Semi-insulating Surface-plasmon Waveguide Research Articles

Fabrication of 4.4 THz quantum cascade laser and its demonstration in high-resolution digital holographic imaging

Abstract In this work, we present our research on the fabrication of high power terahertz quantum cascade laser (THz QCL) with frequency at 4.4 THz and its demonstration in digital holographic imaging. The lasers described in this work are based upon the resonant-phonon active region design and semi-insulating surface-plasmon waveguide. The peak power varies with the size of the ridge and the maximum output power of 95 mW at 10 K in pulsed mode is obtained. The maximum working temperature is 80 K. Interestingly, by changing the injection current, the transverse mode of the laser can be switched between TM00 mode and TM01 mode. With such THz QCL as the light source, the lateral resolution of a home-built digital holographic imaging system can reach 80 μm.

High power-efficiency terahertz quantum cascade laser**Project supported by the National Basic Research Program of China (Grant Nos. 2014CB339803 and 2013CB632801), the Special-funded Program on National Key Scientific Instruments and Equipment Development, China (Grant No. 2011YQ13001802-04), and the National Natural Science Foundation of China (Grant No. 61376051).

We demonstrate continuous-wave (CW) high power-efficiency terahertz quantum cascade laser based on semi-insulating surface-plasmon waveguide with epitaxial-side down (Epi-down) mounting process. The performance of the device is analyzed in detail. The laser emits at a frequency of ∼ 3.27 THz and has a maximum CW operating temperature of ∼ 70 K. The peak output powers are 177 mW in pulsed mode and 149 mW in CW mode at 10 K for 130-μm-wide Epi-down mounted lasers. The record wall-plug efficiencies in direct measurement are 2.26% and 2.05% in pulsed and CW mode, respectively.

High-Power and High-Efficiency Operation of Terahertz Quantum Cascade Lasers at 3.3 THz**Supported by the National Basic Research Program of China under Grant Nos 2014CB339803 and 2013CB632801, and the National Natural Science Foundation of China under Grant No 61376051.

A high-power and high-efficiency GaAs/AlGaAs-based terahertz (THz) quantum cascade laser structure emitting at 3.3 THz is presented. The structure is based on a hybrid bound-to-continuum transition and resonant-phonon extraction active region combined with a semi-insulating surface-plasmon waveguide. By optimizing material structure and device processing, the peak optical output power of 758 mW with a threshold current density of 120 A/cm2 and a wall-plug efficiency of 0.92% at 10K and 404mW at 77K are obtained in pulsed operation. The maximum operating temperature is as high as 115 K. In the cw mode, a record optical output power of 160 mW with a threshold current density of 178 A/cm2 and a wall-plug efficiency of 1.32% is achieved at 10 K.

High-Power Single-Mode Tapered Terahertz Quantum Cascade Lasers

We demonstrate a series of high output power single-mode terahertz quantum cascade lasers based on semi-insulating surface-plasmon waveguide with tapered geometry structure. The taper half-angle is ranging from 0° to 3°. The tapered waveguide effects on the output power and the laser beam quality are experimentally studied. High single longitudinal mode edge-emission peak powers (92.3 mW) are realized. The peak power monolithically increases with the increasing of the taper angle. All distributed feedback (DFB) lasers lased in the temperature range of 70–75 K. The highest brightness is obtained from the 2° laser. Stable single-mode emissions are realized for DFB lasers with the side-mode suppression ratio $\sim 18$ dB.

Material growth and device fabrication of terahertz quantum-cascade laser based on bound-to-continuum structure

The terahertz quantum-cascade laser (THz QCL) based on bound-to-continuum structure is demonstrated. The X-ray diffraction measurement of the material shows a high crystalline quality of the active region. A THz QCL device was fabricated with semi-insulating surface-plasmon waveguide. The test device is lasing at about 3 THz and operating up to 60 K. It shows a single frequency property under different drive currents and temperatures. At 9 K, the maximum output power is greater than 2 mW with a threshold current density of 159 A/cm2.

High power terahertz quantum cascade lasers with symmetric wafer bonded active regions

We increased the active region/waveguide thickness of terahertz quantum cascade lasers with semi-insulating surface plasmon waveguides by stacking two symmetric active regions on top of each other, via a direct wafer bonding technique. In this way, we enhance the generated optical power in the cavity and the mode confinement. We achieved 470 mW peak output power in pulsed mode from a single facet at a heat sink temperature of 5 K and a maximum operation temperature of 122 K. Furthermore, the devices show a broad band emission spectrum over a range of 420 GHz, centered around 3.9 THz.

Continuous-Wave Operation of Terahertz Quantum Cascade Lasers at 3.2 THz

We demonstrate continuous-wave (cw) operation of terahertz (THz) quantum cascade lasers emitting at 3.2 THz based on bound-to-continuum active region and semi-insulating surface-plasmon waveguide design. Optical power of 62 mW with a threshold current density of 285 A/cm2 is obtained at 10 K from a 130-μm-wide and 1.5-mm-long laser in cw operation. Maximum cw operation temperature is up to 60 K. In pulsed mode, peak optical power more than 100 mW at 10 K and 2.1 mW at 85 K are observed from a 230-μm-wide and 2-mm-long device.

Terahertz quantum cascade lasers with thin resonant-phonon depopulation active regions and surface-plasmon waveguides

We report three-well, resonant-phonon depopulation terahertz quantum cascade lasers with semi-insulating surface-plasmon waveguides and reduced active region (AR) thicknesses. Devices with thicknesses of 10, 7.5, 6, and 5 μm are compared in terms of threshold current density, maximum operating temperature, output power, and AR temperature. Thinner ARs are technologically less demanding for epitaxial growth and result in reduced electrical heating of devices. However, it is found that 7.5-μm-thick devices give the lowest electrical power densities at threshold, as they represent the optimal trade-off between low electrical resistance and low threshold gain.

Study of 2.9 THz quantum cascade laser based on bound-to-continuum transition

The material of GaAs/AlGaAs bound-to-continuum terahertz quantum-cascade laser (THz QCL) was grown by gas source molecular beam epitaxy. A THz QCL device was fabricated with semi-insulating surface-plasmon waveguide. Its spectrum and light intensity-current-voltage characteristics were studied. The device emits about 2.95 THz, and yields a maximum temperature of 67 K in pulse mode. In continuous-wave mode, it displays a threshold current density of 230 A/cm2 at 9 K with maximum emitted power of 1.2 mW and lases up to 30 K.

Optimization of THz semi-insulating surface plasmon waveguide structures of GaSb/AlSb quantum cascade lasers

We designed and analyzed the semi-insulating surface plasmon waveguide structures of GaSbbased quantum cascade lasers (QCLs) operating in the terahertz (THz) region of 2.6–6.5 THz using the finite element method. For improving the guiding property of THz QCL waveguide structures, a low threshold gain (gth) and a high optical confinement (Γ) could be achieved by optimizing the doping concentration and the thickness of plasma layer, the waveguide width, the substrate thickness, and the number of stages in active core region at various operating frequencies (2.6–6.5 THz). The effect of different substrates on the gth and Γ of THz QCL waveguide structures was also investigated.

Terahertz Quantum Cascade Laser Operating at 2.94 THz

The development of quantum cascade laser at 2.94 THz is reported. The laser structure is based on a bound-to-continuum active region and a semi-insulating surface-plasmon waveguide. Lasing is observed up to a heat-sink temperature of 70 K in pulsed mode with light power of 4.75 mW at 10 K and 1 mW at 70 K. A threshold current density of 296.5 A/cm2 and an internal quantum efficiency of 1.57 × 10−2 per cascade period are also observed at 10 K. The characteristic temperature of this laser is extracted to be T0 = 57.5 K.

High-power terahertz quantum-cascade lasers

Demonstration of quantum-cascade lasers at ∼4.4 THz (λ∼68 µm), which are measured to emit 248 mW peak power in pulsed mode, and 138 mW continuous-wave power at heatsink temperatures of 10 K, is reported. These lasers are based on a resonant-phonon depopulation scheme, and use a semi-insulating surface-plasmon waveguide.

Terahertz quantum-cascade laser operating up to 137 K

We report operation of a terahertz quantum-cascade laser at 3.8 THz (λ≈79 μm) up to a heat-sink temperature of 137 K. A resonant phonon depopulation design was used with a low-loss metal–metal waveguide, which provided a confinement factor of nearly unity. A threshold current density of 625 A/cm2 was obtained in pulsed mode at 5 K. Devices fabricated using a conventional semi-insulating surface-plasmon waveguide lased up to 92 K with a threshold current density of 670 A/cm2 at 5 K.