Broadband semiconductor terahertz laser based on heterogeneous cascades

We present a development of a broad gain heterogeneous quantum cascade laser for terahertz frequencies. By placing appropriate different active-regions based on a four-quantum-well design into a double-metal waveguide we obtained laser emitting gapless over a bandwidth of 1 THz, between 3.2 to 2.2 THz. This means that this single-device source covers an emission range of nearly 40 % around the center frequency. In pulsed mode operation, our devices show threshold current density as low as 285 A/cm2 and they operate up to 125 K. We also report on continuous wave measurements. © 2011 SPIE

Magnetically assisted quantum cascade laser emitting from 740 GHz to 1.4 THz

In this paper we show that by applying a perpendicular magnetic field to a quantum cascade structure it is possible to enhance the gain of different optical transitions. The combination of magnetic confinement with a broadband, cutoff-free optical resonator allows the demonstration of laser action over a large bandwidth, from 733 GHz to 1.38 THz together with the emission at 3.2 THz. A different lasing scheme is revealed that does not rely on resonant tunneling as the main injection mechanism. In combination with the magnetically enhanced gain laser emission at 1 THz is observed up to a temperature of 115 K, which corresponds to a ratio kB T/hv=2.3 between the lattice thermal energy and the laser photon energy. © 2010 American Institute of Physics

Operation of a Wideband Terahertz Superconducting Bolometer Responding to Quantum Cascade Laser Pulses

We make use of a niobium film to produce a micrometric vacuum-bridge superconducting bolometer responding to THz frequency. The bolometer works anywhere in the temperature range 2-7 K, which can be easily reached in helium bath cryostats or closed-cycle cryocoolers. In this work the bolometer is mounted on a pulse tube refrigerator and operated to measure the equivalent noise power (NEP) and the response to fast (μs) terahertz pulses. The NEP above 100 Hz equals that measured in a liquid helium cryostat showing that potential drawbacks related to the use of a pulse tube refrigerator (like mechanical and thermal oscillations, electromagnetic interference, noise) are irrelevant. At low frequency, instead, the pulse tube expansioncompression cycles originate lines at 1 Hz and harmonics in the noise spectrum. The bolometer was illuminated with THz single pulses coming either from a Quantum Cascade Laser operating at liquid nitrogen temperature or from a frequencymultiplied electronic oscillator. The response of the bolometer to the single pulses show that the device can track signals with a rise time as fast as about 450 n

Improving the Out-Coupling of a Metal-Metal Terahertz Frequency Quantum Cascade Laser Through Integration of a Hybrid Mode Section into the Waveguide

A hybrid mode section is integrated into the end of the metal-metal waveguide of a terahertz (THz) frequency quantum cascade laser (QCL) by removing sub-wavelength portions of the top metal layer. This allows a hybrid mode to penetrate into the air, which reduces the effective index of the mode and improves the out-coupling performance at the facet. The transmission of the hybrid section is further increased by ensuring its length fulfills the criterion for constructive interference. These simple modifications to a 2.5-THz metal-metal QCL waveguide result in a significant increase in the output emission power. In addition, simulations show that further improvements in out-coupling efficiency can be achieved for lower frequencies with effective refractive indices close to the geometric mean of the indices of the metal-metal waveguide and air