Engineering the spectral and temporal properties of a GHz-bandwidth heralded single-photon source interfaced with an on-demand,broadband quantum memory

Photonics offers a route to fast and distributed quantum computing in ambient conditions, provided that photon sources and logic gates can be operated deterministically. Quantum memories, capable of storing and re-emitting photons on demand, enable quasi-deterministic operations by synchronizing stochastic events. Interfaced source–memory systems are thus a key building block in photonics-based quantum information processors. We discuss the design of the single-photon source in this type of light–matter interface and present an experimental system based on a Raman-type quantum memory. In addition to the spectral purity of the produced heralded single photons, we find that their temporal distinguishability also becomes important due to the implicit temporal binning derived from photon storage in the memory. When aiming to operate the source–memory system at high repetition rates, a practical compromise between both of these requirements needs to be found. Our implemented photon source system demonstrates such a solution and enables passive stability, high brightness in a single-pass configuration, high purity as well as good mode matching to our Raman memory.     

Measurement of radio-frequency temporal phase modulation using spectral interferometry

We present an optical method to measure radio-frequency electro-optic phase modulation profiles by employing spectrum-to-time mapping realized by highly chirped optical pulses. We directly characterize temporal phase modulation profiles of up to 12.5 GHz bandwidth, with temporal resolution comparable to high-end electronic oscilloscopes. The presented optical set-up is a valuable tool for direct characterization of complex temporal electro-optic phase modulation profiles, which is indispensable for practical realization of deterministic spectral-temporal reshaping of quantum light pulses     

Parametric down-conversion devices: The coverage of the mid-infrared spectral range by solid-state laser sources

The development of parametric down-conversion devices operating in the mid-infrared, from 3 μm to about 20 μm, based on non-oxide nonlinear optical crystals is reviewed. Such devices, pumped by solid-state laser systems operating in the near-infrared, fill in this spectral gap where no solid-state laser technology exists, on practically all time scales, from continuous-wave to femtosecond regime. The vital element in any frequency-conversion process is the nonlinear optical crystal and this represents one of the major limitations with respect to achieving high energies and average powers in the mid-infrared although the broad spectral tunability seems not to be a problem. Hence, an overview of the available mid-infrared nonlinear optical materials, emphasizing new developments like wide band-gap, engineered (mixed), and quasi-phase-matched crystals, is also included.     

多通道联合的广义总变分能谱CT重建

基于光子计数探测器的能谱CT在材料分解、组织表征、病变检测等应用中具有巨大的潜力。在重建过程中,通道数的增加会造成单通道中光子数减少,从而导致重建图像质量下降,难以满足实际需求。本文从能谱CT重建的角度出发,将广义总变分向矢量延伸,利用奇异值的稀疏性,促进图像梯度的线性依赖,提出一种基于核范数的多通道联合广义总变分的能谱CT重建算法。在图像重建过程中,多层共享结构信息,同时保留独特的差异。实验结果表明,本文提出的算法在抑制噪声的同时,能够更有效地恢复图像细节及边缘信息。     

有机功能色素类化合物的合成与三阶非线性光学性能

设计并合成了苯并二呋喃酮、双偶氮-9,10-蒽二酮、苯并噻蒽以及偶氮-1,3,4-噻二唑4个系列的有机色素类化合物,并用UV,IR,1H NMR和元素分析表征了结构.采用飞秒激光,运用简并四波混频法,研究了化合物的三阶非线性光学性能.它们的三阶非线性光学极化率χ(3)为2.64～3.95×10-13esu,非线性折射n2为4.85～7.27×10-12esu,分子二阶超极化率γ为2.85～3.72×10-31esu.探索了化合物的分子结构对三阶非线性光学性能的影响.长的共轭链、吸供取代基和杂环的引入、取代基强的供电子或吸电子能力等因素有利于获得较大的三阶非线性光学性能.     

Co-existing of dressed non-linear gain and electromagnetically induced absorption

The dressed parametric four-wave mixing (FWM) process has been investigated in hot atomic rubidium vapor. We use a strong pumping field to generate entangled photon pairs of spontaneous parametric FWM (SP-FWM) which can be enhanced by an external dressing effect. Seeding probe beam into the Stokes or anti-Stokes (SP-FWM) channel will form the parametric amplified FWM (PA-FWM) process, then the non-linear gain and electromagnetically induced absorption (EIA) are observed, caused by the internal dressing effect. However, with scanning of pumping field the absorbing background will vanish, which will result in drastic increase in PA-FWM signal gain.     

Tests of a Two-Photon Technique for Measuring Polarization Mode Dispersion With Subfemtosecond Precision

An investigation is made of a recently introduced quantum interferometric method capable of measuring polarization mode dispersion (PMD) on sub-femtosecond scales, without the usual interferometric stability problems associated with such small time scales. The technique makes use of the extreme temporal correlation of orthogonally polarized pairs of photons produced via type-II phase-matched spontaneous parametric down-conversion. When sent into a simple polarization interferometer these photon pairs produce a sharp interference feature seen in the coincidence rate. The PMD of a given sample is determined from the shift of that interference feature as the sample is inserted into the system. The stability and resolution of this technique is shown to be below 0.2 fs. We explore how this precision is improved by reducing the length of the down-conversion crystal and increasing the spectral band pass of the system.     

The effect of central obstruction on singularities and spectral anomalies of polychromatic,spatially coherent light diffracted by an annular aperture

The singularity changes and spectral behavior of polychromatic spatially fully coherent light diffracted by an annular aperture are studied. Detailed numerical calculation results are presented and illustrated. It is shown that the spectral anomalies take place in the vicinity of singularities in the diffraction field, namely the spectrum is red shifted at some points, blue shifted at others, and is split into two lines with equal height elsewhere, and this kind of spectral change depends on the central obstruction ratio ε and axial distance z. It is found that, with the increasing of the central obstruction ratio ε, axial singularities will shift to the aperture and at the off-axial positions new singularities will appear and split. With the further increasing of ε, the near field finally changes to the far field.     

The theoretical investigation of the impact of two-photon absorption and free-carrier absorption on phase-sensitive amplification in silicon waveguides

We investigate the influence of two-photon absorption (TPA) and free-carrier absorption (FCA) on phase-sensitive amplification (PSA) process in a dispersion engineered silicon strip waveguide at the telecom wavelengths. The phase-dependent gain and phase-to-phase transfer functions as well as the phase regeneration capacity of PSA are numerically analysed. It is found that TPA and FCA will suppress the phase-sensitive extinction ratio (PER) and the efficiency of phase squeezing. Moreover, the phase regeneration capability of silicon waveguide-based PSA is investigated by comparing the regenerated signal waveforms at a different signal power with/without TPA and FCA. Our results have potential application in all-optical signal regeneration.     

Studies on the effect of duty cycle variation in Fibonacci-based quasi-phase matching devices

We have studied the effect of duty cycle variation in the Fibonacci optical superlattice (FOS) structure by analysing the second harmonic generation (SHG) spectrum. Unlike periodic quasi-phase matching (QPM) device, the variation in duty cycle changes the SHG spectrum asymmetrically. The higher intensity and other multiple peaks in the spectrum change asymmetrically when the duty ratio varies away from 0.5. As a result, the SHG spectrum of lower and higher duty ratios from the ideal FOS device generates multiple dissimilar peaks as it was observed from entirely different QPM devices. However, the main phase matching peak shows symmetric nature, irrespective of the duty cycle variation like the periodic one. Further, we have analysed the correlation spectrum to prove the asymmetric nature of the FOS SHG spectrum. Although FOS device exhibits asymmetric spectral nature in duty cycle variation, it shows the same optical interaction regardless of the pump direction.     

Growth and characterization of some novel crystals for nonlinear optical applications

The advent of high intensity lasers coupled with the recent advances in crystal technology has led to rapid progress in the field of nonlinear optics. This article traces the history of materials development that has taken place over the past forty odd years and dwells on the current status in this important area. The materials aspect is discussed under three classes viz. inorganic, organic and semiorganic crystals. In the end, some of the crystal growth work that has been carried out in author’s laboratory is presented.     

基于渐进损伤模型的复合材料斜接接头的拉伸强度

本文首先以斜接角度、搭接长度以及搭接厚度为参数分别建立了复合材料单边斜接接头和双边斜接接头在拉伸载荷下的参数化有限元模型,采用abaqus软件中UMAT子程序功能编写了材料损伤本构模型,并利用cohesive单元模拟胶接界面损伤。通过渐进损伤分析,模拟出斜接接头的三种破坏模式,最终得到了各参数变化对斜接接头拉伸强度的影响,其中母板最终破坏模式对应最大拉伸强度,并对单边斜接接头和双边斜接接头结果进行了对比分析,得出双边斜接接头强度大于单边斜接接头强度的结论。     

Towards variance-matrix characterization of complementarity relations in a continuous variable system

We discuss complementarity relations in a bipartite continuous variable system. Building up from the work done on discrete d-dimensional systems, we prove that for symmetric two-mode states, quantum complementarity relations can be put in a simple relation with the elements of the variance matrix. When this condition is not satisfied, such a connection becomes non-trivial. Our investigation is the first step towards an operative characterization of the complementarity in a scenario that has not been investigated so far.     

光谱响应测试在多结型太阳能电池中的应用

针对在高效率多结型硅薄膜元件结构设计中无法测出各层短路电流的困难,指出光谱响应／量子效率光谱测试在这方面的应用。利用该项技术,使电池各层在短路电流密度上达到最佳匹配,从而改进制程,提高电池的效率。     

Synthesis,structure, vibrational spectral,nonlinear optical and electron-behavioral studies of N-(5-chloro-2-hydroxyphenyl)-(3-hydroxyphenyl)-methalimine

In recent years, the imine-bridged organics has been one of the hot spots in the field of photo-responsive materials. Herein, N-(5-chloro-2-hydroxyphenyl)-(3-hydroxyphenyl)-methalimine (CHPHPMI) was synthesized by a condensation reaction of 3-hydroxybenzaldehyde and 2-amino-4-chlorophen with formic acid as catalyst in ethanol solvent, and characterized by the single crystal X-ray diffraction and FT-IR measurement techniques as well as theoretical calculations. The studied molecule adopts trans configuration about the central CN bond, in which a five-numbered ring is formed by an intramolecular hydrogen bonding OH⋯N. The adjacent molecules are linked by three kinds of intermolecular hydrogen bonds to form a one-dimensional columnar supermolecular structure along b axis in the monoclinic crystal system with space group P2₁. The spectral bands were assigned to the molecular structure in details with the aid of the theoretical calculation. Furthermore, the physical parameters of thermodynamics, NLO, atomic charges, natural bond orbitals and frontier molecular orbitals were presented by the theoretical analysis for the studied molecule. The first-order hyperpolarizability of CHPHPMI is 30 times that of urea. The studied molecule presents the molecular battery characteristics connected by six directional batteries in a parallel–series mode to transport electrons in a long range. The HOMO and LUMO orbitals with an energy gap 3.632 eV are responsible for the nonlinear optical and electron-transfer properties. The electron-behavioral characteristics of the studied molecule indicate the studied compound will be an excellent molecular semiconductor candidate of photovoltaic materials.     

Spectroscopic and nonlinear photophysical characterization of organic octupolar-compounds supported by anodic-alumina nanotube-arrays

Amorphous anodic alumina membranes (AAM) comprising highly ordered nanometric porous arrays (porous anodic aluminas: PAA) with 1D-nanotube dimensions of ∼75 nm in diameter and 45 microns in depth were successfully prepared and used as nanostructured host networks for different functionalized octupolar chromophores (named here Oct-(n)). Atomic force microscopy (AFM) studies performed on the developed hybrid systems confirmed a homogeneous insertion of these organic molecules into the PAA nanotube-arrays. Samples with high structural quality were selected for several photophysical characterizations: Comprehensive X-ray diffraction (XRD) and optical spectroscopic characterizations performed according to UV-vis absorption, photoluminescent (PL) and Raman measurements revealed the structural and optical performance of these molecules within the PAA-confinement. Since the implemented optical chromophores were specifically functionalized for nonlinear optical (NLO) applications, the obtained Oct-(n)/PAA-based amorphous hybrids were also characterized according to cubic NLO-techniques such as third harmonic generation (THG) and the Z-Scan method. PAA-confined octupolar chromophores have shown interesting linear and NLO optical properties which have not yet been intensively investigated in bulk hybrid systems; hence, the obtained hybrid nanostructures represent a promising field of investigation in the route to functional octupolar-based materials, where different self-assembled molecular structures may be formed, giving rise to enhanced linear and NLO-properties.     

Synthesis and characterization of binary chromophore polymers for electro-optic application

We prepared the side-chain electro-optic polymers by the active bonding of cyanate- and hydroxy-functional groups, and employed these side-chain polymers as hosts for the bi-chromophore system to enhance the electro-optic coefficient (r₃₃). In singular chromophore polymer (in PMMA host), the dependence of EO response on loading concentration was examined, r₃₃ about 26 pm/V was obtained. In order to further incorporate the chromophore, the loading behavior in binary chromphore systems was investigated. The optimal r₃₃ more than 40 pm/V was achieved, which was almost the sum of two individual polymers. Furthermore, the temporal stability at room temperature and 85 °C, respectively, over 500 h was evaluated for the potential application on electro-optic devices.     

Synthesis,z-scan and degenerate four wave mixing characterization of certain functionalized photosensitive polyesters containing ortho-hydroxyazo chromophores

The preparation and NLO characterization of photosensitive polyesters containing azoaromatic residues in the molecular backbone, functionalized with orthohydroxy chromophores is presented. Samples were studied for its UV–vis absorption, FT-IR and intensity dependent nonlinear absorption properties. Nonlinear characterization was carried out with z-scan using frequency doubled, Q-switched Nd:YAG laser operating at 532 nm. The closed aperture z-scan spectra reveal the self defocusing effects of the samples with negative nonlinearity coefficient (n₂) showing values as high as −1.28 × 10⁻¹⁰ (esu) for certain samples and the corresponding third order susceptibility coefficient of the order of 29.9 × 10⁻¹² (esu). Degenerate four wave mixing technique was employed to substantiate the findings. The numerical fits show that the molecules exhibit reverse saturable absorption. A study of beam fluence dependence of nonlinear absorption coefficient (β_eff) has been presented. All phenomena indicate that molecules are reverse saturable absorbers whose optical limiting property gets enhanced with increasing conjugation length.     

Analysis of multi-degree of freedom strongly non-linear mechanical systems with random input: Part II: equivalent linear system with random matrices and power spectral density matrix

We present a method for estimating the (power spectral density) PSD matrix of the stationary response of lightly damped randomly excited multi-degree of fredom mechanical systems with strong non-linear asymmetrical restoring forces. The PSD matrix is defined as the mean value of the PSD matrix response of an equivalent linear system (ELS) whose damping and stiffness matrices depend on non-linear vibration modes of the associated conservative system, the frequencies and modes shapes being amplitude dependent. Based on a generalized van der Pol transformation and using a stochastic averaging principle, as developed in a companion paper, a stationary probability density function for the amplitude process is derived to characterize the ELS fully. Some possible simplifications of the method, such as modal reduction and/or local linearization, are also discussed. The results obtained are in good agreement with those of direct numerical simulations taking two typical examples.