All-silicon chiral metasurfaces and wavefront shaping assisted by interference

Chiral metasurfaces have different electromagnetic responses with circularly polarized lights, showing as circular dichroism and optical activity. Here, a novel kind of all-silicon chiral metasurface is proposed by introducing destructive interference between achiral meta-atoms. The maximum value of circular dichroism spectra can reach 0.49. By adding an antireflective layer at the side of the silicon substrate, the maximum circular dichroism reaches 0.54. What is more, the bandwidth of circular dichroism greater than 0.4 reaches 0.15 THz. Two samples are fabricated to verify the feasibility of this scheme, and the experimental results are in good agreement with the simulations. In addition, the proposed scheme can also be used to generate various interesting functions, such as beam control and vortex generator. This flexible and efficient implementation solution of chiral metasurface can bring new ideas to the development of chiral devices in the future.     

Controllable and switchable chiral near-fields in symmetric graphene metasurfaces

A strong chiral near-field plays significant roles in the detection, separation and sensing of chiral molecules. In this paper, a simple and symmetric metasurface is proposed to generate strong chiral near-fields with both circularly polarized light and linearly polarized light illuminations in the mid-infrared region. Owing to the near-field interaction between plasmonic resonant modes of two nanosheets excited by circularly polarized light, there is a strong single-handed chiral near-field in the gap between the two graphene nanosheets and the maximum enhancement of the optical chirality could reach two orders of magnitude. As expected, the intensity and the response wavelength of the chiral near-fields could be controlled by the Fermi level and geometrical parameters of the graphene nanosheets, as well as the permittivity of the substrate. Meanwhile, based on the interaction between the incident field and scattered field, the one-handed chiral near-field in the gap also could be generated by the linearly polarized light excitation. For the two cases, the handedness of the chiral near-field could be switched by the polarized direction of the incident light. These results have potential opportunities for applications in molecular detection and sensing.     

Laser diode beam shaping by optical interference

We recently proposed a novel beam shaping technique that employs Lloyd’s mirror interference. In this study, we apply this technique to three commercial laser diodes: laser diodes used for optical pumping of solid-state lasers, for laser beam printers, and for laser displays. The elliptical output beams from these laser diodes could be transformed into nearly circular beams by inserting a mirror-polished GaAs substrate below the active layer of each laser diode and adjusting its height. The experimentally observed far-field patterns were predicted fairly well by numerical calculations based on Huygens’ integral. We confirmed that our beam shaping technique is applicable to laser diodes with various wavelengths and vertical beam divergence angles. We also describe the monolithic configuration of the beam shaping system, which can be fabricated by dry etching.     

Surface‐plasmon wavefront and spectral shaping by near‐field holography

Surface‐plasmon‐polariton waves are two‐dimensional electromagnetic surface waves that propagate at the interface between a metal and a dielectric. These waves exhibit unusual and attractive properties, such as high spatial confinement and enhancement of the optical field, and are widely used in a variety of applications, such as sensing and subwavelength optics. The ability to precisely control the spatial and spectral properties of the surface‐plasmon wave is required in order to support the growing interest in both research and applications of plasmonic waves, and to bring it to the next level. Here, we review the challenges and methods for shaping the wavefront and spectrum of plasmonic waves. In particular, we present the recent advances in plasmonic spatial and spectral shaping, which are based on the realization of plasmonic holograms for the optical nearfield.

     

Recent advances in wavefront shaping techniques for biomedical applications

Due to the highly inhomogeneous distributions of refractive indexes, light propagation in complex media such as biological tissue experiences multiple light scattering events. The suppression and control of multiple light scattering events are investigated because they offer the possibility of optical focusing and imaging through biological tissues, and they may open new avenues for diagnosis and treatment of several human diseases. In order to provide insight into how new optical techniques can address the issues of multiple light scattering in biomedical applications, the recent progress in optical wavefront-shaping techniques is summarized.     

Photoinjector beam quality improvement by shaping the wavefront of a drive laserwith oblique incidence

To increase the quantum efficiency (QE) of a copper photocathode and reduce the thermal emittance of an electron beam, a drive laser with oblique incidence was adopted in a BNL type photocathode rf gun. The disadvantageous effects on the beam quality caused by oblique incidence were analyzed qualitatively. A simple way to solve the problems through wavefront shaping was introduced and the beam quality was improved.     

Photoinjector beam quality improvement by shaping the wavefront of a drive laser with oblique incidence

To increase the quantum efficiency (QE) of a copper photocathode and reduce the thermal emittance of an electron beam, a drive laser with oblique incidence was adopted in a BNL type photocathode rf gun. The disadvantageous effects on the beam quality caused by oblique incidence were analyzed qualitatively. A simple way to solve the problems through wavefront shaping was introduced and the beam quality was improved.     

Synthetic holography in microscopy: opportunities arising from advanced wavefront shaping

The advent of computer-generated or synthetic holography has created a wealth of possibilities for wavefront shaping in optics. We discuss the impact this has had on optical microscopy. Synthetic Holographic Microscopy utilises wavefront shaping by a computer-generated ‘hologram’ (CGH) to modify light on the illumination or the detection side, or both. This enables modifications of the general sample appearance concerning contrast, resolution and other aspects. Multiplexing CGHs can perform several tasks at once, for instance splitting the image into sub-images corresponding to different depths in the sample, or displaying differently contrasted images of the sample, e.g. bright field, darkfield or (spiral) phase contrast, in different sub-images. We give an overview of the options and discuss the advantages and disadvantages of using programmable holographic elements inside an optical microscope.     

Some algorithms for data analysis in interference wavefront detectors

Conclusions Several comments may be made. Two method of organizing interference-pattern scanning may be organized. The first entails invoking the points of a scan from a dense Cartesian raster stored in a computer memory, while for each subsequent point the given scan-selection condition must be verified. The second involves controlling the scanning aperture, when the satisfaction of the selection condition is verified by a servosystem, changing the scanning line as necessary. No excess information is stored in the computer here. The comparative characteristics of these two versions remain to be elucidated.Some difficulties are involved in obtaining a priori information on the position of the extremal points of the phase and the form of the extremum. These complications are not specific to any specific recovery algorithm but are characteristic for interferometry in general. It is evidently necessary to count on the possibility of introducing a linear phase shift between the reference and object waves, which is necessary to eliminate extrema. In addition, it is possible to construct a flexible mirror such that the phase extrema will coincide with the points of force application.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 96–105, November, 1985.     

Sub-wavelength interference in the field assisted by surface plasmons

We present a theoretical study of the optical transmission from a thin metallic double slit. The second-order correlation function as a function of the displacement of the detectors for different values of slit separation is studied. It is shown that surface plasmons excited at one slit and propagating to the other slit modulate the coincidence counts with the variation of slit separation. Sub-wavelength interference effect has also been observed for the field assisted by surface plasmons. It is also shown that the second order interference-diffraction pattern changes with slit separation and at some particular value of slit separation it changes into the Hanbury Brown and Twiss (HBT) effect.     

Efficient manipulation of terahertz waves by multi-bit coding metasurfaces and further applications of such metasurfaces

Benefiting from the unprecedented superiority of coding metasurfaces at manipulating electromagnetic waves in the microwave band,in this paper,we use the Pancharatnam-Berry(PB)phase concept to propose a high-efficiency reflectivetype coding metasurface that can arbitrarily manipulate the scattering pattern of terahertz waves and implement many novel functionalities.By optimizing the coding sequences,we demonstrate that the designed 1-,2-,and 3-bit coding metasurfaces with specific coding sequences have the strong ability to control reflected terahertz waves.The two proposed1-bit coding metasurfaces demonstrate that the reflected terahertz beam can be redirected and arbitrarily controlled.For normally incident x-and y-polarized waves,a 10 d B radar cross-section(RCS)reduction can be achieved from 2.1 THz to5.2 THz using the designed 2-bit coding metasurface.Moreover,two kinds of orbital angular momentum(OAM)vortex beams with different moduli are generated by a coding metasurface using different coding sequences.Our research provides a new degree of freedom for the sophisticated manipulation of terahertz waves,and contributes to the development of metasurfaces towards practical applications.     

波前函数法在光波干涉教学中的应用

本文介绍了波前函数的概念,并利用波前函数法分析讨论了光的杨氏双缝干涉和薄膜干涉现象.传统光程法在讨论两列波干涉时比较直观简单,但在讨论多列波干涉的情况下,光程法简单性的优势将无从体现.和传统的光程法相比较,在讨论多列波相互叠加时,波前函数法的优势凸显,另一方面,波前函数法充分体现了波动的物理思想,刻画出电磁波的传播和叠加的物理图像,有助于学生深层次理解波动概念.     

平行光波阵面调制下的双频干涉条纹形貌测量

传统形貌测量多采用光栅投影,其投影条纹的非正弦性及条纹密度的限制将影响形貌测量的精度,为了实现高密度正弦条纹投影的可调性,采用平行光干涉投影形成明亮且对比度高的正弦条纹,避免条纹的正弦畸变。在平行光波阵面调制下,通过相控阵调节两光束交会角度得到所需的条纹频率,实现条纹投影的可调性,再将调制后双频应用到解包裹中提高解相精度。对比分析了单频和双频解包裹条件下,一个最大高度为35.80 mm物体形貌的恢复,其最高点恢复相对误差分别为2.7%、1.6%。实验结果表明该方法具有有效性与可行性,具有较高的精度。     

Three-dimensional spatial multi-point uniform light focusing through scattering media based on feedback wavefront shaping

We use feedback wavefront shaping technology to realize the multi-point uniform light focusing in three-dimensional(3 D) space through scattering media only by loading the optimal mask once.General 3 D spatial focusing needs to load the optimal mask multiple times to realize the spatial movement of the focal point and the uniformity of multi-point focusing cannot be guaranteed.First,we investigate the effects of speckle axial correlation and different axial distances on 3 D spatial multi-point uniform focusing and propose possible solutions.Then we use our developed non-dominated sorting genetic algorithm suitable for 3 D spatial focusing(S-NSGA) to verify the experiment of multi-point focusing in 3 D space.This research is expected to have potential applications in the fields of optical manipulation and optogenetics.     

High-efficiency wideband flat focusing reflector mediated by metasurfaces

We propose to achieve a high-efficiency wideband flat focusing reflector using metasurfaces. To obtain the wide band,the polarization conversion mechanism is introduced into the reflector design, based on the fact that the reflection phases of cross-polarized waves are linear in quite a wide band. This facilitates the design of wideband parabolic reflection phase profile. As an example, we design two reflective focusing metasurfaces with one- and two-dimensional in-plane parabolic reflection phase profiles based on elliptical split ring resonators(ESRRs). Both the simulation and experiment verify the wideband focusing performance in 10.0–22.0 GHz of the flat reflectors. Due to the wide operating band, such reflectors have important application values in communication, detection, measurement, imaging, etc.     

Functional and nonlinear optical metasurfaces

Optical metasurfaces are thin‐layer subwavelength‐patterned structures that interact strongly with light. Metasurfaces have become the subject of several rapidly growing areas of research, being a logical extension of the field of metamaterials towards their practical applications. Metasurfaces demonstrate many useful properties of metadevices with engineered resonant electric and magnetic optical responses combined with low losses of thin‐layer structures. Here we introduce the basic concepts of this rapidly growing research field that stem from earlier studies of frequency‐selective surfaces in radiophysics, being enriched by the recent development of metamaterials and subwavelength nanophotonics. We review the most interesting properties of photonic metasurfaces, demonstrating their useful functionalities such as frequency selectivity, wavefront shaping, polarization control, etc. We discuss the ways to achieve tunability of metasurfaces and also demonstrate that nonlinear effects can be enhanced with the help of metasurface engineering.

     

High-contrast imaging based on wavefront shaping to improve low signal-to-noise ratio photoacoustic signals using superpixel method

Photoacoustic(PA) imaging has drawn tremendous research interest for various applications in biomedicine and experienced exponential growth over the past decade. Since the scattering effect of biological tissue on ultrasound is two-to three-orders magnitude weaker than that of light, photoacoustic imaging can effectively improve the imaging depth.However, as the depth of imaging further increases, the incident light is seriously affected by scattering that the generated photoacoustic signal is very weak and the signal-to-noise ratio(SNR) is quite low. Low SNR signals can reduce imaging quality and even cause imaging failure. In this paper, we proposed a new wavefront shaping and imaging method of low SNR photoacoustic signal using digital micromirror device(DMD) based superpixel method. We combined the superpixel method with DMD to modulate the phase and amplitude of the incident light, and the genetic algorithm(GA) was used as the wavefront shaping algorithm. The enhancement of the photoacoustic signal reached 10.46. Then we performed scanning imaging by moving the absorber with the translation stage. A clear image with contrast of 8.57 was obtained while imaging with original photoacoustic signals could not be achieved. The proposed method opens new perspectives for imaging with weak photoacoustic signals.