Nonconvex optimization for optimum retrieval of the transmission matrix of a multimode fiber

Transmission matrix (TM) allows light control through complex media such as multimode fibers (MMFs), gaining great attention in areas like biophotonics over the past decade. The measurement of a complex-valued TM is highly desired as it supports full modulation of the light field, yet demanding as the holographic setup is usually entailed. Efforts have been taken to retrieve a TM directly from intensity measurements with several representative phase retrieval algorithms, which still see limitations like slow or suboptimum recovery, especially under noisy environment. Here, a modified non-convex optimization approach is proposed. Through numerical evaluations, it shows that the nonconvex method offers an optimum efficiency of focusing with less running time or sampling rate. The comparative test under different signal-to-noise levels further indicates its improved robustness for TM retrieval. Experimentally, the optimum retrieval of the TM of a MMF is collectively validated by multiple groups of single-spot and multi-spot focusing demonstrations. Focus scanning on the working plane of the MMF is also conducted where our method achieves 93.6% efficiency of the gold standard holography method when the sampling rate is 8. Based on the recovered TM, image transmission through the MMF with high fidelity can be realized via another phase retrieval. Thanks to parallel operation and GPU acceleration, the nonconvex approach can retrieve an 8685$\times$1024 TM (sampling rate=8) with 42.3 s on a regular computer. In brief, the proposed method provides optimum efficiency and fast implementation for TM retrieval, which will facilitate wide applications in deep-tissue optical imaging, manipulation and treatment

Vortex-induced aggregation in superfluid helium droplets

The formation of Ag nanoparticles by the addition of Ag atoms to helium droplets has been investigated. The resulting nanoparticles were then imaged by transmission electron microscopy after being deposited on a thin solid surface. In large helium droplets chains of Ag nanorods were observed similar to recently reported track-like deposits [Gomez et al., Phys. Rev. Lett., 2012, 108, 155302]. However, by adjusting the experimental conditions chains of spherical nanoparticles could also be seen with a nearly uniform inter-particle spacing. Given that spherical Ag nanoparticles have no intrinsic anisotropy, the only viable explanation is that these particles mus

Wavefront shaping: A versatile tool to conquer multiple scattering in multidisciplinary fields

Optical techniques offer a wide variety of applications as light-matter interactions provide extremely sensitive mechanisms to probe or treat target media. Most of these implementations rely on the usage of ballistic or quasi-ballistic photons to achieve high spatial resolution. However, the inherent scattering nature of light in biological tissues or tissue-like scattering media constitutes a critical obstacle that has restricted the penetration depth of non-scattered photons and hence limited the implementation of most optical techniques for wider applications. In addition, the components of an optical system are usually designed and manufactured for a fixed function or performance. Recent advances in wavefront shaping have demonstrated that scattering- or component-induced phase distortions can be compensated by optimizing the wavefront of the input light pattern through iteration or by conjugating the transmission matrix of the scattering medium. This offers unprecedented opportunities in many applications to achieve controllable optical delivery or detection at depths or dynamically configurable functionalities by using scattering media to substitute conventional optical components. In this article, the recent progress of wavefront shaping in multidisciplinary fields is reviewed, from optical focusing and imaging with scattering media, functionalized devices, modulation of mode coupling, and nonlinearity in multimode fiber to multimode fiber-based applications. Apart from insights into the underlying principles and recent advances in wavefront shaping implementations, practical limitations and roadmap for future development are discussed in depth. Looking back and looking forward, it is believed that wavefront shaping holds a bright future that will open new avenues for noninvasive or minimally invasive optical interactions and arbitrary control inside deep tissues. The high degree of freedom with multiple scattering will also provide unprecedented opportunities to develop novel optical devices based on a single scattering medium (generic or customized) that can outperform traditional optical components

Stereoscopic 3D Web: From idea to implementation

In recent years so called 3D web has emerged that gives people to view 3D web contents in a standard web browser, and the representative technologies are Web3D, WebGL and O3D. But since the standard web browser has only a single view in which the 3D contents are shown on a 2D display device, therefore the 3D web is essentially a kind of monoscopic 3D. Nowadays with the rapid development of stereoscopic 3D display device, especially glasses-free auto-stereoscopic 3D display device, we think stereoscopic 3D web will become a hot research field, as that can be used to stereoscopic 3DTV GUI, stereoscopic 3D games, stereoscopic 3D photo gallery, stereoscopic 3D virtual reality, and so forth. In this paper we propose our idea and its implementation for stereoscopic 3D web, and give an example that has been used as a stereoscopic 3DTV GUI.Computer Science, Information SystemsEngineering, Electrical & ElectronicEICPCI-S(ISTP)

HOT MOLECULES IN HELIUM NANODROPLETS: A NEW ROUTE TO OPTICAL SPECTRA

Author Institution: Department of Chemistry, University of Leicester; University Road, Leicester, LE1 7RH, UKA new mechanism is described for recording optical spectra of molecules in helium nanodroplets. This "hot molecule" technique is applicable when optical excitation leads to a long-lived metastable excited state inside a helium droplet, which in turn changes the electron impact ionization cross section of specific ion product channels. This is illustrated by electronic excitation of toluene to its S1 state, which undergoes intersystem crossing into a long-lived triplet state with high quantum yield. By monitoring different ions, spectra in both depletion and enhancement modes have been obtained. The technique has potential for applications whenever optical excitation delivers a relatively long-lived (ms or longer) excited state and therefore complements existing depletion techniques, which require fast dissipation of energy into the helium matrix

Synthesis of Sodium Acrylate and Acrylamide Copolymer/GO Hydrogels and Their Effective Adsorption for Pb²⁺ and Cd²⁺

A serial of cross-linked sodium acrylate and acrylamide copolymer/graphene oxide (P­(AANa-<i>co</i>-AM)/GO) hydrogels with different mass ratios of acrylic acid (AA) to acrylamide (AM) and different contents of <i>N</i>,<i>N</i>-methylenebis­(acrylamide) (MBA) were prepared via dispersive polymerization. The prepared hydrogels were characterized by XRD, FT-IR, TGA, SEM, and BET in detail. The removal efficiency of Pb²⁺ and Cd²⁺ on the P­(AANa-<i>co</i>-AM)/GO hydrogel have been improved significantly due to the addition of 1.0 wt % of GO. The P­(AANa-<i>co</i>-AM)/GO hydrogel with AA/AM = 1 and 0.8 wt % of MBA has the highest removal efficiency for Pb²⁺ and Cd²⁺ and a suitable swelling ratio, which was selected as the adsorbent for systematic adsorption of Pb²⁺ and Cd²⁺ from aqueous solutions. The adsorption capacity and removal efficiency of Pb²⁺ and Cd²⁺ on the P­(AANa-<i>co</i>-AM)/GO hydrogel were dependent on pH, initial heavy metal ion concentration, and adsorbent dosage. The presence of 0.01–0.05 M Mg²⁺ or Ca²⁺ has an obvious effect on the adsorption of Cd²⁺. The adsorption kinetics and adsorption isotherms of Pb²⁺ and Cd²⁺ on the P­(AANa-<i>co</i>-AM)/GO hydrogel followed a pseudo-first order model and the Langmuir isotherm model, respectively. The maximum adsorption capacity of Pb²⁺ and Cd²⁺ on the P­(AANa-<i>co</i>-AM)/GO hydrogel were calculated to be 452.3 and 196.4 mg/g at 298 K, respectively. The XPS analysis revealed that the adsorption mechanism of Pb²⁺ and Cd²⁺ on the P­(AANa-<i>co</i>-AM)/GO hydrogel was ascribed to the chelation of −COO^– with Pb²⁺ and the ion-exchange of −COO^–Na⁺ with Cd²⁺, respectively. The P­(AANa-<i>co</i>-AM)/GO hydrogel possesses good reusability and excellent fixed-bed column operation performance, which suggests that the P­(AANa-<i>co</i>-AM)/GO hydrogel can be considered as a promising adsorbent for removing heavy metal ions from practical wastewater

A high-speed, eight-wavelength visible light-infrared pyrometer for shock physics experiments

An eight-channel, high speed pyrometer for precise temperature measurement is designed and realized in this work. The addition of longer-wavelength channels sensitive at lower temperatures highly expands the measured temperature range, which covers the temperature of interest in shock physics from 1500K-10000K. The working wavelength range is 400-1700nm from visible light to near-infrared (NIR). Semiconductor detectors of Si and InGaAs are used as photoelectric devices, whose bandwidths are 50MHz and 150MHz respectively. Benefitting from the high responsivity and high speed of detectors, the time resolution of the pyrometer can be smaller than 10ns. By combining the high-transmittance beam-splitters and narrow-bandwidth filters, the peak spectrum transmissivity of each channel can be higher than 60%. The gray-body temperatures of NaI crystal under shock-loading are successfully measured by this pyrometer

Vortex-induced aggregation in superfluid helium droplets.

The formation of Ag nanoparticles by the addition of Ag atoms to helium droplets has been investigated. The resulting nanoparticles were then imaged by transmission electron microscopy after being deposited on a thin solid surface. In large helium droplets chains of Ag nanorods were observed similar to recently reported track-like deposits [Gomez et al., Phys. Rev. Lett., 2012, 108, 155302]. However, by adjusting the experimental conditions chains of spherical nanoparticles could also be seen with a nearly uniform inter-particle spacing. Given that spherical Ag nanoparticles have no intrinsic anisotropy, the only viable explanation is that these particles must be guided into position by interaction with a quantized vortex spanning the diameter of the helium droplet. Furthermore, addition of Si to the droplets immediately after Ag resulted in Si inserting between the Ag nanoparticles to form continuous nanowires. This eliminates the possibility that the segmented Ag nanostructures are the result of nanowire fragmentation when the helium droplets collide with the deposition substrate. Thus segmented Ag chains are shown to be an intrinsic feature of Ag aggregation in helium droplets in the presence of a quantized vortex