Over the past three decades a new spectroscopic technique with unique possibilities has emerged. Based on coherent and time-resolved detection of the electric field of ultrashort radiation bursts in the far-infrared, this technique has become known as terahertz time-domain spectroscopy (THz-TDS). In this review article the authors describe the technique in its various implementations for static and time-resolved spectroscopy, and illustrate the performance of the technique with recent examples from solid-state physics and physical chemistry as well as aqueous chemistry. Examples from other fields of research, where THz spectroscopic techniques have proven to be useful research tools, and the potential for industrial applications of THz spectroscopic and imaging techniques are discussed.

Terahertz spectroscopy and imaging – Modern techniques and applications

This review is focused on the latest developments in continuous-wave (CW) photomixing for Terahertz (THz) generation. The first part of the paper explains the limiting factors for operation at high frequencies ∼ 1 THz, namely transit time or lifetime roll-off, antenna (R)-device (C) RC roll-off, current screening and blocking, and heat dissipation. We will present various realizations of both photoconductive and p-i-n diode–based photomixers to overcome these limitations, including perspectives on novel materials for high-power photomixers operating at telecom wavelengths (1550 nm). In addition to the classical approach of feeding current originating from a small semiconductor photomixer device to an antenna (antenna-based emitter, AE), an antennaless approach in which the active area itself radiates (large area emitter, LAE) is discussed in detail. Although we focus on CW photomixing, we briefly discuss recent results for LAEs under pulsed conditions. Record power levels of 1.5 mW average power and conversion efficiencies as high as 2 × 10−3 have been reached, about 2 orders of magnitude higher than those obtained with CW antenna-based emitters. The second part of the paper is devoted to applications for CW photomixers. We begin with a discussion of the development of novel THz optics. Special attention is paid to experiments exploiting the long coherence length of CW photomixers for coherent emission and detection of THz arrays. The long coherence length comes with an unprecedented narrow linewidth. This is of particular interest for spectroscopic applications, the field in which THz research has perhaps the highest impact. We point out that CW spectroscopy systems may potentially be more compact, cheaper, and more accurate than conventional pulsed systems. These features are attributed to telecom-wavelength compatibility, to excellent frequency resolution, and to their huge spectral density. The paper concludes with prototype experiments of THz wireless LAN applications. For future telecommunication systems, the limited bandwidth of photodiodes is inadequate for further upshifting carrier frequencies. This, however, will soon be required for increased data throughput. The implementation of telecom-wavelength compatible photomixing diodes for down-conversion of an optical carrier signal to a (sub-)THz RF signal will be required.

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Tunable, continuous-wave Terahertz photomixer sources and applications

Terahertz (THz) spectroscopy, and especially THz imaging, holds large potential in the field of nondestructive, contact-free testing. The ongoing advances in the development of THz systems, as well as the appearance of the first related commercial products, indicate that large-scale market introduction of THz systems is rapidly approaching. We review selected industrial applications for THz systems, comprising inline monitoring of compounding processes, plastic weld joint inspection, birefringence analysis of fiber-reinforced components, water distribution monitoring in polymers and plants, as well as quality inspection of food products employing both continuous wave and pulsed THz systems.

Terahertz imaging: applications and perspectives

Ultrashort terahertz pulses derived from femtosecond table-top sources have become a valuable tool for time-resolved spectroscopy during the last two decades Until recently, the pulse energies and field strengths of these pulses have been generally too low to allow for the use as pump pulses or the study of nonlinear effects in the terahertz range In this review article we will describe methods of generation of intense single cycle terahertz pulses with emphasis on optical rectification using the tilted-pulse-front pumping technique We will also discuss some applications of these intense pulses in the emerging field of nonlinear terahertz spectroscopy

/pdf/intense-ultrashort-terahertz-pulses-generation-and-1ewsfynvhb.pdf

Intense ultrashort terahertz pulses: generation and applications

Photoconductive antennas (PCAs) have been extensively utilized for the generation and detection of both pulsed broadband and single frequency continuous wave terahertz (THz) band radiation. These devices form the basis of many THz imaging and spectroscopy systems, which have demonstrated promising applications in various industries and research fields. The development of THz PCA technology through the last 30 years is reviewed. The key modalities of improving device performance are identified, and literature is reviewed to summarize the progress made in these areas. The goal of this review is to provide a collection of all relevant literature to bring researchers up to date on the current state and remaining challenges of THz PCA technology.

https://www.spiedigitallibrary.org/journals/optical-engineering/volume-56/issue-1/010901/Review-of-terahertz-photoconductive-antenna-technology/10.1117/1.OE.56.1.010901.pdf

Review of terahertz photoconductive antenna technology

The worldwide first all-fiber THz time-domain spectrometer for operation at 1.5 µm is presented. Applications up to 3 THz are demonstrated. Key devices are photoconductive antennas based on novel LT InGaAs/InAlAs multi-layer structures.

All-fiber terahertz time-domain spectrometer operating at 1.5 microm telecom wavelengths.

A fiber-assembled CW THz System operating at 15 microm is presented High speed telecom photodiodes integrated with planar THz antennas serve as THz emitters with power up to 10 microW Photoconductive antennas based on LT InGaAs/InAlAs multi-layer structures allow coherent detection The system operates in a wide frequency range of 01 -16 THz

Continuous wave terahertz systems exploiting 1.5 microm telecom technologies.

All-optical clock recovery from 40-Gb/s nonreturn-to-zero (NRZ) pseudorandom binary sequence data streams based on self-pulsating lasers is presented. A compact preprocessing circuit is utilized to convert an NRZ signal to a pseudoreturn-to-zero sequence before injecting into the optical clock. It comprises a semiconductor optical amplifier followed by a periodical wavelength-division-multiplexing demultiplexer filter. A stable sinusoidal clock signal with a root-mean-square jitter below 700 fs is detected at the output of the self-pulsating laser within data dynamic range of more than 8 dB. The performance of the all-optical clock recovery scheme is investigated by varying the bit rates between 39.81 and 43.02 Gb/s as well as for various wavelengths in the C-band.

Bit rate and wavelength transparent all-optical clock recovery scheme for NRZ-coded PRBS signals

A novel TE/TM convertor is proposed and demonstrated which is fabricated on (001)-InP substrate and can be integrated with TE-TM phase shifters to provide a general polarisation transformation. Some aspects of the technological realisation and first results are given; in a first attempt, a polarisation rotation of about 20 deg by 20 V could be demonstrated.

Integrated-optic polarisation convertor on (001)-InP substrate

We have fabricated GaInAsP/InP rib waveguides with lengths of up to 7 mm and widths between 2.5 and 10 ?m. The waveguides show low losses of ? = 1.38?1.84 cm?1 at a wavelength of 1.3 ?m.

M. Schlak

Papers

All-fiber terahertz time-domain spectrometer operating at 1.5 microm telecom wavelengths.

Continuous wave terahertz systems exploiting 1.5 microm telecom technologies.

Bit rate and wavelength transparent all-optical clock recovery scheme for NRZ-coded PRBS signals

Integrated-optic polarisation convertor on (001)-InP substrate

Passive optical GaInAsP/InP waveguides