Mid-Infrared-Wavelength Generation in 2-mum Pumped Periodically Poled Lithium Niobate

A periodically poled lithium niobate optical parametric oscillator pumped by a Tm:YAG laser at 2.0124-mum wavelength has been demonstrated. A pump pulse energy of 5.1 mJ generated 0.65 mJ of signal and idler pulse energy at a 50-Hz repetition frequency with a 27.8-mum domain-period-length grating. The lithium niobate crystal at a temperature of 180 degrees C yielded 3.61- and 4.55-mum signal and idler wavelengths, respectively. Wavelength tuning over a wide range was achieved with domain-period lengths from 25.5 to 28.2 mum and crystal temperature tuning from 50 to 180 degrees C. Signal wavelengths of 3.26-3.76 mum and idler wavelengths of 4.33-5.34 mum were generated.     

Second-harmonic Generation Efficiencies in Germanium-doped Planar Waveguides: A Normal-mode Analysis

Abstract

Second-harmonic generation (SHG) in germanium-doped silica planar waveguides seeded by a fundamental wave and its second harmonic is investigated using normal-mode analysis. The effects of self- and cross-phase modulation are included in this analysis. A set of two coupled equations that describe the evolution of the amplitude of the writing second-harmonic beam along the direction of propagation is obtained. The solution to these coupled equations is used to write the effective X ⁽²⁾ grating. This grating is then used to determine the second-harmonic power generated when the beam is read with only a fundamental wave propagating in a particular waveguiding mode. Due to diffraction, the conversion efficiency saturates as a function of waveguide length. It is also found that if the reading fundamental wave is in the same mode as the writing fundamental wave, relatively significant SHG is obtained with the generated second-harmonic wave propagating primarily in the same mode as the seed second-harmonic wave. The power generated in any of the higher order modes is of the same order as that for the lowest order mode.     

Second Harmonic Generation in a Free Electron Laser

It is pointed out that in a free electron laser higher frequencies, which are very nearly equal to the harmonics, are generated. Second harmonic generation, in stimulated emission in a spatially periodic static magnetic field, is studied using the Feynman diagram technique. The ratio of gain for the second harmonic compared to that of the fundamental harmonic is of the order of 10 ^?9 for the conditions present in the free electron laser of Deacon et al. [1].     

Imaging through Nonlinear Metalens Using Second Harmonic Generation

The abrupt phase change of light at metasurfaces provides high flexibility in wave manipulation without the need for accumulation of propagating phase through dispersive materials. In the linear optical regime, one important application field of metasurfaces is imaging by planar metalenses, which enables device miniaturization and aberration correction compared to conventional optical microlens systems. With the incorporation of nonlinear responses into passive metasurfaces, optical functionalities of metalenses are anticipated to be further enriched, leading to completely new application areas. Here, imaging with nonlinear metalenses that combine the function of an ultrathin planar lens with simultaneous frequency conversion is demonstrated. With such nonlinear metalenses, imaging of objects with near infrared light while the image appears in the second harmonic signal of visible frequency range is experimentally demonstrated. Furthermore, the functionality of these nonlinear metalenses can be modified by switching the handedness of the circularly polarized fundamental wave, leading to either real or virtual nonlinear image formation. Nonlinear metalenses not only enable infrared light imaging through a visible detector but also have the ability to modulate nonlinear optical responses through an ultrathin metasurface device while the fundamental wave remains unaffected, which offers the capability of nonlinear information processing with novel optoelectronic devices.     

近化学计量比铌酸锂晶体的研究进展

介绍了近化学计量比铌酸锂晶体的主要生长方法及其特点,总结了近化学计量比铌酸锂晶体的主要性能,通过与同成分铌酸锂晶体的比较,指出了近化学计量比铌酸锂晶体的主要应用优势,明确了该类晶体今后的研究重点.     

Metrology of Multiphoton Microscopes Using Second Harmonic Generation Nanoprobes

In multiphoton microscopy, the ongoing trend toward the use of excitation wavelengths spanning the entire near‐infrared range calls for new standards in order to quantify and compare the performances of microscopes. This article describes a new method for characterizing the imaging properties of multiphoton microscopes over a broad range of excitation wavelengths in a straightforward and efficient manner. It demonstrates how second harmonic generation (SHG) nanoprobes can be used to map the spatial resolution, field curvature, and chromatic aberrations across the microscope field of view with a precision below the diffraction limit and with unique advantages over methods based on fluorescence. KTiOPO4 nanocrystals are used as SHG nanoprobes to measure and compare the performances over the 850–1100 nm wavelength range of several microscope objectives designed for multiphoton microscopy. Finally, this approach is extended to the post‐acquisition correction of chromatic aberrations in multicolor multiphoton imaging. Overall, the use of SHG nanoprobes appears as a uniquely suited method to standardize the metrology of multiphoton microscopes.     

二次谐波在二维材料结构表征中的应用

二次谐波作为非线性光学的重要分支,逐渐成为表征晶体结构的重要手段之一。在众多表征方法中,二次谐波因其无损检测、高稳定性、可调谐性、超快响应、偏振敏感性、通用性、操作简单等特点被广泛应用于二维材料结构表征,为二维材料的物性研究和功能应用提供了重要信息,大大推动了二维材料基础研究的快速发展。本文综述了近几年二次谐波在二维材料结构表征中的研究,简述了二次谐波产生原理,介绍了飞秒激光器接入共聚焦拉曼光谱仪产生二次谐波测试装置,分别讨论了二次谐波在二维材料的层间堆垛层数、层间堆垛角度、单层二维材料晶界及晶体取向表征方面的应用。同时,本文还介绍了采用二次谐波强度直接、灵敏地检测晶体中应变幅度以及通过二次谐波信号变化跟踪材料中的缺陷变化,接着讨论了二次谐波与拉曼光谱、光致发光的多维度关联分析在材料全面深度表征方面的重要性。最后展望了二次谐波未来在材料结构表征中的潜在研究方向。     

Quasicrystal Photonic Metasurfaces for Radiation Controlling of Second Harmonic Generation

Photonic metasurfaces, a kind of 2D structured medium, represent a novel platform to manipulate the propagation of light at subwavelength scale. In linear optical regime, many interesting topics such as planar meta‐lenses, metasurface optical holography, and so on have been widely investigated. Recently, metasurfaces have gone into the nonlinear optical regime. While it is recognized that the local symmetry of the meta‐atoms plays a vital role in determining the polarization, phase, and intensity of the nonlinear waves, much less attention has been paid to the global symmetry of the nonlinear metasurfaces. According to the Penrose tiling and the newly proposed hexagonal quasicrystalline tiling, nonlinear optical quasicrystal metasurfaces are designed and fabricated based on the geometric‐phase‐controlled plasmonic meta‐atoms with local rotational symmetry. It is found that the far‐field radiation behavior of second harmonic generation waves are determined by both the tiling schemes of quasicrystal metasurfaces and the local symmetry of meta‐atoms they consist of. The proposed concept may open new avenues for designing nonlinear optical sources with metasurface crystals.     

Second Harmonic Generation in Optical Fibres: Multiphoton Absorption and Translation-inversion Symmetry

Abstract

Multiphoton absorption provides a route to x ₍₂₎ grating formation in optical fibres. It explains the efficacy of ‘seeding’ the fibre with green light to induce fibre second harmonic generation. The process depends on absorption sites with spatially inverted configurations having different multiphoton absorption rates.     

Structural Defects and Photoluminescence in Zinc-Doped Lithium Niobate Crystals

Inorganic Materials - We demonstrate that increasing the zinc concentration in Zn-doped lithium niobate crystals to their first concentration threshold (~3.0 mol % ZnO) leads to a decrease in...     

Review of Second Harmonic Generation Measurement Techniques for Material State Determination in Metals

This paper presents a comprehensive review of the current state of knowledge of second harmonic generation (SHG) measurements, a subset of nonlinear ultrasonic nondestructive evaluation techniques. These SHG techniques exploit the material nonlinearity of metals in order to measure the acoustic nonlinearity parameter, \(\beta \). In these measurements, a second harmonic wave is generated from a propagating monochromatic elastic wave, due to the anharmonicity of the crystal lattice, as well as the presence of microstructural features such as dislocations and precipitates. This article provides a summary of models that relate the different microstructural contributions to \(\beta \), and provides details of the different SHG measurement and analysis techniques available, focusing on longitudinal and Rayleigh wave methods. The main focus of this paper is a critical review of the literature that utilizes these SHG methods for the nondestructive evaluation of plasticity, fatigue, thermal aging, creep, and radiation damage in metals.     

Light Propagation in Singly and Doubly Periodic Planar Waveguides

Abstract

Light propagation in singly and doubly periodic planar waveguides is investigated with respect to future applications in integrated optics. The waveguides used in our experiments reveal, in the vicinity of Bragg reflection, a strong difference between the directions of phase and group velocities, the beam steering. A clear graphical representation of the observable propagation effects is given in wave-vector diagrams, showing the directional dispersion of the elementary waves in periodic structures, the Floquet-Bloch waves. The dispersion phenomena were measured with high accuracy, using selective wave excitation. In conjunction with straight tapered transitions to smooth planar waveguides, the periodic structures show a great variety of strong frequency and direction-dependent effects such as lateral beam shifting and focusing with a frequency-variant focal length. Ray optics of Floquet-Bloch waves is used to describe these phenomena. Complex interference patterns observable in the vicinity of Bragg reflection in doubly periodic structures are explained by the superposition of Floquet-Bloch waves. Examples of applications of planar periodic structures as highly selective frequency filters, optical multiplexers as well as frequency-tunable beam narrowing, focusing and expanding devices are given together with measured data.     

Photoelectric Fields and Band Gap in Doped Lithium Niobate Crystals

Photoinduced light scattering and optical spectroscopy have been used to study the photorefractive effect and determine the band gap in nominally undoped congruent and stoichiometric lithium niobate crystals, as well as in a series of congruent LiNbO₃ crystals doped with Mg, Zn, B, Gd, Y, and Er cations and LiNbO₃ single crystals codoped with Mg:Gd, Mg:Fe, Mg:Y, and Mg:Ta.     

Photoinduced Second Harmonic Generation of Bi2S3 Microcrystallite Doped Silica Glass

Silica glasses doped with Bi2S3 microcystallite was prepared by the sol-gel process. Photoinduced second harmonic generation (SHG) was observed in the glass when it was irradiated with intense 1.06 μm and frequency doubled laser beams from a mode-locked Nd: YAG laser. It was found that the signal intensity increased with the irradiating time and approached a saturation gradually. The effect may be explained reasonably by the DC field model.     

Investigation of Refractive-index Profiles of Ion-exchanged Planar Waveguides in Glass

Abstract

New data concerning the refractive-index profiles in Ag/Na ion-exchanged waveguides are obtained using a parabolic-segment reconstruction method. The profiles can be described as second-order polynomials, but with a mean curvature b = 2·8 which is significantly greater than the originally suggested value 0·64. The profile so obtained agrees very well with the common diffusion equation for the silver ion concentration only if one assumes a simple linear relationship between the diffusion coefficient and the concentration. This fact necessitates changes in present ideas about the ion-exchange mechanism in sodalime glasses.     

Improved Long-Term DC Drift in OH-Reduced Lithium Niobate Optical Intensity Modulators

Long-term dc drift of lithium niobate optical intensity modulators is successfully reduced by exclusion of OH(-)ions from the lithium niobate substrate. In this regard, the titanium-indiffused waveguide is prepared on a commercial OH-free substrate under a dry gas atmosphere.The dc drift measured for these hermetically sealed modulators indicates a possibility for continuous operation over 20 years at 50°C.