Terahertz optics of semiconductor nanostructures near and far from equilibrium

In semiconductor nanostructures a few tens of nanometers wide, the separation between quantized states, or subbands, can be a few terahertz. In the past year, a consistent experimental picture of intersubband relaxation in wide, doped quantum wells has emerged. Terahertz radiation has also been found to dramatically alter the emission and absorption of semiconductor nanostructures near their band-edges.     

A study of near and far field effects in photoelastic stress intensity determination

A technique known as the Taylor Series Correction Method (TSCM) for extracting the stress intensity factor from photoelastic data is reviewed. The need for ‘artificial’ flaws is identified and an approach due to Savin is used to near field effects of various practical flaw shapes upon the apparent stress intensity factor. Using the Sneddon-Srivastav solution for a line crack in a finite width plate, the constriction of the singular zone is demonstrated as the crack tip approaches the free edge. Results indicate that care must be taken in applying TSCM to obtain photoelastic data at appropriate distances from the crack tip.     

Terahertz polariton propagation in patterned materials

Generation and control of pulsed terahertz-frequency radiation have received extensive attention, with applications in terahertz spectroscopy, imaging and ultrahigh-bandwidth electro-optic signal processing. Terahertz 'polaritonics', in which terahertz lattice waves called phonon-polaritons are generated, manipulated and visualized with femtosecond optical pulses, offers prospects for an integrated solid-state platform for terahertz signal generation and guidance. Here, we extend terahertz polaritonics methods to patterned structures. We demonstrate femtosecond laser fabrication of polaritonic waveguide structures in lithium tantalate and lithium niobate crystals, and illustrate polariton focusing into, and propagation within, the fabricated waveguide structures. We also demonstrate a 90 degrees turn within a structure consisting of two waveguides and a reflecting face, as well as a structure consisting of splitting and recombining elements that can be used as a terahertz Mach-Zehnder interferometer. The structures permit integrated terahertz signal generation, propagation through waveguide-based devices, and readout within a single solid-state platform.     

Short gamma-ray bursts near and far

Progress in understanding the nature of short gamma-ray bursts (GRBs) has been rapid since the discovery of the first afterglows in mid-2005. The emerging picture appears to be of short GRBs, which originate at moderate redshift (a few tenths) and appear in galaxies of all ages. This discovery has been used to argue for their origin in compact binary mergers. However, this population does not describe all short bursts. Here, I will present results of observations of several short GRBs, which challenge the conclusions drawn from the early observations. The observations show that some short GRBs originate in the very low redshift Universe (below 100Mpc), while some may also lie at redshifts comparable with the long GRBs (i.e. z>2). I will discuss the properties of these bursts and the implications they have for the progenitors of short GRBs.     

Terahertz wave propagation in one-dimensional periodic dielectrics

We demonstrate the temporal evolution of terahertz (THz) wave propagation in one-dimensional periodic dielectrics. Distributed Bragg reflectors and a resonant cavity are investigated: The structures involve air gaps interleaved between polymer films. Transmitted and reflected broadband THz waves are measured by means of THz time-domain spectroscopy. The experimental results agree well with transfer matrix calculations.     

深孔台阶爆破近远区振动特征的试验研究

为了探索深孔台阶爆破在近区和远区的振动特征,以现场试验为基础,利用回归分析、随机分析及小波包分解技术,从爆破振动信号的衰减规律、三分量特征、随机特征及能量分布等方面对二者进行了对比研究,结果显示:振速三分量在近区的K、α值均大于远区,远区各分量的α值更为接近。振速三分量在远区的相关系数大于近区,且更接近1,三分量中垂向分量的相关系数最大。近区当比例距离小于5时,径向振速最大,比例距离大于等于5时,垂向振速最大。远区的垂向振速最大。无论在近区还是远区,切向振速一般最小。振速三分量在近区的变异系数大于远区,径向振速的变异系数在近区和远区较为接近。最大段药量或爆心距增加时,近区及远区的能量均向低频带流动,近区的移动速度更快,同时近区及远区能量分布的频带宽度也将趋于集中,近区的频带宽度更宽。     

等离子体中太赫兹波传输及成像探测特性研究

本文根据散射矩阵方法模拟等离子体并建立了非均匀等离子体理论模型,并在此基础上计算了0.1 THz^10 THz频段的全波段太赫兹波在其中的传输特性。根据介质阻挡放电原理在实验室环境下搭建等离子体射流产生装置并产生非均匀等离子体,进行了太赫兹时域光谱(THz-TDS)以及宽带太赫兹源在等离子体中的透射光谱测量以及太赫兹波对等离子体遮挡下目标物的反射成像的试验。理论和实验结果均表明,较高频太赫兹波在等离子体中有良好的穿透性,这为太赫兹波在黑障区的通信以及雷达探测应用打下研究基础。     

Broadly tunable femtosecond pulse generation in the near and mid-infrared

High-repetition-rate (80-MHz) femtosecond infrared pulses are generated by difference frequency mixing (DFM) a femtosecond Ti:sapphire laser with a phase-locked synchronized cw mode-locked Nd:YAG picosecond laser. This DFM scheme is of particular interest for generating ultrashort near-IR pulses (~10 fs) because group velocity mismatch with a pump pulse can be ignored. The simplicity and the broad wavelength tunability (from the near IR to the mid-IR) of this scheme is demonstrated. Short (125-fs FWHM) optical pulses in the near IR around 1.5 mum are obtained with noncritical type-I phase-matched LiB(3) O(5). We also used a similar scheme to generate mid-infrared pulses at 3.0 mum with type-II phase-matched KTiOPO(4).     

Femtosecond laser pulse propagation in a uniaxial crystal

Abstract

The wave equation describing the vector propagation of a femtosecond laser pulse of a few optical cycles in a uniaxial crystal is solved numerically by the method of unidirectional waves. Propagation of the pulse in the direction normal to the optical axis is studied, taking into account both second- and third-order nonlinearities of the crystal. Conversion efficiency as a function of crystal length, pump intensity and pulse duration is studied. As an example, the propagation of femtosecond laser pulse of τ = 10 fs duration at λ = 810 nm in a LiNbO₃ crystal 12 μm thick is studied numerically.     

Heat pulse propagation in liquid4He revisited

We have studied the propagation of heat pulses in superfluid helium at 0.1 K. Our experiments agree with a simple picture of a propagating cloud of interacting excitations, rather than with the surface rotons invoked by the Exeter group. The “Q-peak” observed in double pulse experiments is interpreted as a shock wave propagating in the cloud. We have observed for the first time that theQ-peak splits up in two distinct modes at intermediate helium pressures. We believe this is evidence fo a decoupling of the phonon and roton gasses as predicted by Castaing.     

Equilibrium near and far electromagnetic field structures at a flat boundary of the half-space filled with a homogeneous dielectric (magnetic) medium

The most general expression for the density of electromagnetic field modes at a flat interface between a semi-infinite homogeneous dielectric (magnetic) medium and vacuum is obtained. The medium is characterized by the frequency-dependent dielectric permittivity and magnetic permeability.     

Ultrashort pulse propagation in uniform and nonuniform waveguide long-period gratings

We conduct a detailed theoretical analysis of ultrashort pulse propagation through waveguide long-period grating (LPG) structures operating in the linear regime. We first consider the case of uniform LPGs, and we also investigate the effect of the typical grating nonuniformities, e.g., grating profile apodization, grating period chirping, and discrete phase shifts, on the spectral and temporal behavior of LPG structures. The two interacting modes are analyzed separately, and advanced representation tools, namely, space-wavelength and space-time diagrams (where space refers to the longitudinal grating dimension), are used to provide a deeper insight into the physics that determines the pulse evolution dynamics through the grating structures under analysis. In addition to its intrinsic physical interest, our study reveals the strong potential of LPG-based devices for optical pulse reshaping operations in the subpicosecond regime.     

The effect of spatiotemporal fluctuations on the pulse propagation

We have studied the influence of uncorrelated Gaussian fluctuations on the propagation of pulses in an excitable medium modeled by the FitzHugh-Nagumo system under an external point action. Depending on the properties of this medium, one of the two possible scenarios is realized in the noise-induced suppression of propagating pulses. The first scenario can be classified as the noise-induced incoherence (breakage of the links) between adjacent elements of the medium. The second scenario is related to a spontaneous generation of pulses in the medium under the action of fluctuations. These models are applied to analysis of some recent biological experiments devoted to the dynamics of calcium waves.     

Wavelet operators for nonlinear optical pulse propagation

A method that uses discrete wavelet transforms for the solution of evolution equations that describe optical pulse propagation in nonlinear media is presented. The theory of orthogonal wavelet transforms is outlined and applied to the representation of optical pulses. Wavelet transform representations of propagation operators are presented and applied to the nonlinear Schr?dinger equation, yielding results that are indistinguishable from traditional Fourier-based simulations. The compression properties of wavelet representations of optical pulses permit significant improvement in execution speed compared with that of the split-step Fourier method.     

Terahertz pulse generation using one-dimensional photonic crystals via optical rectification effect

Generation in one-dimensional photonic crystals of a single-cycle terahertz (THz) pulse via the optical rectification effect was studied by using the Green's function method. The bandwidth and the carrier frequency of the generated THz pulse are, respectively, in the range 2–8?THz and 2–18?THz. By decreasing the duration of the input pulses these ranges can be enlarged, but pulse shape is also deformed. The results show that by setting the carrier frequency of the input pulse on the edges of the gap, the amplitude of the generated THz pulse is maximized and its bandwidth is narrowed, but for carrier frequencies located inside the gap the situation is reversed. Moreover, by adjusting the carrier frequency of the input pulse on upper gaps few-cycles THz pulses are generated.     

Identifying the subwavelength-aperture-width variation in the far field with tunable asymmetrically-embedded-aperture interferometer configuration

A tunable asymmetrically-embedded-aperture interferometer configuration is proposed to enhance the measurement sensitivity of subwavelength variation. With this configuration, an aperture of reference was posited asymmetrically, relative to the test aperture, that exhibited subwavelength variation. By a shift in the relative position of the reference aperture, the detection sensitivity of measuring the subwavelength variation in the far field can be enhanced to a desired value at any specific detection width. We discuss the underlying mechanism of optimization and address the difference between the embedded-aperture interferometry and tunable asymmetrically-embedded-aperture interferometry with respect to tolerance.     

Evanescent plane waves and the far field: Resolution of a controversy

Abstract

We show that the evanescent part of the plane wave representation of the free-space dyadic Green function contributes to the far field only along a distinguished axis. The claim of contribution of evanescence in all directions is incorrect as it arises from a flawed procedure, but a limited version of that claim may have merit.     

Optimizing the near field around silver tips

Owing to their promise of obtaining optical as well as topographic information in nanometer scale, apertureless near-field scanning optical microscopy (NSOM) and apertureless near-field scanning optical spectroscopy have drawn much attention recently. However, NSOM is still not a mature technique. A proper understanding of and the ability to tune the near field around the tip end is critically important in NSOM instrumentation and in NSOM image interpretation. On the basis of reflection geometry, we systematically studied the effects of a number of parameters pertinent in the application of apertureless NSOM, e.g., polarization, incident angle, wavelength of the incident laser, tip material, and tip length, by using the generalized field propagator technique. Our results show that all the above parameters have a significant influence on near-field enhancement and that care must be taken in the design of the experiment in order to maximize the near field. In addition to apertureless NSOM and spectroscopy, apertureless near-field lithography can benefit from these simulation results.