Terahertz Electromagnetic Pulses Research Articles

Monte Carlo simulation of the Dember effect in n-InAs exposed to femtosecond pulse laser excitation

The Monte Carlo simulation is used to calculate the Dember photovoltage and study the generation of terahertz electromagnetic pulses excited in indium arsenide by femtosecond laser radiation. The dynamics of the electric field and the charge-carrier transport are treated self-consistently. It is shown that the excitation of a semiconductor by laser pulses with the photon energy of ħω≲1.5 eV, the photovoltage attains its maximum within 50–100 fs after excitation and then falls off, oscillating with the plasma frequency. The photovoltage peak can be as much as tens times higher than the typical Dember photovoltages induced by steady-state illumination. On excitation of the semiconductor by radiation with shorter wavelengths (ħω≳1.6 eV), the photoelectrons are scattered to the L and X valleys side; as a result, the photovoltage and the efficiency of the generation of the terahertz emission decrease.

Terahertz paintmeter for noncontact monitoring of thickness and drying progress in paint film

We propose a paintmeter for noncontact and remote monitoring of the thickness and drying progress of a paint film based on the time-of-flight measurement of the echo signal of a terahertz (THz) electromagnetic pulse. The proposed method is effectively applied to two-dimensional mapping of the painting thickness distribution for single-layer and multilayer paint films. Furthermore, adequate parameters for the drying progress are extracted from the THz pulse-echo signal and effectively applied to monitor the wet-to-dry transformation. The THz paintmeter can be a powerful tool for quality control of the paint film on the in-process monitoring of car body painting.

Ultrafast semiconductor spectroscopy using terahertz electromagnetic pulses

Terahertz electromagnetic pulses can serve as a new and unique tool for various types of spectroscopy. We first characterized the temporal and spatial properties of THz pulses generated from a large-aperture photoconductive antena, and then used them for the study of the ultrafast dynamics of electrons in semiconductros. We studied the dynamics of electrons generated by femtosecond optical pulses with positive and negative excess energies in GaAs and InP by observing the waveform of the emitted THz radiation. Subpicosecond intraband relaxation was observed with positive excess energies. With negative excess energies, a picosecond transition from the Urbach state to free carrier states was observed.

Coherent terahertz emission from ferromagnetic films excited by femtosecond laser pulses

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.

Time reversal three-dimensional imaging using single-cycle terahertz pulses

We demonstrate three-dimensional imaging using single-cycle terahertz electromagnetic pulses. Reflection-mode imaging is performed with a photoconductive transmitter and receiver and a reconstruction algorithm based on time reversal. A two-dimensional array is synthesized from ten concentric ring annular arrays with numerical apertures ranging from 0.27 to 0.43. The system clearly distinguishes image planes separated by 1.5 mm and achieves a −6 dB lateral resolution of 1.1 mm. In terms of the illuminating terahertz power spectrum, the lateral resolution is 38% and 81% of the peak and mean wavelengths, respectively.

테라헤르츠 전자기파 펄스의 변조를 이용한 이미징의 해상도 연구

테라헤르츠 전자기파 펄스의 변조를 이용한 이미징의 해상도 연구

Non-contact Measurement of Local Water Content in Skin Using a Terahertz Electromagnetic Pulse

Non-contact Measurement of Local Water Content in Skin Using a Terahertz Electromagnetic Pulse

A terahertz molecular switch.

We present time-dependent results describing the current through a molecular device, modeled as a complex with two active centers connected to leads under bias. We show that, at a properly adjusted external voltage, a passing terahertz electromagnetic pulse may cause a transition between states of finite and negligible current, suggesting that the system might be useful as a nanoscopic switch in the terahertz range. A phase diagram defining the bias region in which the transition takes place within a short time is given. As described, the physical processes involved are of an entirely different nature than those in ordinary photodetectors.

Ring formation of focused half-cycle terahertz pulses

Studies using a terahertz (THz) imaging system have shown that focused THz electromagnetic pulses exhibit a ring-like spatial intensity distribution. In the study, almost half-cycle THz pulses were generated from a large-aperture photoconductive antenna and focused by an off-axis parabolic mirror. Time-resolved spatial distribution of the THz electric field on the focal plane was found to form a ring before and after the time region where a single peak is formed around the time origin. On the other hand, with the delay time fixed at the origin, the field distribution was also found to exhibit a ring-like profile at positions away from the focal plane. Existence of relatively low-frequency components in the power spectrum of the THz field gives these observable phenomena. Numerical simulations using diffraction integral calculations reproduced the observed features. A simpler numerical analysis based on the Gaussian beam approximation was shown to yield the observed features and also predicted the dependence on the beam parameters.

좁은 간격의 테라헤르츠 다이폴 안테나를 이용한 이진신호의 발생과 검출

<TEX>$12{\mu}{\textrm}{m}$</TEX> 길이, <TEX>$5{\mu}{\textrm}{m}$</TEX> gap의 dipole 안테나에 입사되는 펨토초 레이저의 위치에 따른 테라헤르트 전자기 펄스의 크기를 관찰하였다. 비록 11volt의 작은 전압을 인가 시켰음에도 불구하고 테라헤르츠 전자기 펄스가 최대 4.7nA의 전류로 발생되었는데 이는 dipole 안테나 구조가 아닌 두 전송라인(transmission line)의 간격이 <TEX>$300{\mu}{\textrm}{m}$</TEX> 떨어진 구조의 송신기(transmitter chip)에 비해 6배 작은 전압을 인가하여 3.4배 더 큰 전류가 발생됨을 알 수 있었다. 또한 팸토초 레이저로 여기 되는 dipole 안테나 구조의 송신기에 구형 교류전압을 최소 0.11volt에서 최대 10volt까지 변화시켜 테라헤르트 전자기 펄스의 최대값 크기에 따른 이진신호를 발생시켰다. We present the magnitude of terahertz electromagnetic pulses when femtosecond laser pulses are moving along to a dipole antenna. The dipole antenna chip has maximum 4.7 nA THz current, generated at 11 DC volt. This current is 3.4 times bigger than the current of a <TEX>$300{\mu}{\textrm}{m}$</TEX> separated transmission line structure chip that has 70 DC volt. We also apply an AC square wave voltage to the dipole antenna from 0.1 volt to 10 volt for binary signals using the terahertz electromagnetic pulses.

Gaussian Beam Analysis of Temporal Waveform of Focused Terahertz Pulses

Temporal waveform transformation of terahertz electromagnetic pulses near the focus was studied by experiments and simulations. The terahertz pulses were generated by a large-aperture photoconductive emitter, and focused by an off-axis parabolic mirror. Changes of the temporal waveforms due to the propagation near the focus were measured using the electro-optic sampling method. Experimentally observed waveforms were compared with those of simulations based on the Gaussian beam model, and good agreement between them was obtained. A simple analytical expression describing the amplitude and the temporal shape of the focused THz waveform was also derived.

Terahertz Time Domain Spectroscopy and Imaging.

The technology of generating terahertz electromagnetic pulses from various devices and materials by exciting them with ultrashort laser pulses has recently made a remarkable progress. This technology has been applied to spectroscopy and imaging. In this paper, the various methods of spectroscopy, imaging and their application to various samples are described.

Optical tunneling of single-cycle terahertz bandwidth pulses.

We report time-domain measurements with subpicosecond resolution of optical tunneling of terahertz electromagnetic pulses undergoing frustrated total internal reflection. Measurements of the transmitted electromagnetic pulses over a 3 THz bandwidth permits direct determination of frequency-dependent phase and amplitude changes in both the thin and opaque barrier limits in a single measurement. A complex frequency response function describing propagation through the barrier is developed based upon linear dispersion theory and the Fresnel coefficients at complex angles in the optical barrier. Our measurements are in excellent agreement with this theoretical model across the experimentally determined bandwidth; the model makes no assumptions about the beam path through the barrier and has no adjustable parameters. The theory is shown to satisfy the Kramers-Kronig relations, indicating causal propagation across the barrier.

Compression of terahertz radiation in resonant systems with a quantum superlattice

It is shown theoretically that ultrashort terahertz electromagnetic pulses can be generated in resonant systems containing a quantum semiconductor superlattice. This effect is due to the compression of radiation incident on a cavity as a result of ultrafast self-modulation of the cavity Q factor upon cyclic switching of the superlattice from the screening to the transparent state in a pump field.

FDTD analysis of propagation characteristics of terahertz electromagnetic pulses

A three dimensional finite difference time domain (FDTD) technique is applied to analyze the propagation characteristics of terahertz (THz) antenna whose dimension is smaller than the wavelength. A diffraction theory is also utilized with the FDTD technique for observing the far-field dispersion and attenuation of radiation from the THz antenna with a hyper-hemispherical silicon lens which is much larger than the wavelength. The signal degradation effects due to misalignment between the antenna and silicon lens are also characterized.

Single-shot measurement of terahertz electromagnetic pulses by use of electro-optic sampling

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.

Excitation of a Three-Dimensionally Localized Atomic Electron Wave Packet

We have prepared an atomic electron in a three-dimensionally localized wave packet that travels along a highly eccentric orbit between the nucleus and a turning point approximately 1000 \AA{} away. State preparation consisted of two distinct steps. First the electron wave function was localized to the desired orbit by exciting a stationary extreme Stark state from the $n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}30$ manifold of a sodium atom in a dc electric field. Then the dynamic wave packet was formed by applying a terahertz electromagnetic half-cycle pulse polarized along the direction of the dc field. The localized state is an ideal starting point for further quantum control proceeding along classical orbits.

Electrical generation of terahertz electromagnetic pulses by hot-electrons in quantum wells

We present a theoretical study on the electrical generation of terahertz (THz) electromagnetic pulses by hot-electrons in AlGaAs/GaAs-based two-dimensional semiconductor systems (2DSSs). The intensity of THz blackbody radiation has been calculated as a function of photon frequency and electron temperature. We consider that the electrons are heated by pulsed electric fields and that the hot-electron interactions with electromagnetic fields are mediated by electron--phonon scattering.

Generation and detection of terahertz electromagnetic pulses from semiconductors with femtosecond optics

The basic concepts and preliminary applications of THz photoconductive radiation from semiconductors are presented. We have studied THz radiation from different metallic electrodes with different geometries. We also report the experimental results from unbiased semiconductors with different surface and interfacial conditions, including chemically etched surfaces and metal/semiconductor interfaces.

Terahertz electromagnetic pulses as probes for transient velocity overshoot in GaAs and Si

Ultrafast dynamics of the transient velocity overshoot in GaAs and Si have been studied through the use of measurements of radiated and guided terahertz pulses whose time-domain waveforms are proportional to the acceleration and the velocity of carriers, respectively. These measurements have been compared with each other and also with a Monte Carlo simulation. The field-dependent free-space and guided waveforms are qualitatively consistent with Monte Carlo calculations. By correlation of the calculated velocities with a sampling gate function the observed ratio of peak-to-steady-state velocities is found to be in reasonable quantitative agreement. Also, by detecting the free-space radiation from a Si p-i-n structure, we have made the first, to our knowledge, direct experimental observation of transient velocity overshoot in Si.