Atomically phase-matched second-harmonic generation in a 2D crystal

Second-harmonic generation(SHG)has found extensive applications from hand-held laser pointers to spectroscopic and microscopic techniques.Recently,some cleavable van der Waals(vdW)crystals have shown SHG arising from a single atomic layer,where the SH light elucidated important information such as the grain boundaries and electronic structure in these ultra-thin materials.However,despite the inversion asymmetry of the single layer,the typical crystal stacking restores inversion symmetry for even numbers of layers leading to an oscillatory SH response,drastically reducing the applicability of vdW crystals such as molybdenum disulfide(MoS_(2)).Here,we probe the SHG generated from the noncentrosymmetric 3R crystal phase of MoS_(2).We experimentally observed quadratic dependence of second-harmonic intensity on layer number as a result of atomically phase-matched nonlinear dipoles in layers of the 3R crystal that constructively interfere.By studying the layer evolution of the A and B excitonic transitions in 3R-MoS_(2) using SHG spectroscopy,we also found distinct electronic structure differences arising from the crystal structure and the dramatic effect of symmetry and layer stacking on the nonlinear properties of these atomic crystals.The constructive nature of the SHG in this 2D crystal provides a platform to reliably develop atomically flat and controllably thin nonlinear media.

Diode pumped Nd: YVO_4 laser at 671 nm with a type-Ⅱ noncritical phase-matched LBO

A diode pumped intracavity frequency-doubled Nd: YVO4 laser emitting at 671 nm with a type- Ⅱ noncritical phase-matched LBO crystal is reported. A maximum output power at 671 nm of 404 mW was obtained under an incident pump power of 5.16 W, with an optical-to-optical conversion efficiency up to 7.8%. The output power fluctuates less than 5% while the temperature change of the LBO crystal is maintained at ± 0.5℃ at the phase-matched temperature.

Phase-matching quantum key distribution with imperfect sources

The huge discrepancies between actual devices and theoretical assumptions severely threaten the security of quantum key distribution.Recently,a general new framework called the reference technique has attracted wide attention in defending against the imperfect sources of quantum key distribution.Here,the state preparation flaws,the side channels of mode dependencies,the Trojan horse attacks,and the pulse classical correlations are studied by using the reference technique on the phase-matching protocol.Our simulation results highlight the importance of the actual secure parameters choice for transmitters,which is necessary to achieve secure communication.Increasing the single actual secure parameter will reduce the secure key rate.However,as long as the parameters are set properly,the secure key rate is still high.Considering the influences of multiple actual secure parameters will significantly reduce the secure key rate.These actual secure parameters must be considered when scientists calibrate transmitters.This work is an important step towards the practical and secure implementation of phase-matching protocol.In the future,it is essential to study the main parameters,find out their maximum and general values,classify the multiple parameters as the same parameter,and give countermeasures.

Noncollinear phase-matching geometries in ultra-broadband quasi-parametric amplification

Optical parametric chirped pulse amplification(OPCPA)shows great potential in producing ultrashort high-intensity pulses because of its large gain bandwidth.Quasi-parametric chirped pulse amplification(QPCPA)may further extend the bandwidth.However,behavior of QPCPA at a limited pump intensity(e.g.,≤5 GW/cm^(2) in a nanosecond pumped QPCPA)has not yet been investigated fully.We discuss detailedly the ultra-broadband amplification and the noncollinear phasematching geometry in QPCPA,model and develop a novel noncollinear geometry in QPCPA,namely triple-wavelength phase-matching geometry,which provides two additional phase-matching points around the phase-matching point at the central wavelength.Our analysis demonstrates that the triple-wavelength phase-matching geometry can support stable,ultra-broadband amplification in QPCPA.The numerical simulation results show that ultrashort pulse with a pulse duration of 7.92 fs can be achieved in QPCPA when the pump intensity is limited to 5 GW/cm^(2),calculated using the nonlinear coefficient of YCa;O(BO;);.

Phase-matching quantum key distribution with light source monitoring

The transmission loss of photons during quantum key distribution(QKD)process leads to the linear key rate bound for practical QKD systems without quantum repeaters.Phase matching quantum key distribution(PM-QKD)protocol,an novel QKD protocol,can overcome the constraint with a measurement-device-independent structure,while it still requires the light source to be ideal.This assumption is not guaranteed in practice,leading to practical secure issues.In this paper,we propose a modified PM-QKD protocol with a light source monitoring,named PM-QKD-LSM protocol,which can guarantee the security of the system under the non-ideal source condition.The results show that our proposed protocol performs almost the same as the ideal PM-QKD protocol even considering the imperfect factors in practical systems.PMQKD-LSM protocol has a better performance with source fluctuation,and it is robust in symmetric or asymmetric cases.

Phase-Matching and Parametric Conversion for the Mid-Infrared in As₂S₃Waveguides

We illustrate two As2S3 waveguide designs for four-wave mixing, which can generate 3.03 μm mid-infrared light from a 1.55 μm near-infrared signal source and a 2.05 μm pump source. Through simulations, we verify that four-wave mixing phase-matching efficiencies up to 100% can be achieved using dispersion engineering to maintain the dispersion at 2.05 μm near to zero. The best conversion efficiency is –10 dB. When the waveguide length is 1 cm, the parametric conversion bandwidth is 1525 nm. We also evaluated the shift of 100% phase-matching efficiency wavelengths based upon fabrication tolerances.

Tailoring OAM spectrum of high-order harmonic generation driven by two mixed Laguerre–Gaussian beams with nonzero radial nodes

The extreme ultraviolet(XUV)light beam carrying orbital angular momentum(OAM)can be produced via high-order harmonic generation(HHG)due to the interaction of an intense vortex infrared laser and a gas medium.Here we show that the OAM spectrum of vortex HHG can be readily tailored by varying the radial node(from 0 to 2)in the driving laser consisting of two mixed Laguerre-Gaussian(LG)beams.We find that due to the change in spatial profile of HHG,the distribution range of the OAM spectrum can be broadened and its shape can be modified by increasing the radial node.We also show that the OAM mode range becomes much wider and its distribution shape becomes more symmetric when the harmonic order is increased from the plateau to the cutoff when the driving laser has the nonzero radial nodes.Through the map of coherence length and the evolution of harmonic field in the medium,we reveal that the favorable off-axis phase-matching conditions are greatly modified due to the change of intensity and phase distributions of driving laser with the radial node.We anticipate this work to stimulate some interests in generating the XUV vortex beam with tunable OAM spectrum through the gaseous HHG process achieved by manipulating the mode properties of the driving laser beam.

Controllable transitions among phase-matching conditions in a single nonlinear crystal

Entangled photon pairs are crucial resources for quantum information processing protocols.Via the process of spontaneous parametric downconversion(SPDC),we can generate these photon pairs using bulk nonlinear crystals.Traditionally,the crystal is designed to satisfy a specific type of phase-matching condition.Here,we report controllable transitions among different types of phase matching in a single periodically poled potassium titanyl phosphate crystal.By carefully selecting pump conditions,we can satisfy different phase-matching conditions.This allows us to observe first-order Type-II,fifth-order Type-I,third-order Type-0,and fifth-order Type-II SPDCs.The temperature-dependent spectra of our source were also analyzed in detail.Finally,we discussed the possibility of observing more than nine SPDCs in this crystal.Our work not only deepens the understanding of the physics behind phase-matching conditions,but also offers the potential for a highly versatile entangled biphoton source for quantum information research.

Intersubband absorption with difference-frequency generation in GaAs asymmetric quantum wells

An asymmetric quantum well （AQW） is designed to emit terahertz （THz） waves by using difference frequency generation （DFG） with the structure of GaAs/Al0.2Ga0.8As/Al0.5Ga0.sAs. The characteristics of absorption coefficients are analysed under the parabolic and non-parabolic energy-band conditions in detail. We find that the absorption coefficients vary with the two pump optical intensities, and they reach the maxima when the pump wavelengths are given as λp1 = 9.70 μm and λp2 = 10.64 μm, respectively. Compared with non-parabolic conditions, the total absorption coefficient under parabolic conditions shows a blue shift, which is due to the increase in the energy difference between the ground and excited states. By adjusting the two pump optical intensities, the wave vector phase-matching condition inside the AQW is satisfied.

Polarisation-sensitive four-wave mixing and soliton self-frequency shift effect in the highly birefringent photonic crystal fibre

By adjusting the polarisation state of the pump at 805 nm parallel to slow （x） and fast （y） axes of the highly birefringent photonic crystal fibre with zero dispersion wavelengths 790 nm and 750 nm, this paper demonstrates the efficient polarisation-sensitive four wave mixing involved in pump, anti-Stokes and Stokes signals and soliton self- frequency shift effects induced by the phase-matching between red-shifted solitons and blue-shifted dispersive waves. If the reduction of coupling efficiency to the circular pump laser mode or other circular fibres due to asymmetry of the core is neglected, more than 98% of the total input power is kept in a single linear polarisation. Controlled dispersion characteristic of the doublet of fundamental guided-modes results in achieving light field strongly confined in principal axes of photonic crystal fibre, and enhancing the corresponding nonlinear-optical process through the remarkable nonlinear birefringence.

Tunable Terahertz Source Based on Optics Pumped Nonlinear Crystals

Recent progresses about optical pumped tunable terahertz （THz） sources are interviewed, including THz parametric oscillation （TPO） and difference frequency generation （DFG）. We develop high efficiency and high power surface-emitted TPO, as well as DFG with nonlinear crystals. A novel scheme for the high efficiency DFG source based on the Cherenkov phase-matching technology is comprehensively investigated in both bulk crystals. The widely tunable optical THz radiation is also researched based on the organic nonlinear 4-N,N-dimethylamino-4＇-N＇- methylstilbazolium 2,4,6-trimethylbenzenesulfonate （DSTMS） crystal.

Selection of odd or even harmonics by a multi-color laser field with macroscopic phase-matching

We experimentally and theoretically demonstrate that ＊~he property （odd or even） of generated harmonics can be selected by manipulating the macroscopic phase-matching conditions based on a three-color laser field. Only odd or even harmonics can be made dominant by changing the focal position and adjusting the gas pressure. These results indicate that the odd-even property of the generated harmonics can be controlled by using the mult i-color laser field with macroscopic phase-matching.

Evaluating refractive index and birefringence of nonlinear optical crystals:Classical methods and new developments

Nonlinear optical(NLO)materials play an increasingly important role in laser technology.Birefringence is one of the most important parameters for NLO materials to realize angle phase-matching conditions.In comparison with other desirable optical properties,the availability of birefringence and refractive index dispersion is especially problematic owing to the strict requirements for single crystals.In this review,we described how to obtain the refractive index and birefringence of NLO materials from crystals sub-millimeters to centimeters in size.Espe-cially,recently developed methods including the minimum deflection angle method,auto-collimation method,prism coupling method,oil immersion technique,interference color method,and theoretical calculation(DFT)for rapid assessment of birefringence are summarized,the contents of which are mainly focused on the principles and typical applications,together with the advantages and drawbacks.In addition,representative examples of bire-fringent measurements were presented.The purpose of this work is to provide a useful perspective on the characterization of birefringence for NLO materials.It is hoped that this review can give a clear description of the birefringence measurements and accelerate the discovery of new NLO crystals.

Carrier-Envelope Phase-Dependent Phase Matching of Terahertz Emission in Laser Plasma

We investigate off-axis phase-matched terahertz(THz)radiation in laser plasma pumped by few-cycle laser pulses.We find that the THz amplitude and angular distributions in the far field are sensitively dependent on the pump pulse’s focal carrier-envelope phase(CEP).Ring-like profiles of THz radiation are obtained at CEP values of 0.5πand 1.5π,due to the inversely symmetric local THz waveforms emitted before and after laser focus.Off-axis phase-matched THz radiation offers a tool to accurately measure the CEP of few-cycle pulses at the center of a medium.

S-wave Q structure of the crust and upper mantle beneath Yunnan from surface waves

A phase-matched filtering technique is applied to extract fundamental mode signals from Rayleigh waves recorded at 62 digital seismic stations in the Yunnan and Sichuan regions.We use the fundamental mode of vibrations at two stations that are located on the same great circle as the focus to calculate an inter-station attenuation coefficient of the Rayleigh wave with periods between 0.40 and 80.64 s,and invert for the inter-station S-wave Q-factor (Qβ) at depths of 0-200 km.The results indicate that Qβ in Yunnan is 20-140,presenting a low Qβ background with apparent lateral variation.Taking the Honghe Fault as the boundary,Qβ of the crust is only 20 on the west side,extending to a depth of 120 km.The distribution of Qβ is consistent with large-scale Cenozoic volcanic and intrusive rocks in western Yunnan,implying that the crust and mantle are in the thermally active state.In the eastern Yunnan Block,east of the Xiaojiang Fault,Qβ in the upper 120 km is 140 in the south but only 20 in the north.Additionally,around the Dukou-Chuxiong in the mid-Yunnan Block,Qβ in the lithosphere is relatively high at 60-100,corresponding to a stiff crust.This is because the suture between the Indian and Eurasian plates reversed the tension in the rifting stage into the compression of orogenesis,leading to the closure of a gap in the crust.After some time,interstitial fluids gradually disappeared,resulting in a high velocity layer in the crust and low heat flow on the surface.The Yunnan region consists of an obvious block of elevated Qβ,distributed within the low background,consistent with the distribution of heat-flow values on the surface.The Honghe and Xiaojiang faults are tectonic boundaries in addition to being boundaries between regions of high and low crustal Qβ.The low Qβ is probably the result of crustal rupture and disturbance caused by strong earthquakes and the upwelling of hot substances along the deep fault zones.

Enhanced nonlinear optical functionality in birefringence and refractive index dispersion of the deep-ultraviolet fluorooxoborates

As a promising candidate,the fluorooxoborate has enkindled new explorations of nonlinear optical materials to meet the deep-ultraviolet criteria.However,big challenges and open questions still remain facing this exciting new field,especially the birefringence and dispersion of refractive index which are fundamental parameters for determining the phasematching second harmonic generation wavelength.Here we designed possible anionic groups in fluorooxoborates,and analyzed the optical anisotropy to check its influence on birefringence,which was proved further by the response electronic distribution anisotropy approximation.The functional modules modulating birefringence in fluorooxoborates were explored systematically.We developed an approach for evaluating the behavior of the refractive index dispersions and found that the fluorooxoborates had small refractive index dispersions owing to the introduction of fluorooxoborate modules.Our results demonstrate that fluorooxoborates can be utilized to realize short phase-matching wavelength markedly and offer a path toward novel performance-driven materials design.

The Fabrication and SHG Test of QPM Periodically Poled KTiOPO_4

The Quasi-phase-matching periodically poled flux-grown KTP by high electrical field method is researched. A 8×5×1mm3,∧=9.0μm PPKTP wafer is successfully fabricated for the first order QPM SHG. The interactive length of the sample is about 3mm. The SHG scheme of Nd: YAG at 1064nm tested that the output power of cw 532nm green light is 0.2mw at room temperature with fundamental power of 1.2w. The normalized conversion efficiency is about 0.09% (W·cm)-1.

Enhancement of Sensitivity by Initial Phase Matching in SU(1,1) Interferometers

We derive a general phase-matching condition(PMC) for enhancement of sensitivity in SU(1,1) interferometers. Under this condition, the quantum Fisher information(QFI) of two-mode SU(1,1) interferometry becomes maximal with respect to the relative phase of two modes, for the case of an arbitrary state in one input port and an even(odd) state in the other port, and the phase sensitivity is enhanced. We also find that optimal parameters can let the QFI in some areas achieve the Heisenberg limit for both pure and mixed initial states. As examples, we consider several input states: coherent and even coherent states, squeezed vacuum and even coherent states, squeezed thermal and even coherent states. Furthermore, in the realistic scenario of the photon loss channel, we investigate the effect of photon losses on QFI with numerical studies. We find the PMC remains unchanged and is not affected by the transmission coefficients for the above input states. Our results suggest that the PMC can exist in various kinds of interferometers and the phase-matching is robust to even strong photon losses.

Coherent X-ray mirage: discovery and possible applications

In the far field of the intensity distribution of the beam delivered by a two-stage transient–collisional excitation X-ray laser(XRL), a non-expected interference pattern that is stable from shot to shot has been discovered. It is demonstrated that the interference is caused by the emergence of an imaginary source in the amplifying plasma, which is phase matched to the radiation of the generator. The observed phenomenon is called an X-ray coherent mirage. To explain the obtained results, a new theoretical approach is developed. The basic essential conditions for formation of the X-ray mirage are formulated, and possible applications are discussed. This paper details the experiments, including the formulation of the necessary and sufficient conditions for formation of the X-ray mirage, and possible applications are discussed.