### Comparison of state-of-the-art Computer Generated Holography algorithms and a machine learning approach

Author(s):

Abstract:

“This work studies the use of machine learning and, in particular, a Convolutional Neural Network (CNN) to generate digital holograms and how such a network compares to state-of-the-art iterative methods, both in terms of reconstruction quality and computation time. Since CNNs only require a single pass through the network by a target image to generate a result, and not tens or hundreds of expensive iterations as in the iterative methods, they may be able to accomplish real-time hologram generation; an ability that could open the technology to proper commercial use.

In this work, a CNN built on the UNet architecture, capable of hologram generation, is presented. The network is trained on 4096 images of varying spatial frequencies, both user-generated and from the DIV2K dataset. It is compared to the most common iterative method for hologram generation, namely the Gerchberg–Saxton(GS) algorithm and its modern and improved implementations. In reconstruction quality, the neural network outperforms the original implementation of GS when evaluating Mean Square Error (MSE), geometric error (GE), Structural Similarity Index Measurement (SSIM), and Peak Signal-Noise Ratio (PSNR) of 64 unseen test images. However, on the same test images, the network lacks behind the modern, optimized GS implementations in all error and accuracy measurements. The network does, however, achieve these results at a rate 70–280 times faster than the iterative methods, depending on the particular implementation of the GS algorithm, which corresponds to a possible generation rate of the network of 32 FPS on average.”

Publication: Optics Communications
Issue/Year: Optics Communications, Pages 127590; 2021
DOI: 10.1016/j.optcom.2021.127590

### Synthetic helical dichroism for six-dimensional optical orbital angular momentum multiplexing

Author(s):

Ouyang, Xu; Xu, Yi; Xian, Mincong; Feng, Ziwei; Zhu, Linwei; Cao, Yaoyu; Lan, Sheng; Guan, Bai-Ou; Qiu, Cheng-Wei; Gu, Min & Li, Xiangping

Abstract:

“Optical multiplexing by creating orthogonal data channels has offered an unparalleled approach for information encoding with substantially improved density and security. Despite the fact that the orbital angular momentum (OAM) of light involves physical orthogonal division, the lack of explicit OAM sensitivity at the nanoscale prevents this feature from realizing nanophotonic information encoding. Here we demonstrate the viability of nanoscale information multiplexing utilizing the OAM of light. This is achieved by discovering OAM-dependent polarization ellipses in non-paraxial focusing conditions and hence synthetic helical dichroism resulting from the distinct absorption of achiral nanoparticles to the different order of OAM beams. Leveraging this mechanism, the application of subwavelength-scale focused OAM beams to self-assemble plasmonic nanoaggregates further enables six-dimensional optical information multiplexing, in conjunction with wavelength, polarization and three spatial dimensions. Our results suggest the possibility of multiplexing OAM division as an unbounded degree of freedom for nanophotonic information encoding, security imprinting and beyond.”

Publication: Nature Photonics
Issue/Year: Nature Photonics, 2021
DOI: 10.1038/s41566-021-00880-1

### Fast calculation of computer generated hologram based on single Fourier transform for holographic three-dimensional display

Author(s):

Chang, Chenliang; Zhu, Dongchen; Li, Jiamao; Wang, Di; Xia, Jun & Zhang, Xiaolin

Abstract:

“We present an efficient method for the fast calculation of computer generated hologram (CGH). The 3D object is split into sub-layers according to its depth information. A 2D all-in-focus image is generated by sequential tiling all the layers in one plane. A Fourier hologram that contains all the information of 3D object is calculated from the fast Fourier transform (FFT) of the reassembled 2D image. By multiplying a pre-calculated multifocal off-axis digital phase mask (DPM) to the Fourier hologram, the content of each layer is axially relocated to different depth in the Fourier transform optical system to reconstruct the 3D object. The computation speed of the proposed method is greatly improved with only single FFT calculation process. Both of simulation and experimental results proves the validation of the proposed method.”

Publication: Displays
Issue/Year: Displays, Volume 69; Pages 102064; 2021
DOI: 10.1016/j.displa.2021.102064

### Recognizing fractional orbital angular momentum using feed forward neural network

Author(s):

Jing, Guoqing; Chen, Lizhen; Wang, Peipei; Xiong, Wenjie; Huang, Zebin; Liu, Junmin; Chen, Yu; Li, Ying; Fan, Dianyuan & Chen, Shuqing

Abstract:

“Fractional vortex beam (FVB) possessing helical phase can be applied in the shift-keying communication due to its fractional orbital angular momentum (FOAM) mode, which theoretically allows an infinite increase of the transmitted capacity. However, the discontinuity of spiral phase makes FVB more likely to be disturbed in turbulence environment, and the precise measurement of distorted FOAM modes is crucial for practical FOAM-based communication application. Here, we proposed a FOAM mode recognition method with feedforward neural network (FNN). Employing the diffraction preprocessing of a two-dimensional fork grating, the original optical features of FVBs can be extended along the far-field diffraction order, endowing FNN more feature information and saving calculation time, and enlarging the detection range to conjugate FOAM modes. The simulation results show that the 9-layer FNN can identify FOAM mode with interval of 0.1 with an accuracy of 99.1% under the turbulences of $$C^n_2=1×10^{–14}m^{–2/3}$$ and Δz=10m. Furthermore, we experimentally constructed a 102-ary FOAM shift-keying communication link to transmit gray image, and the signals are successfully demodulated by the FNN model with the pixel-error-rate of 0.07160. It is anticipated that the proposed FNN-based FOAM recognition method will break the limitation of precision measurement under turbulence environment in practical FOAM applications.”

Publication: Results in Physics
Issue/Year: Results in Physics, Volume 28; Pages 104619; 2021
DOI: 10.1016/j.rinp.2021.104619

### Experimental investigation in Airy transform of Gaussian beams with optical vortex

Author(s):

Xu, Yi-Qing; Li, Xia; Zhou, Lu; Zhou, Yi-Min; Wang, Fei & Zhou, Guo-Quan

Abstract:

“The Airy transform was first introduced for a Gaussian beam, and the output beam is an Airy beam. When the Gaussian beam is extended to the Gaussian beam with optical vortex, what kind of output beam will be achieved by executing the Airy transformation. Therefore, the experimental research on Airy transformation of a Gaussian beam with optical vortex is carried out, including the generation of Gaussian beams with optical vortex, the realization of Airy transform, and the related measurements of the output beams. The phase pattern is indirect measured and is recovered from the intensity pattern which is the interference result of a plane wave and the output beam. The experimental measurement results of the light intensity and the phase patterns of transformed Gaussian beams with the optical vortex are consistent with the corresponding numerical simulation results.

Based on the first and the second moments of light intensity, the centroid and the beam size are measured. According to the hyperbolic law of the beam width along the axial propagation distance, the propagation factor of the output beam is measured. The influences of the Airy coefficients and the topological charge on the intensity pattern, the phase pattern, the centroid, the beam size, and the propagation factor of transformed Gaussian beams with optical vortex are experimentally investigated, respectively. The intensity pattern, the phase pattern, the centroid, the beam size, and the propagation factor of a transformed Gaussian beam with optical vortex are also compared with those of the corresponding transformed Gaussian vortex beam. This experiment fully proves the effect of the optical vortex on the Airy transformation of Gaussian beams. Meanwhile, this study offers an optional method to generate Airy-like beams from Gaussian beams with optical vortex, which is beneficial to the applications of Gaussian beams with optical vortex.”

Publication: Results in Physics
Issue/Year: Results in Physics, Volume 28; Pages 104588; 2021
DOI: 10.1016/j.rinp.2021.104588

### Representation of total angular momentum states of beams through a four-parameter notation

Author(s):

Fu, Shiyao; Hai, Lan; Song, Rui; Gao, Chunqing & Zhang, Xiangdong

Abstract:

“It has been confirmed beams carrying total angular momentums (TAMs) that consist of spin angular momentums (SAMs) and orbital angular momentums (OAMs) are widely used in classical and quantum optics. Here we propose and demonstrate a new kind of representation consisting of four real numbers to describe the TAM states of arbitrary beams. It is shown that any homogeneous polarization, scalar vortices and complex vectorial vortex field, all of which result from the TAMs of photons, can be well represented conveniently using our proposed four-parameter representation. Furthermore, the proposed representation can also reveal the internal change of TAMs as the conversion between SAMs and OAMs. The salient properties of the proposed representation is to give a universal form of TAMs associated with complicated polarizations and more exotic vectorial vortex beams, which offer an important basis for the future applications”

Publication: New Journal of Physics
Issue/Year: New Journal of Physics, Volume 23; Number 8; Pages 083015; 2021
DOI: 10.1088/1367-2630/ac1695

### Optically Manipulated Microtools to Measure Adhesion of the Nanoparticle-Targeting Ligand Glutathione to Brain Endothelial Cells

Author(s):

Tamás Fekete, Mária Mészáros, Zsolt Szegletes, Gaszton Vizsnyiczai, László Zimányi, Mária A. Deli, Szilvia Veszelka*, and Lóránd Kelemen

Abstract:

“Targeting nanoparticles as drug delivery platforms is crucial to facilitate their cellular entry. Docking of nanoparticles by targeting ligands on cell membranes is the first step for the initiation of cellular uptake. As a model system, we studied brain microvascular endothelial cells, which form the anatomical basis of the blood–brain barrier, and the tripeptide glutathione, one of the most effective targeting ligands of nanoparticles to cross the blood–brain barrier. To investigate this initial docking step between glutathione and the membrane of living brain endothelial cells, we applied our recently developed innovative optical method. We present a microtool, with a task-specific geometry used as a probe, actuated by multifocus optical tweezers to characterize the adhesion probability and strength of glutathione-coated surfaces to the cell membrane of endothelial cells. The binding probability of the glutathione-coated surface and the adhesion force between the microtool and cell membrane was measured in a novel arrangement: cells were cultured on a vertical polymer wall and the mechanical forces were generated laterally and at the same time, perpendicularly to the plasma membrane. The adhesion force values were also determined with more conventional atomic force microscopy (AFM) measurements using functionalized colloidal probes. The optical trapping-based method was found to be suitable to measure very low adhesion forces (≤ 20 pN) without a high level of noise, which is characteristic for AFM measurements in this range. The holographic optical tweezers-directed functionalized microtools may help characterize the adhesion step of nanoparticles initiating transcytosis and select ligands to target nanoparticles.”

Publication: ACS Applied Materials & Interfaces
Issue/Year: ACS Applied Materials & Interfaces, Volume 13; Number 33; Pages 39018–39029; 2021
DOI: 10.1021/acsami.1c08454

### Experimental analysis of adaptive optics correction methods on the beam carrying orbital angular momentum mode through oceanic turbulence

Author(s):

Zhan, Haichao; Wang, Le; Wang, Wennai & Zhao, Shengmei

Abstract:

“In recent years, as the demand for underwater communication has increased, underwater wireless optical communication (UWOC) has attracted a lot of attentions. Meanwhile, orbital angular momentum (OAM) has been applied in UWOC system to increase communication link capacity. However, the aberrations caused by oceanic turbulence (OT) is unavoidable, which results in serious intermodal crosstalk. Usually, adaptive optics (AO) is used to compensate these distortion aberrations. In this work, we experimentally demonstrate the influence of the distortion caused by OT, and evaluate the performance of AO correction algorithms, including Shack-Hartmann (SH), Stochastic-Parallel-Gradient-Descent (SPGD) and Gerchberg-Saxton (GS) algorithms. During the experiment, OT is simulated by using improved random phase screen model. We discuss the influence of various parameters of OT, including relative strength of temperature and salinity, strength of OT , and propagation distance, on the compensation effect with and without correction algorithms. The results show that all the three AO algorithms have good compensation effect on the distortion caused by OT, and GS algorithm has a better capability than the other two algorithms. Additionally, when the iteration numbers is less than 100, the compensation effect of GS and SPGD algorithms becomes more obvious as the iteration numbers increases, and GS algorithm is superior to SPGD algorithm. This work is beneficial to aberration correction of OAM-based UWOC system.”

Publication: Optik
Issue/Year: Optik, Volume 240; Pages 166990; 2021
DOI: 10.1016/j.ijleo.2021.166990

### Predicting optical transmission through complex scattering media from reflection patterns with deep neural networks

Author(s):

Skarsoulis, Kyriakos; Kakkava, Eirini & Psaltis, Demetri

Abstract:

“Deep neural networks (DNNs) are used to reconstruct transmission speckle intensity patterns from therespective reflection speckle intensity patterns generated by illuminated parafilm layers. The dependence ofthe reconstruction accuracy on the thickness of the sample is examined for different illumination patterns ofvarious feature sizes. High reconstruction accuracy is obtained even for large parafilm thicknesses, for whichthe memory effect of the sample is vanishingly small. The generalization capability of the DNN is also studiedfor unseen scatterers of the same type.”

Publication: Optics Communications
Issue/Year: Optics Communications, Volume 492; Pages 126968; 2021
DOI: 10.1016/j.optcom.2021.126968

### Focusing light into scattering media with ultrasound-induced field perturbation

Author(s):

Cheng, Zhongtao & Wang, Lihong V.

Abstract:

“Focusing light into scattering media, although challenging, is highly desirable in many realms. With the invention of time-reversed ultrasonically encoded (TRUE) optical focusing, acousto-optic modulation was demonstrated as a promising guidestar mechanism for achieving noninvasive and addressable optical focusing into scattering media. Here, we report a new ultrasound-assisted technique, ultrasound-induced field perturbation optical focusing, abbreviated as UFP. Unlike in conventional TRUE optical focusing, where only the weak frequency-shifted first-order diffracted photons due to acousto-optic modulation are useful, here UFP leverages the brighter zeroth-order photons diffracted by an ultrasonic guidestar as information carriers to guide optical focusing. We find that the zeroth-order diffracted photons, although not frequency-shifted, do have a field perturbation caused by the existence of the ultrasonic guidestar. By detecting and time-reversing the differential field of the frequency-unshifted photons when the ultrasound is alternately ON and OFF, we can focus light to the position where the field perturbation occurs inside the scattering medium. We demonstrate here that UFP optical focusing has superior performance to conventional TRUE optical focusing, which benefits from the more intense zeroth-order photons. We further show that UFP optical focusing can be easily and flexibly developed into double-shot realization or even single-shot realization, which is desirable for high-speed wavefront shaping. This new method upsets conventional thinking on the utility of an ultrasonic guidestar and broadens the horizon of light control in scattering media. We hope that it provides a more efficient and flexible mechanism for implementing ultrasound-guided wavefront shaping.”