Collection-mode near-field imaging with 0.5-THz pulses

High spatial resolution imaging with terahertz pulses is implemented with a novel collection-mode near-field probe. The spatial resolution capabilities of the system are in the range of few micrometers. We demonstrate resolution of 7 μm using 0.5-THz pulses and discuss performance of the collection-mode near-field probes and image properties     

Ultrafast imaging of terahertz electric waveforms using quantum dots

Microscopic electric fields govern the majority of elementary excitations in condensed matter and drive electronics at frequencies approaching the Terahertz(THz)regime.However,only few imaging schemes are able to resolve sub-wavelength fields in the THz range,such as scanning-probe techniques,electro-optic sampling,and ultrafast electron microscopy.Still,intrinsic constraints on sample geometry,acquisition speed and field strength limit their applicability.Here,we harness the quantum-confined Stark-effect to encode ultrafast electric near-fields into colloidal quantum dot luminescence.Our approach,termed Quantum-probe Field Microscopy(QFIM),combines far-field imaging of visible photons with phase-resolved sampling of electric waveforms.By capturing ultrafast movies,we spatio-temporally resolve a Terahertz resonance inside a bowtie antenna and unveil the propagation of a Terahertz waveguide excitation deeply in the sub-wavelength regime.The demonstrated QFIM approach is compatible with strong-field excitation and sub-micrometer resolution—introducing a direct route towards ultrafast field imaging of complex nanodevices inoperando.     

基于USB的主动太赫兹成像电路设计

简要介绍了一套针对双波段主动太赫兹无损检测系统研制的基于USB通信的电路系统.该电路系统可通过控制二维扫描机构实现太赫兹无损检测系统的二维扫描,利用双波段太赫兹探测器接收经目标反射的由双波段太赫兹源辐射的太赫兹信号,并对该信号利用过采样方式实现数据采集,通过USB实现数据的上位机传输,并最终利用频域滤波和小波变换等图像处理技术实现主动太赫兹成像.成像结果表明,电路系统符合实际需求,双波段太赫兹无损检测系统可实现分辨率3 mm的无损检测,能很好地满足泡沫等非极性材料的无损检测的应用需求.     

Fighting terrorism with terahertz

The terahertz region of the electromagnetic spectrum is emerging as a powerful weapon in the war on terrorism and illicit drug trafficking. There is still much work to be done in developing the technology, but it shows an amazing ability to single out explosives, fertilizer bombs, chemical and biological agents, and narcotics by their characteristic transmission and reflection properties in the terahertz range.     

Contrast-modified gradient echo imaging using rotary echo preparatory pulses

The use of on-resonance binomial composite pulses in two- or three-dimensional magnetization-prepared gradient-recalled echo magnetic resonance imaging experiments generates rotary echoes, leading to an increase in contrast range that is, in part, determined by the ratio ofT ₂ toT ₁. In comparison with other fast gradient-recalled echo imaging techniques designed for enhancedT ₂ contrast, this method is more robust with respect to radiofrequency field inhomogeneity and less sensitive with respect to motion artifacts. Three-dimensional parametric images may be calculated using least-squares fitting based on a simple model for steady-state longitudinal magnetization during the imaging sequences.     

Interferometric near-field imaging technique for phase andrefractive index profiling in large-area planar-waveguide optoelectronicdevices

A versatile, interferometric optical technique is described for nondestructively imaging the near-field output phase uniformity and refractive index profile in broad-area optoelectronic waveguide devices or heterostructure materials. In active traveling-wave optical power amplifier devices, measurements are presented for thermal lensing, solder bond inhomogeneities, heatsink impedance, and carrier-lensing effects due to nonuniform gain saturation by the amplifier input beam, transverse amplified spontaneous emission, or intensity filaments. The thermal performance of diamond and copper heatsinks for high-power optical amplifiers is compared. In passive devices, the technique is used to observe heteroepitaxial material compositional uniformity, defects, photoelastic stress, and intentional structural waveguide index modifications. The technique has a phase and spatial resolution as low as λ/100 and 1 μm. The corresponding refractive index and temperature resolutions (dependent on device length) are as low as Δn=10^-5 and ΔT=0.025°C for 1000-μm-long devices     

室内实测数据太赫兹合成孔径雷达成像研究

合成孔径雷达实现高分辨率成像要求平台作理想匀速直线运动,相对传统微波SAR而言,太赫兹波的波长更短,SAR平台高频微小振动对常规微波频段 SAR影响几乎可以忽略,对太赫兹合成孔径雷达（Terahertz synthetic aperture radar ,T Hz‐SAR）的影响必须精细处理,研究适用于 T Hz‐SAR的成像补偿算法是必要的。本文建立 T Hz‐SAR非理想情况下的回波模型,分别从时域和频域详细分析运动误差对回波的影响。提出了一种基于回波数据的T Hz‐SAR成像运动补偿算法,采用中心频率0．3 T Hz的SAR系统进行实验,使用RD算法对目标进行二维高分辨成像,得到3个角反射器的二维SAR图像。实验结果验证了系统的可行性和所给处理算法的有效性,为外场车载或机载的T Hz‐SAR成像奠定了基础。     

基于回旋管的星载太赫兹成像雷达设计与仿真

太赫兹技术应用和关键器件研制是当今学术界的重要研究课题。太赫兹雷达具有很高的系统带宽和成像精度,可广泛应用于雷达探测和成像领域。回旋管能在较高的频段上实现大功率发射,是目前广泛采用的一种太赫兹频率源。根据国内外大功率回旋管的研究现状,提出了一种基于回旋管的星载太赫兹成像雷达系统方案,对雷达的关键技术指标,如作用距离、发射功率和天线增益等进行了详细论证,并进行了ISAR成像仿真试验,仿真结果验证了成像系统的高分辨率特性,为太赫兹频段高分辨率成像雷达系统设计提供理论依据和技术支持。     

PRESS-based proton single-voxel spectroscopy and spectroscopic imaging with very short echo times using asymmetric RF pulses

Modified point-resolved spectroscopy (PRESS) sequences for single voxel spetroscopy (MRS) and spectroscopic imaging (SI) with very short echo time (T _E ) are described using asymmetric radio-frequency (RF) pulses as well as an optimized design and timing of the PRESS sequence. The proposed sequences were implemented on a standard 4.7 T imaging system yielding a T _E of 6.0 ms only. Simulations and experimental data measured on phantoms and the rat brain in vivo are presented for MRS and SI showing a high signal-to-noise ratio and hardly any phase distortions caused by J-coupling.     

Improvement of continuous‐wave terahertz imaging system for cellulose acetate membrane electrophoresis

We have successfully achieved terahertz imaging of cellulose acetate membrane electrophoresis of egg albumin using continuous‐wave imaging at 0.189 THz. A sample holder has been devised that can eliminate the membrane crook generated in the drying process after electrophoresis. A probe has been also fabricated, which was assembled with a Schottky barrier diode detector to detect the terahertz signal. A higher spatial resolution of 0.3 mm was achieved, which is 6.83 times the 2.05‐mm resolution without using the probe. Terahertz images of cellulose acetate membrane electrophoresis of egg albumin of 2 µl was obtained, in which the positions of protein were perfectly in accordance with the stained images. The technology can be used instead of the staining method for cellulose acetate membrane electrophoresis. © 2012 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

In-situ diagnostic of femtosecond laser probe pulses for high resolution ultrafast imaging

Ultrafast imaging is essential in physics and chemistry to investigate the femtosecond dynamics of nonuniform samples or of phenomena with strong spatial variat...     

Small field of view imaging using wavelet encoding with 2 dimensional RF pulses and gradient echo: phantom results

Object  

The objective of this work is to propose an imaging sequence based upon the wavelet encoding approach to provide MRI images free from folding artifacts, in the small field of view (FOV) regime, such as dynamic magnetic resonance imaging (MRI) studies.     

Molecular Interferometric Imaging for Biosensor Applications

Molecular interferometric imaging (MI2) is a common-path interferometric imaging technique for detecting protein binding to surfaces. The experimental metrology limit is 10 pm/pixel longitudinal resolution at 0.4-mum diffraction-limited lateral resolution, corresponding to 1.7 attogram of protein, which is only 8 antibody molecules per pixel, near to single-molecule detection. The scaling mass sensitivity at the metrology limit is 5 fg/mm. We demonstrate a protein microarray application in a 128-multiplex immunoassay. Assay applications include prostate specific antigen (PSA) at a detection limit of 60 pg/mL and the cytokine interleukin-5 (IL-5) at a detection limit of 50 pg/mL. Realtime binding assays using MI2 enable the study of reaction kinetics of antibodies exposed to antigen, and the binding of antibody Fc regions to protein G.     

Quasi-automatic phase-control technique for chirp compensation of pulses with over-one-octave bandwidth-generation of few- to mono-cycle optical pulses

This paper introduces our self-recognition type of the computer-controlled spectral phase compensator (SRCSC), which consists of a greatly accurate phase manipulator with a spatial light modulator (SLM), a highly sensitive phase characterizer using a modified spectral phase interferometry for direct electric field reconstruction (M-SPIDER), and a computer for phase analysis and SLM control operating in the immediate feedback (FB) mode. The application of the SRCSC to adaptive compensation of various kinds of complicated spectral phases such as nonlinear chirped pulses with a weak intensity, induced-phase modulated pulses, photonic-crystal-fiber (PCF) output pulses, and nonlinear chirped pulses exceeding a 500-rad phase variation over-one-octave bandwidth demonstrated that the SRCSC is significantly useful for compensation of arbitrary nonlinear chirp and hence enables us to generate quasi-monocycle transform-limited (TL) pulses with a 2.8-fs duration. To the best of our knowledge, this 1.5-cycle pulse is the shortest single pulse with a clean temporal profile in the visible to near-infrared region.     

Generating millimeter and terahertz waves

Research on millimeter waves and/or terahertz waves, which cover the frequency range from 30 GHz to 10 THz, has lately increased since the nature of these electromagnetic waves is well suited to spectroscopic sensing [1]-[3] as well as ultrabroadband wireless communications [4]-[6]. One of the obstacles to developing applications of millimeter waves and terahertz waves is a lack of solid-sate signal sources, rather than detectors [7]. In fact, the frequency band in this region is often referred to as the terahertz gap [8].     

低噪声太赫兹Microbolometer读出电路

常温Nb5 N6 Microbolometer器件对THz(terahertz)信号有很好的响应而且噪声很低.为了实现探测器芯片的实际应用,就要求配备低噪声的读出电路.以LINEAR公司的超低噪声、精准型高速运放LT1028为主放大器,设计了包括阻抗变换、放大、低通滤波和工频陷波在内的读出电路.经过仿真分析和实际调试,实现了在220 Hz～3.6 kHz的频带上对200 nV至mV量级信号的放大读出,增益稳定在940左右,噪声可低至10 nV/Hz1/2量级,达到了Nb5N6 Mierobolometer的噪声性能,为阵列芯片的应用提供了技术基础.     

Exploring new instrumentation parameters for electrochemotherapy.Attacking tumors with bursts of biphasic pulses instead of single pulses

Electrochemotherapy using rectangular biphasic stimuli and bursts of stimuli, as compared to using monophasic pulses obtained by capacitor discharge, are at least of the same efficacy. The use of small surface electrodes in dermatological treatment yields electrode-skin impedances in the range of 200-400 ohms, with the impedances being almost entirely resistive. Therefore, generator output resistances can be in the range of 10 to 20 ohms. The use of a single burst of 50+50 μsec biphasic pulses, with an overall duration of about 7 msec, is well tolerated by the patients, causing just one short stimulus sensation instead of a succession of eight stimuli. The treatment results with the above-described instrumentation are comparable to those reported in the literature. Further studies are necessary in order to better understand the process of electropermeabilization in vivo, so as to determine optimal pulse duration, frequency, and waveform, and to eventually lower field intensity     

Nonlinear terahertz properties of n-type quantum-wellheterostructures

In this letter, we study the effect of many body interactions on the collective response of confined electrons in doped quantum-well (QW) heterostructures to intense far-infrared radiation. Absorption lineshapes are computed both by numerically integrating the equations of motion and by using the appropriately time-averaged equations. For a two-subband double-QW system optical bistability is observed and its parameter range is given. For an asymmetric triple-QW structure driven at ω≈E₂-E₀ Hopf bifurcations occur which generate a strong response at a frequency incommensurate with the drive frequency or any natural frequency of the system