Two-dimensional non-spatial filtering based on holographic Bragg gratings

The filter made up of two gratings performs as a two-dimensional non-spatial filtering. This paper reports that the volume Bragg gratings are fabricated by interfering two collimated coherent laser beams in photopolymer. Diffraction efficiency of a single grating is up to 78% in Bragg＇s condition, then a two-dimensional non-spatial filter, which consists of two volume Bragg gratings and a half-wave plate, enables the laser beam filtered in two dimensions with the diffraction efficiency of 54%. The Bragg＇s condition and effect of polarisation on performances of the two-dimension filter are also discussed.

Two-Dimensional Talbot Effect with Atomic Density Gratings

We report the experimental observation of two-dimensional Talbot effect when a resonance plane wave interacts with a two-dimensional atomic density grating generated by standing wave manipulation of ultracold Bose gases. Clear self-images of the grating and sub-images with reversed phase or fractal patterns are observed. By calculating the autocorrelation functions of the images, the behavior of periodic Talbot images is studied. The Talbot effect with two-dimensional atomic density grating expands the applications of the Talbot effect in a wide variety of research fields.

Influence of Surface Geometry of Grating Substrate on Director in Nematic Liquid Crystal Cell

The director in nematic liquid crystal cell with a weak anchoring grating substrate and a strong anchoring planar substrate is relative to the coordinates x and z.The influence of the surface geometry of the grating substrate in the cell on the director profile is numerically simulated using the two-dimensional finite-difference iterative method under the condition of one elastic constant approximation and zero driven voltage.The deepness of groove and the cell gap affect the distribution of director.For the relatively shallow groove and the relatively thick cell gap,the director is only dependent on the coordinate z.For the relatively deep groove and the relatively thin cell gap,the director must be dependent on the two coordinates x and z because of the increased elastic strain energy induced by the grating surface.

Polarization diversity two-dimensional grating coupler on x-cut lithium niobate on insulator

We propose and demonstrate a polarization diversity two-dimensional grating coupler based on the lithium niobate on insulator platform, for the first time, to the best of our knowledge. The optimization design, performance characteristics,and fabrication tolerance of the two-dimensional grating coupler are thoroughly analyzed utilizing the three-dimensional finite-difference time-domain method. Experimentally,-7.2 d B of coupling efficiency is achieved with 1 d B bandwidth of64 nm. The polarization-dependent loss is about 0.4 d B around 1550 nm. Our work provides new polarization multiplexing approaches for the lithium niobate on insulator platform, paving the way for critical applications such as high-speed polarization multiplexed electro-optical modulators.

Optical true time-delay for two-dimensional phased array antennas using compact fiber grating prism

A two-dimensional(2D) optical true-time delay(TTD) beam-forming system using a compact fiber grating prism(FGP) for a planar phased array antenna(PAA) is proposed. The optical beam-forming system mainly consists of a TTD unit based on the same compact FGP, one tunable laser for elevation beam steering, and a controlled wavelength converter for azimuth beam steering. A planar PAA using such 2D optical TTD unit has advantages such as compactness, low bandwidth requirement for tunable laser sources, and potential for large-scale system implementations. The proof-of-concept experiment results demonstrate the feasibility of the proposed scheme.

Atomically thin optical lenses and gratings

Two-dimensional(2D)materials have emerged as promising candidates for miniaturized optoelectronic devices due to their strong inelastic interactions with light.On the other hand,a miniaturized optical system also requires strong elastic light–matter interactions to control the flow of light.Here we report that a single-layer molybdenum disulfide(MoS2)has a giant optical path length(OPL),around one order of magnitude larger than that from a single-layer of graphene.Using such giant OPL to engineer the phase front of optical beams we have demonstrated,to the best of our knowledge,the world’s thinnest optical lens consisting of a few layers of MoS2 less than 6.3 nm thick.By taking advantage of the giant elastic scattering efficiency in ultra-thin high-index 2D materials,we also demonstrated high-efficiency gratings based on a single-or few-layers of MoS2.The capability of manipulating the flow of light in 2D materials opens an exciting avenue towards unprecedented miniaturization of optical components and the integration of advanced optical functionalities.More importantly,the unique and large tunability of the refractive index by electric field in layered MoS2 will enable various applications in electrically tunable atomically thin optical components,such as micro-lenses with electrically tunable focal lengths,electrical tunable phase shifters with ultra-high accuracy,which cannot be realized by conventional bulk solids.

Angular filtering by volume Bragg grating in photothermorefractive glass for nanosecond laser pulse

The two-dimensional angular filter based on volume Bragg gratings in photothermorefractive glass for a nanosecond(ns) laser pulse is demonstrated. The experimental results show that the near-field beam quality of the laser pulse was effectively improved. The near-field modulation and contrast ratio were improved by 1.75 and4.48 times, respectively. The power spectral density curves showed that the spatial frequencies more than0.9 mm^(-1)in the x direction and 1.2 mm^(-1)in the y direction were effectively suppressed.

Dual-polarization multiplexing amorphous Si:H grating couplers for silicon photonic transmitters in the photonic BiCMOS backend of line

In this paper,we report on polarization combining two-dimensional grating couplers(2D GCs)on amorphous Si:H,fabricated in the backend of line of a photonic BiCMOS platform.The 2D GCs can be used as an interface of a hybrid silicon photonic coherent transmitter,which can be implemented on bulk Si wafers.The fabricated 2D GCs operate in the telecom C-band and show an experimental coupling efficiency of−5 dB with a wafer variation of±1.2 dB.Possibilities for efficiency enhancement and improved performance stability in future design generations are outlined and extension toward O-band devices is also investigated.

Extraction of terahertz emission from a grating-coupled high-electron-mobility transistor

In a grating-coupled high-electron-mobility transistor, weak terahertz emission with wavelength around 400um was observed by using a Fourier-transform spectrometer. The absolute terahertz emission power was extracted from a strong background blackbody emission by using a modulation technique. The power of terahertz emission is proportional to the drain-source current, while the power of blackbody emission has a distinct relation with the electrical power. The dependence on the drain-source bias and the gate voltage suggests that the terahertz emission is induced by accelerated electrons interacting with the grating.

单轴晶体中垂直光轴方向上的二维泰伯效应

基于单轴晶体光束传输理论,从理论上推导了二维正交周期结构光场沿垂直单轴晶体光轴方向传播产生泰伯效应的公式。给出了泰伯效应的产生条件和常规泰伯距离的表达式,常规泰伯距离的大小决定于单轴晶体寻常光的折射率n_(o)与非寻常光折射率n_(e)之比和二维周期结构的x方向周期p_(x)和y方向周期p_(y)之比。模拟了二维正弦光栅和二维项链光栅的光场在单轴晶体中因各向异性衍射产生的泰伯效应。两种情况都能在更短的约化泰伯距离处产生与原光栅相同的泰伯像,在半约化泰伯距离产生与原光栅互补的像;在1/4约化泰伯距离和3/4约化泰伯距离产生的泰伯子像强度分布的周期为原光栅的1/2,相位变化的周期与原光栅相同,但是在空间上有半个周期的移动。本研究将泰伯效应拓展到各向异性介质中的传播动力学,加深了人们对周期性光场的传输理论的理解。