Showing papers on "Terahertz radiation published in 1993"
TL;DR: The coherent electromagnetic radiation originating from Bloch oscillations of charge carriers in an electrically biased semiconductor superlattice structure is detected directly and the detection limit of the measurement system is reached.
Abstract: We directly detect the coherent electromagnetic radiation originating from Bloch oscillations of charge carriers in an electrically biased semiconductor superlattice structure. The oscillation frequency can be tuned with the applied bias field from 0.5 THz to more than 2 THz, the detection limit of our measurement system.
598 citations
TL;DR: The pulse energy of the far-infrared radiation is found to be a quadratic function of bias field and a nonmonotonic function of laser intensity.
Abstract: We have generated sub-single-cycle pulses of electromagnetic radiation with pulse energies as high as 0.8 μJ and pulse lengths < 500 fs. The 10-dB width of the spectrum is 1.5 THz. The transmitter is a GaAs wafer illuminated at normal incidence by 120-fs, 770-nm pulses from a Ti:sapphire chirped-pulse amplifier system while a pulsed electric field is applied across the surface. The pulse energy of the far-infrared radiation is found to be a quadratic function of bias field and a nonmonotonic function of laser intensity.
316 citations
TL;DR: In this article, the Andreev reflection of electrons and weak electron-phonon coupling at low temperatures is exploited to produce a large temperature rise for a small input power (≊10 mK/fW).
Abstract: We present measurements on a novel power detector which can be used as an ultrasensitive detector of millimeter and submillimeter radiation. The absorbing element consists of a thin film resistor strip which is connected to superconducting electrodes. This device exploits the Andreev reflection of electrons and the weak electron‐phonon coupling at low temperatures to produce a large temperature rise for a small input power (≊10 mK/fW). The temperature rise of the electrons is detected by a tunnel junction where part of the metal strip forms the normal electrode. We have measured a voltage responsivity of approximately 109 V/W and an amplifier‐limited electrical noise equivalent power ≊3×10−18 W Hz−1/2 at an operating temperature of 100 mK. If infrared radiation were efficiently coupled to the absorbing element with an antenna or a waveguide, then the sensitivity of this detector would be at least a factor of 10 better than the best available direct detector operating at the same temperature.
179 citations
TL;DR: It is shown that emission of electromagnetic radiation from currently available superlattices can occur due to multiphoton transitions from almost zero up to terahertz frequencies.
Abstract: We report on a theoretical treatment of the Esaki-Tsu superlattice oscillator emphasizing a profound link between the dynamics of charge carriers in a superlattice and the dynamics of Josephson junctions. Using a balance-equation approach we calculate the oscillator efficiency taking account of the negative effective mass that carriers can have in a miniband and of dissipation. We show that emission of electromagnetic radiation from currently available superlattices can occur due to multiphoton transitions from almost zero up to terahertz frequencies
146 citations
TL;DR: It is demonstrated that this work can enhance, weaken, and also phase shift terahertz (THz) radiation emitted by optically excited quantum beats in a coupled quantum well.
Abstract: We demonstrate that we can enhance, weaken, and also phase shift terahertz (THz) radiation emitted by optically excited quantum beats in a coupled quantum well. The changes in the evolution of the THz radiation are induced by exciting the sample with a second optical pulse, phase locked with the first. We observe phase shifts in the emitted THz radiation of 330 fs for a change in the optical-pulse separation of only 1.33 fs
138 citations
TL;DR: The current-voltage characteristic of semiconductor superlattices in the presence of intense terahertz electric fields produced by free-electron lasers is measured and the nonlinear I-V curves exhibit new structure that is attributed to photon-mediated sequential resonant tunneling.
Abstract: We have measured the current-voltage (I-V) characteristic of semiconductor superlattices in the presence of intense terahertz electric fields produced by free-electron lasers. The nonlinear I-V curves exhibit new structure that we attribute to photon-mediated sequential resonant tunneling. This tunneling process consists of well to well sequential tunneling into photon sidebands induced by the terahertz electric fields.
102 citations
TL;DR: In this article, the authors demonstrate a new ultrafast pump-probe technique using terahertz pulses to investigate carrier transport and screening in semiconductors, where the external bias field is screened on a time scale comparable to the duration of the THz pulse, giving rise to changes in the shape and bandwidth of radiated pulses.
Abstract: We demonstrate a new ultrafast pump‐probe technique using terahertz pulses to investigate carrier transport and screening in semiconductors. As an example we have studied the temporal evolution of the local electric field in a dipole antenna, used for generation of ultrafast terahertz pulses. Ultrafast screening effects are shown to be important for both carrier transport and the emission of THz radiation. At high carrier densities the external bias field is screened on a time scale comparable to the duration of the THz pulse, giving rise to changes in the shape and bandwidth of the radiated pulses.
89 citations
TL;DR: In this article, a 1.5μm tensile-strained quantum-well optical amplifier has been studied by highly nondegenerate four-wave mixing at detuning frequencies up to 1.7 THz, and frequency response data indicate the presence of two ultrafast physical processes with characteristic relaxation lifetimes of 650 fs and <100 fs.
Abstract: Ultrafast dynamics in a 1.5‐μm tensile‐strained quantum‐well optical amplifier has been studied by highly nondegenerate four‐wave mixing at detuning frequencies up to 1.7 THz. Frequency response data indicate the presence of two ultrafast physical processes with characteristic relaxation lifetimes of 650 fs and <100 fs. The longer time constant is believed to be associated with the dynamic carrier heating effect. This is in agreement with previous time‐domain pump‐probe measurements using ultrashort optical pulses.
87 citations
TL;DR: It is demonstrated that active control of terahertz radiation due to charge oscillations caused by quantum beats in a coupled-quantum-well structure is possible using two-femtosecond optical pulses with a time delay shorter than the exciton dephasing time.
Abstract: Analytical and numerical solutions for coherent excitations of quantum beats in a coupled quantum well by two phase-locked optical pulses are shown using the density-matrix formulation. It is demonstrated that active control of terahertz radiation due to charge oscillations caused by quantum beats in a coupled-quantum-well structure is possible using two-femtosecond optical pulses with a time delay shorter than the exciton dephasing time. Our theoretical results agree very well with experimental observations.
86 citations
TL;DR: In this paper, the authors proposed an antenna coupled microbolometer which makes use of the weak coupling between electrons and phonons in a metal at low temperatures, where the radiation is collected by a planar lithographed antenna and thermalized in a thin metal strip.
Abstract: The authors propose a novel antenna coupled microbolometer which makes use of the weak coupling between electrons and phonons in a metal at low temperatures. The radiation is collected by a planar lithographed antenna and thermalized in a thin metal strip. The resulting temperature rise of the electrons is detected by a tunnel junction, where part of the metal strip forms the normal electrode. The active area of the bolometer is thermally coupled by its small volume, by the thermal resistance between the electrons and phonons in the strip, and by the reflection of quasi-particles at the interface between the strip and the superconducting antenna. Design calculations based on a metal volume of 2 mu m*6 mu m*0.05 mu m at an operating temperature of 100 mK give an NEP of about 3*10/sup -19/ WHz/sup -1/2/, a time constant of about 10 mu s, and a responsivity of about 10/sup 9/ V/W. The calculated sensitivity is almost two orders of magnitude higher than that of the best available direct detectors of millimeter and submillimeter radiation operated at the same temperature. >
52 citations
TL;DR: For biased InP emitters, the dependence of the generated terahertz radiation on bias field and optical fluence for optical fluences of 0.01-1.0 mJ/cm2 and bias fields as high as 12 kV/cm was studied in this paper.
Abstract: We present, for biased InP emitters, the dependence of the generated terahertz radiation on bias field and optical fluence for optical fluences of 0.01–1.0 mJ/cm2 and bias fields as high as 12 kV/cm. The radiated electric field scales linearly with the bias field up to 12 kV/cm and exhibits monotonic saturation behavior, radiating half the maximum field at an excitation fluence of 0.058 mJ/cm2.
TL;DR: In this paper, a diode fabrication process based on the electrolytic pulse etching of GaAs in combination with an in situ platinum plating for the formation of the Schottky contacts is described.
Abstract: GaAs Schottky barrier diodes with near-ideal electrical and noise characteristics for mixing applications in the terahertz frequency range are described. The conventional formulas describing these characteristics are valid only in a limited forward bias range, corresponding to currents much smaller than the operating currents under submillimeter mixing conditions. Therefore, generalized analytical expressions for the I-V and C-V characteristics of the metal-semiconductor junction in the full bias range are given. A new numerical diode model is presented which takes into account not only the phenomena occurring at the junction, such as current dependent recombination and drift/diffusion velocities, but also the variations of electron mobility and electron temperature in the undepleted epi-layer. A diode fabrication process based on the electrolytic pulse etching of GaAs in combination with an in situ platinum plating for the formation of the Schottky contacts is described. Schottky barrier diodes with a diameter of 1 mu m fabricated by this process have already shown excellent results in a 650-GHz waveguide mixer at room temperature. >
TL;DR: In this paper, the crystallographic orientation dependence of the far-infrared (FIR) light generated at the (001) surface of a zincblende semiconductor is derived principally from bulk difference-frequency mixing.
Abstract: The crystallographic orientation dependence of the far‐infrared (FIR) light generated at the (001) surface of a zincblende semiconductor is shown to derive principally from bulk difference‐frequency mixing. A strong modulation is observed for 1‐GW/cm2 pulses on InP, which demonstrates that the radiated FIR wave produced by bulk optical rectification is comparable to that generated by the transport of photoinjected carriers. Using the bulk rectification light as a clock, we show that more than 95% of the light produced from an InP (111) crystal by 100‐fs, 100‐μJ pulses is generated in a time shorter than the excitation pulse.
TL;DR: In this paper, the authors introduced the notion of significantly modulating the physical characteristics of a solid on a terahertz time scale with coherent lattice vibrations, and showed that the induced coherent phonons are optically excited in single-crystal Ti2O3, which is a narrow-gap semiconductor at 300 K and gradually transforms into a semimetal at 600 K.
Abstract: This letter introduces the general notion of significantly modulating the physical characteristics of a solid on a terahertz time scale with coherent lattice vibrations. We show, as an example, experiments in which coherent phonons are optically excited in single‐crystal Ti2O3, which is a narrow‐gap semiconductor at 300 K and gradually transforms into a semimetal at 600 K. Quantitative comparison of previous equilibrium measurements to our new transient measurements suggests that the induced coherent lattice vibration is significantly modulating the semiconductor‐to‐semimetal transition at 7 THz.
TL;DR: It is shown that dipole radiation in the terahertz range should be detectable for many members of the GaAs/Al[sub [ital x],Ga[sub 1[minus][ital x]]As system.
Abstract: Avoiding any truncation of the Hamiltonian for independent electrons in both ideal and imperfect superlattices subject to a uniform electric field, we show that dipole radiation in the terahertz range should be detectable for many members of the GaAs/Al[sub [ital x]]Ga[sub 1[minus][ital x]]As system. The radiation can be attributed to periodic Bloch oscillations in the case of ideal superlattices, and to almost-periodic oscillations, with the dominant frequencies on the order of the Bloch frequency, in the case of imperfect superlattices.
Patent•
30 Dec 1993TL;DR: In this article, a terahertz optical emission device with first and second electrodes with LT-GaAs dielectric material between them is described, and a corresponding terahethertz radiation detector is disclosed.
Abstract: A terahertz optical emission device comprises a substrate (3) with first and second electrodes (1, 2) one overlying the other with LT-GaAs dielectric material between them. When a femtosecond laser pulse is applied, terahertz radiation is produced in the plane of the substrate. A corresponding terahertz radiation detector is disclosed, together with a sensor system which uses an emitter and detector for detecting and sensing an operational parameter of a system under test.
01 Apr 1993
TL;DR: The Submillimeter Technology Laboratory (STL) at the University of Massachusetts Lowell has developed a methodology of tailoring the complex refractive index for artificial dielectrics at terahertz frequencies as mentioned in this paper.
Abstract: The Submillimeter Technology Laboratory(STL) at the University of Massachusetts Lowell has developed a methodology of tailoring the complex refractive index for artificial dielectrics at terahertz frequencies. A wide range of precisely controlled optical properties have been achieved for materials such as vinyl acetate, silicone, polyethylene and epoxy resin when combined with powdered loading agents such as boron nitride, silicon, graphite, iron oxide and stainless steel flake. Using this technology, STL has successfully fabricated both narrow-band and wide-band anechoic structures. The method of characterizing materials for the purpose of tailoring their dielectric properties at terahertz frequencies is presented along with several demonstrated applications. 2. Introduction Because of the increase in the number of optical and quasi-optical measurement systems operating at terahertz frequencies, new materials providing alternative solutions to the design of system components are required. In response to this, STL’s recent activities include an aggressive program to evaluate a wide range of materials for terahertz frequency applications(1-3). Using inhouse expertise in the technique of tailoring artificial dielectrics and the polarimetric characterization of materials, a variety of composite structures have been created as far-infrared radiation absorbing material (FIRAM)(4,5). metal substrate dielectric layer { { incident radiation
TL;DR: A spectrum of coherent transition radiation (TR) emitted from 150-MeV electron bunches from a linear accelerator has been observed in the wavelength range from 0.6 to 5 mm as discussed by the authors.
Abstract: A spectrum of coherent transition radiation (TR) emitted from 150-MeV electron bunches from a linear accelerator has been observed in the wavelength range from 0.6 to 5 mm. The intensity at a wavelength of 4 mm is enhanced by a factor of about 6\ifmmode\times\else\texttimes\fi{}${10}^{5}$ in comparison with that of ordinary TR. The factor is about a half of the number of electrons in a bunch. The intensity shows nearly a quadratic dependence on the electron-beam current. The interference between TR from adjacent bunches has been observed. The electron distribution in a bunch has been derived from the observed spectrum. It has a full width at half maximum of 0.56 mm and the shape is similar to a Gaussian function.
TL;DR: In this paper, a hybrid full-wave frequency domain technique which uniquely synthesizes well-known integral equation and mode-matching methods is shown to be applicable to the study of microstrip-ridge structures.
Abstract: Microstrip-ridge structures, i.e., conducting strips which are mounted on ridges and are in the close proximity of other conductors on other ridges, are found in submillimeter/terahertz monolithic circuits in conjunction with layered, ridged dielectric waveguides; in millimeter-wave monolithic circuits as microslab lines; in microwave monolithic circuits as integrated traveling-wave optical modulators; and in VLSI circuits as interconnects. A hybrid full-wave frequency domain technique which uniquely synthesizes well-known integral equation and mode-matching methods is shown to be applicable to the study of microstrip-ridge structures. Unlike most other integral equation techniques, the integral equation-mode matching (IEMM) technique is capable of characterizing a wide variety of nonplanar structures. Time domain results are obtained by utilizing a Fourier transform and an equivalent circuit model to evaluate the response at each frequency point. To introduce this method, several two-dimensional structures-specifically, coupled microstrips on ridges, coupled microstrip with an etched groove, and an electrooptic modulator-are examined. >
TL;DR: In this article, the influence of electric and magnetic fields on optically induced THz electromagnetic radiation from semiconductors at normal incidence has been investigated, and it has been shown that electric fields strongly influence the amplitude and phase of transient carrier generated THz radiation, but do not affect THz optical rectification generated by the bulk second order nonlinear susceptibility.
Abstract: We report measurement of the influence of electric and magnetic fields on optically induced THz electromagnetic radiation from semiconductors at normal incidence. The measurements show that electric and magnetic fields strongly influence the amplitude and phase of transient carrier‐generated THz radiation, but do not affect THz optical rectification generated by the bulk second order nonlinear susceptibility, for static electric fields up to 104 V/cm.
TL;DR: The experimental results are compared with a quantum-mechanical model for the low-frequency dielectric response of LiTaO 3 and find that at 300 K the polariton dispersion and damping are determined by a strong resonance at 6 THz and a weak resonance at 1 THz.
Abstract: We study the temperature dependence of the low-frequency dielectric properties of the ferroelectric LiTaO 3 by generating phonon polaritons with frequencies in the terahertz regime. The phonon polaritons are impulsively excited and phase-sensitively detected with 60-fs laser pulses. The propagation and damping of the polaritons are investigated as a function of frequency and temperature. The experimental results are compared with a quantum-mechanical model for the low-frequency dielectric response of LiTaO 3 . In this model the lowest-energy A 1 mode is described with a one dimensional anharmonic potential in a single unit cell. We find that at 300 K the polariton dispersion and damping are determined by a strong resonance at 6 THz and a weak resonance at 1 THz
Patent•
23 Sep 1993
TL;DR: In this article, an array of N photoconductive switches biased across a common voltage source and an optical path difference from a common optical pulse of repetition rate f 0 providing a different optical delay to each of the switches is presented.
Abstract: A submillimeter wave-generating integrated circuit includes an array of N photoconductive switches biased across a common voltage source and an optical path difference from a common optical pulse of repetition rate f0 providing a different optical delay to each of the switches. In one embodiment, each incoming pulse is applied to successive ones of the N switches with successive delays. The N switches are spaced apart with a suitable switch-to-switch spacing so as to generate at the output load or antenna radiation of a submillimeter wave frequency f on the order of Nf0. Preferably, the optical pulse has a repetition rate of at least 10 GHz and N is of the order of 100, so that the circuit generates radiation of frequency of the order of or greater than 1 Terahertz.
TL;DR: In this article, the authors demonstrate the application of terahertz electromagnetic transients to the study of the dynamical conductivity of a two-dimensional, high mobility electron gas in modulation-doped GaAs quantum wells.
Abstract: We demonstrate the application of terahertz electromagnetic transients to the study of the dynamical conductivity of a two‐dimensional, high mobility electron gas in modulation‐doped GaAs quantum wells. The transient response of such an electron system on a picosecond time scale occurs in a regime where the implications of electron–electron interaction effects need be considered.
TL;DR: In this article, a silicon optical switch photo-excited by a Nd∶YAG laser was used for cavity-dumping a submillimeter radiation high-Q, zig-zag optically-pumped resonator.
Abstract: Results are reported of cavity-dumping a submillimeter radiation high-Q, zig-zag optically-pumped resonator which utilizes a silicon optical switch photo-excited by a Nd∶YAG laser. Peak powers at 152 μm approaching 10 kW in temporally smooth pulses of 5 ns (FWHM) duration have been obtained at a pulse repetition rate of 12 Hz. The far-infrared laser radiation, as measured with a scanning metal-mesh Fabry-Perot interferometer and averaged over many pulses, is, upon deconvolution of the 125 MHz instrumental linewidth, found to oscillate in a predominantly single longitudinal mode of width 250 MHz. Theoretical investigations of the transient far-infrared reflectivity of the silicon optical switch predict a rise in the Brewster-angle reflectivity from 0% to nearly 80% in 1 ns, when activated by a Q-switched, frequency-doubled Nd∶YAG laser providing an incident energy density of 50 mJ/cm2 in a pulsewidth of 10 ns.
TL;DR: Schottky varactor diodes with 4 THz cutoff frequencies were fabricated using 1 μm lithography and selfaligned RIE sidewall etching as mentioned in this paper, which were incorporated into nonlinear transmission line pulse generators that produced 3 V steps with 0.68 ps 10-90% fall time.
Abstract: Schottky varactor diodes with 4 THz cutoff frequencies were fabricated using 1 μm lithography and selfaligned RIE sidewall etching. These diodes were incorporated into nonlinear transmission line pulse generators that produced 3 V steps with 0.68 ps 10-90% fall time. The lines were measured with integrated sampling circuits that had 515 GHz bandwidth
TL;DR: In this article, the saturation and power scaling of terahertz radiation produced by large-aperture photoconducting antennas under high electric fields and high optical fluences are described.
Abstract: The saturation and power scaling of terahertz radiation produced by large-aperture photoconducting antennas under high electric fields and high optical fluences are described. From the saturation behavior, a large-aperture transmitter can be designed to produce the maximum pulse energy of terahertz radiation for a given photoconductor, optical pulse energy and electric field.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
TL;DR: In this article, the amplitude and shape of the radiated pulses change with the applied bias and the wavelength of the optical pulses illuminating the p−i−n diode, showing evidence of velocity overshoot in the transient response of optically injected electrons in GaAs.
Abstract: Subpicosecond electromagnetic pulses having tetrahertz (THz) bandwidths have been generated from large aperture GaAs p‐i‐n diodes under bias by illumination with femtosecond optical pulses. The amplitude and shape of the radiated pulses change with the applied bias and the wavelength of the optical pulses illuminating the p‐i‐n diode. Comparing this radiation with THz radiation from large aperture Si p‐i‐n diodes provides evidence of velocity overshoot in the transient response of optically injected electrons in GaAs.
30 Aug 1993
TL;DR: In this article, a photlithographic technique has been developed for the fabrication of fundamental mode metal pipe waveguide for operation at frequencies between 300GHz and 2THz, where active devices such as Schottky or Resonant Tunnelling Diodes (RTD) may be incorporated within them.
Abstract: A novel photlithographic technique has been developed for the fabrication of fundamental mode metal pipe waveguide foroperation at frequencies between 300GHz and â€â€2THz. The guides are formed on a semiconductor substrate and henceactive devices such as Schottky or Resonant Tunnelling Diodes ( RTD ) may be incorporated within them.
TL;DR: In this paper, a centimetre-sized single grain of the perfect AlPdMn icosahedral phase can be grown by slow cooling from the liquid by means of inelastic measurements on triple axis spectrometers.
Abstract: A centimetre-sized single grain of the perfect AlPdMn icosahedral phase can be grown by slow cooling from the liquid. This phase has been shown to have perfect icosahedral symmetry, i.e. without any frozen phason strain. The dynamical properties of this icosahedral phase have been studied by mean of inelastic measurements on triple axis spectrometers. The dispersion relations around several Bragg reflections were measured. Pseudo-zone boundaries can be defined, similarly to those previously obtained in the AlLiCu phase. A dispersionless optic mode was identified around 4 THz.
TL;DR: In this article, a system for measuring the absolute frequency of a far-infrared (FIR) laser is described, where Josephson point contacts are used as a frequency harmonic mixer connecting microwaves and optically pumped CH3OH laser lines.
Abstract: A system for measuring the absolute frequency of a far-infrared (FIR) laser is described. Josephson point contacts have been utilized in the system as a frequency harmonic mixer connecting microwaves and optically pumped CH3OH laser lines. The Josephson point contacts are capable of generating beat signals of 90 GHz microwaves and FIR waves of up to 4.25 THz. To measure the frequency of the beat signals from the Josephson junction with a frequency counter, tracking oscillators have been developed, which tracks the beat signals by phase locking and regenerate clean signals for frequency counting. It is shown that the absolute frequency can be measured to an accuracy of about 100 Hz by using the tracking oscillators.