Generation of squeezed light upon frequency self-doubling in active nonlinear crystals with regular domain structure

A quantum theory of self-convers of the lasing frequency in active nonlinear crystals placed in a cavity is developed. The generation of nonclassical (quadrature squeezed) light upon self-doubling of the laser radiation frequency is studied in detail, with no limitations on the relations between the relaxation times of the inverse population and the polarization of the medium and the lifetime of a photon in the cavity. The fluctuation spectra of the quadrature components of the fields of the laser radiation and its second harmonic are studied in dependence on the pumping parameters and the parameters of the cavity. Numerical calculations are performed for a Nd:Mg:LiNbO₃ crystal with a regular domain structure.     

Degenerate parametric frequency conversion of phase-modulated femtosecond laser pulses in crystals with a regular and chirped domain structure

A systematic theoretical analysis of the degenerate parametric frequency conversion in a LiNbO₃ crystal with a regular domain structure and a linearly varying domain thickness (chirped crystal) is presented for the pulses of a titanium-sapphire laser with a wavelength of 0.8 μm and durations of 100 and 50 fs in the presence and in the absence of phase modulation. The results are obtained with regard to the difference in the group velocities of interacting pulses and the group velocity dispersion. For an effective frequency conversion of the phase-modulated (PM) pump pulse, it is expedient to employ chirped crystals in which the domain thickness decreases from the entrance to the exit of the crystal. The pump energy is effectively converted into subharmonic energy when the pump carrier frequency decreases with time. It is demonstrated that the efficiency of the energy conversion to a subharmonic of 80% can be realized for PM pumping with a pulse duration of 100 fs in the chirped LiNbO₃ crystal. The efficiency of the parametric frequency conversion depends on the pump intensity as well as on the phase modulation of the pulse and the chirp of the crystal. Note that a variation in one of these parameters causes variations in the remaining parameters needed for the maximum efficiency of the parametric frequency conversion.     

Statistical properties of four-mode quantum states in coupled optical parametric down-conversions

We study the quantum properties of four coupled parametric down-conversion processes which can be realized in an aperiodical nonlinear photonic crystal. In the interaction under consideration, each mode is created by two parametric processes. The photon statistics of particular modes and correlation functions for photon numbers and their fluctuations are studied in detail. It is shown that photon number correlations are nonclassical at small interaction lengths. We calculate the covariance matrix for continuous quantum variables of the generated modes and make use of the criterion of separability based on the partial scaling transform to reveal the entanglement between the variables. It is established that cluster entangled states are created in these coupled parametric processes.     

Entanglement in cascaded-crystal parametric down-conversion

We use spontaneous parametric down-conversion in a cascade of crystals, driven by a single monochromatic cw pump laser, to study the interference of entangled photon pairs. By changing the distance between the crystals, the observed quantum interference pattern varies continuously from that associated with a longer single crystal to that associated with independent emissions from two distinct crystals. Postselection via spectral filtering suppresses this phenomenon. These findings are expected to advance the field of quantum-state engineering.     

Classical and quantum aspects of multimode parametric interactions

Parametric interactions in nonlinear crystals represent a powerful tool in the optical manipulation of information, both in the classical and the quantum regime. Here, we analyze in detail classical and quantum aspects of three-and five-mode parametric interactions in x⁽²⁾ nonlinear crystals. The equations of motion are explicitly derived and then solved within the parametric approximation. We describe several applications, including holography, all-optical gates, generation of entanglement, and telecloning. Experimental results on the photon distributions and correlations of the generated beams are also reported and discussed.     

Frequency doubling of phase-modulated femtosecond laser pulses in periodically poled and chirped nonlinear crystals

The second harmonic generation (SHG) at a wavelength of 0.8 μm by 50-and 10-fs pulses with and without phase modulation (PM) was systematically studied in LiNbO₃ crystals with regular domain structure and linearly varied domain thickness. The main results were obtained by numerical method, taking into account the difference between group velocities of interacting pulses and the group-velocity dispersion. In the approximation of the given field of the fundamental radiation, an analytical expression was derived for the spectral density of the second harmonic in the periodically poled nonlinear crystal (PPNC) under nonstationary excitation conditions. It was numerically found that the conversion efficiency of about 90% can be achieved by doubling the frequency of 50-fs laser pulses without PM in the LiNbO₃ PPNC. The maximum conversion efficiency for the SHG by PM pulses is achieved at a certain optimum chirp step in the crystal domain length, which depends on both the value and sign of frequency modulation.     

Generation of arbitrary frequency-entangled states of two-photon light

We consider a new method for the generation of polarization-frequency entangled states of photon pairs. We use a frequency-nondegenerate regime of spontaneous parametric down conversion where the photon pairs (biphotons) are produced with identical polarizations, propagate mostly in the same direction, but differ in frequency. Entanglement is achieved by a coherent superposition of pairs emitted from two nonlinear crystals, with the polarization of the biphotons from the first crystal being changed by a transformer placed between the two crystals. We show that this scheme allows the degree of entanglement to be controlled by the choice of biphoton frequencies.     

Quantum vacuum radiation spectra from a semiconductor microcavity with a time-modulated vacuum Rabi frequency

We develop a general theory of the quantum vacuum radiation generated by an arbitrary time modulation of the vacuum Rabi frequency of an intersubband transition in a doped quantum well system embedded in a planar microcavity. Both nonradiative and radiative losses are included within an input-output quantum Langevin framework. The intensity and the spectral signatures of the extra-cavity emission are characterized versus the modulation properties. For realistic parameters, the photon pair emission is predicted to largely exceed the blackbody radiation in the mid and far infrared. For strong and resonant modulation a parametric oscillation regime is achievable.     

Interaction of light waves in active nonlinear and periodically poled nonlinear crystals

The results of classical and quantum studies of the laser-radiation self-frequency conversion processes in periodically poled active nonlinear crystals are overviewed. The theoretical and experimental results of studying quasi-phase-matched self-frequency doubling and summation of the laser and pump frequencies in an active nonlinear periodically poled Nd:Mg:LiNbO₃ crystal are presented. The possibility of producing frequency-and polarization-entangled states and the sub-Poisson field statistics through a consecutive nonlinear optical frequency conversion in periodically poled nonlinear crystals is considered.     

A ring optical generator of terahertz and far-infrared radiation

Specific features of difference-frequency generation of terahertz radiation in the regime of parametric microwave generation in a nonlinear crystal are investigated. It is suggested to use a periodic stripe structure formed in the crystal to achieve phase matching. The dynamics of the wave at difference frequency, which falls into terahertz or far-IR spectral region, is determined based on solution of the set of parametric interaction equations taking into consideration quadratic and cubic nonlinearities.     

Hanbury Brown-Twiss correlations to probe the population statistics of GHz photons emitted by conductors

We present the first study of the statistics of GHz photons in quantum circuits, using Hanbury Brown and Twiss correlations. The super-Poissonian and Poissonian photon statistics of thermal and coherent sources, respectively, made of a resistor and a radio frequency generator, are measured down to the quantum regime at milli-Kelvin temperatures. As photon correlations are linked to the second and fourth moments of current fluctuations, this experiment, which is based on current cryogenic electronics, may become a standard for probing electron/photon statistics in quantum conductors.     

Dynamics of difference frequency radiation generation in the field of few-cycle laser pulse propagating in GaAs crystal with domain structure

We consider the process of generation of difference frequency in a GaAs crystal with periodic domain structure during propagation of a few-cycle laser pulse in the crystal in the regime of weakly expressed chromatic dispersion. Method of lines is used to obtain numerical solution to the system of coupled nonlinear differential equations in partial derivatives describing the evolution of the electric field of a few-cycle laser pulse in GaAs crystal both with periodic and aperiodic domain structure. It is shown that employing GaAs crystal with domain structure allows controlling the instantaneous frequency of broadband pulse radiation at difference frequency.     

基于周期极化晶体MgO:PPLN的纠缠光子对制备过程的研究

基于二阶非线性效应的自发参量转换技术制备纠缠光子对过程中,以掺5 mol%MgO:PPLN周期极化晶体为研究对象,将光参量变换过程中的动量守恒和能量守恒条件与该晶体的色散方程,以及晶体极化周期随温度变化的热膨胀方程相联系,得到了355 nm、405 nm、532 nm、780 nm和1 064 nm这5个实验室常用波长点在制备纠缠光子对时的周期调谐特性和温度调谐特性。研究过程中发现,出现了晶体极化周期过小和产生两对纠缠光子对问题,总结并归纳了各波长点在一定极化周期和温度下与非线性晶体作用所产生的纠缠光波段范围。当选用其他非线性周期极化晶体进行实验时,改变QPM动量守恒条件中的极化周期项,同时根据具体使用的晶体改变色散方程。该研究方案可直接推广到使用不同非线性晶体产生通信光波段或红外光波段的纠缠光子对研究中,在制备量子光源等领域具有重要研究价值。     

A Broadband Infrared Laser Source (2.5–17 μm) for Plasma Diagnostics

This paper presents the results of studies aimed at the creation of a hybrid laser system which is composed of a gas lasers and a nonlinear crystal and appreciably broadens and enriches the radiation spectrum of these lasers. A highly efficient conversion (37%) is attained when generating the second harmonic in a ZnGeP₂ crystal owing to an increase in the peak power of CO laser radiation in the mode locking regime. The two-cascade conversion (generation of both sum and difference frequencies) of radiation of a broadband CO laser in the single sample of such nonlinear crystals as ZnGeP₂ and AgGaSe₂ is demonstrated. In this case, the radiation spectrum is broadened by nearly a factor of two, and the number of detected spectral lines grows by a factor of four. The use of a comparatively simple laser system of gas-discharge CO and CO₂ lasers to conversion in AgGaSe₂ results in laser radiation tunable over a set of narrow spectral lines within a range from 2.5 to 16.6 μm (more than two and a half octaves).     

Measurement of the spectral brightness of terahertz radiation in the process of nonlinear-optical detection

A method has been proposed for the absolute measurement of the spectral brightness of terahertz radiation in the process of the parametric frequency conversion to the optical range. The method has a low threshold flux and does not require extrinsic references. The method simultaneously allows for the calibration of thermal fluctuations and the determination of the absolute local temperature of a nonlinear converter crystal.     

非线性光子晶体中原子辐射光场的非经典性质

讨论了非线性光子晶体微腔中二能级原子在相干场驱动下腔场的频谱特性及光子的统计性质.研究结果表明,当光子晶体的态密度很大时,如果腔场模与原子共振荧光Mollow峰的中心峰共振,则腔场的涨落压缩到量子散弹噪声之下,且与线性光子晶体微腔的情况相比其谱线峰值变大.当驱动场频率较大时光子服从亚Poisson分布,且非常接近Poisson分布. 关键词： 光子晶体 参量振荡 非弹性谱 二阶相关函数     

非线性光子晶体中原子辐射光场的非经典性质

讨论了非线性光子晶体微腔中二能级原子在相干场驱动下腔场的频谱特性及光子的统计性质.研究结果表明,当光子晶体的态密度很大时,如果腔场模与原子共振荧光Mollow峰的中心峰共振,则腔场的涨落压缩到量子散弹噪声之下,且与线性光子晶体微腔的情况相比其谱线峰值变大.当驱动场频率较大时光子服从亚Poisson分布,且非常接近Poisson分布.     

光学超晶格中级联参量过程制备纠缠光子对

探索制备纠缠光子对的新途径是量子光学领域的一个研究热点.从理论上证明在准周期极化 的光学超晶格晶体中的级联参量过程能够产生纠缠光子对.分别利用脉冲和连续的532nm抽运 光在一块准周期极化的 LiTaO3晶体中同时实现非简并参量下转换（产生信号光与闲置光 ）与和频（闲置光与抽运光和频生成和频光）的级联参量过程，获得630nm的信号光与460nm 的和频光.指出信号光与和频光之间存在纠缠关联性质.该方案的优点是可产生短波长的纠缠 光子对. 关键词： 准相位匹配 级联参量过程 光量子纠缠态     

差频可调谐太赫兹技术的新进展

太赫兹技术在最近30年来得到快速发展, 并在医学、生物、农业、材料、安检、通信、天文等领域得到广泛应用. 从太赫兹源的频谱特性可以分为窄带(单频)太赫兹源和宽带太赫兹源. 从频谱技术方面来说, 相干的宽带和窄带太赫兹谱是一种互补性关系, 具有各自的技术特点和应用范围. 宽带太赫兹谱可以用于快速获取较宽频谱范围的分子振转谱, 实现混合特征谱的快速检测或成像. 窄带太赫兹源具有很好的光谱灵敏度和分辨率, 适用于太赫兹抽运-探测、分子振转能级谱精细结构分辨 以及太赫兹远程探测和成像. 因此研制具有可调谐的高峰值功率的窄带太赫兹源是适用于探测和识别分子振转能级指纹谱的应用需求, 而差频技术是获得高功率和宽调谐窄带太赫兹源最重要的技术之一. 为了突出该技术的最新进展, 本综述引证论文仅仅限于近5 年来基于差频技术产生太赫兹波的研究进展, 分为光学激光差频源和量子级联激光器差频源两大部分. 对于光学激光差频源, 分别对目前文献报道的各种双波长差频源和太赫兹产生用的非线性晶体进行分类介绍, 并给出所采用的技术和实验结果; 对于量子级联激光器差频源, 分别介绍了量子级联激光器中的差频产生技术和波长调谐技术的最新进展. 量子级联激光器差频太赫兹源是目前实现量子级联激光器在太赫兹波段室温运转的惟一技术, 是实现小型化、窄带宽调谐和室温运转太赫兹源的新发展领域, 值得关注.     

三共振准相位匹配光学参量振荡器反射抽运场的正交位相压缩

利用全固化单频Nd∶YVO₄激光器抽运由periodically poled lithium niobate(PPLN)准相位匹配晶体组成的光学参量振荡器,通过调节晶体的温度,在信号模与闲置模近简并的情况下,完成了抽运场与两个亚谐波模在腔内同时共振,实现了光学参量振荡器的极低阈值运转(阈值为1.2mW);当抽运功率为阈值抽运功率的2.8倍时,观察到反射抽运光的正交位相压缩,压缩度达1.1dB.数值计算与实验结果一致