Single-shot spatiotemporal terahertz field imaging

Using an optically chirped pulse in an electro-optic detection system, we image the spatiotemporal distribution of a terahertz (THz) pulse. We demonstrate single-shot THz field imaging of photoconductive emitters.     

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.     

Approach to broadband terahertz pulse field detection and recovery with electro-optic sampling measurement

Electro-optic sampling (EOS) measurement of terahertz (THz) pulses inherently leads to distortions, especially for relatively broadband THz pulses. In this paper, we proposed an approach to broadband THz pulse field detection and recovery with EOS measurement. In our approach, we use an EOS measurement utilizing a thick GaP crystal, which has a high transverse optical phonon frequency, to reduce the distortions caused by dispersive propagation, interface reflection and Fabry-Perot effect (multiple reflections within an EO crystal). And a retrieval algorithm is designed to eliminate the distortion of broadband THz pluses after passing through the thick GaP crystal (The distortion in a thick GaP crystal is mainly attributed to phase mismatching between the THz pulse and the sampling laser pulse.), in which preconditioned conjugated gradient method is used.     

Ultrashort pulse characterization with a terahertz streak camera

A phase-locked terahertz transient is exploited as an ultrafast phase gate for femtosecond optical pulses. We directly map out the group delay dispersion of a low-power near-infrared pulse by measuring the electro-optically induced polarization rotation as a function of wavelength. Our experiment covers the spectral window from 1.0 to 1.4 μm and reaches a temporal precision better than 1 fs. A quantitative analysis of the detector response confirms that this streaking technique requires no reconstruction algorithm and is also well suited for the characterization of pulses spanning more than one optical octave.     

Tailoring terahertz near-field enhancement via two-dimensional plasmons

We suggest a novel possibility for electrically tunable terahertz near-field enhancement in flatland electronic materials supporting two-dimensional plasmons, including recently discovered graphene. We employ electric-field effect modulation of electron density in such materials and induce a periodic plasmonic lattice with a defect cavity. We demonstrate that the plasmons resonantly excited in such a periodic plasmonic lattice by an incident terahertz radiation can strongly pump the cavity plasmon modes leading to a deep subwavelength concentration of terahertz energy, beyond λ/1000, with giant electric-field enhancement factors up to 10(4), which is 2 orders of magnitude higher than achieved previously in metal-based terahertz field concentrators.     

Implementation of electro-optic spectral shearing interferometry for ultrashort pulse characterization

Electro-optic spectral shearing interferometry is implemented for the highly sensitive characterization of ultrashort pulses from a free-running source. A simple phase-locked-loop clock recovery circuit is used to extract the high-frequency microwave clock signal to drive the phase modulator for spectral shearing. We demonstrate accurate full temporal characterization of 200-fs pulses from an optical parametric oscillator at 5-pW average power.     

ZnTe晶体中光学整流产生的THz辐射及其电光探测研究

借助抽运-探测技术研究了ZnTe晶体中光学整流产生的太赫兹(THz)辐射，利用ZnTe晶体的线性电光效应探测THz辐射场分布，观察到了较窄(约为0.2 ps)的THz场分布及相应较宽(响应超过4 THz，半峰宽约为2.4 THz)的THz频谱，并运用琼斯矩阵对实验结果进行了理论拟合. 研究了飞秒激光脉冲波长(750—850 nm)、脉冲宽度(56—225 fs)和晶体旋转与THz辐射产生的关系. 同时改变探测光偏振方向进行偏振调制，并从理论上分析了偏振调制对THz辐射探测的影响. 关键词： THz辐射 光学整流 电光探测 ZnTe     

Single-shot UV-laser ablation of polyimide with variable pulse lengths

Single-shot laser ablation of polyimide has been investigated with UV-Ar⁺-laser radiation ( = 270-315 nm) for pulse lengths between 140 ns and 5 µs. The irradiated polymer surface was studied with respect to its morphology and ablated depth by means of atomic force microscopy. The dependence of the ablation threshold on laser pulse-length and intensity can be tentatively interpreted on the basis of a thermal process and a (thermal or non-thermal) mechanism which diminishes the activation energy for the desorption of species from the surface.     

Generation and detection of terahertz radiation with multilayered electro-optic polymer films

We use optical rectification and electro-optic sensing to generate and detect terahertz radiation, using individual and multilayered stacks of electro-optic polymer films. For comparable thickness, the polymer films are more efficient generators and detectors of terahertz radiation than ZnTe crystals.     

High-power terahertz pulse sensor with overmoded structure

Based on the hot electron effect in a semiconductor, an overmoded resistive sensor for 0.3-0.4 THz band is investi-gated. The distribution of electromagnetic field components, voltage standing wave ratio （VSWR）, and the average electric field in the silicon block are obtained by using the three-dimensional finite-difference time-domain （FDTD） method. By adjusting several factors （such as the length, width, height and specific resistance of the silicon block） a novel sensor with optimal structural parameters that can be used as a power measurement device for high power terahertz pulse directly is proposed. The results show that the sensor has a relative sensitivity of about 0.24 kW 1, with a fluctuation of relative sensitivity of no more than ±22%, and the maximum of VSWR is 2.74 for 0.3-0.4 THz band.     

Efficiency of terahertz detection in electro-optic polymer sensors with interdigitated coplanar electrodes

We discuss the efficiency of an electro-optic （EO） polymer sensor with interdigitated coplanar electrodes. The developed EO sensor is used to detect terahertz radiation via EO sampling. Results show that the sensor improves more significantly detection sensitivity than does a sensor with sandwich configurations.     

Terahertz real-time imaging with balanced electro-optic detection

We propose a method for two-dimensional terahertz (THz) real-time imaging which systemically integrates the THz balanced electro-optic sampling technique and the dynamic subtraction technique. The proposed method can effectively improve the signal to noise ratio and the spectrum measurement accuracy of the imaging system. Experiment results demonstrate the advantage of the method.     

Single-shot dual-energy x-ray imaging with a flat-panel sandwich detector for preclinical imaging

We describe a multi-layer (“sandwich”) configuration detector consisting of two x-ray imaging flat-panel detectors for single-shot (single-kV) dual-energy imaging. An intermediate copper filter is used to increase spectral separation between the two detectors to improve contrast at the expense of image noise. Monte Carlo and cascaded-systems analyses of the signal and noise performance are described that quantify performance characteristics. Image quality of dual-energy images obtained from a prototype sandwich-detector system is evaluated using a figure of merit (FOM), defined as the squared contrast-to-noise ratio normalized by x-ray exposure for a mouse phantom for preclinical imaging. Demonstration dual-energy bone and soft-tissues images of a postmortem mouse are obtained using the prototype system. While the FOM with the single-shot detector is lower than that achieved using a conventional dual-shot (dual-kV) method, the single-shot approach may be preferable when imaging speed or insensitivity to motion artifacts is a primary concern.     

Generation and characterization of terahertz pulse trains from biased, large-aperture photoconductors

The saturation properties of terahertz emission from biased, large-aperture photoconductors excited by trains of amplified femtosecond optical pulses are presented. A direct comparison is made of the multiple-pulse saturation properties of terahertz emission from semi-insulating GaAs and low-temperature-grown GaAs emitters with different carrier lifetimes. When the carrier lifetime is less than or comparable with the interpulse spacing, a significant enhancement of the narrow-band terahertz output is observed. The enhancement is not observed for emitters with long carrier lifetimes, consistent with the results of a previously derived saturation theory [Opt. Lett. 18, 1340 (1993)].     

TBOR-shaped electro-optic modulator with dual output based on double-layer graphene

We proposed an electro-optic modulator with two-bus one-ring (TBOR) structure to improve the extinction ratio and reduce insert loss. It has a dual output compared with one-bus one-ring structure. In addition, double-layer graphene makes it possible for the modulation in the visible to mid-infrared wavelength range. It shows that this new electro-optic modulator can present two switching states well with low insertion loss, high absorption and high extinction ratio. At $\lambda =1550\text{ nm}$, when the switching states are based on the chemical potential, ${\mu }_{\mathrm{c}}=0.38\text{ eV}$ and ${\mu }_{\mathrm{c}}=0.4\text{ eV}$, the insertion losses of both output ports are less than 2 dB, the absorption of the output port coupled via a micro-ring reaches 45 dB and the extinction ratio reaches 14 dB. When the refractive index of the dielectric material is 4.2, the applied voltage will be less than 1.2 V, thus can be used in low-voltage CMOS technology.     

Single-shot extreme ultraviolet laser imaging of nanostructures with wavelength resolution

We have demonstrated near-wavelength resolution microscopy in the extreme ultraviolet. Images of 50 nm diameter nanotubes were obtained with a single ~1 ns duration pulse from a desktop-size 46.9 nm laser. We measured the modulation transfer function of the microscope for three different numerical aperture zone plate objectives, demonstrating that 54 nm half-period structures can be resolved. The combination of near-wavelength spatial resolution and high temporal resolution opens myriad opportunities in imaging, such as the ability to directly investigate dynamics of nanoscale structures.     

Complete characterization of a spatiotemporal pulse shaper with two-dimensional Fourier transform spectral interferometry

Spatiotemporal pulse shaping is characterized with two-dimensional Fourier transform spectral interferometry. A deformable-mirror-based bidimensional pulse shaper is used to create simple spatiotemporal structures on a femtosecond pulse, structures that are directly calculated from the measured spatiospectral phases and intensities.     

Single-shot laser-induced fluorescence imaging of formaldehyde with XeF excimer excitation

Single-shot formaldehyde laser-induced fluorescence (LIF) imaging measurements in a technical scale turbulent flame have been obtained using XeF excimer laser excitation in the ?¹A₂-˜X¹A₁ transition at 353.2 nm. Measurements have been carried out in a 150 kW natural gas swirl burner where formaldehyde distribution fields have the potential, in combination with OH concentration fields, to visualize the heat release distribution and therefore give an optimal visualization of flame-front positions. The extended areas where formaldehyde was detected in the swirl flame indicates the presence of low temperature chemistry in preheated gas pockets before ignition. Received: 31 January 2000 / Revised version: 2 March 2000 / Published online: 5 April 2000     

Spectral editing of 13C NMR spectra via two-dimensional pulse techniques

Two-dimensional pulse techniques for subspectral editing in ¹³C NMR spectroscopy are described. The experiments are compared with existing one-dimensional editing methods with respect to sensitivity, information content, and practical performance. In combination with a computer program for fully automatic extraction of one-dimensional edited subspectra and radiofrequency field strength information, the two-dimensional presented experiments are useful as setup experiments in ¹³C NMR.     

Temperature dependent narrow-band terahertz pulse generation in periodically poled crystals via difference frequency generation

Femtosecond optical pulse is used to generate narrow-band terahertz pulses depending on a quasi-phase-matched condition in periodically poled lithium niobate (PPLN) and stoichiometric lithium tantalate (PPSLT) crystals by difference frequency generation. The origin of narrow-band THz generation proved that the two frequency components of the fs pulse contribute to the frequency mixing. By cryogenic cooling, the absorption of THz waves in the crystal is significantly reduced which results in efficient THz generation. Simultaneously generated forward and backward THz pulses were 1.38 and 0.65 THz with as narrow as the bandwidth of 32 GHz in the PPSLT sample. Temperature dependence of the generated THz waveforms had good agreement with the simulation result using one dimensional plane-wave propagation model.