Time-domain differentiation of terahertz pulses

We report on the time-domain differentiation of light waves by metallic transmission gratings. Time-resolved terahertz experiments show that the first time derivative of an arbitrary waveform can be achieved by use of gratings of subwavelength period. The results are in accord with classical diffraction theory and may permit novel applications for tailoring few-cycle light pulses and ultrahigh-frequency optoelectronics.     

Superluminal terahertz pulses

In femtosecond terahertz-pulse (T-ray) imaging of metal structures with dimensions of the order of the wavelength, it is observed that the T rays propagate faster than the vacuum speed of light. In the case of apertures this can be understood as a waveguide effect in which superluminal velocities are expected close to the cutoff frequency. However, the effect is also observed close to knife edges and in propagation past thin metal wires.     

Whispering-gallery mode terahertz pulses

We report what is to our knowledge the first observation of a subpicosecond terahertz (THz) pulse propagating as a superposition of the whispering-gallery modes (WGMs) of a dielectric cylinder. The WGM THz pulses are coupled into and out of a 5-mm-diameter silicon cylinder via a 100-mum -thick, single-mode silicon slab waveguide. We observed two round-trip cylindrical cavity pulses from this coupling structure, which cover a continuous frequency range from 0.4 to 1.8 THz. The coupled-mode equations are used to analyze this system and give reasonably good agreement with the experiment in both the frequency and the time domains.     

Nonlinear reshaping of terahertz pulses with graphene metamaterials

We study the propagation of electromagnetic waves through a slab of graphene metamaterial composed of the layers of graphene separated by dielectric slabs. Starting from the kinetic expression for two-dimensional electric current in graphene, we derive a novel equation describing the nonlinear propagation of terahertz electromagnetic pulses through the layered graphene-dielectric structure in the presence of losses and non-linearities. We demonstrate strong nonlinearity-induced reshaping of transmitted and reflected terahertz pulses through the interaction with the thin multilayer graphene metamaterial structure.     

Spatiotemporal shaping of terahertz pulses

We report temporal shaping of few-cycle terahertz pulses, using a slit in a conductive screen as a high-pass filter. The filter's cutoff frequency was tuned by changing the width of the slit; the slope of the cutoff transition was altered by changing the thickness of the screen. We measured the transmission function of the filters, using large-aperture photoconducting antennas to create and detect the incident and transmitted electric field. Our experimental results were in excellent agreement with the performed finite-difference time-domain simulations of the propagation of the pulse through the slit. When the screen thickness was greater than the slit width, the filter was well modeled by a short, planar waveguide. Using a simple transfer function, we accurately describe the sharp cutoff and dispersion of such a filter.     

Nonlinear cross-phase modulation with intense single-cycle terahertz pulses

We have demonstrated nonlinear cross-phase modulation in electro-optic crystals using intense, single-cycle terahertz (THz) radiation. Individual THz pulses, generated by coherent transition radiation emitted by subpicosecond electron bunches, have peak energies of up to 100 microJ per pulse. The time-dependent electric field of the intense THz pulses induces cross-phase modulation in electro-optic crystals through the Pockels effect, leading to spectral shifting, broadening, and modulation of copropagating laser pulses. The observed THz-induced cross-phase modulation agrees well with a time-dependent phase-shift model.     

Interaction of strong single-cycle terahertz pulses with semiconductor quantum wells

An experiment-theory comparison is presented to demonstrate terahertz-induced extreme-nonlinear transients in a GaAs/AlGaAs quantum-well system. The terahertz-pump and optical-probe experiments show pronounced spectral modulations of the light- and heavy-hole excitonic resonances. Excellent agreement with the results of microscopic many-body calculations is obtained, identifying clear ponderomotive contributions and the generation of terahertz harmonics.     

Third order harmonic imaging for biological tissues using three phase-coded pulses

Compared to the fundamental and the second harmonic imaging, the third harmonic imaging shows significant improvements in image quality due to the better resolution, but it is degraded by the lower sound pressure and signal-to-noise ratio (SNR). In this study, a phase-coded pulse technique is proposed to selectively enhance the sound pressure of the third harmonic by 9.5 dB whereas the fundamental and the second harmonic components are efficiently suppressed and SNR is also increased by 4.7 dB. Based on the solution of the KZK nonlinear equation, the axial and lateral beam profiles of harmonics radiated from a planar piston transducer were theoretically simulated and experimentally examined. Finally, the third harmonic images using this technique were performed for several biological tissues and compared with the images obtained by the fundamental and the second harmonic imaging. Results demonstrate that the phase-coded pulse technique yields a dramatically cleaner and sharper contrast image.     

Undistorted guided-wave propagation of subpicosecond terahertz pulses

We report efficient quasi-optic coupling of a freely propagating beam of terahertz (THz) pulses into a parallel-plate copper waveguide (with a plate separation of 108mum) and subsequent low-loss, single-TEM-mode propagation with virtually no group-velocity dispersion. Undistorted, low-loss propagation of the incoming 0.3-ps FWHM THz pulses was observed within the bandwidth from 0.1 to 4 THz for a length of 24.4 mm. We compare experimentally derived values for the absorption and phase velocity with theory to show consistency. This demonstration is direct proof of the excellent performance of the parallel-plate waveguide as a wideband THz interconnect.     

Study of terahertz pulses at an edge

The propagation behaviour of terahertz （THz） pulses at an edge is characterized. The phenomenon that the amplitude oscillates periodically in the frequency spectrum is similar to Young＇s interference, if the absorption effect is neglected. The oscillation cycle is shorter for a thicker sample. THz pulses at an edge are analyzed by the broadband Huygens-Fresnel diffraction integral. The experimental results are in agreement with the simulation results approximately. The simulation errors are also analyzed.     

Single-shot spatiotemporal measurements of high-field terahertz pulses

The electric field profiles of broad-bandwidth coherent terahertz (THz) pulses, emitted by laser-wakefield-accelerated electron bunches, are studied. The near-single-cycle THz pulses are measured with two single-shot techniques in the temporal and spatial domains. Spectra of 0-6 THz and peak fields up to approximately or = 0.4 MV cm(-1) are observed. The measured field substructure demonstrates the manifestation of spatiotemporal coupling at focus, which affects the interpretation of THz radiation as a bunch diagnostic and in high-field pump-probe experiments.     

Mobile source of high-energy single-cycle terahertz pulses

The Teramobile laser facility was used to realize the first mobile source of high-power THz pulses. The source is based on a tilted-pulse-front pumping THz generation scheme optimized for application of terawatt laser pulses. Generation of 50-μJ single-cycle electromagnetic pulses centered at 0.19 THz with a repetition rate of 10 Hz was obtained for incoming 700-fs 120-mJ near-infrared laser pulses. The corresponding laser-to-THz photon conversion efficiency is approximately 100%.     

High-temporal-resolution,single-shot characterization of terahertz pulses

A technique for noncollinear cross correlation of electro-optic modulated optical pulses is presented for the single-shot characterization of terahertz waveforms and is compared to established electro-optic terahertz characterization methods. This technique is free from the limitations on time resolution and faithful reproduction of previously demonstrated single-shot amplitude modulation spectral encoding.     

频率步进太赫兹脉冲成像技术研究

针对太赫兹频段具有大带宽的特点,阐述了太赫兹波段进行高分辨力成像的原理,按照微波上变频的方法,将微波源通过混频和倍频方式得到太赫兹频率源;采用频率步进脉冲体制,设计和实现了220 GHz太赫兹主动成像系统;并在暗室和外场进行了分辨力测试和拉距实验,获得的纵向分辨力达4 cm,测试距离达10 m。     

频率步进太赫兹脉冲成像技术研究

针对太赫兹频段具有大带宽的特点,阐述了太赫兹波段进行高分辨力成像的原理,按照微波上变频的方法,将微波源通过混频和倍频方式得到太赫兹频率源;采用频率步进脉冲体制,设计和实现了220 GHz太赫兹主动成像系统;并在暗室和外场进行了分辨力测试和拉距实验,获得的纵向分辨力达4 cm,测试距离达10 m。     

Generation of sub-mJ terahertz pulses by optical rectification

Recent theoretical calculations predicted an order-of-magnitude increase in the efficiency of terahertz pulse generation by optical rectification in lithium niobate when 500 fs long pump pulses are used, rather than the commonly used ~100 fs pulses. Even by using longer than optimal pump pulses of 1.3 ps duration, 2.5× higher THz pulse energy (125 μJ) was measured with 2.5× higher pump-to-THz energy conversion efficiency (0.25%) than reported previously with shorter pulses. These results verify the advantage of longer pump pulses and support the expectation that mJ-level THz pulses will be available by cooling the crystal and using large pumped area.     

On focusing of terahertz radiation beams and pulses

We show that, for terahertz (THz) radiation, the effective focal length of a lens differs significantly from its geometrical-optics (nominal) focal length. As a pulse of THz radiation with a bell-shaped (Gaussian) transverse profile is incident on a lens, the field-amplitude distribution in the nominally focal plane of the lens may radically change, up to the formation of dips in the paraxial region. These changes in the spatiotemporal shape of the beam (pulse) may considerably distort the results of pulsed THz spectroscopy. These distortions can be minimized by using media with anomalous dispersion, including metamaterials.     

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.