Sorting infrared optical vortices with a nonlinear angular lens.

Analogous to the regular lens, which spatially maps plane waves in the space domain to distinct points in the Fourier domain, the angular lens establishes the mapping relations between an angular mode and angular position, thus providing an effective toolkit for detecting an optical vortex. However, using the angular lens to sort infrared optical vortex modes via nonlinear optical processes remains relatively unexplored. Here, we design a nonlinear optical version of the angular lens to map the various infrared optical vortex modes to different angular positions in the visible region. We successfully sort nine infrared optical vortex modes of different topological charges with a visible camera, showing the cost-effective ability to sort infrared vortices compared to a relatively expensive infrared camera. Our scheme holds promise for infrared remote sensing, infrared vortex-encoded optical communications, and so on.

Non-orthogonal polarization encoding/decoding assisted by structured optical pattern recognition.

The complex vector beams yield up an abundance of polarization information that has not yet been well utilized in information encoding. In this paper, we propose a polarization encoding scheme with the non-orthogonal polarization states using a stationary vector beam. Recognizing those non-orthogonal polarization states is assisted by the structured patterns of the single vector beams under different polarization projections. We show that one can achieve different capacities of encoding bits by changing the step of the polarization angle with the single vector beam. We also demonstrate the non-orthogonal polarization encoding scheme can be well decoded with the machine learning classification algorithm. A 64×64 gray image is successfully transmitted by using 4 bits/symbol encoding-decoding scheme with 99.94 % transmission accuracy. Besides, by extending the encoding-decoding scheme to 8 bits/symbol based on the same single vector beam, we achieve a higher transmission rate with 65.58% transmission accuracy. Our work holds promise for small-angle non-orthogonal polarization encoding for free-space optical communications.

Thermal lens effect with light's orbital angular momentum.

Thermal lens effect has been well developed and exploited for decades by using the Gaussian intensity distribution of a laser beam. In this paper, a new thermal lens effect by using a laser beam with Orbital Angular Momentum (OAM) is proposed. We find that the dynamic process for the formation of the OAM-thermal lens has reda rapid change towards the evolution direction at the beginning but then slowly approaches to a steady state for a while. This phenomenon is significantly different from the traditional Gaussian-thermal lens, thus it may be used to improve the sensitivity of the absorption spectrum for the chemical and biomedical analysis. Besides, theoretically and experimentally, the factors affecting the steady state of the OAM-thermal lens are also studied, hoping these may provide a useful reference for the research community. We also find a potential slow thermal-optical gate that can control of light passing through or blocking by changing the OAM of the heating beam. Our work opens the door which utilizes the structured light beam to study the thermal-optical effect, and more interesting phenomena remain to be explored.

Preparation and Application of Quaternized Chitosan- and AgNPs-Base Synergistic Antibacterial Hydrogel for Burn Wound Healing.

Infection is the major reason that people die from burns; however, traditional medical dressings such as gauze cannot restrain bacterial growth and enhance the healing process. Herein, an organic- and inorganic-base hydrogel with antibacterial activities was designed and prepared to treat burn wounds. Oxidized dextran (ODex) and adipic dihydrazide grafted hyaluronic acid (HA-ADH) were prepared, mixed with quaternized chitosan (HACC) and silver nanoparticles to fabricate Ag@ODex/HA-ADH/HACC hydrogel. The hydrogel, composed of nature biomaterials, has a good cytocompatibility and biodegradability. Moreover, the hydrogel has an excellent antibacterial ability and presents fast healing for burn wounds compared with commercial Ag dressings. The Ag@ODex/HA-ADH/HACC hydrogel will be a promising wound dressing to repair burn wounds and will significantly decrease the possibility of bacterial infection.

Implementing selective edge enhancement in nonlinear optics.

Recently, it has been demonstrated that a nonlinear spatial filter using second harmonic generation can implement a visible edge enhancement under invisible illumination, and it provides a promising application in biological imaging with light-sensitive specimens. But with this nonlinear spatial filter, all phase or intensity edges of a sample are highlighted isotropically, independent of their local directions. Here we propose a vectorial one to cover this shortage. Our vectorial nonlinear spatial filter uses two cascaded nonlinear crystals with orthogonal optical axes to produce superposed nonlinear vortex filtering. We show that with the control of the polarization of the invisible illumination, one can highlight the features of the samples in special directions visually. Moreover, we find the intensity of the sample arm can be weaker by two orders of magnitude than the filter arm. This striking feature may offer a practical application in biological imaging or microscopy, since the light field reflected from the sample is always weak. Our work offers an interesting way to see and emphasize the different directions of edges or contours of phase and intensity objects with the polarization control of the invisible illumination.

Influence of pump coherence on the generation of position-momentum entanglement in optical parametric down-conversion.

We examine experimentally how the degree of position-momentum entanglement of photon pairs depends on the transverse coherence of the pump beam that excites them in a process of spontaneous parametric down-conversion. Using spatially incoherent light from a light-emitting diode, we obtain strong position correlation of the photons, but we find that transverse momentum correlation, and thus entanglement, is entirely absent. When we continuously vary the degree of spatial coherence on the pump beam, we observe the emergence of stronger momentum correlations and entanglement. We present theoretical arguments that explain our experimental results. Our results shed light on entanglement generation and can be applied to control entanglement for quantum information applications.

Realization of the Einstein-Podolsky-Rosen Paradox Using Radial Position and Radial Momentum Variables.

As is well known, angular position and orbital angular momentum (OAM) of photons are a conjugate pair of variables that have been extensively explored for quantum information science and technology. In contrast, the radial degrees of freedom remain relatively unexplored. Here we exploit the radial variables, i.e., radial position and radial momentum, to demonstrate Einstein-Podolsky-Rosen correlations between down-converted photons. In our experiment, we prepare various annular apertures to define the radial positions and use eigenmode projection to measure the radial momenta. The resulting correlations are found to violate the Heisenberg-like uncertainty principle for independent particles, thus manifesting the entangled feature in the radial structure of two-photon wave functions. Our work suggests that, in parallel with angular position and OAM, the radial position and radial momentum can offer a new platform for a fundamental test of quantum mechanics and for novel application of quantum information.

Seeing infrared optical vortex arrays with a nonlinear spiral phase filter.

We demonstrate a new method to detect infrared optical vortex arrays efficiently, which is based on simultaneous up-conversion imaging and spiral phase contrast via second-harmonic generation (SHG) in the Fourier domain. In our experiment, we use a spatial light modulator to prepare a variety of 1064 nm structured vortex arrays and employ a vortex phase plate of different topological charges to serve as the nonlinear orbital angular momentum (OAM) filter. The SHG is done by mixing the Fourier spectra of input-structured vortices with a single OAM beam in a type-II potassium titanyl phosphate crystal. Then we can convert the input invisible vortex arrays into the visible SHG light fields, and the vortex cores are mapped and seen by bright Gaussian spots, revealing both their positions and topological charges. Our work has potential in the field of infrared imaging and monitoring.

Structured-Pump-Enabled Quantum Pattern Recognition.

We report a new scheme of ghost imaging by using a spatially structured pump in the Fourier domain of spontaneous parametric down-conversion for quantum-correlation-based pattern recognition. We exploit the mathematical feature of Laguerre-Gaussian mode's Fourier transform to describe the pump-modulated formation of a ghost image. Of particular interest is the experimental demonstration of a quantum equivalence of a Vander Lugt filter, based on which the nonlocal spiral phase contrast for vortex mapping and quantum-correlation-based human face recognition are implemented successfully. The photons used for probing a test object, scanning the database, and producing a correlation signal can belong to three different light beams, which suggests some security applications where low-light-level illumination and covert operation are desired.

LED-based visible light communication for color image and audio transmission utilizing orbital angular momentum superposition modes.

Twisted light has recently gained enormous interest in communication systems ranging from fiber-optic to radio frequency regimes. Thus far, the light-emitting diode (LED) has not yet been exploited for orbital angular momentum (OAM) encoding to transmit data, which, however, could open up an opportunity towards a new model of secure indoor communication. Here, by multiplexing and demultiplexing red, green and blue (RGB) twisted beams derived from a white light emitting diode, we build a new visible light communication system with RGB colors serving as independent channels and with OAM superposition modes encoding the information. At the sender, by means of theta-modulation, we use a computer-controlled spatial light modulator to generate two-dimensional holographic gratings to encode a large alphabet with 16 different OAM superposition modes in each RGB channel. At the receiver, based on supervised machine learning, we develop a pattern recognition method to identify the characteristic mode patterns recorded by CCD cameras, and therefore, decoding the information. We succeed in demonstrating the transmission of color images and a piece of audio over a 6-meter indoor link with the fidelity over 96%.

HIV-1 CRF01_AE strain is associated with faster HIV/AIDS progression in Jiangsu Province, China.

The goal of this study was to assess risk factors associated with HIV/AIDS progression. Between May 2007 and December 2014, 114 subjects were enrolled in Wuxi City and examined every 6 months. The pol gene sequence was amplified to ascertain the HIV-1 subtype. A Cox proportional hazards regression model was used to estimate the factors associated with HIV/AIDS progression. The median follow-up time for all 114 subjects was 26.70 months (IQR: 18.50-41.47), while the median progression time of the 38 progressed subjects was 24.80 months (IQR: 14.13-34.38). Overall, the CRF01_AE subtype was correlated with a significant risk of accelerated progression compared to non-CRF01_AE subtypes (HR = 3.14, 95%CI: 1.39-7.08, P = 0.006). In addition, a lower CD4 count (350-499) at baseline was associated with a risk of accelerated HIV/AIDS progression compared to higher CD4 count (≥500) (HR = 4.38, 95%CI: 1.95-9.82, P < 0.001). Furthermore, interaction analyses showed that HIV-1 subtypes interacted multiplicatively with transmission routes or CD4 count at baseline to contribute to HIV/AIDS progression (P = 0.023 and P < 0.001, respectively). In conclusion, the CRF01_AE subtype and a lower CD4 count at baseline tend to be associated with the faster progression of HIV/AIDS. Understanding the factors affecting HIV/AIDS progression is crucial for developing personalized management and clinical counselling strategies.

Theta-modulated generation of chromatic orbital angular momentum beams from a white-light source.

We demonstrate a simple experiment to produce chromatic orbital angular momentum (OAM) beams from a white-light source based on theta-modulation. The array of pixels of a single spatial light modulator (SLM) is divided into four sub-domains, each of which individually displays a holographic grating both for producing the desired OAM beam and for modulating the orientation of the blazed grating. In the Fourier plane of a 4f optical system, a tailor-made spatial filter is employed for the coloration of various OAM beams with different colors. In the imaging plane, we record the generated chromatic OAM beams of different topological charges as well as their superpositions, which appear as mixtures of red, green and blue (RGB) in different proportions. Our work may find direct application in optical information processing and in free-space optical communications.

Encoding and decoding of digital spiral imaging based on bidirectional transformation of light's spatial eigenmodes.

Digital spiral imaging has been demonstrated as an effective optical tool to encode optical information and retrieve topographic information of an object. Here we develop a conceptually new and concise scheme for optical image encoding and decoding toward free-space digital spiral imaging. We experimentally demonstrate that the optical lattices with ℓ=±50 orbital angular momentum superpositions and a clover image with nearly 200 Laguerre-Gaussian (LG) modes can be well encoded and successfully decoded. It is found that an image encoded/decoded with a two-index LG spectrum (considering both azimuthal and radial indices, ℓ and p) possesses much higher fidelity than that with a one-index LG spectrum (only considering the ℓ index). Our work provides an alternative tool for the image encoding/decoding scheme toward free-space optical communications.

Shaping the Arago-Poisson spot with incomplete spiral phase modulation.

The Arago-Poisson spot played an important role in the discovery of the wave nature of light. We demonstrate a novel way to shape the Arago-Poisson spot by partially twisting the phase fronts of the incident light beam. We use a spatial light modulator to generate the holographic gratings both for mimicking the circular opaque objects and for modulating the spiral phase profiles. For incomplete spiral phase of five- and tenfold symmetry, we observe the gradual formation of the on-axis bright spots upon propagation. Our results show that two fundamental but seemingly independent optical phenomena, namely, the Arago-Poisson spot and the orbital angular momentum (OAM) of light, can be well connected by changing the phase height ϑ gradually from 0 to 2π. The experimental results are well interpreted visually by plotting the Poynting vector flows. In addition, based on the decomposed OAM spectra, the observations can also be understood from the controllable mixture of a fundamental Gaussian beam and an OAM beam. Our work is an elegant demonstration that spiral phase modulation can add to the optical tool to effectively shape the diffraction of light and may have potential applications in the field of optical manipulations.

Gradual edge enhancement in spiral phase contrast imaging with fractional vortex filters.

In the spiral phase contrast imaging, the integer spiral phase plate (SPP) are generally employed to perform the radial Hilbert transform on the object. Here we introduce fractional SPP filters, instead of the integer ones, to investigate the gradual formation of edge enhancement for pure phase objects. Two spatial light modulators are used in our experimental configuration. One is addressed to display the pure phase object of a five-pointed star, while the other serves as a dynamic filter of fractional topological charge Q. Of interest is the observation of the complete reversal of the edge and background brightness by gradually changing the fractional vortices from Q = 0 to 1. The experimental results were well interpreted based on the OAM spectra of fractional SPP, which indicates that the filtered output image can be considered as a coherent superposition of all possible images that are individually resulted from the integer OAM filtering. Besides, we show that the spiral phase contrast effect can still be observed in real time for a rotating three-leaf clover. Our results may find potential applications in the optical microscopic imaging.

Revisiting the which-way experiment with twisted light beams.

We report an experiment of which-way information and a quantum eraser based on polarization-controlled interference of two twisted light beams carrying high orbital angular momentum (OAM) up to ℓ=±50 and ±100, respectively. By changing the polarization plane of one OAM beam from 0° to 90° with respect to that of the other OAM beam, we observe the gradual disappearance of the interference petal-like patterns into the noninterference single bright rings. Subsequently, we use the eraser of a diagonal polarizer to retrieve the characteristic petal-like interference. The experimental results can be well explained in the frame of single-photon Greenberger-Horne-Zeilinger-like (GHZ-like) entanglement. Our work may be beneficial to understanding the wave-particle duality of light.

Polarization singularities and orbital angular momentum sidebands from rotational symmetry broken by the Pockels effect.

The law of angular momentum conservation is naturally linked to the rotational symmetry of the involved system. Here we demonstrate theoretically how to break the rotational symmetry of a uniaxial crystal via the electro-optic Pockels effect. By numerical method based on asymptotic expansion, we discover the 3D structure of polarization singularities in terms of C lines and L surfaces embedded in the emerging light. We visualize the controllable dynamics evolution of polarization singularities when undergoing the Pockels effect, which behaves just like the binary fission of a prokaryotic cell, i.e., the splitting of C points and fission of L lines are animated in analogy with the cleavage of nucleus and division of cytoplasm. We reveal the connection of polarization singularity dynamics with the accompanying generation of orbital angular momentum sidebands. It is unexpected that although the total angular momentum of light is not conserved, the total topological index of C points is conserved.

Mimicking Faraday rotation to sort the orbital angular momentum of light.

The efficient separation of the orbital angular momentum (OAM) is essential to both the classical and quantum applications with twisted photons. Here we devise and demonstrate experimentally an efficient method of mimicking the Faraday rotation to sort the OAM based on the OAM-to-polarization coupling effect induced by a modified Mach-Zehnder interferometer. Our device is capable of sorting the OAM of positive and negative numbers, as well as their mixtures. Furthermore, we report the first experimental demonstration to sort optical vortices of noninteger charges. The possibility of working at the photon-count level is also shown using an electron-multiplying CCD camera. Our scheme holds promise for quantum information applications with single-photon entanglement and for high-capacity communication systems with polarization and OAM multiplexing.