Using the method of time-domain spectroscopy, we measure the far-infrared absorption and dispersion from 0.2 to 2 THz of the crystalline dielectrics sapphire and quartz, fused silica, and the semiconductors silicon, gallium arsenide, and germanium. For sapphire and quartz, the measured absorptions are consistent with the earlier work below 0.5 THz. Above 1 THz we measure significantly more absorption for sapphire, while for quartz our values are in reasonable agreement with those of the previous work. Our results on high-purity fused silica are consistent with those on the most transparent fused silica measured to date. For the semiconductors, we show that many of the previous measurements on silicon were dominated by the effects of carriers due to impurities. For high-resistivity, 10-kΩ cm silicon, we measure a remarkable transparency together with an exceptionally nondispersive index of refraction. For GaAs our measurements extend the precision of the previous work, and we resolve two weak absorption features at 0.4 and 0.7 THz. Our measurements on germanium demonstrate the dominant role of intrinsic carriers; the measured absorption and dispersion are well fitted by the simple Drude theory.

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Far-infrared time-domain spectroscopy with terahertz beams of dielectrics and semiconductors

Hydrogels have been developed to respond to a wide variety of stimuli, but their use in macroscopic systems has been hindered by slow response times (diffusion being the rate-limiting factor governing the swelling process) However, there are many natural examples of chemically driven actuation that rely on short diffusion paths to produce a rapid response It is therefore expected that scaling down hydrogel objects to the micrometre scale should greatly improve response times At these scales, stimuli-responsive hydrogels could enhance the capabilities of microfluidic systems by allowing self-regulated flow control Here we report the fabrication of active hydrogel components inside microchannels via direct photopatterning of a liquid phase Our approach greatly simplifies system construction and assembly as the functional components are fabricated in situ, and the stimuli-responsive hydrogel components perform both sensing and actuation functions We demonstrate significantly improved response times (less than 10 seconds) in hydrogel valves capable of autonomous control of local flow

Functional hydrogel structures for autonomous flow control inside microfluidic channels

Time-resolved, pulsed terahertz spectroscopy has developed into a powerful tool to study charge carrier dynamics in semiconductors and semiconductor structures over the past decades. Covering the energy range from a few to about 100 meV, terahertz radiation is sensitive to the response of charge quasiparticles, e.g., free carriers, polarons, and excitons. The distinct spectral signatures of these different quasiparticles in the THz range allow their discrimination and characterization using pulsed THz radiation. This frequency region is also well suited for the study of phonon resonances and intraband transitions in low-dimensional systems. Moreover, using a pump-probe scheme, it is possible to monitor the nonequilibrium time evolution of carriers and low-energy excitations with sub-ps time resolution. Being an all-optical technique, terahertz time-domain spectroscopy is contact-free and noninvasive and hence suited to probe the conductivity of, particularly, nanostructured materials that are difficult or impossible to access with other methods. The latest developments in the application of terahertz time-domain spectroscopy to bulk and nanostructured semiconductors are reviewed.

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Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

Introductory Fourier transform spectroscopy

Terahertz (THz) radiation, which occupies a relatively unexplored portion of the electromagnetic spectrum between the mid-infrared and microwave bands, offers innovative sensing and imaging technologies that can provide information unavailable through conventional methods such as microwave and X-ray techniques. With the advancement of THz technologies, THz sensing and imaging will impact a broad range of interdisciplinary fields, including chemical and biological detections and identifications. In particular, THz radiation offers the opportunity for transformational advances in defense and security. Recent work shows that THz technologies are promising for the standoff detection and identification of explosive targets.

Terahertz Spectroscopy and Imaging for Defense and Security Applications

The optical constants of some common low-loss polymers between 4 and 40 cm -1

The far-infrared optical constants of polypropylene, PTFE and polystyrene

The far infrared optical constants of polyethylene

The refractive index of water vapour: A comparison of measurement and theory

An interferometer for the determination of the temperature variation of the complex refraction spectra of reasonably transparent solids at near-millimetre wavelengths

J.R. Birch

Papers

The optical constants of some common low-loss polymers between 4 and 40 cm -1

The far-infrared optical constants of polypropylene, PTFE and polystyrene

The far infrared optical constants of polyethylene

The refractive index of water vapour: A comparison of measurement and theory

An interferometer for the determination of the temperature variation of the complex refraction spectra of reasonably transparent solids at near-millimetre wavelengths