transmission mode at a standoff distance of 25meters. Lasing frequency was selected for optimum transmission within an atmospheric window at ∼4.9THz. Coarse frequency selection was made by design of the QCL gain medium. Finer selection (to within 0.1THz) was made by judicious choice of laser cavity length to adjust facet losses and therefore lasing threshold bias, in order to overlap the peak frequency of the Stark-shifted gain spectrum with the atmospheric window. Images are shown using an uncooled 320×240 microbolometer camera.

Real-time imaging using a 4.3-THz quantum cascade laser and a 320 /spl times/ 240 microbolometer focal-plane array

We report the use of a ~50-mW peak power 4.3-THz quantum cascade laser (QCL) as an illumination source for real-time imaging with a 320 times 240 element room-temperature microbolometer focal-plane array detector. The QCL is modulated synchronously with the focal-plane array for differential imaging. Signal-to-noise ratios of ~340 are achieved at a 20-frame/s acquisition rate, and the optical noise equivalent power of the detector array at 4.3 THz is estimated to be ~320 pW/radicHz. Both reflection and transmission mode imaging are demonstrated

Electromagnetic modeling of terahertz quantum cascade laser waveguides and resonators

Finite-element numerical modeling and analysis of electromagnetic waveguides and resonators used in terahertz quantum cascade lasers are presented. Both metal-metal and semi-insulating (SI) surface-plasmon ridge waveguide geometries were investigated. Simulations and analysis of two types were performed: two-dimensional waveguides (eigenmode calculation), and two- and three-dimensional resonators (facet reflectivity calculation for infinite width and finite width waveguides, respectively). Waveguide simulations extend previous transverse one-dimensional analyses to two dimensions (for the lateral and transverse dimensions), and quantify the breakdown of the one-dimensional approximation as the ridge width is reduced. Resonator simulations in two and three dimensions are presented and are used to obtain facet reflectivities and output radiation patterns. For the metal-metal waveguide structures, these resonator simulations quantitatively show strong deviations for terahertz facet reflectivities from those ...

Terahertz heterodyne receiver based on a quantum cascade laser and a superconducting bolometer

We report the first demonstration of an all solid-state heterodyne receiver that can be used for high-resolution spectroscopy above 2THz suitable for space-based observatories. The receiver uses a NbN superconducting hot-electron bolometer as mixer and a quantum cascade laser operating at 2.8THz as local oscillator. We measure a double sideband receiver noise temperature of 1400K at 2.8THz and 4.2K, and find that the free-running QCL has sufficient power stability for a practical receiver, demonstrating an unprecedented combination of sensitivity and stability.

Continuous-wave operation of terahertz quantum-cascade lasers above liquid-nitrogen temperature

We report cw operation of a quantum-cascade laser at 3.2 THz (λ≈94 μm) up to a heat-sink temperature of 93 K. Resonant longitudinal-optical phonon scattering is used to depopulate the lower radiative state and a low-loss metal–metal waveguide is used to provide high modal confinement. Optical powers of ∼1.8 mW at 10 K and ∼400 μW at 78 K are observed from a single facet of a 40-μm-wide and 1.35-mm-long laser device. A threshold current density of 432 A/cm2 at 10 K and 552 A/cm2 at 78 K was obtained in cw mode. The same device lased up to 129 K in pulsed mode with a threshold current density of 419 A/cm2 at 5 K.

Surface-emitting distributed feedback terahertz quantum-cascade lasers in metal-metal waveguides

Single-mode surface-emitting distributed feedback terahertz quantum-cascade lasers operating around 2.9 THz are developed in metal-metal waveguides. A combination of techniques including precise control of phase of reflection at the facets, and use of metal on the sidewalls to eliminate higher-order lateral modes allow robust single-mode operation over a range of approximately 0.35 THz. Single-lobed far-field radiation pattern is obtained using a pi phase-shift in center of the second-order Bragg grating. A grating device operating at 2.93 THz lased up to 149 K in pulsed mode and a temperature tuning of 19.7 GHz was observed from 5 K to 147 K. The same device lased up to 78 K in continuous-wave (cw) mode emitting more than 6 mW of cw power at 5 K. In general, maximum temperature of pulsed operation for grating devices was within a few Kelvin of that of multi-mode Fabry-Perot ridge lasers.

Measurement of subband electronic temperatures and population inversion in THz quantum-cascade lasers

We compare the electronic temperatures and the population inversion both below and above the lasing threshold in three quantum-cascade lasers (QCLs) operating at 2.8THz, 3.2THz, and 3.8THz using microprobe band-to-band photoluminescence. In the lasing range, while the ground-state temperature remains close to the lattice one (90K–100K), the upper radiative state heats up to ∼200K. From the measured thermal resistance and the power dependence of the ground-state electronic temperature, we get a value of the electron-lattice energy relaxation rate comparable with that typical of midinfrared QCLs.