Showing papers on "Terahertz radiation published in 2000"
TL;DR: A new four-wave rectification method for the generation of intense, ultrafast terahertz pulses from gases that is sensitive to the relative phases of the fundamental and second-harmonic pulses.
Abstract: We describe a new four-wave rectification method for the generation of intense, ultrafast terahertz (THz) pulses from gases. The fundamental and second-harmonic output of an amplified Ti:sapphire laser is focused to a peak intensity of ~5x10(14)W/cm (2) . Under these conditions, peak THz fields estimated at 2 kV/cm have been observed; the measured power spectrum peaks near 2 THz. Phase-dependent measurements show that this is a coherent process and is sensitive to the relative phases of the fundamental and second-harmonic pulses. Comparable THz signals have been observed from nitrogen and argon as well as from air.
915 citations
TL;DR: The multifrequency high-field EMR facility is revealed to be extremely advantageous while solving such problems as observation of AFMR transitions in spin-ordered systems, g-factor resolution enhancement in complex organic radicals, and resonance signal detection in EMR-silent spin systems having integer spin and large zero field splitting.
303 citations
TL;DR: In this article, femtosecond optical pulses are propagated through a periodically poled lithium-niobate crystal, where the domain length is matched to the walk-off length between the optical and THz pulses.
Abstract: We demonstrate a promising technique for generating narrow-band terahertz electromagnetic radiation. Femtosecond optical pulses are propagated through a periodically poled lithium-niobate crystal, where the domain length is matched to the walk-off length between the optical and THz pulses. The bandwidth of the THz wave forms is 0.11 at 1.7 THz. Optical rectification gives rise to a THz wave form which corresponds to the domain structure of the periodically poled lithium niobate.
300 citations
TL;DR: By introduction of an optical gating beam on a semiconductor wafer, near-field terahertz imaging with a dynamic aperture has been realized and THz imaging with subwavelength spatial resolution (better than 50mum) is demonstrated.
Abstract: By introduction of an optical gating beam on a semiconductor wafer, near-field terahertz (THz) imaging with a dynamic aperture has been realized The spatial resolution is determined by the focus size of the optical gating bean and the near-field diffraction effect THz imaging with subwavelength spatial resolution (better than 50 µm) is demonstrated
271 citations
TL;DR: In this paper, a single-crystal sapphire fiber with sub-ps terahertz (THz) pulses was used to demonstrate the dominance of the single HE11 mode, despite the fiber dimensions allowing for multimode propagation.
Abstract: Waveguide propagation of sub-ps terahertz pulses in single-crystal sapphire fibers is reported. An incident THz pulse of approximately 0.6 ps duration undergoes considerable reshaping due to the absorptive and dispersive waveguide propagation, resulting in transmitted chirped pulse durations of 10–30 ps. Good agreement between theory and experiment is obtained by analyzing the propagation in terms of the single HE11 waveguide mode. The dominance of the single HE11 mode, despite the fiber dimensions allowing for multimode propagation, is attributed to the free-space to waveguide coupling.
245 citations
TL;DR: Time-domain transmission imaging of an opaque structure in pork-fat tissue obtained with a terahertz (THz) field sampling technique shows significantly enhanced contrast, as a result of low scattering.
Abstract: We present time-domain transmission imaging of an opaque structure in pork-fat tissue obtained with a terahertz (THz) field sampling technique. Compared with imaging with near-infrared pulses, the terahertz sampling technique shows significantly enhanced contrast, as a result of low scattering. For enhanced spatial resolution, we show mid-infrared THz imaging of onion cells. Water absorption of THz pulse in muscle tissues is discussed.
238 citations
TL;DR: The detection of linear polarization from Sgr A* at 750, 850, 1350, and 2000 µm which confirms the contribution of synchrotron radiation and implies that the excess arises in an extremely compact source of approximately 2 Schwarzschild radii.
Abstract: We report the detection of linear polarization from Sgr A* at 750, 850, 1350, and 2000 µm which confirms the contribution of synchrotron radiation. From the lack of polarization at longer wavelengths, it appears to arise in the millimeter/submillimeter excess. There are large position angle changes between the millimeter and submillimeter results, and these are discussed in terms of a polarized dust contribution in the submillimeter and various synchrotron models. In the model that best explains the data, the synchrotron radiation from the excess is self-absorbed in the millimeter region and becomes optically thin in the submillimeter. This implies that the excess arises in an extremely compact source of approximately 2 Schwarzschild radii.
186 citations
TL;DR: In this paper, an experimental demonstration of the generation of far-infrared (terahertz) pulses by photoionization of electrically biased air with amplified laser pulses is presented.
Abstract: We present an experimental demonstration of the generation of far-infrared (terahertz) pulses by photoionization of electrically biased air with amplified laser pulses. The current surge following photoionization of the air with an applied bias field of 10.6 kV/cm leads to the emission of THz pulses with an intensity which can be almost as high as that of THz pulses radiated from a large-area intrinsic-field GaAs emitter. The spectra peak at higher frequency than those of biased large-area GaAs emitters.
173 citations
TL;DR: A simple scheme for capturing the temporal waveforms of a freely propagating terahertz electromagnetic transient in a single shot that provides time resolution that is comparable to that of conventional electro-optic sampling measurements.
Abstract: We demonstrate a simple scheme for capturing the temporal waveforms of a freely propagating terahertz electromagnetic transient in a single shot. The method relies on electro-optic sampling in a noncollinear geometry for the terahertz radiation and the visible probe beam, coupled with multichannel detection. The approach provides time resolution that is comparable to that of conventional electro-optic sampling measurements.
169 citations
TL;DR: The use of the organic crystal DAST as a freely propagating electromagnetic-wave sensor at terahertz (THz) frequency demonstrates a sixfold enhancement of radiation at 7-20 THz.
Abstract: We report the use of the organic crystal DAST as a freely propagating electromagnetic-wave sensor at terahertz (THz) frequency. We also report the result of using a DAST crystal as a mid-IR THz emitter. Compared with the optical rectification from our best ZnTe emitter, that obtained by use of DAST demonstrates a sixfold enhancement of radiation at 7–20 THz.
161 citations
TL;DR: Using a mixed type-I/type-II GaAs/AlAs multiple-quantum-well sample, an optically controllable and tunable terahertz (THz) filter was demonstrated in this paper.
Abstract: Using a mixed type-I/type-II GaAs/AlAs multiple-quantum-well sample, we have demonstrated an optically controllable and tunable terahertz (THz) filter. Long-lived electron–hole pairs in the quantum wells allow for efficient THz attenuation over a large THz spot size (2 mm) for extremely low optical cw power. This sample can also be used as an optically tunable THz phase shifter. The optically induced change of the GaAs quantum wells from a dielectric to a conducting material leads to the observed attenuation and the shifting of the THz wave forms.
TL;DR: In this paper, a low-temperature grown GaAs photoconductive dipole antenna gated with 15 fs light pulses was used for the detection of radiation in wavelengths spanning from far to mid-infrared.
Abstract: We report on the ultrabroadband coherent detection of radiation in wavelengths spanning from far to midinfrared with a low-temperature-grown GaAs photoconductive dipole antenna gated with 15 fs light pulses. The detected spectral frequency exceeds 20 THz.
TL;DR: In this article, a low-temperature grown GaAs (LT-GaAs) photoconductive antenna probed with a 1.55 μm probe laser was used to detect THz radiation.
Abstract: THz radiation is detected by a low-temperature-grown GaAs (LT-GaAs) photoconductive antenna probed with a 1.55 μm probe laser. The detection efficiency is found to be approximately 10% of that obtained with a 780 nm probe. From the nonquadratic dependence of photoconductivity on laser intensity, two-step photoabsorption mediated by midgap states in LT-GaAs is suggested, instead of the two-photon absorption, as the primary process for the photoconductivity.
TL;DR: In this article, the contribution of various error sources to uncertainty in the far-infrared optical constants (refractive index and absorption coefficient) measured by terahertz (THz) time-domain spectroscopy is analyzed.
Abstract: We analyze the contributions of various error sources to uncertainty in the far-infrared optical constants (refractive index and absorption coefficient) measured by terahertz (THz) time-domain spectroscopy We focus our study on the influence of noise This noise study is made with a thick slab of transparent material for which the THz transmitted signal exhibits temporal echoes owing to reflections in the sample Extracting data from each of these time-windowed echoes allows us to characterize the noise sources In THz time-domain spectroscopy experiments in which photoswitches are used as antennae, the transmitting antenna constitutes the principal noise source The uncertainty in the far-infrared optical constants can be strongly reduced when the extraction is performed with THz echoes that have encountered many reflections in the sample
01 Jan 2000
TL;DR: In this article, the authors present an overview of recent achievements in the field of condensedmatter physics, including the detection of coherent phonons and phonon-polaritons with femtosecond time resolution.
Abstract: The impulsive excitation and phase-sensitive detection of coherent phonons and phonon-polaritons provide a detailed insight into the dynamical properties of matter. The experiments are based on optical pump-probe techniques with femtosecond time resolution. These techniques enable the detection of amplitude and phase of the coherent lattice motion simultaneously. Frequencies in the terahertz range and dephasing times in the picosecond range can be obtained with high accuracy. Especially in semiconductors and semiconductor heterostructures, where a coherent phonon mode and free carriers are excited simultaneously, important information about carrier-phonon interaction far away from equilibrium is obtained. This chapter presents an overview of recent achievements in this lively field of condensedmatter physics.
TL;DR: In this paper, a terahertz-wave parametric oscillator (TPO) based on laser light scattering from the lowest A/sub 1/symmetry polariton mode of LiNbO/sub 3/ was used to generate coherent tunability and coherency.
Abstract: Coherent tunable terahertz waves were generated successfully using a terahertz-wave parametric oscillator (TPO) based on laser light scattering from the A/sub 1/-symmetry polariton mode of LiNbO/sub 3/. This method has several advantages, such as continuous and wide tunability (frequency: 0.9-3.1 THz), a relatively high peak power (more than a few milliwatts), and compactness of its system (tabletop size). In addition, the system simply requires a fixed-wavelength pump source and it is easy to tune. This paper deals with the general performance of this terahertz-wave source using the prism output-coupler method as well as the development and applications of the system. Its tunability, coherency, power, and polarization were measured, and this tunable source was used for terahertz spectroscopy to measure the absorption spectra of LiNbO/sub 3/ and water vapor. Also, the use of MgO-doped LiNbO/sub 3/ (MgO:LiNbO/sub 3/) in our terahertz regime, as well as its far-infrared properties, is described. We found that the MgO:LiNbO/sub 3/ TPO is almost five times more efficient than the undoped LiNbO/sub 3/ TPO, and we have proven that the enhancement mechanism originates from the enhanced scattering cross section of the lowest A/sub 1/-symmetry mode in a spontaneous Raman experiment.
TL;DR: By use of dynamic subtraction it is feasible to adopt phase-sensitive detection with a CCD camera to reduce long-term optical background drift and introduce a modified detection geometry that realizes near-field imaging capability with greatly improved spatial resolution.
Abstract: By use of dynamic subtraction it is feasible to adopt phase-sensitive detection with a CCD camera to reduce long-term optical background drift. We report on a two-order improvement of the signal-to-noise ratio. The improved system is used to image terahertz field distribution generated by an optically rectified electro-optic crystal with a modulation depth as small as 10-4. We also introduce a modified detection geometry that realizes near-field imaging capability with greatly improved spatial resolution.
TL;DR: In this article, the authors used thin films of parylene as an anti-reflection layer for silicon optics and showed low-loss behavior well above 1 THz with a relatively modest absorption coefficient.
Abstract: A method for reducing the reflections from silicon optics at terahertz frequencies has been investigated. In this study, we used thin films of parylene as an anti-reflection (AR) layer for silicon optics and show low-loss behavior well above 1 THz. Transmittance spectra are acquired on double-sided-parylene-coated, high-resistivity, single-crystal silicon etalons between 0.45 THz and 2.8 THz. Modeling the optical behavior of the three-layer system allowed for the determination of the refractive index and absorption coefficient of parylene at these frequencies. Our data indicate a refractive index, n, of 1.62 for parylene C and parylene D, and a reasonably modest absorption coefficient make these materials a suitable AR coating for silicon at terahertz frequencies. Coatings sufficiently thick for AR performance reduced the average transmittance of the three-layer system by <10% compared to a lossless AR coating with an ideal refractive index.
TL;DR: In this article, a semiconductor quantum device which enables the phase modulation of terahertz signals is described, where the modulation-doped parabolic quantum wells enable the electronic control of dispersion at tera-hertz frequencies.
Abstract: A semiconductor quantum device which enables the phase modulation of terahertz signals is described. Modulation-doped parabolic quantum wells enable the electronic control of dispersion at terahertz frequencies. Device characterisation with terahertz time-domain spectroscopy shows a phase modulation for frequencies up to 4 THz.
TL;DR: In this article, the authors reported the calculations of terahertz radiation from biased photoconductive antennas based on the Drude-Lorentz theory of carrier transport in semiconductors.
Abstract: In this paper we report the calculations of terahertz radiation from biased photoconductive antennas. The calculations are based on the Drude-Lorentz theory of carrier transport in semiconductors. The calculation model takes into account the interaction between electrons and holes, trapping of carriers in mid-gap states, scattering of carriers and dynamical space-charge effects. Our calculation results indicate that, when a biased photoconductive antenna is pumped by femtosecond laser pulses to generate THz radiation, a major portion of the radiation results from the ultrafast change of the carrier density. The results also show that the local electrical field oscillates and induces electromagnetic radiation at high carrier generation density. When a photoconductive antenna is used as a detector, the detected THz signal deviates considerably from the THz pulse incident upon the photoconductive antenna.
TL;DR: In this paper, a comparative study of magnetic-field-enhanced THz generation in semiconductor surfaces of InSb, InAs, InP, GaAs, and GaSb is reported.
Abstract: A comparative study of magnetic-field-enhanced THz generation in semiconductor surfaces of InSb, InAs, InP, GaAs, and GaSb is reported. Applying an external magnetic field, the power of the generated THz radiation is increased for all examined semiconductor materials. The use of time-resolved measurements of the THz waveform allows to distinguish between the fraction of radiation originating from the surface depletion field and the fraction that is additionally generated by the magnetic field. It turns out that the power enhancement factor due to the magnetic field is inversely proportional to the effective electron mass.
TL;DR: In this article, the authors present the measurements and the theoretical model of the frequency-dependent noise temperature of a superconductor lattice-cooled hot-electron bolometer mixer in the terahertz frequency range.
Abstract: We present the measurements and the theoretical model of the frequency-dependent noise temperature of a superconductor lattice-cooled hot-electron bolometer mixer in the terahertz frequency range. The increase of the noise temperature with frequency is a cumulative effect of the nonuniform distribution of the high-frequency current in the bolometer and the charge imbalance, which occurs at the edges of the normal domain and at the contacts with normal metal. We show that under optimal operation the fluctuation sensitivity of the mixer is determined by thermodynamic fluctuations of the noise power, whereas at small biases there appears additional noise, which is probably due to the flux flow. We propose the prescription of how to minimize the influence of the current distribution on the mixer performance.
TL;DR: In this paper, a collection mode near-field probe for the terahertz spectral range is proposed. But the aperture size of the probe is determined by the probe aperture.
Abstract: We report on the development of a collection mode near-field probe for the terahertz spectral range. The near-field detector is based on an aperture type probe with dimensions of 30×30 μm2. The collection mode technique provides higher sensitivity and higher resolution than the similar illumination mode approach. Spatial resolution better than 40 μm is demonstrated for a broad spectrum of 300–600 μm, which equals to λ/15 for the longest wavelength. The observed resolution is determined by the size of the probe aperture.
TL;DR: In this article, the authors have reviewed and discussed the generation of continuous-wave terahertz radiation by the photomixing of dual-mode lasers, which includes a multi-mode laser diode (LD), a dualmode microchip laser, a monolithic dual mode DBR LD, and a widely tunable dual mode external cavity LD.
Abstract: We have reviewed and discussed the generation of continuous-wave terahertz radiation by the photomixing of dual-mode lasers, which includes a multi-mode laser diode (LD), a dual-mode microchip laser, a monolithic dual-mode DBR LD, and a widely tunable dual-mode external cavity LD It is demonstrated by several experiments that the beat frequency of a dual-mode laser is more stable than the original laser modes because of the common-mode rejection effect We also suggest a novel scheme by which the optical beat is stabilized to less than 1 kHz and its frequency can be precisely determined as well
TL;DR: In this article, a periodically poled lithium niobate crystal is used to generate narrow-band terahertz wave forms via optical rectification, and the authors obtain bandwidths as narrow as 18 GHz at a carrier frequency of 1.8 THz.
Abstract: Femtosecond optical pulses are used to generate narrow-band terahertz wave forms via optical rectification in a periodically poled lithium niobate crystal. By cooling the crystal to reduce losses due to phonon absorption, we are able to obtain bandwidths as narrow as 18 GHz at a carrier frequency of 1.8 THz. Temperature-dependent measurements show insignificant bandwidth broadening between 10 and 120 K, although the terahertz power substantially decreases as the temperature increases. Absolute power measurements indicate a conversion efficiency of at least 10−5.
TL;DR: In this paper, an overview of an experimental search for an ultrawide-band transmission channel with low dispersion and loss is presented, together with phase-coherent Cherenkov radiation in the form of an electromagnetic shock wave from these propagating electrical pulses.
Abstract: An overview is presented of an experimental search for an ultrawide-band transmission channel with low dispersion and loss. Such a terahertz (THz) interconnect will soon be required by the insatiable demand for higher speed devices and wider bandwidth communication. Starting with the early optoelectronic generation and detection of single-mode, subpicosecond electrical pulses on coplanar transmission lines, their complete characterization by THz time-domain spectroscopy (THz-TDS) is described. The consequent discovery of phase-coherent Cherenkov radiation in the form of an electromagnetic shock wave from these propagating electrical pulses is discussed together with its dominant role in the large measured propagation loss of these pulses. Various techniques to reduce this radiation are presented. The importance of dielectric materials characterization is explained and illustrated by THz-TDS measurements of high T/sub c/ substrates. Newly obtained THz waveguide results are presented and compared to the performance of coplanar transmission lines.
TL;DR: In this article, the fabrication and characterization of micromachined reduced-height air-filled rectangular waveguide components suitable for integration is reported, and the fabrication technique used permits structures with heights of up to 100 /spl mu/m to be successfully constructed in a repeatable manner.
Abstract: The fabrication and characterization of micromachined reduced-height air-filled rectangular waveguide components suitable for integration is reported in this paper. The lithographic technique used permits structures with heights of up to 100 /spl mu/m to be successfully constructed in a repeatable manner. Waveguide S-parameter measurements at frequencies between 75-110 GHz using a vector network analyzer demonstrate low loss propagation in the TE/sub 10/ mode reaching 0.2 dB per wavelength. Scanning electron microscope photographs of conventional and micromachined waveguides show that the fabrication technique can provide a superior surface finish than possible with commercially available components. In order to circumvent problems in efficiently coupling free-space propagating beams to the reduced-height G-band waveguides, as well as to characterize them using quasi-optical techniques, a novel integrated micromachined slotted horn antenna has been designed and fabricated, E-, H-, and D-plane far-field antenna pattern measurements at different frequencies using a quasi-optical setup show that the fabricated structures are optimized for 180-GHz operation with an E-plane half-power beamwidth of 32/spl deg/ elevated 35/spl deg/ above the substrate, a symmetrical H-plane pattern with a half-power beamwidth of 23/spl deg/ and a maximum D-plane cross-polar level of -33 dB. Far-field pattern simulations using HFSS show good agreement with experimental results.
TL;DR: The transmission characteristics of an air-GaAs interface and the transient absorption and index spectra of the thin, photoexcited surface layer are investigated subsequent to excitation by a femtosecond laser pulse to find that the total phase change and transmission of a terahertz probe pulse are dominated by interface effects.
Abstract: The transmission characteristics of an air–GaAs interface and the transient absorption and index spectra of the thin, photoexcited surface layer are investigated subsequent to excitation by a femtosecond laser pulse. We find that the total phase change and transmission of a terahertz (THz) probe pulse are dominated by interface effects. This observation has important implications in the interpretation of THz time-domain spectroscopy data of absorbing media. We also observe that the THz pulse apparently arrives at the detector as much as 60 fs earlier when it is transmitted through an optically excited GaAs wafer. This effect is fully explained in terms of a frequency-dependent transmission and phase shift at the air–GaAs interface and is not associated with superluminal propagation.
Patent•
23 Feb 2000
TL;DR: In this article, a two-dimensional array of pixels is subdivided into two dimensions, and radiation from each pixel is detected over a plurality of frequencies, and an image is generated from the radiation detected in step (b) preferably using a frequency or a selection of frequencies from the plurality in the pulsed electro-magnetic radiation.
Abstract: In an apparatus and method for imaging a sample: a) the sample to be imaged is irradiated with pulsed electro-magnetic radiation with a plurality of frequencies in the range from 50 GHz to 84 THz; b) an area of the sample is subdivided into a two dimensional array of pixels, and radiation from each pixel is detected over a plurality of frequencies; and c) an image is generated from the radiation detected in step (b) preferably using a frequency or a selection of frequencies from the plurality of frequencies in the pulsed electro-magnetic radiation. The method can be used as a medical imaging technique and can be used to image cancer tumours.