Terahertz Pulse Emission Research Articles

Scanning laser THz imaging system

The laser terahertz (THz) emission microscope (LTEM) is a unique inspection tool which can directly two-dimensionally map the THz pulse emission from a variety of electric materials and devices. Thus, two-dimensional mapping directly yields various physical information by visualizing the distributions of electric field, supercurrent, ferroelectric domain structures, magnetic fluxes, etc. Based on techniques created for conventional LTEM, we have developed a more highly functional laser THz imaging system, including a scanning laser THz (near-field) imaging system having the performance of high-speed and high-resolution, a scanning probe LTEM coupled with an atomic force microscope for high-resolution imaging, and a dynamic THz emission microscope to investigate the ultrafast carrier dynamics two-dimensionally in the sample. These systems offer diverse characteristics, making them suitable for various applications. We take a look at these systems and some typical applications.

Enhanced terahertz emission by coherent optical absorption in ultrathin semiconductor films on metals

We report on the surprisingly strong, broadband emission of coherent terahertz pulses from ultrathin layers of semiconductors such as amorphous silicon, germanium and polycrystalline cuprous oxide deposited on gold, upon illumination with femtosecond laser pulses. The strength of the emission is surprising because the materials are considered to be bad (amorphous silicon and polycrystalline cuprous oxide) or fair (amorphous germanium) terahertz emitters at best. We show that the strength of the emission is partly explained by cavity-enhanced optical absorption. This forces most of the light to be absorbed in the depletion region of the semiconductor/metal interface where terahertz generation occurs. For an excitation wavelength of 800 nm, the strongest terahertz emission is found for a 25 nm thick layer of amorphous germanium, a 40 nm thick layer of amorphous silicon and a 420 nm thick layer of cuprous oxide, all on gold. The emission from cuprous oxide is similar in strength to that obtained with optical rectification from a 300 μm thick gallium phosphide crystal. As an application of our findings we demonstrate how such thin films can be used to turn standard optical components, such as paraboloidal mirrors, into self-focusing terahertz emitters.

Terahertz photoconductive antenna with metal nanoislands

This work presents a nanoplasmonic photoconductive antenna (PCA) with metal nanoislands for enhancing terahertz (THz) pulse emission. The whole photoconductive area was fully covered with metal nanoislands by using thermal dewetting of thin metal film at relatively low temperature. The metal nanoislands serve as plasmonic nanoantennas to locally enhance the electric field of an ultrashort pulsed pump beam for higher photocarrier generation. The plasmon resonance of metal nanoislands was achieved at an excitation laser wavelength by changing the initial thickness of metal film. This nanoplasmonic PCA shows two times higher enhancement for THz pulse emission power than a conventional PCA. This work opens up a new opportunity for plasmon enhanced large-aperture THz photoconductive antennas.

Terahertz emission from cubic semiconductor induced by a transient anisotropic photocurrent

Terahertz emission from the surfaces of narrow-gap semiconductors excited by femtosecond laser pulses was described in terms of a transient interband photoconductivity. It has been found that the nonparabolicity of the electron dispersion law as well as the optical alignment of the photoexcited carrier momenta result in anisotropic photocurrent with a component perpendicular to the surface dc electric field even in semiconductors with a cubic symmetry. This lateral transient photocurrent component is the strongest during the first few hundreds of femtoseconds after the photoexcitation and causes the emission of terahertz radiation pulses with an amplitude dependent on the angle between the optical field and the crystallographic axes. In the case of InAs the contribution of this component explains experimental results of both the azimuthal anisotropy of the emitted terahertz pulse amplitude and its dependence on the exciting photon energy.

Terahertz Pulse Emission from Nanostructured (311) Surfaces of GaAs

In the present work, azimuthal angle dependences of the terahertz (THz) pulse emission from lower symmetry, (311) planes of porous GaAs samples have been studied. GaAs porous layers were prepared by pulsed anodic electrochemical etching of n-type (311) GaAs wafers in mixed acidic fluoride-iodide electrolyte. It has been discovered that the anodic electrochemical etching of the GaAs sample significantly enhances its terahertz radiation emissivity. It was shown theoretically that for this crystallographic plane the contributions of both optical rectification (OR) and electric-field-induced optical rectification (EFIOR) effects are characterized by different azimuthal angle dependences. Experimental measurements were compared with the theoretical calculations of the azimuthal angle dependencies; it has been shown that both nonlinear optical effects are necessary to take into account when explaining the experimental observations.

Enhancement of Terahertz Pulse Emission by Optical Nanoantenna

Bridging the gap between ultrashort pulsed optical waves and terahertz (THz) waves, the THz photoconductive antenna (PCA) is a major constituent for the emission or detection of THz waves by diverse optical and electrical methods. However, THz PCA still lacks employment of advanced breakthrough technologies for high-power THz emission. Here, we report the enhancement of THz emission power by incorporating optical nanoantennas with a THz photoconductive antenna. The confinement and concentration of an optical pump beam on a photoconductive substrate can be efficiently achieved with optical nanoantennas over a high-index photoconductive substrate. Both numerical and experimental results clearly demonstrate the enhancement of THz wave emission due to high photocarrier generation at the plasmon resonance of nanoantennas. This work opens up many opportunities for diverse integrated photonic elements on a single PCA at THz and optical frequencies.

Terahertz emission from Indium Oxide films grown on MgO substrates using sub-bandgap photon energy excitation

Indium oxide (In2O3) films grown by thermal oxidation on MgO substrates were optically excited by femtosecond laser pulses having photon energy lower than the In2O3 bandgap. Terahertz (THz) pulse emission was observed using time domain spectroscopy. Results show that THz emission saturates at an excitation fluence of ~400 nJ/cm2. Even as two-photon absorption has been excluded, the actual emission mechanism has yet to be confirmed but is currently attributed to carriers due to weak absorption from defect levels that are driven by a strain field at the interface of the substrate and the grown film.

Bi-directional terahertz emission from gold-coated nanogratings by excitation via femtosecond laser pulses

We report on the investigation of terahertz (THz) emission from gold-coated nanogratings (500 nm grating constant) upon femtosecond laser irradiation (785 nm, 150 fs, 1 kHz, ≤1 mJ/pulse). Unlike common assumptions, THz emission is not only observed in case of rear side irradiation (through substrate (Welsh et al. in Phys. Rev. Lett. 98:026803, 2007; Welsh and Wynne in Opt. Express 17:2470–2480, 2009)) of the nanograting, but also in case of front side excitation (through air). Furthermore in both cases, THz emission propagates in the direction of laser beam propagation and reverse. Based on these findings, we suggest a new approach to describe the newly observed phenomena. Using a highly sensitive and fast superconducting transition edge sensor (TES) as calorimeter, it was possible to directly measure the absolute energy of the emitted THz pulses in a defined spectral and spatial range, enabling for the first time a quantitative analysis of the THz emission process.

Enhanced Center Frequency of Terahertz Pulse Emission from Photoconductive Antenna

The center frequency of terahertz (THz) pulse radiated from photoconductive antenna is located at the low-frequency range of THz band. None of the antenna designs have been done to enhance the center frequency without decreasing antenna's dimension, which would largely decrease the radiation power. We introduce here the idea of separating antennas from the transmission lines to reduce the current distribution on them, showing superior performance for both the center frequency (∼ 90% increased) and the radiation power from the simulations of the dipole antennas.

On the theory of the generation of terahertz radiation accompanying the optical breakdown of air by femtosecond laser pulses containing the second harmonic

A theoretical model is presented for the generation of a quasi-constant current accompanying the optical breakdown of air by femtosecond laser pulses containing the second harmonic, along with the fundamental. Analytical estimates of the current are made. Results of the modelling of the emission of terahertz pulses from a plasma column are presented.

Terahertz chemical microscope for label-free detection of protein complex

A terahertz (THz) chemical microscope (TCM) has been proposed and developed for label-free analysis of the interactions of proteins, including immune reactions. A SiO2/Si/sapphire plate was fabricated and emitted THz pulses from the sapphire side using a femtosecond laser. The amplitude of a THz pulse depends on the chemical potential of the SiO2 surface. As an initial demonstration, a biotin–avidin complex was analyzed using TCM. Results indicate that biotin concentrations of up to 10−12 mol/l could be detected, suggesting that a high sensitivity immunoassay system can be realized using this method. The distribution of the THz peak amplitude was measured, reflecting the distribution of the biotin–avidin complex on the plate.

Terahertz pulse emission optimization from tailored femtosecond laser pulse filamentation in air

We study the generation of intense terahertz pulses produced by two-color laser pulse filamentation in air. We tailor the filamentation process and the produced plasma strings and study how the generated terahertz field is modified. An important terahertz pulse shortening is found for plasma strings with uniform electron density.

Terahertz pulse emission from semiconductor surfaces illuminated by femtosecond Yb:KGW laser pulses

Abstract The amplitudes of terahertz pulses emitted from the surfaces of InAs, InSb, InGaAs, GaAs and Ge after their excitation by femtosecond 1 μm laser pulses was compared. It has been found that this effect is most efficient in p-type InAs. The mechanisms leading to the terahertz emission are investigated and discussed. It has been concluded that in the majority of the investigated semiconductors the main contribution to THz pulse emission comes from the electrical-field-induced optical rectification effect.

THz emission from semiconductor surfaces

We provide a review of experimental and theoretical work on electromagnetic terahertz pulse emission from semiconductor surfaces excited by femtosecond laser radiation. The main terahertz emission mechanisms are analysed. The terahertz emission from InAs and Ge is explained by the photo-Dember effect and electric field induced optical rectification. Electronic band structure and carrier scattering mechanisms are investigated by means of terahertz emission and absorption spectroscopy in InAs, InSb and Ge. To cite this article: V.L. Malevich et al., C. R. Physique 9 (2008).

Measurement of submilliwatt, picosecond terahertz emission from a femtosecond-laser-pumped solid-state dc to ac radiation converter based on a ZnSe crystal

We measured the terahertz pulse emission from a femtosecond-laser-pumped solid-state dc to ac radiation converter using a 150fs Ti:sapphire laser pulse for dense plasma diagnostics. The laser-produced ionization front was directly modulated from a periodic electrostatic field to pulsed emission. The central frequency of the emission was measured to be 0.13THz having a bandwidth of 0.1THz and a peak power of 0.2mW. This emission source is suitable for use in various novel diagnostic techniques, such as dense plasma diagnostics.

Distinctive features of photoionized plasma from short x-ray-pulse interaction with gaseous medium

The study of a photoionized plasma created through the interaction of a short linearly polarized x-ray pulse with a gaseous medium is presented in the context of the future design of experiments involving femtosecond free-electron laser pulses. The dispersion properties and instabilities of such x-ray-produced plasma are studied. It is shown that a two-stream type of instability can be obtained with a growth rate comparable to the plasma frequency. This is expected to have a dramatic effect on the evolution of an x-ray laser-produced plasma. A model for describing electromagnetic-field generation and emission resulting from the interaction of a short x-ray laser pulse with a gas jet is proposed. Issues addressed include the effect of the thermal electromotive force at the edge of a plasma driven by the anisotropic electron energy distribution that arises from the photoionization of a gas. Terahertz pulse emission from these photoionized plasma is also predicted.

Emission and detection of terahertz pulses from a metal-tip antenna

We investigate the antenna characteristics of a metal tip coupled to terahertz (THz) pulses generated from a photoconductive switch. Enhanced terahertz pulse emission is observed with the metal tip in contact with one of the electrodes of the photoconductive switch. Measurements of the angular dependence of the emitted THz radiation show that the metal tip acts as a highly directional antenna with radiation patterns well described by the theory for long-wire traveling-wave antennas. Similar behavior is observed for the metal tip acting as a THz pulse receiver, in accordance with the reciprocity principle. Effects related to the broadband nature of the THz pulses are discussed.

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.

Ultrafast carrier dynamics in optically excited YBa/sub 2/Cu/sub 3/O/sub 7-δ/ thin films diagnosed by a pump-and-probe terahertz experiment

A pump-and-probe terahertz emission technique is reported to investigate the nonequilibrium process of ultrafast generation and relaxation of hot quasiparticles and their recombination to form Cooper pairs in superconducting YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin films following femtosecond laser excitation. Subpicosecond electromagnetic pulses emitted by an YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// microbridge change after an excitation of the material due to an additional laser pulse, the so-called pump pulse. The modulation of the emitted terahertz pulse amplitude is used to measure the time resolved evaluation of the hot carrier excitation and relaxation after a time delay to the pump laser pulse. The results are compared to terahertz pump-and-probe experiments on GaAs photoswitches and can be explained by avalanche Cooper pair breaking, supercurrent modulation followed by carrier acceleration and Cooper pair recombination accompanied with deceleration of the supercurrent carriers.

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.