Showing papers on "Terahertz radiation published in 2004"
TL;DR: It is shown that magnetic response at terahertz frequencies can be achieved in a planar structure composed of nonmagnetic conductive resonant elements and suggested that artificial magnetic structures, or hybrid structures that combine natural and artificial magnetic materials, can play a key role in teraHertz devices.
Abstract: We show that magnetic response at terahertz frequencies can be achieved in a planar structure composed of nonmagnetic conductive resonant elements. The effect is realized over a large bandwidth and can be tuned throughout the terahertz frequency regime by scaling the dimensions of the structure. We suggest that artificial magnetic structures, or hybrid structures that combine natural and artificial magnetic materials, can play a key role in terahertz devices.
1,453 citations
TL;DR: Terahertz irradiation and sensing is being applied for the first time to a wide range of fields outside the traditional niches of space science, molecular line spectroscopy, and plasma diagnostics as discussed by the authors.
Abstract: Terahertz irradiation and sensing is being applied for the first time to a wide range of fields outside the traditional niches of space science, molecular line spectroscopy, and plasma diagnostics. This paper surveys some of the terahertz measurements and applications of interest in the biological and medical sciences.
1,090 citations
TL;DR: It is shown how a simple waveguide, namely a bare metal wire, can be used to transport terahertz pulses with virtually no dispersion, low attenuation, and with remarkable structural simplicity.
Abstract: Sources and systems for far-infrared or terahertz (1 THz = 10(12) Hz) radiation have received extensive attention in recent years, with applications in sensing, imaging and spectroscopy. Terahertz radiation bridges the gap between the microwave and optical regimes, and offers significant scientific and technological potential in many fields. However, waveguiding in this intermediate spectral region still remains a challenge. Neither conventional metal waveguides for microwave radiation, nor dielectric fibres for visible and near-infrared radiation can be used to guide terahertz waves over a long distance, owing to the high loss from the finite conductivity of metals or the high absorption coefficient of dielectric materials in this spectral range. Furthermore, the extensive use of broadband pulses in the terahertz regime imposes an additional constraint of low dispersion, which is necessary for compatibility with spectroscopic applications. Here we show how a simple waveguide, namely a bare metal wire, can be used to transport terahertz pulses with virtually no dispersion, low attenuation, and with remarkable structural simplicity. As an example of this new waveguiding structure, we demonstrate an endoscope for terahertz pulses.
1,047 citations
TL;DR: A novel type of Fourier-transform infrared spectrometer based on two Ti:sapphire lasers emitting femtosecond pulse trains with slightly different repetition frequencies that superimposed upon a detector to produce purely time-domain interferograms that encode the infrared spectrum is demonstrated.
Abstract: A novel type of Fourier-transform infrared spectrometer (FTIR) is demonstrated. It is based on two Ti:sapphire lasers emitting femtosecond pulse trains with slightly different repetition frequencies. Two mid-infrared beams-derived from those lasers by rectification in GaSe-are superimposed upon a detector to produce purely time-domain interferograms that encode the infrared spectrum. The advantages of this spectrometer compared with the common FTIR include ease of operation (no moving parts), speed of acquisition (100 micros demonstrated), and not-yet-shown collimated long-distance propagation, diffraction-limited microscopic probing, and electronically controllable chemometric factoring. Extending time-domain frequency-comb spectroscopy to lower (terahertz) or higher (visible, ultraviolet) frequencies should be feasible.
697 citations
TL;DR: The far-infrared absorption and index of refraction of high resistivity, float-zone, crystalline silicon has been measured by terahertz time-domain spectroscopy.
Abstract: The far-infrared absorption and index of refraction of high-resistivity, float-zone, crystalline silicon has been measured by terahertz time-domain spectroscopy. The measured new upper limit for the absorption of this most transparent dielectric material in the far infrared shows unprecedented transparency over the range from 0.5 to 2.5 THz and a well-resolved absorption feature at 3.6 THz. The index of refraction shows remarkably little dispersion, changing by only 0.0001 over the range from 0.5 to 4.5 THz.
412 citations
TL;DR: In this article, the authors reviewed the most recent results in terahertz fields and focused on the physical principles of tera-hertz generators and receivers, underlining the link between terahethertz devices and modern technologies such as micromachining and nanotechnology.
405 citations
TL;DR: A threshold for THz generation is identified that proves that generation of a plasma is required and that the nonlinearity of air is insufficient to explain measurements, and the plasma emitter is competitive with other state-of-the-art THz emitters.
Abstract: Intense radiation in the terahertz (THz) frequency range can be generated by focusing of an ultrashort laser pulse composed of both a fundamental wave and its second-harmonic field into air, as reported previously by Cook
[Opt. Lett.25, 1210 (2000)]. We identify a threshold for THz generation that proves that generation of a plasma is required and that the nonlinearity of air is insufficient to explain our measurements. An additional THz field component generated in the type I β-barium borate crystal used for second-harmonic generation has to be considered if one is to avoid misinterpretation of this kind of experiment. We conclude with a comparison that shows that the plasma emitter is competitive with other state-of-the-art THz emitters.
388 citations
TL;DR: In this paper, the resonant, voltage tunable emission of terahertz radiation (0.4 − 1.0 THz) from a gated two-dimensional electron gas in a 60 nm InGaAs high electron mobility transistor was investigated.
Abstract: We report on the resonant, voltage tunable emission of terahertz radiation (0.4–1.0 THz) from a gated two-dimensional electron gas in a 60 nm InGaAs high electron mobility transistor. The emission is interpreted as resulting from a current driven plasma instability leading to oscillations in the transistor channel (Dyakonov–Shur instability).
321 citations
TL;DR: In this article, the photoresponse to sub-THz (120GHz) radiation of Si field effect transistors (FETs) with nanometer and sub-micron gate lengths at 300K was investigated.
Abstract: We report on experiments on photoresponse to sub-THz (120GHz) radiation of Si field-effect transistors (FETs) with nanometer and submicron gate lengths at 300K. The observed photoresponse is in agreement with predictions of the Dyakonov–Shur plasma wave detection theory. This is experimental evidence of the plasma wave detection by silicon FETs. The plasma wave parameters deduced from the experiments allow us to predict the nonresonant and resonant detection in THz range by nanometer size silicon devices—operating at room temperature.
315 citations
TL;DR: In this article, a femtosecond laser-based optical frequency synthesizer is used to demonstrate the generation and control of the frequency of electromagnetic fields over 100 terahertz of bandwidth with fractional uncertainties approaching 1 part in 1019.
Abstract: A femtosecond laser–based optical frequency synthesizer is referenced to an optical standard, and we use it to demonstrate the generation and control of the frequency of electromagnetic fields over 100 terahertz of bandwidth with fractional uncertainties approaching 1 part in 1019. The reproducibility of this performance is verified by comparison of different types of femtosecond laser–based frequency synthesizers from three laboratories.
285 citations
TL;DR: In this paper, the phase and amplitude of a terahertz time-domain spectroscopy (THz-TDS) pulse at each frequency can be determined by using a digital holography method.
Abstract: Terahertz time-domain spectroscopy (THz-TDS) is a coherent measurement technology. Using THz-TDS, the phase and amplitude of the THz pulse at each frequency can be determined. Like radar, THz-TDS also provides time information that allows us to develop various three-dimensional THz tomographic imaging modalities. The three-dimensional THz tomographic imagings we investigated are: terahertz diffraction tomography (THz DT), terahertz computed tomography (THz CT), THz binary lens tomography and THz digital holography. THz DT uses the THz wave as a probe beam to interact with a target, and then reconstructs the three-dimensional image of the target using the THz waves scattered by the target. THz CT is based on geometrical optics and inspired by x-ray CT. THz binary lens tomography uses the frequency dependent focal length property of binary lenses to obtain tomographic images of an object. THz three-dimensional holography combines radar and conventional holography technology. By separating the multiple scattered THz waves of different scattering orders, we used a digital holography method to reconstruct the sparsely distributed scattering centres. Three-dimensional THz imaging has potential in such applications as non-destructive inspection. The interaction between a coherent THz pulse and an object provides rich information about the object under study; therefore, three-dimensional THz imaging is a very useful tool to inspect or characterize dielectric and semiconductor objects. For example, three-dimensional THz imaging can be used to detect and identify the defects inside a space shuttle insulation tile.
TL;DR: In this paper, it was shown that the laser induced ultrafast demagnetization of ferromagnetic films results in the emission of a terahertz electromagnetic pulse, which was detected from Ni films using free-space electro-optic sampling.
Abstract: It is shown that the laser induced ultrafast demagnetization of ferromagnetic films results in the emission of a terahertz electromagnetic pulse. This emission has been detected from Ni films using free-space electro-optic sampling. The radiated electric field E(t) is explained by Maxwell equations (radiation from a time dependent magnetic dipole), and is expected to be proportional to the second time derivative of the magnetization d2M/dt2, as measured in the far field. This technique opens appealing perspectives in the context of measuring and understanding the ultrafast spin dynamics as well as the interaction of electrons (both charge and spin) with electromagnetic fields.
TL;DR: A wide-bandwidth, phase-stabilized femtosecond laser is used to monitor the real-time dynamic evolution of population transfer and the mechanical action of the optical frequency comb on the atomic sample is explored and controlled, leading to precision spectroscopy with an appreciable reduction in systematic errors.
Abstract: Ultrashort laser pulses have thus far been used in two distinct modes. In the time domain, the pulses have allowed probing and manipulation of dynamics on a subpicosecond time scale. More recently, phase stabilization has produced optical frequency combs with absolute frequency reference across a broad bandwidth. Here we combine these two applications in a spectroscopic study of rubidium atoms. A wide-bandwidth, phase-stabilized femtosecond laser is used to monitor the real-time dynamic evolution of population transfer. Coherent pulse accumulation and quantum interference effects are observed and well modeled by theory. At the same time, the narrow linewidth of individual comb lines permits a precise and efficient determination of the global energy-level structure, providing a direct connection among the optical, terahertz, and radio-frequency domains. The mechanical action of the optical frequency comb on the atomic sample is explored and controlled, leading to precision spectroscopy with an appreciable reduction in systematic errors.
TL;DR: A number of preliminary applications are shown that cover various areas: nondestructive real-time testing for plastic tubes and packaging seals; biological terahertz imaging of fresh, frozen, or freeze-dried samples; paraffin-embedded specimens of cancer tissue; and measurement of the absorption coefficient of water by use of a wedge-shaped cell.
Abstract: We present an imaging system designed for use in the terahertz range. As the radiation source a backward-wave oscillator was chosen for its special features such as high output power, good wave-front quality, good stability, and wavelength tunability from 520 to 710 GHz. Detection is achieved with a pyroelectric sensor operated at room temperature. The alignment procedure for the optical elements is described, and several methods to reduce the etalon effect that are inherent in monochromatic sources are discussed. The terahertz spot size in the sample plane is 550 microm (nearly the diffraction limit), and the signal-to-noise ratio is 10,000:1; other characteristics were also measured and are presented in detail. A number of preliminary applications are also shown that cover various areas: nondestructive real-time testing for plastic tubes and packaging seals; biological terahertz imaging of fresh, frozen, or freeze-dried samples; paraffin-embedded specimens of cancer tissue; and measurement of the absorption coefficient of water by use of a wedge-shaped cell.
TL;DR: In this paper, the application of terahertz pulsed spectroscopy to polymorphic, liquid crystalline and amorphous forms of pharmaceutical compounds has been investigated and it was shown that the modes observed are due to crystalline phonon and possibly hydrogen-bonding vibrations.
TL;DR: In this paper, the authors demonstrate the construction of reasonably long and non-polarization changing photonic fiber waveguide using Teflon which is a readily available and highly flexible material.
Abstract: We demonstrate the construction of reasonably long and non-polarization changing photonic fiber waveguide using Teflon which is a readily available and highly flexible material. Due to its relatively low loss coefficient, the possibility of preparing longer photonic fiber waveguide, which has the potential of guiding intense THz radiation, can be easily attained.
TL;DR: In this article, the authors reported the room-temperature operation of an electrically controlled terahertz modulator, which achieved a maximum modulation depth of 3% for a pulse of terahethertz radiation covering the range of frequencies from 0.1 to 2 THz.
Abstract: In this letter, we report the room-temperature operation of an electrically controlled THz modulator. The modulation is achieved by reducing the electron density in a gated two-dimensional electron gas structure, which leads to an increase in the transmitted intensity of an incident beam of THz radiation. By depleting an electron gas of density 1012 cm−2, we achieved a maximum modulation depth of 3% for a pulse of terahertz radiation covering the range of frequencies from 0.1 to 2 THz.
TL;DR: Flexible, hollow polycarbonate waveguides with interior Cu coatings for broadband THz transmission using simple liquid-phase chemistry techniques and the losses for these hollow-core guides were measured using a tunable, cw single-mode far IR laser.
Abstract: Terahertz (THz) radiation has important applications in spectroscopy, imaging, and space science. Fiber optics for the THz region have been limited to rigid hollow metallic waveguides or short lengths of solid-core transparent dielectrics such as sapphire and plastic. We have fabricated flexible, hollow polycarbonate waveguides with interior Cu coatings for broadband THz transmission using simple liquid-phase chemistry techniques. The losses for these hollow-core guides were measured using a tunable, cw single-mode far IR laser. The losses for the best guides were found to be less than four dB/m and the single mode of the laser was preserved for the smaller bore waveguides.
TL;DR: In this paper, the authors used a custom ErAs:GaAs tunable photomixing spectrometer to measure millimeter-wave and terahertz electromagnetic transmission through cloth samples from eight types of fabrics common in garments and baggage.
Abstract: This letter reports electromagnetic transmission measurements through cloth samples from eight types of fabrics common in garments and baggage. The transmission at millimeter-wave and terahertz frequencies was measured with a custom ErAs:GaAs tunable photomixing spectrometer. The IR transmission between 3 and 8μm was measured with a Fourier-transform infrared spectrometer. All samples were usefully transparent at millimeter-wave frequencies (up to 300GHz) based on a 3dB criterion, but became progressively opaque at higher frequencies in a highly sample-dependent manner. This is explained by the samples becoming “optically dense” in the THz region, so that the transmission becomes exponentially dependent on sample thickness. The attenuation in the IR region is very high (⩾25dB) except in two samples (rayon and nylon), whose exceptional transparency (e.g., −12dB in nylon) is attributed to pores intrinsic to the material.
TL;DR: In this article, field-resolved detection of ultrabroadband infrared pulses is implemented with GaSe crystals via phase matching, extending the detectable frequency range toward the near infrared (λ=2.5μm) and directly record transform-limited pulses as short as 28fs with a 3dB bandwidth of 41THz.
Abstract: Field-resolved detection of ultrabroadband infrared pulses is implemented with GaSe crystals. Via phase matching, we extend the detectable frequency range toward the near infrared (λ=2.5μm) and directly record transform-limited pulses as short as 28fs with a 3dB bandwidth of 41THz. The continuous tunability of the center frequency over a wide interval ranging from 31THz to the far-infrared is demonstrated using thick sensors.
TL;DR: Waveguide terahertz time-domain spectroscopy is demonstrated to have the sensitivity to characterize nanometer-thick water layers on the surfaces of a parallel-plate metal waveguide.
Abstract: Waveguide terahertz time-domain spectroscopy is demonstrated to have the sensitivity to characterize nanometer-thick water layers on the surfaces of a parallel-plate metal waveguide. The measured far-infrared absorption and index of refraction of the 20-nm water layers are in reasonable self-consistent agreement with those of bulk water.
TL;DR: In this paper, the design and projected performance of quantum-well infrared photodetectors (QWIP) for the terahertz (1-10 THz) or the very-far-infrared region are presented together with the initial demonstration of a GaAs/AlGaAs QWIP working at photon energies below the optical phonons.
Abstract: The design and projected performance of quantum-well infrared photodetectors (QWIP) for the terahertz (1–10 THz) or the very-far-infrared region are presented together with our initial demonstration of a GaAs/AlGaAs QWIP working at photon energies below the optical phonons. We point out the problem with this initial device, discuss possible causes, and suggest areas of improvement.
TL;DR: In this paper, a quantum-cascade laser was reported to operate at 3.2 THz up to a heat-sink temperature of 93 K. Resonant longitudinal-optical phonon scattering was used to depopulate the lower radiative state and a low-loss metal-metal waveguide was employed to provide high modal confinement.
Abstract: We report cw operation of a quantum-cascade laser at 3.2 THz (λ≈94 μm) up to a heat-sink temperature of 93 K. Resonant longitudinal-optical phonon scattering is used to depopulate the lower radiative state and a low-loss metal–metal waveguide is used to provide high modal confinement. Optical powers of ∼1.8 mW at 10 K and ∼400 μW at 78 K are observed from a single facet of a 40-μm-wide and 1.35-mm-long laser device. A threshold current density of 432 A/cm2 at 10 K and 552 A/cm2 at 78 K was obtained in cw mode. The same device lased up to 129 K in pulsed mode with a threshold current density of 419 A/cm2 at 5 K.
TL;DR: Experimental results of the transmission magnitude and phase change of terahertz pulses through thin metallic films patterned with subwavelength hole arrays on silicon wafers determine that the aperture shape has a notable effect on the transmission properties of two-dimensional hole arrays.
Abstract: We present experimental results of the transmission magnitude and phase change of terahertz pulses through thin metallic films patterned with subwavelength hole arrays on silicon wafers. Terahertz time-domain spectroscopy measurements reveal a sharp phase peak centered on the surface plasmon resonance. Correspondingly, and consistent with the Kramers-Kronig relations, the measured transmission magnitude has the shape of the derivative of this peak. In addition, we determine that the aperture shape has a notable effect on the transmission properties of two-dimensional hole arrays.
TL;DR: In this article, a GaAs/AlGaAs-based far-infrared quantum well infrared photodetector at a wavelength of λ=84 μm was demonstrated for zero bias operation, which profits from a relatively low dark current and a good noise behavior.
Abstract: We demonstrated a GaAs/AlGaAs-based far-infrared quantum well infrared photodetector at a wavelength of λ=84 μm. The relevant intersubband transition is slightly diagonal with a dipole matrix element of 3.0 nm. At 10 K, a responsivity of 8.6 mA/W and a detectivity of 5×107 cm √Hz/W have been achieved; and successful detection up to a device temperature of 50 K has been observed. Being designed for zero bias operation, this device profits from a relatively low dark current and a good noise behavior.
Journal Article•
TL;DR: The development of technology in the THz frequency band has seen rapid progress recently as mentioned in this paper, with the development of sources and detectors for this frequency range driven by other applications such as spectroscopy, imaging, and impulse ranging.
Abstract: The development of technology in the THz frequency band has seen rapid progress recently. Considered as an extension of the microwave and millimeter wave bands, the THz frequency offers greater communications bandwidth than is available at microwave frequencies. The development of sources and detectors for this frequency range has been driven by other applications such as spectroscopy, imaging, and impulse ranging. Only recently modulators and filters have been added to enable the development of communications applications. APL's contributions to date in THz research have been primarily in the areas of spectroscopy and imaging. This article gives an overview of THz technology for communications and sensing applications, with some discussion of the sources, detectors, and modulators needed for a practical THz communications system.
TL;DR: In this article, a finite difference-time-domain simulation was developed to model the propagation of a terahertz (THz) pulse and incorporate double Debye theory to model water subject to THz radiation.
Abstract: Studies in terahertz (THz) imaging have revealed a significant difference between skin cancer (basal cell carcinoma) and healthy tissue. Since water has strong absorptions at THz frequencies and tumor affects the water content of tissue, a likely contrast mechanism is variation in water content. Modeling the propagation of a THz pulse through water is the first step toward understanding the origin of contrast in terahertz pulsed images of skin cancer. In this letter, we develop a finite-difference-time-domain simulation to model the propagation of a THz pulse and incorporate double Debye theory to model the behavior of water subject to THz radiation. Furthermore, we apply this model to skin.
TL;DR: Based on phase-matched collinear difference-frequency generation in a single GaSe crystal, continuously tunable and coherent radiation in the extremely wide ranges of 2.7-38.4 and 58.2-3540 μm has been achieved.
Abstract: Based on phase-matched collinear difference-frequency generation in a single GaSe crystal, continuously tunable and coherent radiation in the extremely wide ranges of 2.7–38.4 and 58.2–3540 μm has been achieved. This unique source has the additional advantages of high coherence (narrow linewidth) and simple alignment. The peak output power for the terahertz radiation reaches 209 W at the wavelength of 196 μm (1.53 THz), which corresponds to a power conversion efficiency of 0.055%. Moreover, the terahertz transmission spectra on DNA macromolecules and protein were directly measured, demonstrating some potential and important applications of this terahertz source.
TL;DR: The most promising microfabrication techniques applicable to high-frequency RF devices and examples of recent applications are described.
Abstract: Advances in manufacturing technology for microstructures are allowing new opportunities for vacuum electron devices producing radio-frequency (RF) radiation. Specifically, the capability to produce small circuit structures is allowing development of RF devices at frequencies impractical with traditional machining technology. This is generating increased interest in applications in the submillimeter and terahertz frequency range. High-power RF devices in this frequency range are needed for medical, communications, defense, and homeland security applications. This paper describes the most promising microfabrication techniques applicable to high-frequency RF devices and examples of recent applications.
TL;DR: In this article, the authors reported the coherent generation and detection of ultrabroadband terahertz (THz) radiation using low-temperature-grown GaAs photoconductive antennas as both emitters and receivers.
Abstract: We report the coherent generation and detection of ultrabroadband terahertz (THz) radiation using low-temperature-grown GaAs photoconductive antennas as both emitters and receivers. THz radiation with frequency components over 15THz was obtained, the highest reported for a THz time-domain system based on photoconductive antennas. Such a system has a smooth spectral distribution between 0.3 and 7.5THz, ideal for spectroscopic applications. In addition, sharp spectral features at 8.0 and 8.8THz were observed, and explained in terms of optical phonon resonances in the photoconductive antennas.