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

We present what is to our knowledge the first imaging system based on optoelectronic terahertz time-domain spectroscopy Terahertz time-domain waveforms are downconverted from the terahertz to the kilohertz frequency range, and the waveform for each pixel is frequency analyzed in real time with a digital signal processor to extract compositional information at that point We demonstrate applications to package inspection and chemical content mapping in biological objects

Imaging with terahertz waves

The use of terahertz pulses for imaging has opened new possibilities for scientific and industrial applications in the terahertz frequency range. In this article, we describe the technology necessary to perform terahertz "T-ray" imaging, novel imaging techniques, and commercial applications of T-ray imaging.

T-ray imaging

This paper introduces a novel method that allows fast and reliable extraction of material parameters in terahertz time-domain spectroscopy. This method could be applied for most materials and requires neither simplifying assumptions nor samples of different thickness for the extraction. The presented extraction procedure operates either on truncated terahertz signals when temporal windowing is possible, or on full ones otherwise. Some experimental examples covering all practical cases are given. In particular, the extraction procedure treats the tedious case of samples for which internal reflections of the terahertz pulse slightly overlap.

A reliable method for extraction of material parameters in terahertz time-domain spectroscopy

We report the frequency-dependent optical constants, n(ν) and α(ν), or, equivalently, the complex permittivity e(ω) = e‘(ω) − ie‘‘(ω), over the frequency range from 2 to 50 cm-1 for water, methanol, ethanol, 1-propanol, and liquid ammonia. These spectra have been measured with femtosecond terahertz pulse transmission spectroscopy. These liquids exhibit multiple-Debye behavior, making their frequency-dependent dielectric constants valuable benchmarks for molecular dynamics simulations and other theoretical treatments of liquids.

Far-infrared dielectric properties of polar liquids probed by femtosecond terahertz pulse spectroscopy

A newly developed terahertz time-domain spectrometer based on photoconductive dipole antennas driven by femtosecond laser pulses was used to measure the absorption and index of refraction for benzene, carbon tetrachloride, and cyclohexane in the far infrared. The spectra cover the region of 3-66 cm/sup -1/, where the dielectric response of the nonpolar liquids is dominated by collision induced dipole moments. With terahertz time-domain spectroscopy, information on both the real and imaginary part of the frequency response is obtained in a range difficult to access by standard techniques. >

THz time-domain spectroscopy of nonpolar liquids

We report an unexpected observation of strong terahertz pulses, radiated perpendicular to semiconductor‐air interfaces illuminated by femtosecond laser pulses at normal incidence. Symmetry arguments preclude radiation emitted strictly along the direction of the surface electric field and the dipoles, normal to the interface. We attribute our observations to the strong interface‐induced angular redistribution of the radiation being collected by a small dielectric lens. For detection, a small symmetry breaking displacement within the terahertz diffraction spot is sufficient.

Terahertz pulses from semiconductor-air interfaces

We report on the observation of unstable two-wave mixing in a Yb-doped optical fiber amplifier induced by frequency modulation of a single-frequency laser. What is believed to be a reflection of the main signal experiences a gain much higher than that provided by the optical pumping and potentially limits power scaling under frequency modulation. We propose an explanation for the effect based on the dynamic population and refractive index gratings formed by the interference between the main signal and its slightly frequency-detuned reflection.

Observation of two-wave mixing in a single-frequency fiber amplifier induced by frequency modulation.

Advanced digital signal processing techniques in combination with ultra-wideband balanced coherent detection have enabled a new generation of ultra-high speed fiber-optic communication systems, by moving most of the processing functionalities into digital domain. In this paper, we demonstrate how digital signal processing techniques, in combination with ultra-wideband balanced coherent detection can enable optical frequency comb noise characterization techniques with novel functionalities. We propose a measurement method based on subspace tracking, in combination with multi-heterodyne coherent detection, for independent phase noise sources identification, separation and measurement. Our proposed measurement technique offers several benefits. First, it enables the separation of the total phase noise associated with a particular comb-line or -lines into multiple independent phase noise terms associated with different noise sources. Second, it facilitates the determination of the scaling of each independent phase noise term with comb-line number. Our measurement technique can be used to: identify the most dominant source of phase noise; gain a better understanding of the physics behind the phase noise accumulation process; and confirm, already existing, and enable better phase noise models. In general, our measurement technique provides new insights into noise behavior of optical frequency combs.

J. E. Pedersen

Papers

THz time-domain spectroscopy of nonpolar liquids

Terahertz pulses from semiconductor-air interfaces

Observation of two-wave mixing in a single-frequency fiber amplifier induced by frequency modulation.

Subspace tracking for independent phase noise source separation in frequency combs