Simplified setup for electro-optic sampling of terahertz pulses

We present a simplified setup for the generation and electro-optic sampling of terahertz pulses employing a nonamplified pump source and a single ZnTe crystal, which is simultaneously used for generation and detection. The setup is characterized, and first steps of its application for gas-phase spectroscopy are presented.     

Propagation of terahertz pulses in random media

We describe measurements of single-cycle terahertz pulse propagation in a random medium. The unique capabilities of terahertz time-domain spectroscopy permit the characterization of a multiply scattered field with unprecedented spatial and temporal resolution. With these results, we can develop a framework for understanding the statistics of broadband laser speckle. Also, the ability to extract information on the phase of the field opens up new possibilities for characterizing multiply scattered waves. We illustrate this with a simple example, which involves computing a time-windowed temporal correlation between fields measured at different spatial locations. This enables the identification of individual scattering events, and could lead to a new method for imaging in random media.     

Time-frequency signal processing of terahertz pulses

We demonstrate that some signal-processing techniques, such as the fractional Fourier transform and the spectrogram, which are known to be effective for optical signals, can be implemented at terahertz frequencies and with available terahertz devices. These techniques, contrary to the frequency-resolved optical-gating method for pulse characterization, do not require nonlinear media, which do not exist at terahertz frequencies. Thus the fractional Fourier transform or the spectrogram offers the only possibility of characterizing terahertz pulses simultaneously in time and frequency.     

Velocity matching of a GaAs electro-optic modulator

New designs for the velocity matching of a deep-etched semiconductor electro-optic modulator are presented. A tantalum pentoxide (Ta2O5) coating is considered here for achieving velocity matching between the microwave and the optical signals. The effects of the velocity mismatch, the conductor loss, the dielectric loss, and the impedance mismatch are studied in relation to the optical bandwidth of a high-speed semiconductor modulator. It is shown that both the dielectric loss and the impedance matching play key roles for velocity-matched high-speed modulators with low conductor loss. The effects of Ta2O5 thickness on the overall bandwidth and on the half-wave voltage-length product VpiL are also reported.     

Brewster's angle attenuator for terahertz pulses

A variable attenuator for terahertz (THz) pulses is developed on the basis of the change in reflectivity of lithium niobate wafers with incident angle in a Brewster configuration. We can vary the THz field transmission from 22% to 54%, a change of a factor of 2.5, while preserving the shape of the THz pulse spectrum. Changes in the THz spectrum are shown to be much smaller when the Brewster attenuator is used than when either the near-infrared pump power or the bias voltage on a THz photoconductive antenna is varied. The Brewster attenuator should prove especially useful for varying THz field strength in nonlinear optical studies that use broadband THz pulses.     

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.     

Generation of picosecond sampling pulses

Translated from Izmeritel'naya Tekhnika, No. 3, pp. 59–62, March, 1993.     

Component spatial pattern analysis of chemicals by use of two-dimensional electro-optic terahertz imaging

We demonstrate a method of chemical mapping by using the transillumination terahertz (THz) images obtained by two-dimensional electro-optic THz imaging. The images and spectral data were measured between 0.1 and 1.0 THz. An experimental sample consisting of three chemicals was prepared, with one in two concentrations. By introducing the component spatial pattern analysis based on the least-squares method, the chemical composition, spatial distribution, and difference in concentration were clearly determined.     

A review of non-linear terahertz spectroscopy with ultrashort tabletop-laser pulses

Over the past decade, breakthroughs in the generation and control of ultrafast high-field terahertz (THz) radiation have led to new spectroscopic methodologies for the study of light-matter interactions in the strong-field limit. In this review, we will outline recent experimental demonstrations of non-linear THz material responses in materials ranging from molecular gases, to liquids, to varieties of solids – including semiconductors, nanocarbon, and correlated electron materials. New insights into how strong THz fields interact with matter will be discussed in which a THz field can act as either a non-resonant electric field or a broad bandwidth pulse driving specific resonances within it. As an emerging field, non-linear THz spectroscopy shows promise for elucidating dynamic problems associated with next generation electronics and optoelectronics, as well as for demonstrating control over collective material degrees of freedom.     

Reconfigurable optical logic unit with a terahertz optical asymmetric demultiplexer and electro-optic switches

A reconfigurable optical logic unit that can execute any binary logic and arithmetic operation on the same hardware with different configurations is proposed. The design, based on cascaded terahertz optical asymmetric demultiplexer switches, introduces reconfigurability with the help of electro-optic switches. The model is simple and practicable.     

Propagation of ultrashort RF pulses in a polluted atmosphere

The effect of absorption and dispersion on the spectral characteristics and the shape of ultrashort RF pulses propagating in a polluted atmosphere is studied theoretically. The frequency band occupied by the pulses essentially covers the rotational spectrum of gas molecules encountered in a real atmosphere.     

Propagation of Bessel-X pulses in a hybrid photonic crystal

We report the propagation of Bessel-X pulses in a two-dimensional hybrid photonic crystal, investigated by the finite-difference time-domain method, in which broadband super-collimation and the propagation of self-collimated ultrashort pulses were reported. We first show the propagation of Bessel-X pulses in two-dimensional free space, whose transverse branches diverge rapidly with propagation. We then show that Bessel-X pulses propagate with their transverse and longitudinal shapes almost unchanged in the hybrid photonic crystal.     

Measurement of the terahertz-induced phase shift in electro-optic sampling for an arbitrary biasing phase

We present a simple method to correct for nonlinearities in the detection of terahertz (THz) transients by electro-optic sampling. The THz-induced phase shift of the optical probe beam is calibrated with an additional biasing phase from a variable wave plate. All the parameters that are needed for the determination of this phase shift can be directly measured, which makes the method insensitive to errors resulting from residual birefringences of the electro-optic material or imperfections of the wave plate. We show that the THz-induced phase shift is correctly revealed for a wide range of the biasing phase.     

Distortion of terahertz signals due to imperfect synchronization with chirped probe pulses

Terahertz (THz) signals measured by means of the spectral-encoding technique with different temporal discrepancies between probe pulses and THz signals are investigated. It is found that imperfect synchronization between the chirped probe and THz pulses induce a distortion and this distortion affects significantly the retrieved THz spectrum if the temporal discrepancy is large. The distortion becomes more prominent if the probe pulse length is less than the optimal chirped probe pulse duration. A simple approach is proposed to realize the synchronization and minimize the distortion. THz signals from a high-voltage-biased air plasma filament are measured with this approach and distortion similar to the simulation results is observed.     

Propagation of surface acoustic waves inn-type GaAs films

The effect of nonparabolicity on the amplification of surface acoustic waves in n-type GaAs films is investigated quantum mechanically in the GHz frequency region. Numerical results show that the amplification coefficient for the nonparabolic band structure is enhanced due to the nonlinear nature of the energy band in semiconductors. Moreover, the amplification coefficients in semiconductors depend on the temperature, the electronic screening effect, the frequency of sound waves, the applied electric field, and the thickness of the semiconductor film.Partially supported by National Science Council of the Republic of China.     

Propagation of optical pulses with spatial and temporal dependence

A convenient approximate formula is proposed for the study of free-space propagation of spatial and temporal pulses with an identifiable carrier frequency. It does not contain a time derivative operation on the pulse's temporal envelope explicitly. It is shown that once a short (for example, picosecond or subpicosecond) pulse is created with a spatial and a temporal structure, it does not last forever. The approximation discussed is valid over a certain distance as dictated by the wave equation. Beyond this distance, the spatial and temporal characteristics begin to influence each other significantly. Two examples are presented. The first example is that of a pulse with a factored form of a spatial envelope times a temporal envelope. The second example is that of a clear aperture with a grating, by which pulse stretching or temporal distortion is examined and the result is in agreement with that found in the literature.     

Improved interferometer studies of linear electro-optic effects of dye-doped polymers

The basic Mach-Zehnder interferometer was modified for use in in situ temperature-dependent linear electro-optic (LEO) measurements of thin films of 4-dimethylamino-4?-nitrostilbene (DANS) doped into poly(methyl methacrylate) (PMMA). Optimum interferometer phase stability was possible because of an incorporated electronic feedback system. Film thickness variation was compensated for to obtain more accurate LEO coefficient measurements and thus the second-order susceptibility. Moreover, both the alpha relaxation associated with the glass transition, T(g), and beta relaxation associated with the secondary transition occurring below T(g) of PMMA + 2 wt.% DANS was obtained.     

On possibility of high-power terahertz emission from target under the action of powerful laser pulses

The possibility of terahertz (THz) emission from a target irradiated by short (∼0.1 ns) high-intensity (I ∼ 10¹⁸–10¹⁹ W/cm²) laser pulses has been studied by numerical simulations using a relativistic electromagnetic PIC code. The laser pulse action on the target generates plasma and the runaway electrons form a virtual cathode, which oscillates in the intrinsic field of electrons and the field of plasma ions. These oscillations account for the emission of radiation in a THz range. The generation efficiency is about three times as high as that in the absence of ions (according to the conventional reditron mechanism). Explanation of the observed phenomena is proposed.