Pump-probe transmission experiments have been performed on PbSe above the fundamental absorption edge near 4 m in the temperature range 30 to 300 K, using the Dutch ps free-electron laser. For temperatures below 200 K and carrier densities above the threshold for stimulated emission, stimulated recombination represents the most efficient recombination mechanism with relatively fast kinetics in the 50-100-ps regime, in good agreement with earlier reports of photoluminescent emission. Above this temperature Auger recombination dominates, and the Auger coefficient C is determined from the pump-probe decay curves. In the lowtemperature regime the Auger coefficient is determined from the decay curves at times beyond 100 ps. The Auger coefficient is approximately constant ͑with a value of about 8ϫ10 Ϫ28 cm 6 s Ϫ1) between 300 and 70 K, and then drops a value of about 1ϫ10 Ϫ28 cm 6 s Ϫ1 at 30 K, in good agreement with the theory for nonparabolic near-mirror bands and nondegenerate statistics. It is found that C for PbSe is between one and two orders of magnitude lower than for Hg 1Ϫx Cd x Te of comparable band gap. ͓S0163-1829͑98͒07243-9͔

An imaging system has been developed based on pulses of Terahertz (THz) radiation generated and detected using all-optical effects accessed by irradiating semiconductors with ultrafast (fs-ps) pulses of visible laser light. This technique, commonly referred to as T-Ray Imaging or THz Pulse Imaging (TPI), holds enormous promise for certain aspects of medical imaging. We have conducted an initial survey of possible medical applications of TPI and demonstrated that TPI images show good contrast between different animal tissue types (muscle, fat, kidney, skin, cartilage). Moreover, the diagnostic power of TPI has been elucidated by the spectra available at each pixel in the image, which are markedly different for the different tissue types. This suggests that the spectral information inherent in TPI might be used to identify the type of soft and hard tissue at each pixel in an image and provide other diagnostic information not afforded by conventional imaging techniques.Preliminary TPI studies ofpork skin show that 3D tomographic imaging ofthe skin surface and thickness is possible, and data from experiments on models of the human dermis are presented which demonstrate that different constituents of skin have different refractive indices. Lastly, we present the first THz image of human tissue, namely an extracted tooth. The time of flight of THz pulses through the tooth allows the thickness of the enamel to be determined, and is used to create an image showing the enamel and dentine regions. Absorption of THz pulses in the tooth allows the pulp cavity region to be identified. Initial evidence strongly suggests that TPI may be used to provide valuable diagnostic information pertaining to the enamel, dentine, and the pulp cavity.

Image of a human tooth formed from terahertz radiation. The data can be manipulated to provide different terahertz Images, each containing different diagnostic information IMAGING technology is becoming increasingly prevalent in our society. X-ray scanners are routinely used to examine luggage at airports, for example, and most hospitals are equipped with ultrasound scanners and magnetic resonance imaging machines. There is also a wealth of other less well known applications throughout industry. For instance, X-rays are used for package inspection, while the defects or voids in materials on production lines are often probed using microwaves or ultrasound.In spite of their considerable success, X-rays, magnetic resonance imaging and ultrasound all have shortcomings (see table on page 37). Many clinicians and non-medical users feel that fundamentally different physical principles are needed to provide safer and more cost-effective imaging techniques. And physicists are turning to other regions of the electromagnetic spectrum to address these issues.Indeed, a cursory examination reveals that conventional imaging techniques only use the extreme ends of the electromagnetic spectrum: photons with energies greater than 30 keV for X-rays, and around 0.4 jaeV for magnetic resonance imaging. The radiation between these extremes falls largely into the visible, infrared and millimetre or microwave regions.

The improvement in the detection of caries offers the possibility for dramatic improvement in dental healthcare. Current caries detection rates suggest that there may be scope for improvement. This paper describes a preliminary study to examine applications of terahertz pulse imaging (TPI) to caries detection. We present results for the detection of early stage caries in the occlusal enamel layer of a range of human tooth cross sections using TPI. Higher attenuation of terahertz radiation was observed in carious enamel as compared with healthy enamel. Hypomineralised enamel had different absorption spectra and contrast compared to carious enamel in TPI images. These results have important implications for extending TPI to other medical imaging applications where both early diagnosis and safety issues are important.

Articles you may be interested inElectric-field-induced anisotropy of excitonic optical properties in semiconductor quantum dots

Picosecond time-resolved far-infrared measurements are presented of the scattering between conductionband states in a doped quasi quantum dot. These states are created by the application of a magnetic field along the growth direction of an InAs/AlSb quantum well. A clear suppression of the cooling rate is seen, from 10 12 s Ϫ1 when the level spacing is equal to the phonon energy, to 10 10 s Ϫ1 away from this resonance, and thus the results provide unambiguous evidence for the phonon bottleneck. Furthermore, the lifetimes had only weak dependence on temperature between 4 and 80 K. ͓S0163-1829͑99͒50612-7͔Electronic lifetimes of two-dimensional ͑2D͒ systems in magnetic fields are of fundamental interest in part because the quantization effect of the magnetic field mimics the effect of a quantum dot potential with an easily variable degree of confinement. 1 The magnetic field perpendicular to the layers forces the electrons into confined orbits and the density of states becomes a ladder of broadened ␦ functions similar to that of a quantum dot. Recently much work has been carried out on the so-called ''phonon bottleneck'' that has been claimed to inhibit the cooling of carriers in quantum dots when the level separation is not equal to the phonon energy. [2][3][4][5][6] However, partly as a result of different groups using different growth techniques for interband photoluminescence samples and partly on fundamental grounds, this is controversial and is the subject of much debate. 7-12 Indeed several mechanisms have been proposed that may bypass the bottleneck, such as multiphonon scattering, 7 Auger processes, 8 excitonic effects, 9 and defect related processes. 10 In the present work we observe clear phonon suppression in n-type quasi 0D dots ͑i.e., Landau quantized rather than spatially quantized͒ by a time-resolved intraband absorption measurement. This provides unambiguous evidence for the phonon bottleneck effect independently of arguments concerning which processes dominate in the interband photoluminescence measurements in dots 3-6,9-12 and quasi dots 13 such as electron-hole scattering. Further, because of the very clean model system ͑much sharper interfaces and no wetting layer, etc.͒, the interpretation is not complicated by detailed questions about different growth techniques and the quality of different dot sample structures. The results should assist in the understanding of which aspect of these processes is fundamental and which is dependent on sample structure.Resonant absorption of longitudinal optic ͑LO͒ phonons causes a variety of transport properties to oscillate with applied magnetic field, such as the magnetoresistance. 14 Resonant phonon scattering occurs whenwhere ប LO is the LO phonon energy, ប c ϭបeB/m* is the cyclotron energy, and ⌬l is an integer. At these resonances the LO phonon absorption/emission probability is strongly enhanced giving rise to large changes in the electron energy relaxation lifetime. 15,16 In the present work we have used the pump-probe technique, with cyclotron resonan...

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Room-temperature pump–probe transmission experiments have been performed on an arsenic-rich InAs/InAs1−xSbx strained layer superlattice (SLS) above the fundamental absorption edge near 10 μm, using a ps far-infrared free-electron laser. Measurements show complete bleaching at the excitation frequency, with recovery times which are found to be strongly dependent on the pump photon energy. At high excited carrier densities, corresponding to high photon energy and interband absorption coefficient, the recombination is dominated by Auger processes. A direct comparison with identical measurements on epilayers of InSb, of comparable room-temperature band gap, shows that the Auger processes have been substantially suppressed in the superlattice case as a result of both the quantum confinement and strain splittings in the SLS structure. In the nondegenerate regime, where the Auger lifetime scales as τ−1aug=C1N2e, a value of C1 some 100 times smaller is obtained for the SLS structure. The results have been interpreted in terms of an 8×8 k⋅p SLS energy band calculation, including the full dispersion for both k in plane and k parallel to the growth direction. This is the strongest example of room-temperature Auger suppression observed to date for these long-wavelength SLS alloy compositions and implies that these SLS materials may be attractive for applications as room-temperature mid-IR diode lasers.