We report the Hong–Ou–Mandel (HOM) interference, with visibility of 91%, produced from two independent single photons retrieved from collective atomic excitations in two separate cold-atom clouds with high optical depths of 90. The high visibility of the HOM dip is ascribed to the pure single photon in the Fock state that was generated from a dense-cold-atom cloud pumping by a short pulse. The visibility is always the same regardless of the time response of the single-photon detectors. This result experimentally shows that the single photons retrieved are in a separable temporal state with their idler photons.

Holographic head-mounted display (HHMD) is a specific application of holography. The previous conventional computer-generated hologram (CGH) generation method has a large redundancy and suffers from a heavy computing burden in the HHMD. A low redundancy and fast calculation method is presented for a CGH that is suitable for an HHMD with the effective diffraction area recording method. For the limited pupil size of an observing eye, the size of the area producing an effective wavefront is very small, and the calculated amount can be dramatically reduced. A numerical simulation and an augmented virtual reality experimental system are presented to verify the proposed method. 1.5% of the calculation consumption of the conventional CGH generation method is used, and good holographically reconstructed images can be observed.

A self-comparison method with closely interleaved switching states is analyzed and used to evaluate some type-B uncertainties of an Rb87 atomic fountain clock. Free from additional frequency reference, the method can be applied to a running fountain to reach a precision beyond its uncertainty. A verification experiment proves an uncertainty of 9.2×10 16 at an averaging time of 242500 s. Further, the method is applied to measure light shift, and no visible relative frequency shift is found in the fountain within the uncertainty of 2.1×10 15. When applied to the evaluation of a cold collisional shift, the result gives a 2.2×10 15 shift with a 9.5×10 16 uncertainty.

This Letter presents a method of an optical sensor for measuring wavelength shifts. The system consists of a diffraction grating and a total internal reflection heterodyne interferometer. As a heterodyne light beam strikes a grating, the first-order diffraction beam is generated. The light penetrates into a total internal reflection prism at an angle larger than the critical angle. A wavelength variation will affect the diffractive angle of the first-order beam, thus inducing a phase difference variation of the light beam emerging from the total internal reflections inside the trapezoid prism. Both the experimental and theoretical results reveal that, for the first-order diffractive beam, the sensitivity and resolution levels are superior to 5°/nm and 0.006 nm, respectively, in the range of wavelength from 632 to 634 nm, and are superior to 3.1°/nm and 0.0095 nm in the range from 632 to 637 nm. For the theoretical simulation of the fourth-order diffractive beam, they are superior to 6.4 deg/nm and 0.0047 nm in the range from 632 to 637 nm.

A new approach is proposed to accurately determine the thickness of films, especially for ultra-thin films, through spectrum-fitting with the assistance of an interference layer. The determination limit can reach even less than 1 nm. Its accuracy is far better than that of the traditional methods. This determination method is verified by experiments, and the determination limit is at least 3.5 nm compared with the results of atomic force microscope (AFM). Furthermore, a double interference-aided spectra fitting method is proposed to reduce the requirements of the determination instruments, which thus allows one to determine the film’s thickness with a low-precision common spectrometer and to greatly lower the cost. It is a very high-precision determination method for on-site and in-situ applications, especially for ultra-thin films.

We report on the amplification of high-average-power and high-efficiency picosecond pulses in a self-made very-large-mode-area Yb-doped photonic crystal fiber (PCF). The PCF with a core diameter of 105 μm and a core numerical aperture of 0.05 is prepared by the sol-gel method combined with the powder sintering technique. The fiber amplification system produces the highest average power of 255 W at a 10 MHz repetition rate with a 21 ps pulse duration corresponding to a peak power of 1.2 MW. This result exemplifies the high-average-power and high-peak-power potential of this specifically designed fiber.

Incipient plasmonic bubble formation is observed around gold nanopillars with different inter-nanopillar separations. The experimental measurements and theoretical analysis show that the nanobubble formation is due to the enhanced plasmonic resonance rather than from the laser heating. Inter-nanopillar distribution may lead to threshold fluence variations. The lifetime of plasmonic bubbles can reach several minutes. Furthermore, both the radius and the growth rate of the plasmonic nanobubble increase as the inter-nanopillar distribution decreases. Smaller-spacing distributed arrays produced larger bubbles. The maximum growth rate of the bubbles can be reached at about 883.5×10 6 m/s on 1 μm nanopillars, but it is only 56.9×10 6 m/s on 4 μm nanopillars.

Tadpole-shaped Au nano-particles with controllable tails are successfully fabricated by simply using laser fragmentation of separated Au nano-spheres in liquid. The optimum laser power densities (1.5–3 GW/cm2) can enable part of the individual Au nano-sphere to be re-melted, released, and ultra-rapidly recondensed/crystallized on the outside of the original region. We find that the length of the tail in a tadpole-shaped Au nano-particle significantly increases from about 10 to 25 nm by increasing the laser power density. Benefiting from the unique structural features, the localized surface plasmon resonance (LSPR) absorption spectra of the tadpole-shaped Au nano-particles become broader by increasing the tail length. Moreover, the LSPR absorption band also exhibits a noticeable red shift from about 520 to 650 nm. Our results provide a convenient and valuable strategy to fabricated novel anisotropic-shaped nano-structures with fascinating properties.

We demonstrate the generation of Q-switched pulses from an ytterbium-doped fiber laser (YDFL) using quantum dot (QD) CdSe as a passive saturable absorber (SA). The CdSe QD is fabricated by the synthesis of CdO, Se, and manganese acetate and paraffin oil and oleic acid as the solvent and surfactant, respectively. The CdSe QD is then doped into poly-methyl-methacrylate (PMMA) via an emulsion polymerization process. A PMMA-hosted CdSe QD thin flake with a homogeneous end surface is then formed and placed between two ferrules and assembled in a YDFL cavity to achieve the Q-switching operation with a repetition rate of 24.45 to 40.50 kHz while varying the pump power from 975 to 1196 mW. The pulse width changes from 6.78 to 3.65 μs with a maximum calculated pulse energy at 0.77 μJ at a pump power of 1101 mW. This work may be the first demonstration of CdSe QD-based Q-switching in an all-fiber configuration that should give proportional insight into semiconductor QD materials in photonics applications.

A phase-diffractive optical element is designed to measure the topological charge of optical vortices. We use the scalar diffraction theory to calculate the far-field diffraction patterns. The simulation results show that almost all of the power of the incident beams is diffracted to the same diffraction order, and this approach is also effective for multi-ring optical vortices. We upload this phase-diffractive optical element on the liquid crystal spatial light modulator to do the experiment. The observed far-field diffraction patterns fit well with the simulation results.

An InP-based monolithically integrated few-mode transmitter aiming at the combination of wavelength division multiplexing (WDM) and mode division multiplexing (MDM) technologies is proposed. The core elements of the proposed transmitter are mode converters and a wavelength-mode division multiplexer that are all based on multimode interference (MMI) couplers. Simulations show that the wavelength-mode division multiplexer has a large fabrication tolerance of 30 and 0.5 μm for the length and the width of the device, respectively. A low loss below 0.26 dB for the passive parts of the transmitter is obtained in the whole C-band wavelength range.

Face recognition technology has great prospects for practical applications. Three-dimensional (3D) human faces are becoming more and more important in consideration of the limits of two-dimensional face recognition. We propose an active binocular setup to obtain a 3D colorful human face using the band-limited binary patterns (BBLP) method. Two grayscale cameras capture the BBLP projected onto the target of human face by a digital light processing (DLP) projector synchronously. Then, a color camera captures a colorful image of the human face. The benefit of this system is that the 3D colorful human face can be obtained easily with an improved temporal correlation algorithm and the precalibration results between three cameras. The experimental results demonstrated the robustness, easy operation, and the high speed of this 3D imaging setup.

This work proposes a method to concurrently calibrate multiple acoustic speeds in different mediums with a photoacoustic (PA) and ultrasound (US) dual-modality imaging system. First, physical infrastructure information of the target is acquired through a US image. Then, we repeatedly build PA images around a special target to yield the best focused result by dynamically updating the acoustic speeds in a different medium of the target. With these correct acoustic propagation velocities in the according mediums, we can effectively optimize the PA image quality as the experiments proved, which might benefit future research in biomedical imaging science.

Brain regenerative studies require precise visualization of the morphological structures. However, few imaging methods can effectively detect the adult zebrafish brain in real time with high resolution and good penetration depth. Long-term in vivo monitoring of brain injuries and brain regeneration on adult zebrafish is achieved in this study by using 1325 nm spectral-domain optical coherence tomography (SD-OCT). The SD-OCT is able to noninvasively visualize the skull injury and brain lesion of adult zebrafish. Valuable phenomenon such as the fractured skull, swollen brain tissues, and part of the brain regeneration process can be conducted based on the SD-OCT images at different time points during a period of 43 days.

A highly Tm-doped lead germanate glass fiber is developed using the rod-in-tube method. The ～2 μm laser beam quality of the fiber is ～1.5. The lead germanate composite fiber jumpers are homemade for all the fiber laser investigations. When core is pumped by a 1590 nm Yb/Er fiber laser, a maximum laser output of 313 mW is achieved at a 670 mW pump power, and the corresponding slope efficiency is ～52.8%. Moreover, by using a 2 cm-long lead germanate fiber as the gain medium, a 33 mW 1942 nm Tm laser is also demonstrated.

The stochastic resonance based on optical bistability in the semiconductor optical amplifier is numerically investigated to extract a weak pulse signal buried in noise. The output property of optical bistability under different system parameters is analyzed, which determines the performance of the stochastic resonance. Through optimizing these parameters, the noise-hidden signal is extracted via stochastic resonance, in which the maximum cross-correlation gain higher than nine is obtained. This provides a novel technology for detecting a weak optical signal in various signal processing fields.

An ultra-broadband and fabrication-tolerant silicon polarization rotator splitter is proposed in this Letter. Benefitting from the broadband and low-loss characteristics of the bi-level taper and counter-tapered coupler, the designed device has a simulated insertion loss and crosstalk of less than 0.2 and 15 dB in the waveband from 1290 to 1610 nm. These characteristics make it valuable in applications with large bandwidth requirements, such as full-grid Coarse wavelength division multiplexer (CWDM) and diplexer/triplexer fiber-to-the-home systems. The fabrication tolerance of the design is also analyzed, showing that the device performance is quite stable with normal manufacturing errors in silicon photonics foundries.

We report the upconversion luminescence of lithium fluoride single crystals excited by an infrared femtosecond laser at room temperature. The luminescence spectra demonstrate that upconversion luminescence originates from the color center of F3+. The dependence of fluorescence intensity on pump power reveals that a two-photon excitation process dominates the conversion of infrared radiation into visible emission. Simultaneous absorption of two infrared photons is suggested to produce the F3+ center population, which leads to the characteristic visible emission. The results are on the reveal and evaluation of the simultaneous two-photon absorption on the green upconversion process.

Complementary analysis techniques are applied in this work to study the interface structure of Mo/Si multilayers. The samples are characterized by grazing incident x-ray reflectivity, x-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and extreme ultraviolet reflectivity. The results indicate that the layer thickness is controlled well with small diffusion on the interface by forming MoSi2. Considering MoSi2 as the interface composition, simulating the result of our four-layer model fits well with the measured reflectivity curve at 13.5 nm.

Analytical formulas for a class of tunable random electromagnetic beams propagating in a turbulent atmosphere through a complex optical system are derived with the help of a tensor method. One finds that the far field intensity distribution is tunable by modulating the source correlation structure function. The on-axis spectral degree of polarization monotonically increases to the same value for different values of order M in free space while it returns to the initial value after propagating a sufficient distance in turbulence. Furthermore, it is revealed that the state of polarization is closely determined by the initial correlation structure rather than by the turbulence parameters.

Mathematical models for the superimposed orbital angular momentum (OAM) mode of multiple Hankel–Bessel (HB) beams in anisotropic non-Kolmogorov turbulence are developed. The effects of anisotropic turbulence and source parameters on the mode detection spectrum of the superimposed OAM mode are analyzed. Anisotropic characteristics of the turbulence in the free atmosphere can enhance the performance of OAM-based communication. The HB beam is a good source for mitigating the turbulence effects due to its nondiffraction and self-focusing properties. Turbulence effects on the superimposed OAM mode can be effectively reduced by the appropriate allocation of OAM modes at the transmitter based on the reciprocal features of the mode cross talk.