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Phase-matching quantum key distribution (PM-QKD) is a promising protocol that can surpass the linear key-rate bound. Generally, inserting an advantage distillation (AD) step into PM-QKD can improve the secure key rate and transmission distance significantly in the asymptotic case. Here, we investigate the performance of practical PM-QKD systems with AD in the non-asymptotic case. Simulation results show that AD can dramatically improve the performance of practical PM-QKD systems. Moreover, we propose to estimate the information leakage using three mutually unbiased bases, namely X, Y and Z, to further enhance its performance, which demonstrates that using the error rates in X, Y and Z bases to estimate the information leakage can improve the secure key rate at high channel losses and misalignment errors. [ABSTRACT FROM AUTHOR]

The emission of resonant radiation from temporal solitary waves—also known as dispersive wave generation—allows efficient energy transfer to far‐distant spectral domains. This coherent radiation can deliver large spectral densities at selected wavelengths once control over the individual soliton is achieved. Here, the concepts of few‐mode operation and local temperature tuning are combined for precise steering of cascaded dispersive wave generation in liquid‐core optical fibers. By exciting higher‐order TM and TE modes with femtosecond pulses at 1600 nm, the generation of two dispersive waves tunable by up to 33 nm K−1 through adjusting a selected part of the waveguide is observed. Sophisticated soliton‐driven nonlinear dynamics arising from thermally transitioning from anomalous to all‐normal dispersion with temperature changes of only a few Kelvin have been found, including soliton steering, soliton breakdown, and soliton post‐fission tuning. All experimental results are verified by nonlinear simulations and semi‐analytic phase‐matching calculations, overall providing a cost‐effective and practical toolbox for discovering unexplored states of light as well as for developing dynamically tunable broadband light sources. [ABSTRACT FROM AUTHOR]

Mid-infrared nonlinear optical (MIR-NLO) crystals are a key component in the field of high-precision measurements. Among these, phosphide (Pn; Pn = P, As) materials are renowned for their formidable polarization capabilities and hold substantial research potential. In this work, four non-cubic Pn materials (NaGe 3 P 3 , Cd 3 Cu 2 P 10 , Sr 14 Sn 3 As 12 , and Ba 13 Si 6 Sn 8 As 22) were systematically investigated using Density Functional Theory (DFT). Notably, the four materials are all composed of tri-coordinated groups and tetra-coordinated groups ("3 + 4"). The electronic and optical properties of these materials were systematically analyzed. The strong Second Harmonic Generation (SHG) properties of Cd 3 Cu 2 P 10 , Sr 14 Sn 3 As 12 , and Ba 13 Si 6 Sn 8 As 22 were predicted for the first time. Additionally, the contributions of different groups to polarizability and nonlinear polarizability were analyzed. In these "3 + 4" Pn materials, phase-matching was found to primarily arise from tri-coordinated groups, while the contributions to SHG predominantly stemmed from the groups with higher Pn content. The present study provides frontier theoretical guidance for high-performance MIR-NLO crystals in future research. [Display omitted] [ABSTRACT FROM AUTHOR]

Optically pumped nonlinear frequency down conversion is a proven approach for monochromatic terahertz (THz)-wave generation that provides superior properties such as continuous and wide tunability as well as laser-like linewidth and beam quality. Phase-matching (PM) is an important connection between the pump sources and nonlinear crystals and determines the direction of energy flow (as well as the output power). In past decades, a variety of peculiar PM configurations in the THz region have been invented and are different from the traditional ones in the optical region. We summarize the configurations that have been applied in nonlinear THz-wave generation, which mainly fall in two categories: scalar (collinear) PM and vector PM (including macroscopic noncollinear PM and microscopic vector PM). The development of this technique could relax the matching conditions in a wide range of nonlinear crystals and pump wavelengths and could finally promote the improvement of coherent THz sources. [ABSTRACT FROM AUTHOR]

Continuous-variable measurement-device-independent quantum key distribution (CV-MDI-QKD) allows remote parties to share information-theoretical secure keys while defending all the side-channel attacks on measurement devices. However, the secure transmission distance and the secret key rate are quite limited due to the high untrusted equivalent excess noise in the Gaussian modulation. More particularly, extremely high-efficiency homodyne detections are required for even non-zero secure transmission distances, which directly restrict its practical realization. Here, we propose a CV-MDI-QKD protocol by encoding the key information into matched discrete phases of two groups of coherent states, which decreases the required detection efficiency for ideally asymmetric cases, and makes it possible to practically achieve secure key distribution with current low-efficiency homodyne detections. Besides, a proof-of-principle experiment with a locally generated oscillator is implemented, which, for the first time, demonstrates the realizability of CV-MDI-QKD using all fiber-based devices. The discrete-modulated phase-matching method provides an alternative direction of an applicable quantum key distribution with practical security. [ABSTRACT FROM AUTHOR]

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 (MoS2). Here, we probe the SHG generated from the noncentrosymmetric 3R crystal phase of MoS2. 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-MoS2 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.

Optically pumped nonlinear frequency down conversion is a proven approach for monochromatic terahertz (THz)-wave generation that provides superior properties such as continuous and wide tunability as well as laser-like linewidth and beam quality. Phase-matching (PM) is an important connection between the pump sources and nonlinear crystals and determines the direction of energy flow (as well as the output power). In past decades, a variety of peculiar PM configurations in the THz region have been invented and are different from the traditional ones in the optical region. We summarize the configurations that have been applied in nonlinear THz-wave generation, which mainly fall in two categories: scalar (collinear) PM and vector PM (including macroscopic noncollinear PM and microscopic vector PM). The development of this technique could relax the matching conditions in a wide range of nonlinear crystals and pump wavelengths and could finally promote the improvement of coherent THz sources.

In this article we briefly review three generation mechanisms of high-energy photons related, directly or otherwise, to high-intensity laser–matter interactions. The three mechanisms are the "Betatron Radiation" (BR), the "Parametric X-ray Radiation" (PXR) from Laser-WakeField Accelerated Electrons (LWFA), and the all-optical "Compton BackScattering" (CBS). The aim is to highlight similarities and differences, in particular regarding the phase-matching conditions for the high-energy photons to be emitted. • Short review of radiation mechanisms in relativistic laser-matter interactions. • Study of the phase-matching for high-energy photon sources. • Applications of the Fermi–Weizsacker–Williams method. [ABSTRACT FROM AUTHOR]

Ultraviolet (UV) transparency, second harmonic effect and optical birefringence are three vital but mutually restrictive factors in the application of UV nonlinear optical (NLO) materials. It is difficult for traditional phosphates to achieve a good balance among these factors. In this communication, we propose that the structural evolution of the NLO motif from traditional phosphate to oxythiophosphate would enhance the birefringence and second harmonic generation (SHG) effect while maintaining wide UV transparency, which is confirmed by first‐principles calculations and preliminary experimental measurements. Following this strategy, we predict that, compared with the well‐known NLO phosphate KH2PO4, the oxythiophosphate KH2PO3S exhibits better balance for the UV NLO performance, including wide UV transparency (UV cutoff down to 203 nm), strong SHG effect (ca. 0.9 pm V−1), and large birefringence (ca. 0.1 at 1 μm) with short phase‐matching SHG output wavelength (≈214 nm). [ABSTRACT FROM AUTHOR]

Continuous-variable measurement-device-independent quantum key distribution (CV-MDI-QKD) allows remote parties to share information-theoretical secure keys while defending all the side-channel attacks on measurement devices. However, the secure transmission distance and the secret key rate are quite limited due to the high untrusted equivalent excess noise in the Gaussian modulation. More particularly, extremely high-efficiency homodyne detections are required for even non-zero secure transmission distances, which directly restrict its practical realization. Here, we propose a CV-MDI-QKD protocol by encoding the key information into matched discrete phases of two groups of coherent states, which decreases the required detection efficiency for ideally asymmetric cases, and makes it possible to practically achieve secure key distribution with current low-efficiency homodyne detections. Besides, a proof-of-principle experiment with a locally generated oscillator is implemented, which, for the first time, demonstrates the realizability of CV-MDI-QKD using all fiber-based devices. The discrete-modulated phase-matching method provides an alternative direction of an applicable quantum key distribution with practical security.

KDP (Potassium dihydrogen phosphate) single crystals were grown from Amaranth dye and EDTA (Ethylenediaminetetraacetic acid disodium salt) mixed KDP solutions by the temperature lowering method and Sankaranarayanan-Ramasamy technique. The molar concentration of EDTA in the solutions was 1 mol%. The Amaranth concentrations were 0.001, 0.005, 0.01, 0.03 and 0.05 mol%. The growth of KDP crystals was oriented in type I and type II phase-matching directions which bring maximum second harmonic generation (SHG) efficiency to the crystals. Optical properties of Amaranth and EDTA co-doped KDP crystals (KAE) were investigated by Ultraviolet-Visible (UV-Vis) spectroscopy, FT-Raman spectroscopy, and photoluminescence. SHG efficiency of the crystals was measured by shining 1064 nm Nd:YAG laser directly into them and receiving output signals. The most important result in the study showed that KAE crystals had SHG efficiency of 1.7 and 2.4 times higher than EDTA doped KDP crystal (KE) and pure KDP crystal respectively. Then second-order nonlinear optical coefficients of these crystals were calculated. The simultaneous doping of two organic substances into KDP crystals has further enhanced their nonlinear optical characteristics. [ABSTRACT FROM AUTHOR]

High energy single‐ to few‐cycle terahertz pulses enable the exploration of electron acceleration, strong‐field physics, and nonlinear terahertz spectroscopy. One important method of generating such terahertz pulses is using the tilted‐pulse‐front (TPF) technique. However, the needed angular dispersion leads to a spatial and temporal break‐up of the optical pump, reducing the generation efficiency and the electric field quality of the terahertz pulses. To decrease the effects caused by angular dispersion, multiple schemes with discrete pulse‐front‐tilt are suggested. Based on a 2D+1 numerical model, a systematic comparative study of the conventional TPF scheme, three discrete TPF schemes, and a scheme with spatio‐temporally chirped (STC) optical pulses is performed. Smaller optimal interaction lengths and conversion efficiencies are predicted compared to 1D models. For small pump beam sizes, it is concluded that the STC scheme delivers spatially homogeneous terahertz pulses with the highest conversion efficiency, and for short interaction lengths, the discrete TPF schemes cannot outperform the continuous ones. However, for large pump sizes, the nonlinear echelon slab delivers the spatially most homogeneous terahertz beams and has the unique potential of generating high energy terahertz pulses. In general, this work gives guidance to choose the most appropriate setup for a given terahertz experiment. [ABSTRACT FROM AUTHOR]

The twin-field quantum key distribution (TF-QKD) protocol and its variations have been proposed to overcome the linear Pirandola–Laurenza–Ottaviani–Banchi (PLOB) bound. One variation called phase-matching QKD (PM-QKD) protocol employs discrete phase randomization and the phase post-compensation technique to improve the key rate quadratically. However, the discrete phase randomization opens a loophole to threaten the actual security. In this paper, we first introduce the unambiguous state discrimination (USD) measurement and the photon-number-splitting (PNS) attack against PM-QKD with imperfect phase randomization. Then, we prove the rigorous security of decoy state PM-QKD with discrete phase randomization. Simulation results show that, considering the intrinsic bit error rate and sifting factor, there is an optimal discrete phase randomization value to guarantee security and performance. Furthermore, as the number of discrete phase randomization increases, the key rate of adopting vacuum and one decoy state approaches infinite decoy states, the key rate between discrete phase randomization and continuous phase randomization is almost the same.

We newly analyze the Čerenkov-type terahertz wave generation via the optical difference frequency generation in nonlinear dielectric waveguides based on the coupled-mode theory. The nonlinear coupled equations are derived, and the simple analytical solution for the terahertz wave is obtained in a closed form. The validity of our solution is also checked by the nonlinear beam propagation method. The analytical solution shows that for planar waveguides, the coupling coefficient has pole singularities at the propagation constants of the leaky modes at the terahertz frequency of the waveguides, and thus, the terahertz wave generation can be significantly enhanced as the phase-matching point approaches to the propagation constant of the lowest order leaky mode, whose imaginary part is very small compared to the real part of that. [ABSTRACT FROM AUTHOR]

The value of the generated terahertz power limits the major specifications of the THz coherent techniques in imaging and tomography. We report on the effective generation pulse of THz radiation in a nonlinear crystals partially filling the cross section of the rectangular waveguide. The generation of terahertz (THz) radiation in the band 0.1-3 THz via optical rectification of femtosecond laser pulses in LiNbO3, ZnTe, DAST, GaSe nonlinear optical crystals are presented, as well as the results of the numerical calculations for phase-matching. [ABSTRACT FROM AUTHOR]

The effect of THz radiation absorption on the efficiency of generation of coherent THz radiation in a nonlinear optical crystal placed into a metal rectangular waveguide is studied. The efficiency of the nonlinear conversion of optical laser radiation to the THz band is also a function of the phase-matching (PM) condition inside the nonlinear crystal. The method of partial filling of a metal waveguide with a nonlinear optical crystal is used to ensure phase matching. Phase matching was obtained by the proper choice of the thickness of the nonlinear crystal, namely the degree of partial filling of the waveguide. We have studied the THz radiation attenuation caused by the losses in both the metal walls of the waveguide and in the crystal, taking into account the dimension of the cross section of the waveguide, the degree of partial filling and its dielectric constant. [ABSTRACT FROM AUTHOR]

Nonlinear optical (NLO) crystals, especially the mid-infrared nonlinear crystals play important roles in laser guidance and measurement. Among them, the tetra-coordinated structures have strong polarization ability. In this work, calculations for three zero-dimension (0D) tetra-coordinated structures (Na 4 Ca 2 SiP 4 , Cs 2 HgI 2 Cl 2 , Rb 10 Zn 4 Sn 4 S 17) were analyzed by using density functional theory (DFT) simulations. Their crystal structure parameters, band structures, and optical properties are analyzed and compared. Meanwhile, the polarizability (electric dipole moment, dipole polarizability, first dipole hyper-polarizability) of these tetra-coordinated groups were analyzed separately. The results showed that the 0D isolated tetrahedron structure with strong consistent permutation may have excellent second harmonic generation (SHG) effect. It was worth mentioning that Na 4 Ca 2 SiP 4 was predicted to have a large nonlinear effect and phase-matching ability. This enriches the theory of A/M (A = alkali or alkaline-earth metal cations except for Li; M = tetra-coordinated cations). In addition, the relationship between the E gap and electronegativity of the IA-IIA-M-Pn (IA = alkali metal; IIA = alkaline-earth metal; M = tetra-coordinated cations; Pn = P, As) structure was also investigated. This finding may shed light on the future construction of mid-infrared NLO materials with a wide E gap and high SHG effect. Three zero-dimensional (0D) tetra-coordinated structures (Na 4 Ca 2 SiP 4 , Cs 2 HgI 2 Cl 2 , Rb 10 Zn 4 Sn 4 S 17) were analyzed through density functional theory (DFT) simulations. Na 4 Ca 2 SiP 4 was predicted to have a large nonlinear effect and good phase-matching ability. Meanwhile, the isolated 0D tetrahedron structure with strong consistent permutation may have excellent second harmonic generation (SHG) effect. [Display omitted] • Three zero-dimensional (0D) tetra-coordinated structures (Na 4 Ca 2 SiP 4 , Cs 2 HgI 2 Cl 2 , Rb 10 Zn 4 Sn 4 S 17) were analyzed through density functional theory (DFT). • The isolated 0D tetrahedron structure with strong consistent permutation may have excellent second harmonic generation (SHG) effect. • Na 4 Ca 2 SiP 4 was predicted to have a large nonlinear effect and good phase-matching ability. [ABSTRACT FROM AUTHOR]

We realize a high spatially resolved idler imaging with a compact noncollinear optical parametric amplifier, which is based on type-II phase matching (PM). Using an intense femtosecond laser pulse train as the pump, this amplifier can amplify a separate continuous-wave (CW) signal with high gain and broad gain bandwidth with no need for time synchronization between the pump and the signal. In spite of the noncollinear arrangement of the amplifier, the spatial geometric smearing (SGS) is eliminated inside the amplifier due to the collinear Poynting vectors of the signal and the idler, which promotes the horizontal spatial resolution of the idler image to approach its vertical spatial resolution. Our experiments show that this design can improve the 2-D space-bandwidth product (SBP) of the idler imaging by a factor of 2.8, compared with the noncollinear type-I PM amplifier. The measured 2-D SBP is more than 73 000, which, to our knowledge, is the highest value for idler imaging to date.

Single KDP crystals were grown from an aqueous solution by the slow cooling method. They were cut into triangular prism-shaped blocks whose angles were determined by type II phase-matching condition for the second harmonic generation (SHG). The 1064 nm laser was shone into the KDP crystals and was partially converted to the 532 nm light. The power of remaining 1064 nm beam was much higher than that of 532 nm SHG beam. Total internal reflection occurred inside the KDP crystals for the 1064 nm beam. The 532 nm beam passed through the crystals and was recorded by a detector. The KDP crystal prisms have two functions: help to measure the output 532 nm radiation accurately, and adjust the power of 532 nm beam by changing the point of incidence of the 1064 nm beam, which improves convenience of the frequency doubler in optical devices. [ABSTRACT FROM AUTHOR]

In this work, we propose a strategy to design new deep‐ultraviolet (deep‐UV) nonlinear optical (NLO) materials through the combination of borate and phosphate NLO material genome. Accordingly, a series of borophosphates are predicted with very large energy bandgaps, strong second harmonic generation (SHG) effects, and moderate birefringence in the deep‐UV region. In particular, two novel fluoride borophosphates, PBO3F2 and PB3O6F2, are able to achieve the deep‐UV SHG laser output with shorter phase‐matching wavelengths (≈155 and 137 nm) than that of KBe2BO3F2 (KBBF, ≈161 nm). Further calculations show that these two structures are dynamically and thermodynamically stable, thus triggering possible synthesis, growth, and applications of them. The results attest that the proposed strategy is instructive and pave a comprehensive road map for us to discover new NLO crystals, especially in the borophosphate catalog with excellent deep‐UV phase‐matching capabilities beyond KBBF. [ABSTRACT FROM AUTHOR]

We consider a system of two coupled magneto-optical single-mode slab waveguides separated by an isotropic dielectric spacer. In the transverse magneto-optical configuration, the TE/TM states of polarization can be handled separately. The even/odd supermodes are derived in much the same way as in similar non-magnetic coupled systems; perfect phase-matching ensures a total transfer of energy from one waveguide to the other. The coupling constant can be related to the difference between the propagation constants of the even (slow) and odd (fast) supermodes. Besides, while reversing the magnetization in one of the two waveguides does not affect the phase-matching, it does change the coupling mechanism in a subtle way, thus allowing a magnetic control of the supermodes and of the polarization-dependent coupling efficiency. [ABSTRACT FROM AUTHOR]

In cavity-based axion dark matter search experiments exploring high mass regions, multiple-cavity design is under consideration as a method to increase the detection volume within a given magnet bore. We introduce a new idea, referred to as a multiple-cell cavity, which provides various benefits including a larger detection volume, simpler experimental setup, and easier phase-matching mechanism. We present the characteristics of this concept and demonstrate the experimental feasibility with an example of a double-cell cavity. [ABSTRACT FROM AUTHOR]

Conventional axion dark matter search experiments employ cylindrical microwave cavities immersed in a solenoidal magnetic field. Exploring higher frequency regions requires smaller size cavities as the TM 010 resonant frequencies scale inversely with cavity radius. One intuitive way to make efficient use of a given magnet volume, and thereby to increase the experimental sensitivity, is to bundle multiple cavities together and combine their individual outputs, ensuring a phase-matching of the coherent axion signal. We perform an extensive study for realistic design of a phase-matching mechanism for multiple-cavity systems and demonstrate its experimental feasibility using a double-cavity system. [ABSTRACT FROM AUTHOR]

This paper reports on the new Sellmeier and thermo-optic dispersion formulas that provide a good reproduction of the temperature-dependent phase-matching conditions for second-harmonic generation (SHG) and sum-frequency generation (SFG) of a CO2 laser and a Nd:YAG laser-pumped KTiOPO4 (KTP) optical parametric oscillator (OPO) in the 0.8859–10.5910 μm range as well as those for difference-frequency generation (DFG) between the two diode lasers in the 4.9–6.5 μm range and DFG between the two periodically poled LiNbO3 (PPLN) OPOs in the 5–12 μm range thus far reported in the literature.

An experimental study of the scaling laws for optimal driving conditions and output properties of the beam generated through high-order harmonic generation on the f -number of laser focusing is presented. Alteration of experimental parameters for focal lengths from 0.3 to 5 m, in optimal conditions for phase-matching (in terms of driving laser intensity, gas cell length and gas pressure) was carried out, enabling the experimental verification of the scaling laws for output parameters, such as XUV pulse energy, conversion efficiency and divergence of the harmonic beam. In order to improve the yield of the harmonic signal, allowing the source to become convenient for applications that require pulses with high photon numbers, a loose focusing geometry with high peak-power driver pulses was implemented. A high harmonic beam with energy up to 40 nJ per pulse was generated in Ar with a focal length of 5 m. Application of the scaling laws verified within this study allows the estimation of parameters for a future high harmonic source that is being implemented at ELI Beamlines facility. [ABSTRACT FROM AUTHOR]

We demonstrate the full bandwidth measurement of the intermodal four-wave mixing efficiency for a 4.7-km three-spatial-mode few-mode fiber. Large fluctuations of intermodal four-wave mixing between the LP 01 and LP11 modes are experimentally observed. These fluctuations exhibit a strong wavelength dependence that can be persistent in time. The fluctuations are also shown to display some polarization dependence. [ABSTRACT FROM PUBLISHER]

In this study, simultaneous second-harmonic generation (SHG), sum frequency generation (SFG), and Raman conversion based on MgO-doped periodically poled lithium niobate (MgO:PPLN) for multi-wavelength generation is demonstrated. The approach used is based on a single MgO:PPLN crystal poled with a uniform period of 10.2 µm that phase matches SHG and SFG, simultaneously. Using a simplified double-pass geometry, up to 0.8 W of blue light at 487 nm is achieved by a frequency-doubling 974 nm laser diode pump, and 0.5 W of orange light at 598 nm is generated by frequency mixing 974 nm pump with C-band (1527⁻1565 nm) tunable laser source. At high pump powers of the 974 nm laser source, other unexpected peaks at 437, 536, 756, 815 and 1038 nm were observed, of which the 1038 nm line is due to Stimulated Raman Scattering within the MgO:PPLN crystal. The resulting multi-wavelength light source may find a wide range of applications in biomedicine and basic research.

The overview of the recent results for discovery and investigations of a very exotic phenomenon – optical mirage in the X-ray spectral range – is presented. It was found that the mirage could be created in the form of coherent virtual point source, emerging in the vicinity of the second plasma in two-stage oscillator-amplifier X-ray laser. The X-ray source-mirage, rigidly phased with the initial radiation of generator, occurs only when amplification takes place in the amplifier plasma and leads to the appearance of the interference pattern in the form of concentric rings in the spatial profile of the output X-ray laser beam. The equation describing the emergence of X-ray mirage was found, numerical solution of which shows that its formation is similar to that of the optical mirages observed at propagation of light rays through an inhomogeneously heated air. Obtained results have already demonstrated novel comprehension into the physical nature of amplification of X-ray radiation, opening additional opportunities for X-ray interferometry, holography, and other applications, which require multiple rigidly phased sources of coherent radiation. [ABSTRACT FROM PUBLISHER]

We realize a high spatially resolved idler imaging with a compact noncollinear optical parametric amplifier, which is based on type-II phase matching (PM). Using an intense femtosecond laser pulse train as the pump, this amplifier can amplify a separate continuous-wave (CW) signal with high gain and broad gain bandwidth with no need for time synchronization between the pump and the signal. In spite of the noncollinear arrangement of the amplifier, the spatial geometric smearing (SGS) is eliminated inside the amplifier due to the collinear Poynting vectors of the signal and the idler, which promotes the horizontal spatial resolution of the idler image to approach its vertical spatial resolution. Our experiments show that this design can improve the 2-D space–bandwidth product (SBP) of the idler imaging by a factor of 2.8, compared with the noncollinear type-I PM amplifier. The measured 2-D SBP is more than 73 000, which, to our knowledge, is the highest value for idler imaging to date. [ABSTRACT FROM PUBLISHER]

We model two-color high-order harmonic generation using a full 3-D nonadiabatic model and show that the presence of a weak perturbative near infrared pulse assisting a strong pulse of long wavelength increases the high-harmonic yield with one order of magnitude in the whole spectral range up to 160 eV. In addition, we demonstrate that it is possible to obtain a single attosecond pulse at the exit of the interaction region even by starting from two laser pulses of 50 fs each. [ABSTRACT FROM PUBLISHER]

Abstract: We investigate the spontaneous parametric down-conversion (SPDC) process in biaxial nonlinear crystal BiB3O6 (BIBO) with a broadband pump. Under the type I and type II phase-matching conditions, we numerically calculate the influencing factors on SPDC process in BIBO crystal, such as temporal and spatial walk-off, the acceptance angles, and spectral acceptance bandwidth. Comparing the two types of phase-matching, we could conclude that the type II phase-matching in BIBO crystal is better for the SPDC because of zero dispersion and bigger acceptance parameters. These results can be used to construct a system to generate ultra-fast entangled photon pairs. [Copyright &y& Elsevier]

• Good agreement of theoretical model with experimental results for hollow-core gas cells. • HHG efficiency of 10−8–10−7 in cut-off region with orders higher than the 51st. • Stable operation of HHG light source for hours. • HHG system for various applications including actinic EUV mask inspection. We have developed a coherent and compact extreme-ultraviolet light source based on phase-matched high harmonic generation by using a hollow-core gas cell that can be easily aligned and maintain long stability. The gas cell consists of three bodies to minimize gas leakage and maintain stable vacuum. The photon flux was calculated considering the dependence of the gas number density and the pressure on the radii of the hollow core and the hole in the Al plates on both sides of the gas cell, which showed good coincidence with the experimental values. The photon flux is 2 × 1010 photons s−1 near 13.5 nm (91.5 eV) within 10 nm bandwidth. Also, the compact extreme-ultraviolet light source is found to operate stably for hours, which is suitable for practical applications such as metrology and inspection for extreme-ultraviolet mask, spectroscopy, holography, microscopy, etc. [ABSTRACT FROM AUTHOR]

We theoretically investigate the effective nonlinear coefficient deff of spontaneous parametric down-conversion (SPDC) process in biaxial nonlinear crystal BiB3O6 (BIBO). We solve the Fresnel equations in the collinear degenerate down-converted configuration, and get the relations of phase-matching angles. Considering the double phase-matching, we compute the phase-matching angles with different wavelength. Furthermore, we simplify the expression of effective nonlinear coefficient out of the principal planes, and numerically calculate it varying with the angle and wavelength. These results indicate that there are three effective phase-matching schemes for SPDC which can be used to construct a system to generate entangled photon pairs. [ABSTRACT FROM AUTHOR]

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/Al0:5Ga0:5As. 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 p1 = 9:70 m and p2 = 10:64 m, 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. [ABSTRACT FROM AUTHOR]

In order to enhance optical third-harmonic generation (THG) in polymer films, we investigated linear and nonlinear optical properties of polymer films containing ethyl-violet with varying thickness. The THG intensity increased monotonically with thickness, as opposed to the theory for non-phase-matched THG in films thicker than the coherence length. By measuring the refractive indices, we found that the difference between the indices of the fundamental and the third-harmonic was smaller in thicker films. Thus, the coherence lengths for thicker films were evaluated to be larger than those for the thin films, explaining the monotonic increase of the THG signal with thickness. [ABSTRACT FROM AUTHOR]

We present a method to trace quantum trajectories of electrons in a high-order harmonic generation process. The method relies on the saddle-point technique and, unlike previously developed analytical techniques, it enables an arbitrary variation in time of the driving fields. The trajectory phases yield information about phase-matching conditions at the optical cycle level, especially useful in investigating attosecond pulse generation. Specially prepared laser pulses, as well as propagation effects can be included in these phase-matching calculations. The method can handle both one- and two-dimensional electron trajectories produced by corresponding driving fields. We show the potentials of the method by calculating trajectories and phase-matching conditions for a polarization gate configuration proposed recently [Altucci, C.; Esposito, R.; Tosa, V.; Velotta, R. Opt. Lett. 2008, 33, 2943-2945.] to obtain isolated attosecond pulse from multi-cycle laser pulses. [ABSTRACT FROM AUTHOR]

Abstract: A novel optical waveguide isolator based on self-phase compensated TE–TM mode conversion was proposed. In this work, the troublesome phase-match condition for the TE–TM mode conversion is eliminated without degradation on the performances. An isolator based on this mechanism is designed and simulated. An insertion loss of −0.1dB and an isolation of −25dB was obtained. The tolerance of fabrication and bandwidth of the isolator was also discussed. [Copyright &y& Elsevier]

In this paper, we consider second harmonic generation and its phase-matching in an underdense plasma in the presence of a wiggler magnetic field. Wiggler magnetic field plays both a dynamic role in producing the traverse harmonic current and a kinematical role in ensuring phase-matching. The inertial ponderomotive force ρ(u⃗ · ∇⃗)u⃗ is a source of harmonic generation. u⃗ beats with itself to produce a different harmonic. The inertial ponderomotive force can also affect the efficiency of second harmonic generation; and its effect on second harmonic generation is also considered. [ABSTRACT FROM AUTHOR]

Standard quasi-phase-matching (QPM) schemes assume exact momentum relations between the interacting beams, thus necessitating the direct use of a reciprocal lattice vector to satisfy momentum balance. Usage of finite width beams for colinear interactions permits to use a projection of the reciprocal lattice vector along the propagation direction of the beams. We analytically derive this result and exam the new options given by this projection-based QPM for the analysis and design of nonlinear optical devices. [ABSTRACT FROM AUTHOR]

Abstract: We report on recent developments of widely tunable, continuous-wave (CW) laser sources based on difference-frequency generation (DFG) in nonlinear optical materials. The state-of-the-art of CW DFG technology will be reviewed. Applications of the DFG-based laser source to high-resolution molecular spectroscopy and trace gas detection will be presented. New development trends of DFG will be discussed. To cite this article: W. Chen et al., C. R. Physique 8 (2007). [Copyright &y& Elsevier]

In this paper, the properties of nonsilica glasses and the related technology for microstructured fiber fabrication are reviewed. Numerical simulation results are shown using the properties of nonsilica microstructured fibers for mid-infrared (mid-IR) supercontinuum generation when seeding with near-IR, 200-fs pump pulses. In particular, bismuth glass small-core fibers that have two zero-dispersion wavelengths (ZDWs) are investigated, and efficient mid-IR generation is enabled by phase-matching of a 2.0-mum seed across the upper ZDW into the 3-4.5 mum wavelength range. Fiber lengths considered were 40 mm. Simulation results for a range of nonsilica large-mode fibers are also shown for comparison. [ABSTRACT FROM PUBLISHER]

Abstract: We report, for the first time, an efficient intra-cavity second-harmonic generation (SHG) at 1084nm in a nonlinear optical crystal, BiB3O6(BIBO) at the direction of (θ, ϕ)=(170.1°,90°), performed with a LD end-pumped cw Nd:YVO4 laser. With 590mW diode pump power, a continuous-wave (cw) SHG output power of 19mW at 542nm yellow–green color has been obtained using a 1.5mm-thick BIBO crystal. The optical conversion efficiency was 3.22%. It was found that the output wavelength could be 532nm, 537nm or 542nm according to regulating the angle of BIBO. [Copyright &y& Elsevier]

Coherent beams at soft X-ray (SXR) wavelengths can be generated using extreme nonlinear optics by focusing an intense laser into a gas. In this paper, we discuss phase-matching and quasi-phase-matching techniques that use gas-filled modulated waveguides to enhance the frequency conversion process. This leads to the generation of SXR beams that are both spatially and temporally coherent. [ABSTRACT FROM AUTHOR]