Fabio Beltram

Terahertz semiconductor-heterostructure laser

Semiconductor devices have become indispensable for generating electromagnetic radiation in everyday applications. Visible and infrared diode lasers are at the core of information technology, and at the other end of the spectrum, microwave and radio-frequency emitters enable wireless communications. But the terahertz region (1–10u2009THz; 1u2009THz = 1012u2009Hz) between these ranges has remained largely underdeveloped, despite the identification of various possible applications—for example, chemical detection, astronomy and medical imaging. Progress in this area has been hampered by the lack of compact, low-consumption, solid-state terahertz sources. Here we report a monolithic terahertz injection laser that is based on interminiband transitions in the conduction band of a semiconductor (GaAs/AlGaAs) heterostructure. The prototype demonstrated emits a single mode at 4.4u2009THz, and already shows high output powers of more than 2u2009mW with low threshold current densities of about a few hundred Au2009cm-2 up to 50u2009K. These results are very promising for extending the present laser concept to continuous-wave and high-temperature operation, which would lead to implementation in practical photonic systems.

Continuous-wave operation of terahertz quantum-cascade lasers

We report continuous-wave (CW) operation of a 4.4-THz quantum-cascade laser grown in the GaAs-AlGaAs materials system by molecular beam epitaxy. The device operates at 4 K with a threshold current of 160 mA, and an output power of /spl sim/25 /spl mu/W. In pulsed mode, the maximum operating temperature is 52 K, with a threshold current of 108 mA at 4 K. CW lasing was achieved by using a small cavity ridge area (60/spl times/600 /spl mu/m), and by coating one of the laser facets. These two features allowed for a substantial decrease of the threshold current and therefore reduced detrimental heating effects. The role played by the lateral resistance of the 800-nm GaAs layer underneath the active region was also investigated. Experimental data is presented showing that the temperature of the active region, which eventually hinders CW lasing, can be substantially influenced by the value of this lateral resistance.

Terahertz quantum cascade lasers

The terahertz region (1-10 THz) of the electromagnetic spectrum offers ample opportunities in spectroscopy, free space communications, remote sensing and medical imaging. Yet, the use of THz waves in all these fields has been limited by the absence of appropriate sources. Existing solid-state emitters, in fact, lack the general requisites of compactness, integrability and portability necessary for implementation in actual devices, and suffer from low output powers, limited tunability, and/or the necessity of liquid helium cryogenics. Research on THz semiconductor lasers has received new life in the last few years thanks to the development of quantum cascade (QC) lasers of ever increasing wavelength. Yet, fundamental problems related to the transition energy being below the optical phonon resonance and to efficient confinement of the emitted radiation have seriously hampered the realization of THz QC devices. With the help of detailed theoretical modelling and the adoption of a special partially metallic waveguide, we have now demonstrated a monolithic THz heterostructure laser based on interminiband transitions in the conduction band of a GaAs/AlGaAs QC heterostructure. Single mode emission has been achieved at 4.4 THz (/spl lambda/ /spl sim/ 67 /spl mu/m) with high output powers of more than 2 mW. Although operation is presently limited to 60 K, the low threshold current density of less than 300 A/cm/sup 2/ testifies the great development potential for higher temperatures.

Continuous wave terahertz quantum cascade laser

Reports continuous wave operation of a 4.4 THz unipolar injection quantum cascade laser grown in the AlGaAs/GaAs materials system by molecular beam epitaxy. The device operates at 4 K with a threshold current of 160 mA, and an output power of /spl sim/25 /spl mu/W. In pulsed mode the maximum operating temperature is 52 K with a threshold current of 108 mA at 4 K.

An External Cavity 4.7 Terahertz Quantum Cascade Laser

An anti-reflection coated 4.7 terahertz quantum cascade laser coupled to an external cavity formed by a single moving mirror frequency tunes up to 4 wavenumbers with mode hops and around 0.4 wavenumbers without mode hops.

Terahertz heterostructure lasers

Quantum cascade lasers in GaAs/AlGaAs heterostructures are operated in the THz range, with peak powers of several mWs and thresholds of few hundred A/cm/sup 2/ up to 60 K. Single-mode and continuous-wave emission are also achieved at 4.4 THz.

MBE growth of terahertz quantum cascade semiconductor lasers

We report here the growth, fabrication, and operation of two different quantum cascade laser structures that emit in a single cavity mode in the terahertz regime; Both laser designs employ a chirped superlattice active region in a QC structure, comprising seven GaAs quantum wells separated by Al/sub 0.15/Ga/sub 0.85/As barriers, with a novel type of waveguide that provides confinement of the very long wavelength radiation to a very thin, highly doped buried layer.