Harvey E. Beere

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 (1–10 THz; 1 THz = 1012 Hz) 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 4.4 THz, and already shows high output powers of more than 2 mW with low threshold current densities of about a few hundred A cm-2 up to 50 K. 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.

Electrically pumped photonic-crystal terahertz lasers controlled by boundary conditions.

Semiconductor lasers based on two-dimensional photonic crystals generally rely on an optically pumped central area, surrounded by un-pumped, and therefore absorbing, regions. This ideal configuration is lost when photonic-crystal lasers are electrically pumped, which is practically more attractive as an external laser source is not required. In this case, in order to avoid lateral spreading of the electrical current, the device active area must be physically defined by appropriate semiconductor processing. This creates an abrupt change in the complex dielectric constant at the device boundaries, especially in the case of lasers operating in the far-infrared, where the large emission wavelengths impose device thicknesses of several micrometres. Here we show that such abrupt boundary conditions can dramatically influence the operation of electrically pumped photonic-crystal lasers. By demonstrating a general technique to implement reflecting or absorbing boundaries, we produce evidence that whispering-gallery-like modes or true photonic-crystal states can be alternatively excited. We illustrate the power of this technique by fabricating photonic-crystal terahertz (THz) semiconductor lasers, where the photonic crystal is implemented via the sole patterning of the device top metallization. Single-mode laser action is obtained in the 2.55–2.88 THz range, and the emission far field exhibits a small angular divergence, thus providing a solution for the quasi-total lack of directionality typical of THz semiconductor lasers based on metal–metal waveguides.

2.9THz quantum cascade lasers operating up to 70K in continuous wave

We report the operation of a quantum cascade laser emitting at a 103μm wavelength (2.9THz). The active region is based on a bound-to-continuum design allowing a low parasitic leakage current, and a high upper-to-lower-state lifetime ratio. The latter is demonstrated by a pronounced decrease of the differential resistance at threshold, which is visible up to high temperatures, and by a weak temperature dependence of the slope efficiency. At 4K, we report a threshold current density of only 105A∕cm2 both in pulsed and continuous-wave operation, and an emitted peak power of 15mW independent of the duty cycle. Maximum operating temperatures of 95K and 70K are observed in pulsed and continuous wave modes, respectively.

Low-threshold terahertz quantum-cascade lasers

A quantum-cascade laser operating at λ=66 μm is demonstrated. It consists of a three-quantum-well chirped-superlattice active region embedded in a waveguide based on a single interface plasmon and a buried contact. A threshold current density of 210 A/cm2 at T=12 K, a maximum peak optical power of 4 mW, and operation up to T=44 K are achieved in a 2.7 mm long device with a high reflectivity backfacet coating.

Far-infrared (λ≃87 μm) bound-to-continuum quantum-cascade lasers operating up to 90 K

We report terahertz frequency (3.5 THz, λ≃87 μm) emission from quantum-cascade lasers employing a bound-to-continuum transition in the active region. The maximum operating temperature is in excess of 90 K. Peak powers of 20 mW at 20 K and 10 mW at 77 K are achieved. The same devices show continuous-wave operation up to 55 K with measured optical powers of 15 mW at 10 K.

Terahertz range quantum well infrared photodetector

We demonstrated a GaAs/AlGaAs-based far-infrared quantum well infrared photodetector at a wavelength of λ=84 μm. The relevant intersubband transition is slightly diagonal with a dipole matrix element of 3.0 nm. At 10 K, a responsivity of 8.6 mA/W and a detectivity of 5×107 cm √Hz/W have been achieved; and successful detection up to a device temperature of 50 K has been observed. Being designed for zero bias operation, this device profits from a relatively low dark current and a good noise behavior.

Terahertz quantum cascade laser as local oscillator in a heterodyne receiver

Terahertz quantum cascade lasers have been investigated with respect to their performance as a local oscillator in a heterodyne receiver. The beam profile has been measured and transformed in to a close to Gaussian profile resulting in a good matching between the field patterns of the quantum cascade laser and the antenna of a superconducting hot electron bolometric mixer. Noise temperature measurements with the hot electron bolometer and a 2.5 THz quantum cascade laser yielded the same result as with a gas laser as local oscillator.

Ultrabroadband terahertz radiation from low-temperature-grown GaAs photoconductive emitters

Terahertz radiation was generated with a biased and asymmetrically excited low-temperature-grown GaAs photoconductive emitter, and characterized with a 20-μm-thick ZnTe crystal using free-space electro-optic sampling. Using a backward collection scheme, we obtained terahertz radiation with frequency components over 30 THz, the highest ever observed for photoconductive emitters. We present spectra over the whole frequency range between 0.3 and 20 THz, demonstrating the use of this source for ultrabroadband THz spectroscopy.

High-resolution gas phase spectroscopy with a distributed feedback terahertz quantum cascade laser

The quantum cascade laser is a powerful, narrow linewidth, and continuous wave source of terahertz radiation. The authors have implemented a distributed feedback device in a spectrometer for high-resolution gas phase spectroscopy. Amplitude as well as frequency modulation schemes have been realized. The absolute frequency was determined by mixing the radiation from the quantum cascade laser with that from a gas laser. The pressure broadening and the pressure shift of a rotational transition of methanol at 2.519THz were measured in order to demonstrate the performance of the spectrometer.The quantum cascade laser is a powerful, narrow linewidth, and continuous wave source of terahertz radiation. The authors have implemented a distributed feedback device in a spectrometer for high-resolution gas phase spectroscopy. Amplitude as well as frequency modulation schemes have been realized. The absolute frequency was determined by mixing the radiation from the quantum cascade laser with that from a gas laser. The pressure broadening and the pressure shift of a rotational transition of methanol at 2.519THz were measured in order to demonstrate the performance of the spectrometer.

4.35 kW peak power femtosecond pulse mode-locked VECSEL for supercontinuum generation

We report a passively mode-locked vertical external cavity surface emitting laser (VECSEL) producing 400 fs pulses with 4.35 kW peak power. The average output power was 3.3 W and the VECSEL had a repetition rate of 1.67 GHz at a center wavelength of 1013 nm. A near-antiresonant, substrate-removed, 10 quantum well (QW) gain structure designed to enable femtosecond pulse operation is used. A SESAM which uses fast carrier recombination at the semiconductor surface and the optical Stark effect enables passive mode-locking. When 1 W of the VECSEL output is launched into a 2 m long photonic crystal fiber (PCF) with a 2.2 µm core, a supercontinuum spanning 175 nm, with average power 0.5 W is produced.