Self-collimation and beam splitting in low-index photonic crystals

We study self-collimation and beam splitting in low-refractive-index photonic crystals created within chalcogenide glass. We propose a beam splitter structure that allows direct experimental verification of photonic-crystal effects at optical wavelengths in a straightforward and definitive manner. The beam splitter provides angular separation of 90° using a highly compact spatial footprint, thus delivering direct application in highly integrated photonic devices.     

Third-Order Optical Nonlinearity in Two-Dimensional Transition Metal Dichalcogenides

We present a detailed calculation of the linear and nonlinear optical response of four types of monolayer twodimensional (2D) transition-metal dichalcogenides (TMDCs), having the formula MX₂ with M=Mo, W and X=S, Se. The calculations are based on 6-band tight-binding model of TMDCs, and then performing a semi-classical perturbation analysis of response functions. We numerically calculate the linear χ_μν⁽¹⁾ (-ω;ω) and nonlinear surface susceptibility tensors χ_μνζη⁽³⁾(-ω_Σ; ω_r; ω_s; ω_t) with ω_Σ=ω_r+ω_s+ω_t. Both non-degenerate and degenerate cases are studied for thirdharmonic generation and nonlinear refractive index, respectively. Computational results obtained with no external fitting parameters are discussed regarding two recent reported experiments on MoS₂, and thus we can confirm the extraordinarily strong optical nonlinearity of TMDCs. As a possible application, we demonstrate generation of a π/4-rotated squeezed state by means of nonlinear response of TMDCs, in a silica micro-disk resonator covered with the 2D material. Our proposed method will enable accurate calculations of nonlinear optical response, such as four-wave mixing and highharmonic generation in 2D materials and their heterostructures, thus enabling study of novel functionalities of 2D photonic integrated circuits.     

Ultra-broadband modulation instability gain characteristics in As_2S_3 and As_2Se_3 chalcogenide glass photonic crystal fiber

Based on the designed As_2Se_3 and As_2S_3 chalcogenide glass photonic crystal fiber(PCF) and the scalar nonlinear Schrdinger equation,the effects of pump power and wavelength on modulation instability(MI) gain are comprehensively studied in the abnormal dispersion regime of chalcogenide glass PCF.Owing to high Raman effect and high nonlinearity,ultra-broadband MI gain is obtained in chalcogenide glass PCF.By choosing the appropriate pump parameter,the MI gain bandwidth reaches 2738 nm for the As_2Se_3 glass PCF in the abnormal-dispersion region,while it is 1961 nm for the As_2S_3 glass PCF.     

CdSe infiltrated TiO2 based omnidirectional photonic crystals for visible light control

Control of light emission by engineering the electromagnetic vacuum is important in photonics for fundamental understanding as well as in applications. Omnidirectional photonic crystals such as logpile photonic crystals provide a very interesting system to study such phenomena. Here, we describe the first steps towards achieving systematic and reproducible control of light using titanium dioxide (TiO₂) logpile photonic crystals (PC) with incorporated cadmium chalcogenide (CdSe) quantum dots (QDs) (λ  600 nm) to potentially modify QD photoluminescence (PL). We present a technique for coating a monolayer of the QDs on the PC rods based on 3-mercaptopropyltrimethoxysilanol linkage and measured the resulting PL response. Comparison of the PL spectra to a finite difference time domain model shows the need for further localization of QDs and fabrication of additional PC unit cells to observe PL modification.     

光子晶体器件进展

光子晶体对光具有独特的局域、反射、传导、分束、耦合、调制、慢光等操纵能力,使其成为微/纳光电集成的重要材料之一。介绍了光子晶体的几种重要物理特性——带隙特性、慢光特性、自准直和负折射特性,叙述了光子晶体集成器件方面所取得的最新进展,对光子晶体器件的发展动向做了展望。     

Ge-Se基硫系玻璃在通信波段的三阶非线性与光谱特性研究

在Ge-Se二元体系中引入相同摩尔比的Ga, Sn, Sb, Te四种元素, 使用熔融淬冷法制备了一系列硫系玻璃. 利用吸收光谱获得了不同元素引入下硫系玻璃能带结构的变化, 并结合拉曼光谱详细研究了产生光学特性变化的微观表征. 使用Z扫描方法测试了各个硫系玻璃样品在1550 nm波长下的三阶非线性参数, 发现加入Sn的玻璃的三阶非线性折射率n₂最大, 达到了6.36×10^-17 m²/W, 且其品质因子大于23, 表明Sn引入能够增强硫系玻璃在通信波段的三阶非线性, 这一研究结果为以后的高性能红外器件的设计及制备提供了一种环保且性能优良的候选材料.     

非线性一维光子晶体波导光双稳

利用非线性折射率系数较大且非线性时间响应较快的CdS_xSe_1-x玻璃为材料,设计并制备了非线性一维光子晶体波导光双稳器件,该器件的折射率空间分布呈正弦形式。实验测得双稳开关的阈值功率密度为1.60×10⁵W/cm²,开关时间为63ps。采用时域有限差分方法讨论了光子晶体带隙随入射光强变化而移动的情况,随着入射光功率密度的增加,光子晶体的带隙中心向短波方向移动。同时计算了该器件的双稳特性,理论计算得到双稳开关的阈值功率密度为1.40×10⁵W/cm²,开关时间约为50ps。获得了理论与实验基本一致的结果。     

Enhancement of the second harmonic generation from monolayer WS2 coupled with a silica microsphere

Monolayer transition metal dichalcogenides (TMDs) are widely used for integrated optical and photoelectric devices. Owing to their broken inversion symmetry, monolayer TMDs have a large second-order optical nonlinearity. However, the optical second-order nonlinear conversion efficiency of monolayer TMDs is still limited by the interaction length. In this work, we theoretically study the second harmonic generation (SHG) from monolayer tungsten sulfide (WS₂) enhanced by a silica microsphere cavity. By tuning the position, size, and crystal orientation of the material, second-order nonlinear coupling can occur between the fundamental pump mode and different second harmonic cavity modes, and we obtain an optimal SHG conversion efficiency with orders of magnitude enhancement. Our work demonstrates that the microsphere cavity can significantly enhance SHG from monolayer 2D materials under flexible conditions.     

The geometric phase in nonlinear frequency conversion

The geometric phase of light has been demonstrated in various platforms of the linear optical regime, raising interest both for fundamental science as well as applications, such as flat optical elements. Recently, the concept of geometric phases has been extended to nonlinear optics, following advances in engineering both bulk nonlinear photonic crystals and nonlinear metasurfaces. These new technologies offer a great promise of applications for nonlinear manipulation of light. In this review, we cover the recent theoretical and experimental advances in the field of geometric phases accompanying nonlinear frequency conversion. We first consider the case of bulk nonlinear photonic crystals, in which the interaction between propagating waves is quasi-phase-matched, with an engineerable geometric phase accumulated by the light. Nonlinear photonic crystals can offer efficient and robust frequency conversion in both the linearized and fully-nonlinear regimes of interaction, and allow for several applications including adiabatic mode conversion, electromagnetic nonreciprocity and novel topological effects for light. We then cover the rapidly-growing field of nonlinear Pancharatnam-Berry metasurfaces, which allow the simultaneous nonlinear generation and shaping of light by using ultrathin optical elements with subwavelength phase and amplitude resolution. We discuss the macroscopic selection rules that depend on the rotational symmetry of the constituent meta-atoms, the order of the harmonic generations, and the change in circular polarization. Continuous geometric phase gradients allow the steering of light beams and shaping of their spatial modes. More complex designs perform nonlinear imaging and multiplex nonlinear holograms, where the functionality is varied according to the generated harmonic order and polarization. Recent advancements in the fabrication of three dimensional nonlinear photonic crystals, as well as the pursuit of quantum light sources based on nonlinear metasurfaces, offer exciting new possibilities for novel nonlinear optical applications based on geometric phases.     

非线性光学超构表面

在线性光学范畴内,人们已经通过亚波长尺度的超薄超构表面成功实现了对光的众多新颖特性的调控功能.其主要理念是通过对具有亚波长尺度且空间方向变化的超构功能基元进行特定的排列,从而实现对光的偏振、相位和振幅的有效控制.近来,超构表面上的非线性光学特性也引起了大家的广泛关注.在本综述中,我们对非线性光学超构表面的设计、超构功能单元的材料和对称性选择、非线性手性光学、非线性贝里几何相位和非线性波前整形等内容进行了总结;最后对非线性光学超构表面在调控光与物质的相互作用中面临的挑战和前景进行了展望.     

Tm~(3+)掺杂Ge-Ga-S玻璃微球-石英光纤锥耦合系统的荧光回廊模特性

以熔融淬冷法自制了Tm~(3+)掺杂Ge-Ga-S硫系玻璃,并以此为基质材料,用漂浮粉料熔融法制备了直径分布为50—200μm的高品质因数(Q10~4)的有源硫系玻璃微球谐振腔.在显微镜下优选出一颗表面质量好、球形度较高、直径为72.84μm的微球,与氢氧焰扫描拉锥法制备的一根腰锥直径为1.93μm的石英光纤锥进行近场耦合.根据基质材料的吸收光谱特性,选用808 nm的半导体激光器作为抽运源.实验测得光纤锥倏逝波场激发出了掺Tm~(3+)硫系玻璃微球在1460 nm附近的荧光回廊模式,其典型共振峰间隔为4.39 nm.实验测得的荧光回廊模式与米氏散射理论计算结果符合度较高(最大误差仅为0.047%),验证了本文提出的掺Tm~(3+)硫系微球制备及耦合工艺的可行性.     

Bandgap guidance in hybrid chalcogenide-silica photonic crystal fibers

We report a hybrid chalcogenide-silica photonic crystal fiber made by pressure-assisted melt-filling of molten glass. Photonic bandgap guidance is obtained at a silica core placed centrally in a hexagonal array of continuous centimeters-long chalcogenide strands with diameters of 1.45 μm. In the passbands of the cladding, when the transmission through the silica core is very weak, the chalcogenide strands light up with distinct modal patterns corresponding to Mie resonances. In the spectral regions between these passbands, strong bandgap guidance is observed, where the silica core transmission loss is 60 dB/cm lower. The pressure-assisted fabrication approach opens up new ways of integrating sophisticated glass-based devices into optical fiber circuitry with potential applications in supercontinuum generation, magneto-optics, wavelength selective devices, and rare-earth-doped amplifiers with high gain per unit length.     

On‐chip single photon sources using planar photonic crystals and single quantum dots

We review the basic light‐matter interactions and optical properties of chip‐based single photon sources, that are enabled by integrating single quantum dots with planar photonic crystals. A theoretical framework is presented that allows one to connect to a wide range of quantum light propagation effects in a physically intuitive and straightforward way. We focus on the important mechanisms of enhanced spontaneous emission, and efficient photon extraction, using all‐integrated photonic crystal components including waveguides, cavities, quantum dots and output couplers. The limitations, challenges, and exciting prospects of developing on‐chip quantum light sources using integrated photonic crystal structures are discussed.     

掺铒硫系玻璃的制备及其微结构光纤的中红外信号放大特性研究

为进一步揭示硫系玻璃基掺Er³⁺微结构光纤对于中红外波段信号的放大特性, 采用熔融淬火法研制了Er³⁺离子掺杂的Ga₅Ge₂₀Sb₁₀S₆₅硫系玻璃, 测试了玻璃样品的吸收光谱和2.7 μm波段荧光光谱, 利用Judd-Ofelt和Futchbauer-Ladenburg理论分别计算得到了Er³⁺离子的辐射跃迁概率、辐射寿命以及2.7 μm波段受激发射截面. 在此基础上, 建立了一个980 nm抽运下该玻璃基掺Er³⁺微结构光纤2.7 μm波段中红外信号的放大模型, 理论上研究了其作为2.7 μm波段中红外信号增益介质时的光放大特性. 结果显示, 硫系玻璃基掺Er³⁺微结构光纤具有优异的高增益和宽带放大品性. 在200 mW抽运功率激励下的100 cm光纤长度上, 最大小信号增益超过了40 dB, 高于30 dB信号增益的放大带宽达到了120 nm (2696—2816 nm). 研究表明, Ga₅Ge₂₀Sb₁₀S₆₅硫系玻璃基掺Er³⁺微结构光纤是一种理想的可应用于2.7 μm波段中红外宽带放大器的增益介质.     

Periodic,quasi‐periodic,and random quadratic nonlinear photonic crystals

Quadratic nonlinear photonic crystals are materials in which the second order susceptibility χ⁽²⁾ is spatially modulated while the linear susceptibility remains constant. These structures are significantly different than the more common photonic crystals, in which the linear susceptibility is modulated. Nonlinear processes in nonlinear photonic crystals are governed by the phase matching requirements, which are determined by the reciprocal lattice of these crystals. Therefore, the modulation of the nonlinear susceptibility enables to engineer the spatial and spectral response in various three‐wave mixing processes. It enables to support the efficient generation of new optical frequencies at multiple directions. We analyze three wave mixing processes in nonlinear photonic crystals in which the modulation is either periodic, quasi‐periodic, radially symmetric or even random. We discuss both one‐dimensional and two‐dimensional modulations. In addition to harmonic generations, we outline several new possibilities for all‐optical control of the spatial and polarization properties of optical beams in specially designed nonlinear photonic crystals.     

Directly laser-written integrated photonics devices including diffractive optical elements

Femtosecond laser-written integrated devices involving Fresnel Zone Plates (FZPs) and waveguide arrays are demonstrated as built-in optical couplers. These structures were fabricated in borosilicate glass using a direct laser writing technique. The optical properties of these integrated photonic structures were investigated using CW lasers and high-resolution CCDs. For a single FZP coupled to a single waveguide, the overall coupling efficiency was 9%. A multiplexed optical coupler composed of three FZP layers was demonstrated to couple three waveguides simultaneously in a waveguide array. Structures of this type can be used as platforms for multichannel waveguide coupling elements or as microfluidic sensors that require higher light collecting efficiency.     

Ultra broadband flat dispersion tailoring on reversed-rib chalcogenide glass waveguide

In this paper,we introduce a horizontal slot in the reversed-rib chalcogenide glass waveguide to tailor its dispersion characteristics.The waveguide exhibits a flat and low dispersion over a wavelength range of 1080 nm,in which the dispersion fluctuates between-10.6 ps·nm~(-1)·km~(-1) and +11.14 ps·nm~(-1)·km~(-1).The dispersion tailoring effect is due to the mode field transfer from the reversed-rib waveguide to the slot with the increase of wavelength,which results in the extension of the low dispersion band.Moreover,the nonlinear coefficient and the phase-matching condition of the fourwave mixing process in this waveguide are studied,showing that the waveguide has great potential in nonlinear optical applications over a wide wavelength range.     

Wide-angle stop-gap chalcogenide photonic crystals generated by direct multiple-line laser writing

We present the fabrication and the angle-resolved optical characterizations of three-dimensional chalcogenide photonic crystals with a wide-angle stop gap. Multiple-line scanning provides an effective remedy to the elongation of the focal spot in the z direction during direct laser writing fabrication in high refractive index and highly nonlinear chalcogenide glasses. The aspect ratio of the rods is reduced from 4.46 to 1.53, thus allowing the successful fabrication of three-dimensional chalcogenide photonic crystals with a face-centered cubic symmetry and quasi-circular rods. Suppression of the angle-resolved transmission spectra is observed at a wide range of incident angles.     

Ge-As-S硫系玻璃的结构与性能调控

制备了系列具有不同化学配比特征的Ge-As-S硫系玻璃,并研究了玻璃的结构、折射率和光学带隙(Eg).Ge-As-S玻璃具有以[Ge S_4]四面体和[As S_3]三角锥为骨架结构单元相互交联形成的连续网络结构,当S过量时,结构中出现S链或S_8环;当S不足时,结构中形成As_4S_4/As_4S_3分子,甚至出现大量AsAs/Ge-Ge同极键.玻璃的组成元素在2—10μm波段的摩尔折射度分别为R_(Ge)=9.83—10.42 cm~3/mol,RAs=11.72—11.87 cm~3/mol和R_S=7.78—7.86 cm3/mol.Ge-As-S玻璃的折射率与密度和组成元素的摩尔折射度之间存在较好的定量关系,可根据该定量关系在1%偏差内对玻璃的折射率进行预测或调控.提出了采用玻璃粉末的漫反射光谱确定可靠Eg的方法,通过该方法可获得玻璃的强吸收数据用于确定Eg.Ge-As-S玻璃的Eg与玻璃的平均键能之间存在较好的关联,S含量较高的玻璃更倾向于具有较大的平均键能,因此具有较大的Eg.     

All-optical switching in silicon photonic crystal waveguides by use of the plasma dispersion effect

Silicon photonic crystals offer new ways of controlling the propagation of light as well as new tools for the realization of high-density optical integration on monolithic substrates. However, silicon does not possess the strong nonlinearities that are commonly used in the dynamic control of optical devices. Such dynamic control is nevertheless essential if silicon is to provide the higher levels of functionality that are required for optical integration. We demonstrate that the combination of the refractive index change caused by the presence of photoexcited carriers, or so-called plasma dispersion, and photonic crystal properties such as photonic bandgaps, constitutes a powerful tool for active control of light in silicon integrated devices. We show close to 100% modulation depth near the photonic crystal band edge.