Showing papers on "Terahertz radiation published in 2002"
TL;DR: Terahertz spectroscopy and imaging provide a powerful tool for the characterization of a broad range of materials, including semiconductors and biomolecules, as well as novel, higher-power terahertz sources.
Abstract: Terahertz spectroscopy systems use far-infrared radiation to extract molecular spectral information in an otherwise inaccessible portion of the electromagnetic spectrum. Materials research is an essential component of modern terahertz systems: novel, higher-power terahertz sources rely heavily on new materials such as quantum cascade structures. At the same time, terahertz spectroscopy and imaging provide a powerful tool for the characterization of a broad range of materials, including semiconductors and biomolecules.
2,673 citations
TL;DR: The ability to count optical oscillations of more than 1015 cycles per second facilitates high-precision optical spectroscopy, and has led to the construction of an all-optical atomic clock that is expected eventually to outperform today's state-of-the-art caesium clocks.
Abstract: Extremely narrow optical resonances in cold atoms or single trapped ions can be measured with high resolution. A laser locked to such a narrow optical resonance could serve as a highly stable oscillator for an all-optical atomic clock. However, until recently there was no reliable clockwork mechanism that could count optical frequencies of hundreds of terahertz. Techniques using femtosecond-laser frequency combs, developed within the past few years, have solved this problem. The ability to count optical oscillations of more than 1015 cycles per second facilitates high-precision optical spectroscopy, and has led to the construction of an all-optical atomic clock that is expected eventually to outperform today's state-of-the-art caesium clocks.
2,612 citations
TL;DR: A monolithic terahertz injection laser that is based on interminiband transitions in the conduction band of a semiconductor (GaAs/AlGaAs) heterostructure is reported, which is very promising for extending the present laser concept to continuous-wave and high-temperature operation, which would lead to implementation in practical photonic systems.
Abstract: Semiconductor devices have become indispensable for generating electromagnetic radiation in everyday applications. Visible and infrared diode lasers are at the core of information technology, and at the other end of the spectrum, microwave and radio-frequency emitters enable wireless communications. But the terahertz region (1-10 THz; 1 THz = 10(12) Hz) between these ranges has remained largely underdeveloped, despite the identification of various possible applications--for example, chemical detection, astronomy and medical imaging. Progress in this area has been hampered by the lack of compact, low-consumption, solid-state terahertz sources. Here we report a monolithic terahertz injection laser that is based on interminiband transitions in the conduction band of a semiconductor (GaAs/AlGaAs) heterostructure. The prototype demonstrated emits a single mode at 4.4 THz, and already shows high output powers of more than 2 mW with low threshold current densities of about a few hundred A cm(-2) up to 50 K. These results are very promising for extending the present laser concept to continuous-wave and high-temperature operation, which would lead to implementation in practical photonic systems.
2,425 citations
TL;DR: A generally applicable velocity matching method for THz-pulse generation by optical rectification in the range below the phonon frequency of the nonlinear material is proposed and advantages in comparison to the electro-optic Cherenkov effect and non-collinear beam mixing are discussed.
Abstract: We propose a generally applicable velocity matching method for THz-pulse generation by optical rectification in the range below the phonon frequency of the nonlinear material. Velocity matching is based on pulse front tilting of the ultrashort excitation pulse and is able to produce a large-area THz beam. Tuning of the THz radiation by changing the tilt angle is experimentally demonstrated for a narrow line in the range between 0.8-0.97 times the phonon frequency. According to model calculations broadband THz radiation can be generated at lower frequencies. Advantages of the new velocity matching technique in comparison to the electro-optic Cherenkov effect and non-collinear beam mixing are discussed.
730 citations
TL;DR: By studying the terahertz pulse shape in the time domain, TPI is able to differentiate between diseased and normal tissue for the study of basal cell carcinoma (BCC) and inflammation and scar tissue.
Abstract: We demonstrate the application of terahertz pulse imaging (TPI) in reflection geometry for the study of skin tissue and related cancers both in vitro and in vivo. The sensitivity of terahertz radiation to polar molecules, such as water, makes TPI suitable for studying the hydration levels in the skin and the determination of the lateral spread of skin cancer pre-operatively. By studying the terahertz pulse shape in the time domain we have been able to differentiate between diseased and normal tissue for the study of basal cell carcinoma (BCC). Basal cell carcinoma has shown a positive terahertz contrast, and inflammation and scar tissue a negative terahertz contrast compared to normal tissue. In vivo measurements on the stratum corneum have enabled visualization of the stratum corneum-epidermis interface and the study of skin hydration levels. These results demonstrate the potential of terahertz pulse imaging for the study of skin tissue and its related disorders, both in vitro and in vivo.
692 citations
TL;DR: Calculations and measurements are reported that confirm the production of high-power broadband THz radiation from subpicosecond electron bunches in an accelerator, several orders of magnitude higher than any existing source, which could enable various new applications.
Abstract: Terahertz (THz) radiation, which lies in the far-infrared region, is at the interface of electronics and photonics. Narrow-band THz radiation can be produced by free-electron lasers and fast diodes. Broadband THz radiation can be produced by thermal sources and, more recently, by table-top laser-driven sources and by short electron bunches in accelerators, but so far only with low power. Here we report calculations and measurements that confirm the production of high-power broadband THz radiation from subpicosecond electron bunches in an accelerator. The average power is nearly 20 watts, several orders of magnitude higher than any existing source, which could enable various new applications. In particular, many materials have distinct absorptive and dispersive properties in this spectral range, so that THz imaging could reveal interesting features. For example, it would be possible to image the distribution of specific proteins or water in tissue, or buried metal layers in semiconductors; the present source would allow full-field, real-time capture of such images. High peak and average power THz sources are also critical in driving new nonlinear phenomena and for pump-probe studies of dynamical properties of materials.
673 citations
TL;DR: In this paper, the authors present a technique to directly excite Luttinger liquid collective modes in carbon nanotubes at gigahertz frequencies by modeling the nanotube as a nano-transmission line with distributed kinetic and magnetic inductance as well as distributed quantum and electrostatic capacitance.
Abstract: Presents a technique to directly excite Luttinger liquid collective modes in carbon nanotubes at gigahertz frequencies. By modeling the nanotube as a nano-transmission line with distributed kinetic and magnetic inductance as well as distributed quantum and electrostatic capacitance, we calculate the complex frequency-dependent impedance for a variety of measurement geometries. Exciting voltage waves on the nano-transmission line is equivalent to directly exciting the yet-to-be observed one-dimensional plasmons, the low energy excitation of a Luttinger liquid. Our technique has already been applied to two-dimensional plasmons and should work well for one-dimensional plasmons. Tubes of length 100 microns must be grown for gigahertz resonance frequencies. Ohmic contact is not necessary with our technique; capacitive contacts can work. Our modeling has applications in potentially terahertz nanotube transistors and RF nanospintronics.
617 citations
TL;DR: The use of terahertz time-domain spectroscopy (THz-TDS) for recording the far-infrared dielectric function of the four nucleobases and corresponding nucleosides forming the building blocks of deoxyribose nucleic acid (DNA).
Abstract: The far-infrared dielectric function of a wide range of organic molecules is dominated by vibrations involving a substantial fraction of the atoms forming the molecule and motion associated with intermolecular hydrogen bond vibrations. Due to their collective nature such modes are highly sensitive to the intra- and intermolecular structure and thus provide a unique fingerprint of the conformational state of the molecule and effects of its environment. We demonstrate the use of terahertz time-domain spectroscopy (THz-TDS) for recording the far-infrared (0.5-4.0 THz) dielectric function of the four nucleobases and corresponding nucleosides forming the building blocks of deoxyribose nucleic acid (DNA). We observe numerous distinct spectral features with large differences between the molecules in both frequency-dependent absorption coefficient and index of refraction. Assisted by results from density-functional calculations we interpret the origin of the observed resonances as vibrations of hydrogen bonds between the molecules.
556 citations
TL;DR: Continuously tunable and coherent radiation in the wide range 56-1618 mum (0.18-5.27 THz) has been achieved as a novel and promising terahertz source based on collinear phase-matched difference frequency generation in a GaSe crystal.
Abstract: Continuously tunable and coherent radiation in the wide range 56.8–1618 µm (0.18–5.27 THz) has been achieved as a novel and promising terahertz source based on collinear phase-matched difference frequency generation in a GaSe crystal. This source has the advantages of high coherence, simplicity for tuning, simple alignment, and stable output. The peak output power for the terahertz radiation reaches 69.4 W at a wavelength of 196 µm (1.53 THz), which corresponds to a photon conversion efficiency of 3.3%. A simple optimization of the design can yield a compact terahertz source.
401 citations
TL;DR: In this paper, the photoresponse measured as a function of the gate voltage exhibited a maximum near the threshold voltage, which can be explained by the combined effect of exponential decrease of the electron density and the gate leakage current.
Abstract: We present an experimental and theoretical study of nonresonant detection of subterahertz radiation in GaAs/AlGaAs and GaN/AlGaN heterostructure field effect transistors. The experiments were performed in a wide range of temperatures (8–300 K) and for frequencies ranging from 100 to 600 GHz. The photoresponse measured as a function of the gate voltage exhibited a maximum near the threshold voltage. The results were interpreted using a theoretical model that shows that the maximum in photoresponse can be explained by the combined effect of exponential decrease of the electron density and the gate leakage current.
393 citations
02 Jun 2002
TL;DR: Guided-wave singlemode propagation of sub-ps terahertz (THz) pulses in a plastic photonic crystal fiber has been experimentally demonstrated for the first time to the best of our knowledge as discussed by the authors.
Abstract: Guided-wave single-mode propagation of sub-ps terahertz (THz) pulses in a plastic photonic crystal fiber has been experimentally demonstrated for the first time to the best of our knowledge. The plastic photonic crystal fiber is fabricated from high density polyethylene tubes and filaments. The fabricated fiber exhibits low loss and relatively low dispersive propagation of THz pulses within the experimental bandwidth of 0.1 /spl sim/ 3 THz. The measured loss and group velocity dispersion are less than 0.5 cm/sup -1/ and -0.3 ps/THz/spl middot/cm above 0.6 THz, respectively.
TL;DR: A tomographic imaging modality that uses pulsed terahertz (THz) radiation to probe the optical properties of three-dimensional structures in the far-infrared, analogous to conventional CT techniques such as x-ray CT.
Abstract: We demonstrate a tomographic imaging modality that uses pulsed terahertz (THz) radiation to probe the optical properties of three-dimensional (3D) structures in the far-infrared. This THz-wave computed tomography (T-ray CT) system provides sectional images of objects in a manner analogous to conventional CT techniques such as x-ray CT. The transmitted amplitude and phase of broadband pulses of THz radiation are measured at multiple projection angles. The filtered backprojection algorithm is then used to reconstruct the target object, including both its 3D structure and its frequency-dependent far-infrared optical properties.
TL;DR: In this article, a tunable terahertz (THz) wave source with a frequency range between 0.7 and 3 THz was proposed, using a parametric oscillation of LiNbO3 or MgO-doped LiNiBO3 crystal.
Abstract: Widely tunable terahertz (THz) wave generation by optical parametric processes based on laser light scattering from the polariton mode of nonlinear crystals is reviewed. Using parametric oscillation of LiNbO3 or MgO-doped LiNbO3 crystal pumped by a nanosecond Q-switched Nd : YAG laser, we have realized widely tunable coherent THz-wave sources in the range between 0.7 and 3 THz, with a simple configuration. For the efficient coupling of the THz wave, a monolithic grating coupler or an Si-prism array coupler was used. We report the detailed characteristics of the oscillation and the radiation, including tunability, spatial and temporal coherency, uni-directivity, and efficiency. Further, Fourier transform limited THz-wave spectrum narrowing was achieved by introducing the injection seeding method. A linewidth of about 100 MHz (0.0033 cm-1) was assured by the absorption spectrum measurement of low-pressure water vapour. At the same time, the THz-wave output was increased hundreds of times higher than that of a conventional generator which has no injection seeder. In addition, a wider tunability was observed using a tunable diode laser as the injection seeder. This room-temperature-operated, tabletop system promises to be a new widely tunable THz-wave source that is suited to a variety of applications.
TL;DR: In this article, double-quantum well field effect transistors with a grating gate exhibit a sharply resonant, voltage tuned terahertz photoconductivity, determined by the plasma oscillations of the composite structure.
Abstract: Double-quantum-well field-effect transistors with a grating gate exhibit a sharply resonant, voltage tuned terahertz photoconductivity The voltage tuned resonance is determined by the plasma oscillations of the composite structure The resonant photoconductivity requires a double-quantum well but the mechanism whereby plasma oscillations produce changes in device conductance is not understood The phenomenon is potentially important for fast, tunable terahertz detectors
TL;DR: In this article, a three-dimensional semiclassical Monte Carlo model is presented to describe fast carrier dynamics in semiconductors after photoexcitation, and the authors show that a magnetic field enhances THz emission by rotating the emitting dipole with respect to the sample surface, leading to an increased coupling of radiation through the surface.
Abstract: A three-dimensional semiclassical Monte Carlo model is presented to describe fast carrier dynamics in semiconductors after photoexcitation. Far-field terahertz (THz) radiation patterns are calculated for both InAs and GaAs with, and without, application of external magnetic fields. This analysis distinguishes between surface depletion field and photo-Dember mechanisms for generating THz radiation. The theoretical model reproduces experimental data from GaAs and InAs, and demonstrates that a magnetic field enhances THz emission by rotating the emitting dipole with respect to the sample surface, leading to an increased coupling of radiation through the surface.
TL;DR: In this article, the authors compared the polarity of the THz waveforms of InSb and InAS with that of InP, which is a wide band-gap semiconductor.
Abstract: Terahertz radiation from InSb and InAS, which are typical narrow band-gap semiconductors, was investigated using time-resolved THz emission measurements. When we compared between the polarity of the THz waveforms of these narrow band-gap semiconductors with that of InP, which is a wide bandgap semiconductor, we concluded that the ultrafast buildup of the photo-Dember field is the main mechanism for the emission of THz radiation in both InAs and InSb. The emission efficiency of InSb is approximately one-hundredth of that of InAs, although the electron mobility in InSb is higher than in InAs. Wavelength-dependent measurements implied that the anomalously low THz emission efficiency of InSb might be due to a reduction in transient mobility resulting from the scattering of electrons into the low-mobility L valley.
TL;DR: In this article, the authors report on the experiments on resonant photoresponse of the gated two-dimensional electron gas to the terahertz radiation and show that the visible-light-induced, metastable increase of the carrier density in the transistor channel shifts the resonance position to the higher gate voltages, in agreement with plasma wave detection theory.
Abstract: We report on the experiments on resonant photoresponse of the gated two-dimensional electron gas to the terahertz radiation. The visible-light-induced, metastable increase of the carrier density in the transistor channel shifts the resonance position to the higher gate voltages, in agreement with plasma wave detection theory. In this way, an unambiguous proof of the origin of the observed resonant detection is provided. The visible light illumination also leads to an increase of the electron mobility and, as a result, to an increase of the resonant detection quality factor. Resonant detection of the harmonics of the Gunn diode-based emission system is demonstrated up to 1.2 THz.
TL;DR: A device has been invented that produces radiation in the terahertz range that is a considerable feat of semiconductor fabrication, and could be used in a wide range of applications.
Abstract: A device has been invented that produces radiation in the terahertz range It is a considerable feat of semiconductor fabrication, and could be used in a wide range of applications
TL;DR: In this paper, the potential for in vivo medical applications in comparison with existing modalities is discussed, and the potential of terahertz pulsed imaging for medical applications is discussed.
Abstract: Methods have recently been developed that make use of electromagnetic radiation at terahertz (THz) frequencies, the region of the spectrum between millimetre wavelengths and the infrared, for imaging purposes. Radiation at these wavelengths is non-ionizing and subject to far less Rayleigh scatter than visible or infrared wavelengths, making it suitable for medical applications. This paper introduces THz pulsed imaging and discusses its potential for in vivo medical applications in comparison with existing modalities.
TL;DR: In this paper, a sharp copper tip is used to locally distort and concentrate the THz electric field, which is electro-optically measured in an (100) oriented GaP crystal.
Abstract: We report on a method to obtain a subwavelength resolution in terahertz time-domain imaging. In our method, a sharp copper tip is used to locally distort and concentrate the THz electric field. The distorted electric field, present mainly in the near field of the tip, is electro-optically measured in an (100) oriented GaP crystal. By raster scanning the tip along the surface of the crystal, we find the smallest THz spot size of 18 μm for frequencies from 0.1 to 2.5 THz. For our peak frequency of 0.15 THz, this corresponds to a resolution of λ/110. Our setup has the potential to reach a resolution down to a few μm.
TL;DR: In this article, a three-quantum-well chirped-superlattice active region embedded in a waveguide based on a single interface plasmon and a buried contact is demonstrated.
Abstract: A quantum-cascade laser operating at λ=66 μm is demonstrated. It consists of a three-quantum-well chirped-superlattice active region embedded in a waveguide based on a single interface plasmon and a buried contact. A threshold current density of 210 A/cm2 at T=12 K, a maximum peak optical power of 4 mW, and operation up to T=44 K are achieved in a 2.7 mm long device with a high reflectivity backfacet coating.
TL;DR: In this paper, an all-opto-electronic THz imaging system based on photomixing of two continuous-wave laser beams using photoconductive antennas is presented.
Abstract: We present an all-optoelectronic THz imaging system based on photomixing of two continuous-wave laser beams using photoconductive antennas. For a specific biological sample, we compare continuous-wave THz imaging and pulsed THz imaging at 1 THz with respect to data-acquisition time and signal-to-noise ratio, and discuss image formation from both amplitude and phase data. In addition, we introduce the application of hyperboloidal lenses which allow tighter focusing and a corresponding improvement in spatial resolution compared to off-axis paraboloidal mirrors.
TL;DR: In this article, the main aspects of nonequilibrium hot-electron phenomena in superconductors and various theoretical models developed to describe the hotelectron effect are discussed, and the most successful practical devices such as terahertz mixers and direct intensity detectors for far-infrared radiation are presented.
Abstract: The paper reviews the main aspects of nonequilibrium hot-electron phenomena in superconductors and various theoretical models developed to describe the hot-electron effect. We discuss implementation of the hot-electron avalanche mechanism in superconducting radiation sensors and present the most successful practical devices, such as terahertz mixers and direct intensity detectors, for far-infrared radiation. Our presentation also includes the novel approach to hot-electron quantum detection implemented in superconducting x-ray to optical photon counters.
24 May 2002
TL;DR: In this paper, an injection laser based on interminiband transitions in GaAs/AlGaAs heterostructures is operated in the THz range, achieving singlemode emission at 4.4THz, with output powers of 2.5mW and thresholds of few hundred A/cm/sup 2/ up to 50K.
Abstract: Injection lasers based on interminiband transitions in GaAs/AlGaAs heterostructures are operated in the THz range. Single-mode emission is achieved at 4.4THz, with output powers of 2.5mW and thresholds of few hundred A/cm/sup 2/ up to 50K.
TL;DR: In this article, a terahertz-wave (THz-wave) surface-emitted difference frequency generation with nanosecond pulse duration was demonstrated by mixing the radiation of a dual-signal-wave optical parametric oscillator based on a periodically phase-reversed PPLN crystal.
Abstract: We demonstrate terahertz-wave (THz-wave) surface-emitted difference frequency generation with nanosecond pulse duration. A slant-stripe-type periodically poled lithium niobate (PPLN) crystal was used to realize the quasi-phase-matching in two mutually perpendicular directions of optical and THz-wave propagation. A THz-wave with a wavelength near 200 μm was generated by mixing the radiation of a dual-signal-wave optical parametric oscillator based on a periodically phase-reversed PPLN crystal.
10 Nov 2002
TL;DR: The ability of Terahertz Pulse Imaging (TPI) to distinguish between carcinoma and normal tissue is demonstrated and this technique is the first to be used to identify basal cell carcinoma macroscopically in the teraherz frequency regime.
Abstract: We present results which demonstrate the ability of Terahertz Pulse Imaging (TPI) to distinguish between carcinoma and normal tissue. This technique is the first to be used to identify basal cell carcinoma macroscopically in the terahertz frequency regime. The increase in absorption observed in the diseased tissue is attributed to an increase in the amount or a change in the binding of water within the basal cell carcinoma. This makes water an important molecular marker for TPI. Our results represent a new application for the use of terahertz radiation in distinguishing carcinoma from normal tissue.
TL;DR: The first use of differential terahertz time-domain spectroscopy for bioaffinity sensing is reported, pointing the way for a host of biosensor applications using T-rays, or pulsed far-infrared radiation.
Abstract: We report the first use of differential terahertz time-domain spectroscopy for bioaffinity sensing. Binding is observed by measuring the transmission of a thin layer of biotin bound to the sensor protein avidin. We demonstrate the THz wave transmission of a sub-micron-thick film and sensitivity to 0.1 µg cm−2 of biotin. These results point the way for a host of biosensor applications using T-rays, or pulsed far-infrared (FIR) radiation.
TL;DR: In this paper, the principles of various types of terahertz time domain spectroscopies (THz TDSs) and their applications to the characterization of semiconductors are described.
Abstract: Spectroscopies using terahertz (THz) radiation excited by ultrashort laser pulses have been rapidly developing recently. In this paper, the principles of various types of THz time domain spectroscopies (THz TDSs), i.e. transmission-, reflection-and ellipsometry-type THz TDSs, and their applications to the characterization of semiconductors are described. In addition to the THz TDS using a femtosecond laser, a sub-THz TDS system using a cheap and compact continuous multimode laser diode is also described.
TL;DR: How femtosecond near-infrared laser pulses can be converted into broad band terahertz radiation using semiconductor crystals is reviewed, and the optimization of one specific generation mechanism based on ultra-fast transport of electrons and holes at a semiconductor surface is discussed.
Abstract: The terahertz region of the electromagnetic spectrum spans the frequency range between the mid-infrared and the millimetre/microwave. This region has not been exploited fully to date owing to the limited number of suitable (in particular, coherent) radiation sources and detectors. Recent demonstrations, using pulsed near-infrared femtosecond laser systems, of the viability of THz medical imaging and spectroscopy have sparked international interest; yet much research still needs to be undertaken to optimize both the power and bandwidth in such THz systems. In this paper, we review how femtosecond near-infrared laser pulses can be converted into broad band THz radiation using semiconductor crystals, and discuss in depth the optimization of one specific generation mechanism based on ultra-fast transport of electrons and holes at a semiconductor surface. We also outline a few of the opportunities for a technology that can address a diverse range of challenges spanning the physical and biological sciences, and note the continuing need for the development of solid state, continuous wave, THz sources which operate at room temperature.
TL;DR: In this paper, the angular radiation patterns from lens-coupled terahertz antennas fabricated on photoconductive substrates were measured with a novel tera-hertz (THz) time-domain spectrometer in which the femtosecond optical pulses used to gate the emitter and receiver antennas were delivered by optical fiber.
Abstract: We describe measurements of the angular radiation patterns from lens-coupled terahertz antennas fabricated on photoconductive substrates. These measurements were performed with a novel terahertz (THz) time-domain spectrometer in which the femtosecond optical pulses used to gate the emitter and receiver antennas were delivered by optical fiber. We used this system to perform a comparison between the two substrate-lens designs commonly used in THz time-domain spectrometers. We measured both E-plane and H-plane emission patterns for a 90° bow-tie antenna. By comparing these experimental results with simulations based on Fresnel–Kirchoff diffraction, we find that the choice of substrate-lens design is important in determining not only the directivity of the emitted beam but also the spectral bandwidth. These results emphasize the significance of this crucial component in the design of broadband THz spectrometers.