Pulsed Terahertz Radiation Research Articles

A metal-dielectric antenna for terahertz near-field imaging

We present an antenna-based approach to near-field imaging and spectroscopy, which can be used for both continuous-wave and pulsed broadband electromagnetic radiations from microwave to terahertz frequencies. Our near-field antenna consists of a rectangular-shaped block of low-loss dielectric material sharpened to a pyramidal tip which is partially metallized and terminated by a micron-sized plane facet. At this facet the entire energy of the incident wave is concentrated as a very high but strongly localized electric field, which can be used as a sensitive near-field microprobe for electromagnetic radiation. Currently, experiments in reflection geometry with pulsed terahertz radiation and continuous-wave radiation near 80GHz reveal a frequency-independent spatial resolution of about 20μm corresponding to λ∕200 at 80GHz, which is only limited by the size of the facet terminating the tip.

Comparative performance of terahertz emitters in amplifier-laser-based systems

We give an overview of opto-electronic terahertz systems based on femtosecond amplifier lasers, and present a comparison of the relative performance and design issues of the various suitable downconversion photomixers employed for the generation of intense terahertz radiation pulses in such systems. The survey includes large-area biased semiconductor antennas, large-area nonlinear optical crystals and laser-generated gas plasmas as sources for terahertz radiation. The performance of these emitters is reviewed in terms of the terahertz pulse energies achieved and their associated spectral and signal-to-noise properties.

Competition between linear and nonlinear processes during generation of pulsed terahertz radiation in a ZnTe crystal

The generation of terahertz (THz) pulses by the optical rectification of femtosecond laser pulses in a ZnTe crystal is studied. A substantial decrease in the THz radiation power was observed upon tight focusing of laser radiation into the crystal. It is shown that the consideration of competing two-photon absorption and second-harmonic generation processes proceeding simultaneously with optical rectification cannot explain this effect even qualitatively. It is assumed that the observed decrease in the THz radiation power is caused by a decrease in the size of a source of nonlinear polarisation. The conditions are found for the most efficient generation of THz radiation in the ZnTe crystal.

Low-Temperature MBE Grown GaAs for Pulsed THz Radiation Applications

The coherent generation and detection of terahertz (THz) radiation using ultrashort laser pulses and photoconductive antennae have been intensively studied during the last decade. The best results were achieved when low-temperature MBE grown GaAs (LTG GaAs) or ion-implanted GaAs layers were used as a basis for THz emitters and detectors. The ultimate performance of these devices depends on carrier mobilities and their trapping times in semiconductor layers; therefore the determination of those parameters is of a critical importance. In the present work, we will describe the characterization of GaAs layers with ultrafast carrier recombination and the results obtained on THz radiation components manufactured from these layers.

Phase Control of Few-Cycle Coherent Terahertz Radiation Pulses

We report a method to control the carrier-envelope phase shift innearly single-cycle coherent terahertz (THz) radiation pulses, in whichwe used Kerr effect and two-photon absorption in the generated crystalsdue to the pump beam peak intensity. Dependence of relativecarrier-envelope phase shift of THz pulses on the pump beam intensityin the ZnTe crystal was measured experimentally. Under our experimentalcondition, we measured that maximum relative phase shift is 0.29radians.

Propagation of Surface Plasmon Polaritons on Semiconductor Gratings

We present time-domain measurements of terahertz surface plasmon polaritons (SPPs) propagating on gratings structured on silicon surfaces. Using single-cycle pulses of terahertz radiation to excite SPPs in a broad frequency range, we observe that the efficient SPPs scattering on the semiconductor periodic structure introduces significant dispersion and modifies the SPPs propagation. A stop gap, or a frequency range where SPPs are Bragg reflected, is formed by the structure. This gap depends strongly on the Si doping density and type. The resonant scattering at the edge of the gap reduces the group velocity by more than a factor of 2. The measurements show a good agreement with our numerical calculations based on the reduced Rayleigh equation.

On the origin of photogenerated terahertz radiation from current-biased superconducting YBa2Cu3O7−δ thin films

The origin of photogenerated terahertz radiation pulse emitted from current-biased superconducting YBa2Cu3O7−δ thin films excited by femtosecond optical laser pulses is delineated. By investigating the performance of the transient terahertz radiation generated under different operating parameters, pulse reshaping in the measured terahertz electric field caused by the kinetic inductance of the superconducting charge carriers is identified. After recovering the original wave forms of the emitted terahertz pulses, the transient supercurrent density directly correlated to the optically excited quasiparticle dynamics is obtained. A fast decreasing component of about 1.0 ps and a slower recovery process with a value of 2.5 ps are unambiguously delineated in the optically induced supercurrent modulation. The radiation mechanism of the transient terahertz pulse related to nonequilibrium superconductivity is discussed.

Do in vivo terahertz imaging systems comply with safety guidelines?

Techniques for the coherent generation and detection of electromagnetic radiation in the far infrared, or terahertz, region of the electromagnetic spectrum have recently developed rapidly and may soon be applied for in vivo medical imaging. Both continuous wave and pulsed imaging systems are under development, with terahertz pulsed imaging being the more common method. Typically a pump and probe technique is used, with picosecond pulses of terahertz radiation generated from femtosecond infrared laser pulses, using an antenna or nonlinear crystal. After interaction with the subject either by transmission or reflection, coherent detection is achieved when the terahertz beam is combined with the probe laser beam. Raster scanning of the subject leads to an image data set comprising a time series representing the pulse at each pixel. A set of parametric images may be calculated, mapping the values of various parameters calculated from the shape of the pulses. A safety analysis has been performed, based on current guidelines for skin exposure to radiation of wavelengths 2.6 μm–20 mm (15 GHz–115 THz), to determine the maximum permissible exposure (MPE) for such a terahertz imaging system. The international guidelines for this range of wavelengths are drawn from two U.S. standards documents. The method for this analysis was taken from the American National Standard for the Safe Use of Lasers (ANSI Z136.1), and to ensure a conservative analysis, parameters were drawn from both this standard and from the IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields (C95.1). The calculated maximum permissible average beam power was 3 mW, indicating that typical terahertz imaging systems are safe according to the current guidelines. Further developments may however result in systems that will exceed the calculated limit. Furthermore, the published MPEs for pulsed exposures are based on measurements at shorter wavelengths and with pulses of longer duration than those used in terahertz pulsed imaging systems, so the results should be treated with caution.

Carrier dynamics of terahertz emission from low-temperature-grown gaas.

Through theoretical modeling, we find that the dynamics of photogenerated carriers play a very important role in shaping the temporal waveform of terahertz (THz) radiation pulses emitted from biased low-temperature (LT-grown GaAs antenna. Our modeling gives successful analyses for the sharp and short, slow and long negative parts of temporal THz waveforms. By including intraband, carrier relaxation effect in the modeled mobility, we find an obvious dependence of the THz conversion efficiency on the material of THz emitter and experimental parameters such as the optical duration, the center wavelength, and the fluence of the laser pulses. Our research also shows that electron-hole and electron-electron collisions in LT-GaAs contribute to the saturation phenomenon with an increase of laser fluence.

Terahertz responses of high-resistivity YBCO thin films

We used pulse terahertz radiation to study the complex conductivity of high-resistivity YBCO thin films. The complex conductivity of YBCO superconducting thin films in the 50–300 GHz range was obtained. The temperature dependence of λ 2(0)/ λ 2( T) of such films exhibits a T 1 behavior which is closed to the prediction of the d-wave theory in the clean limit. However, its zero-temperature value is larger than that of typical high-quality single crystal YBCO. The temperature dependence of the real part of the conductivity below T c exhibits a frequency-dependent broad peak. Using the two-fluid Drude model for the superconducting state, the temperature dependence of the quasiparticle scattering rate is observed. The experimental result indicated that the quasiparticle scattering rate is higher than that reported on very-high-quality YBCO single crystals in the millimeter wave region.

Production of high power femtosecond terahertz radiation

The terahertz (THz) region of the electromagnetic spectrum is attracting interest for a broad range of applications ranging from diagnosing electron beams to biological imaging. Most sources of short pulse THz radiation utilize excitation of biased semiconductors or electro-optic crystals by high peak power lasers. For example, this was done by using an un-doped InAs wafer irradiated by a femtosecond free-electron laser (FEL) at the Thomas Jefferson National Accelerator Facility. Microwatt levels of THz radiation were detected when excited with FEL pulses at 1.06μm wavelength and 10W average power. Recently substantially higher powers of femtosecond THz pulses produced by synchrotron emission were extracted from the electron beamline. Calculations and measurements confirm the production of coherent broadband THz radiation from relativistic electrons with an average power of nearly 20W, a world record in this wavelength range by a factor of 10,000. We describe the source, presenting theoretical calculations and their experimental verification. Potential applications of this exciting new source include driving new non-linear phenomena, performing pump-probe studies of dynamical properties of novel materials, and studying molecular vibrations and rotations, low frequency protein motions, phonons, superconductor bandgaps, electronic scattering, collective electronic excitations (e.g., charge density waves), and spintronics.

THz imaging of supercurrent distribution in Meissner state of LPE-BSCCO film

In order to apply the terahertz (THz) imaging technique to high-TC superconductor thick film, the THz imaging system was improved to observe the back-scattered THz radiation pulse. Using the improved system we succeeded in the observation of supercurrent distribution in Bi2Sr2CaCu2O8+δ thick film in Meissner state.

Double modulated differential THz-TDS for thin film dielectric characterization

Terahertz differential time-domain spectroscopy (DTDS) is a new technique that uses pulsed terahertz radiation to characterize the optical properties of thin dielectric films. Characterizing thin films in the GHz to THz range is critical for the development of new technologies in integrated circuitry, photonic systems and micro-electro-mechanical systems. There are potential applications for gene and protein chips. This paper shows how DTDS can be combined with double modulation in the pump-probe system to improve sensitivity by an order of magnitude. An iterative algorithm is presented to estimate the optical properties of a given thin film. The technique is experimentally verified using 1-μm-thick samples of silicon dioxide on silicon.

Spectroscopy by pulsed terahertz radiation

Spectroscopies using terahertz (THz) radiation excited by ultrashort laser pulseshave been rapidly developing recently. In this paper, the principles of varioustypes of THz time domain spectroscopies (THz TDSs), i.e. transmission-,reflection-and ellipsometry-type THz TDSs, and their applications tothe characterization of semiconductors are described. In addition to theTHz TDS using a femtosecond laser, a sub-THz TDS system using acheap and compact continuous multimode laser diode is also described.

Generation and detection of terahertzpulsed radiation with photoconductive antennas and its application toimaging

We describe the basic properties of photoconductive (PC)antennas as emitters and detectors of terahertz (THz) pulsedradiation. The efficiency of PC antennas is discussed, taking intoaccount the saturation effect due to field screening byphoto-generated carriers. We show that maximum emission efficiencyunder a constant pump power condition is achieved by adjusting the PCgap size so that the pump intensity (power divided by PC area) isequal to the characteristic saturation intensity. We also discuss thebandwidth of PC antennas and conclude that PC antennas are capable ofgenerating and detecting ultra-broad THz radiation beyond 10 THz whenshort enough laser pulses are used. Our conclusion is demonstratedexperimentally by detecting ultra-broadband THz radiation, whosespectrum distributed up to 40 THz, with a PC antenna. As an exampleof applications of THz pulsed radiation based on a PCemitter-detector system, results for the imaging of objects inpowders are presented and discussed.

Terahertz radiation emitted by dipole antennas of YBa2Cu3O7−δ/PrBa2Cu3O7 multilayers

The emission amplitude of terahertz radiation pulses emitted by YBa2Cu3O7−δ thin film antennas increases if thin PrBa2Cu3O7 layers are introduced into the film. Using the same system configuration, sample layout and excitation conditions, the signal efficiency of the electromagnetic pulses, emitted from the device, appears to be dependent on the number of mobile charge carriers inside the superconducting film. Twenty one PrBa2Cu3O7/YBa2Cu3O7−δ double layers are deposited to form one multilayer structure. The PrBa2Cu3O7 layers are used to reduce the number of mobile holes in YBa2Cu3O7−δ without changing the intrinsic properties of the material itself. The thickness of the PrBa2Cu3O7 layers, dPBCO, is varied. dPBCO influences the mobile hole density, i.e. connected to the content x of Pr ions inside the whole multilayer structure: PrxY1−x. The signal amplitude of the emitted terahertz signal increases with increasing dPBCO within the observed thickness range. Calculations of the dependence of the signal efficiency of the emitted terahertz pulses on the transmission of the multilayer structure in the terahertz frequency range agreed well with the experimental results of Y1−xPrxBa2Cu3O7−δ alloys in the case of a doping x less than 0.3.

T-ray computed tomography

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.

Terahertz multistatic reflection imaging.

We describe a new imaging method using single-cycle pulses of terahertz (THz) radiation. This technique emulates the data collection and image processing procedures developed for geophysical prospecting and is made possible by the availability of fiber-coupled THz receiver antennas. We use a migration procedure to solve the inverse problem; this permits us to reconstruct the location, the shape, and the refractive index of targets. We show examples for both metallic and dielectric model targets, and we perform velocity analysis on dielectric targets to estimate the refractive indices of imaged components. These results broaden the capabilities of THz imaging systems and also demonstrate the viability of the THz system as a test bed for the exploration of new seismic processing methods.

Tetrahertz Pulse Radiation Properties of a Bi2Sr2CaCu2O8+δ Bowtie Antenna by Optical Pulse Illumination

We demonstrate, for the first time, terahertz (THz) pulse radiation properties from a current biased Bi2Sr2CaCu2O8+δ (BSCCO) bowtie antenna by femtosecond laser pulse illumination. The bias current, excitation laser power and temperature dependence of the radiated amplitude of THz pulse are basically in good agreement with those observed on YBa2Cu3O7-δ (YBCO). However, the detailed properties of the THz pulse from BSCCO are considerably different from those for YBCO. The observed amplitude of the terahertz (THz) radiation pulse was very strong due to the small index value of BSCCO thin film even for the small bias current, and the Fourier components of the pulse contains the frequencies lower than 0.4 THz with a central frequency around 0.1–0.16 THz reflecting its long relaxation time of photo-excited quasi-particle in BSCCO.

Propagation of single-cycle terahertz pulses in random media

We describe what are to our knowledge the first measurements of the propagation of coherent, single-cycle pulses of terahertz radiation in a scattering medium. By measuring the transmission as a function of the length L of the medium, we extract the scattering mean free path l(s)(omega) over a broad bandwidth. We observe variations in l(s) ranging over nearly 2 orders of magnitude and covering the entire thin sample regime from L/l(s)<<1 to L/l(s)~10 . We also observe scattering-induced dispersive effects, which can be attributed to the additional path traveled by photons scattered at small angles.