Optoelectronic techniques for the generation and detection of terahertz waves

D. Saeedkia,     TeTechS Inc., Canada

Abstract:

In this chapter, terahertz source and detector technologies are reviewed. Terahertz signal generation and detection in terahertz photoconductive antennas are studied, and their performances under continuous-wave and pulsed mode operation are compared. Terahertz signal generation in nonlinear crystals by parametric interaction and difference frequency mixing techniques are studied.

Key words

terahertz sources

terahertz detectors

photoconductive antennas

parametric sources

nonlinear crystals

1.1 Introduction

Technology advancement in photonics, electronics and communications has entered a new realm: the Tera Era (T-Era). Today's transistors function at teraflops per second, wireless-data communications is reaching to Tb/s speed, and terabyte hard-drive memories are now a reality. Over the past two decades, intense research and development activities in academia and industry have closed the gap between the microwave and infrared spectra. Compact terahertz (THz) sources and detectors have been developed to generate, detect and manipulate coherent terahertz signals. Terahertz sensing and imaging systems are now commercially available, and terahertz wireless communication is on the horizon. Recent innovations offering both powerful and reliable terahertz sources, together with high performance terahertz spectroscopy and imaging systems, have opened remarkable new opportunities in science and technology. Although major gains in performance and functionality are still anticipated, commercially available terahertz devices and systems have already made the terahertz spectrum accessible to many scientists and technologists in diverse areas, ranging from biology and medicine to chemical, pharmaceutical and environmental sciences to revisit their scientific problems under the light of terahertz waves.

1.1.1 Terahertz source technologies

Terahertz sources can be divided into two major categories: electronic and photonic sources. Terahertz electronic sources that are widely used include electron beam and solid-state sources, and frequency multipliers. The common terahertz photonic sources include terahertz semiconductor and gas lasers and terahertz optoelectronic sources.

Electron beam sources

Gyrotrons,1 free electron lasers (FELs),2 and backward wave oscillators (BWOs)3 are electron beam sources that generate relatively high-power signals at the terahertz frequency range. The operation of these devices is based on the interaction of a high-energy electron beam with a strong magnetic field inside resonant cavities or waveguides, which results in an energy transfer between the electron beam and an electromagnetic wave. Gyrotrons with 1 MW power at 140 GHz have been successfully developed.4 An FEL is a gyrotron with very high operating frequencies and wider frequency tuning range.5

BWOs can be electrically tuned over a bandwidth of more than 50% of their operational frequencies, and can generate up to 50 mW of power at 300 GHz going down to a few mW at 1 THz.6 Complete systems are heavy and large and need high bias voltage and usually a water-cooling system.7 Microfabrication and micro-assembly technologies are promising approaches to reduce the size of these devices and to make them more suitable for terahertz applications.8

Solid-state sources

Solid-state sources include resonant tunnelling diodes (RTD),9 Gunn or transferred electron devices (TED),10,11 and transit time devices such as impact avalanche transit time (IMPATT) diodes and tunnel injection transit time (TUNNETT) diodes. Gunn devices generating 0.2-5 µW power at 400-560 GHz frequency range are now feasible.12 TUNNETT diodes with operational frequency as high as 355 GHz with 140 µW output power have been reported.13

Frequency multipliers

In a terahertz frequency multiplier, the frequency of a driver source is multiplied in a nonlinear device to generate higher-order harmonic frequencies. Planar Schottky varactor diodes are commonly used in frequency multipliers, taking the advantage of GaAs substrateless technology to reduce substrate loss. The drive sources can be BWOs or solid-state sources such as Gunn and IMPATT oscillators, with relatively high output power in the range of 50 GHz to 150 GHz. Microwave frequency synthesizers in combination with high-gain power amplifiers fabricated by the monolithic microwave integrated circuit (MMIC) technology can generate high output power above 100 GHz.14 The most efficient terahertz frequency multipliers are realized by series chains of frequency doublers and frequency triplers.15 Signals up to 2 THz are achievable from frequency multipliers.16,17 However, using a hybrid system consisting of a BWO and a chain of frequency multipliers, it is possible to generate terahertz signals with frequencies of more than 2.5 THz.18

Terahertz semiconductor lasers

The most promising terahertz semiconductor lasers are quantum cascade lasers.19,20 A quantum cascade laser is a unipolar laser, in which the conduction band or the valence band is divided into few sub-bands. The carrier transition occurs between these discrete energy levels within the same band. The discrete energy levels are created in a semiconductor heterostructure containing several coupled quantum wells. Quantum cascade lasers with around 10 mW output power at 2 THz have been demonstrated. Operational temperatures as high as 93 K have been reported for a terahertz quantum cascade laser at 3.2 THz.21

Optically pumped far-infrared gas lasers

These terahertz sources consist of a pump laser radiating into a cavity filled with a gas that lases at the terahertz frequency range.22,23 The lasing frequency is dictated by the filling gas. Power levels of 1-20 mW are common for 20-100 W laser-pump power.

1.1.2 Terahertz optoelectronic sources

Terahertz photoconductive antennas (THz-PCAs) are widely used to generate terahertz broadband pulses and terahertz narrowband continuous-wave (cw) signals.24 Since their demonstration as practical THz sources and detectors, THz-PCAs have been the subject of a vast number of scientific and industrial reports investigating their application as terahertz wave transmitters and receivers. In continuous-wave mode, two cw laser beams, with their frequency difference in the THz range, combined either inside an optical fibre or properly overlapped in space, are mixed in a photo-absorbing medium (photomixer) and generate a beat frequency signal.25 Terahertz signals with the frequency linewidth as low as a few kHz can be generated by photomixers. The frequency of the terahertz signal can be tuned by tuning the wavelengths of the lasers. The output power in conventional photomixers falls from 2 µW at 1 THz to below 0.1 µW at 3 THz.

Broadband terahertz pulses can be generated by exciting THz-PCAs with a femto-second short pulse laser. Using a femto-second laser with ~ 100 fs optical pulse duration, terahertz pulses with their frequency content extended up to around 5 THz and an average power of a few µW can be achieved.26 Broadband terahertz pulses can also be generated in electrooptic crystals excited by femto-second short pulse lasers.27 Terahertz signals can be generated via parametric interaction of near-infrared photons and optical vibration modes inside an optical crystal. Using this technique, generation of a quasi-cw terahertz signal with pulse duration of 3.6 ns, an average power 9 nW and a frequency tuning range of 0.7 to 2.4 THz has been reported.28

1.2 Terahertz detector technologies

Highly sensitive cooled terahertz detectors have been traditionally developed for radio astronomy applications. However, developing highly sensitive room temperature terahertz detectors has been challenging. In this section, we review the common terahertz detection techniques.

1.2.1 Homodyne detectors

In homodyne (or direct) detection techniques, the incident terahertz wave on a detector is converted into a measurable electrical signal. Thermal detectors form a large class of the homodyne detectors. The incident terahertz signal on a thermal detector is absorbed by a material whose physical properties such as volume, electric conductivity and dielectric properties change with temperature. Thermal detectors are square-law devices, whose output signals are proportional to the square of the incident field. The phase information of the incident signal is not directly measured by thermal detectors. Most of the thermal detectors have relaxed intrinsic-frequency limitation due to their purely thermal sensing principle, and they are usually slow-response devices. The figure of merit for...