In this article, taking advantage of the special magnetic shieldings and the optimal coil design of a transportable Rb atomic fountain clock, the intensity distribution in space and the fluctuations with time of the quantization magnetic field in the Ramsey region were measured using the atomic magneton-sensitive transition method. In an approximately 310 mm long Ramsey region, a peak-to-peak magnetic field intensity of a 0.74 nT deviation in space and a 0.06 nT fluctuation with time were obtained. These results correspond to a second-order Zeeman frequency shift of approximately (2095.5±5.1)×10-17. This is an essential step in advancing the total frequency uncertainty of the fountain clock to the order of 10-17.

Localized wavefront aberrations would introduce artifacts in biomedical imaging, which, however, are often neglected, as their compensations are at the cost of the field-of-view. Here, we show rarely reported local artifacts in two-photon imaging of dendrites beneath blood vessels in a mouse brain in vivo and interpret the phenomena via numerical simulations. The artifacts of divided parallel structures are found to be induced by coma and astigmatism, resulting from sample tilting and the cylinder shape of vasculatures, respectively. Different from that in single-photon microscopy, such artifacts in nonlinear microscopy show unique characteristics and should be recognized for proper interpretation of the images.

Photoacoustic imaging (PAI) with a handheld linear ultrasound (US) probe is widely used owing to its convenient and inherent dual modality capability. However, the limited length of the linear probe makes PAI suffer from the limited view. In this study, we present a simple method to substantially increase the view angle aided by two US reflectors. Both phantom and in vivo animal study results have demonstrated that the imaging quality can be greatly improved with the reflector without sacrificing the imaging speed.

In this Letter, we developed a robust method for integrating nanodiamonds (NDs) to optical fiber. The NDs, containing nitrogen-vacancy (NV) centers, were uniformly mixed with UV adhesive before coating the end surface of a multimode fiber as a hemispherical film. The excitation and collection efficiency of NV fluorescence can be enhanced by increasing the thickness of UV adhesive film and additional aluminum film deposition. The fiber-based quantum sensor was also experimentally demonstrated for all-optical thermometry application. The variation of the refractive index of UV adhesive under different temperatures will also affect the NV collection efficiency by changing the light confinement. The demonstrated facile integration approach paves the way for developing fiber-based quantum thermometry and magnetometry.

We have proposed and experimentally demonstrated a reconfigurable free space optical interconnect with broadcasting capability based on an eight-channel silicon integrated optical phased array. By using the silicon integrated beam steering and broadcasting device, 10 Gb/s on–off keying data is transmitted over 15 cm in free space for up to three receivers located in three different cards. The experimental results show that the optical phased array can be used with broadcasting capability provided to multi-receivers in the card to card optical interconnects, which can significantly reduce device size, system complexity, and total costs.

Typical single-pixel imaging techniques for edge detection are mostly based on first-order differential edge detection operators. In this paper, we present a novel edge detection scheme combining Fourier single-pixel imaging with a second-order Laplacian of Gaussian (LoG) operator. This method utilizes the convolution results of an LoG operator and Fourier basis patterns as the modulated patterns to extract the edge detail of an unknown object without imaging it. The simulation and experimental results demonstrate that our scheme can ensure finer edge detail, especially under a noisy environment, and save half the processing time when compared with a traditional first-order Sobel operator.

Three-dimensional (3D) refractive index (RI) distribution is important to reveal the object’s inner structure. We implemented terahertz (THz) diffraction tomography with a continuous-wave single-frequency THz source for measuring 3D RI maps. The off-axis holographic interference configuration was employed to obtain the quantitative scattered field of the object under each rotation angle. The 3D reconstruction algorithm adopted the filtered backpropagation method, which can theoretically calculate the exact scattering potential from the measured scattered field. Based on the Rytov approximation, the 3D RI distribution of polystyrene foam spheres was achieved with high fidelity, which verified the feasibility of the proposed method.

High-performance ultra-compact polarization splitter-rotators (PSRs) are designed and experimentally demonstrated, using dual etching and a tapered asymmetrical directional coupler. First, two novel PSRs are designed with nanowire and subwavelength grating cross-port waveguides and verified in simulations. Then, one of the two PSRs is fabricated. Experiment results reveal that the extinction ratio is higher than 28 dB or 32 dB at 1550 nm for the launched fundamental transverse magnetic or the transverse electric modes, while the corresponding insertion loss and polarization conversion loss are 0.33 dB and 0.18 dB, respectively.

The dissipative Kerr soliton microcomb provides a promising laser source for wavelength-division multiplexing (WDM) communication systems thanks to its compatibility with chip integration. However, the soliton microcomb commonly suffers from a low-power level due to the intrinsically limited energy conversion efficiency from the continuous-wave pump laser to ultra-short solitary pulses. Here, we exploit laser injection locking to amplify and equalize dissipative Kerr soliton comb lines, superior gain factor larger than 30 dB, and optical-signal-to-noise-ratio (OSNR) as high as 60 dB obtained experimentally, providing a potential pathway to constitute a high-power chip-integrated WDM laser source for optical communications.

We report Q-switched mode-locked (QML) pulses generation in an Yb-doped multimode fiber (MMF) laser by using a graphene-deposited multimode microfiber (GMM) for the first time, to the best of our knowledge. The single-wavelength QML operation with the central wavelength tunable from 1028.81 nm to 1039.20 nm and the dual-wavelength QML operation with the wavelength spacing tunable from 0.93 nm to 5.79 nm are achieved due to the multimode interference filtering effect induced by the few-mode fiber and MMF structure and the GMM in the cavity. Particularly, in the single-wavelength QML operation, the fifth harmonic is also realized owing to the high nonlinear effect of the GMM. The obtained results indicate that the QML pulses can be generated in the MMF laser, and such a flexible tunable laser has promising applications in optical sensing, measuring, and laser processing.

We demonstrate the non-mechanical beam steering and amplifier operation of a vertical cavity surface emitting laser (VCSEL) integrated Bragg reflector waveguide amplifier with a cut-off wavelength detuning design, which enables unidirectional lateral coupling, continuous electrical beam steering, and diffraction-limited divergence angle. We present the modeling of the proposed structure for unidirectional coupling between a seed single-mode VCSEL and slow-light amplifier. We also present the detailed operating characteristics including the near-field and far-field patterns, light/current characteristics, and lasing spectrum. The experimental measurements exhibit a single-mode output of over 8 mW under CW operation, a continuous beam steering range of 16°, and beam divergence below 0.1° as an optical beam scanner. The integrated amplifier length is as small as 0.9 mm, and thus we could expect much higher powers and higher resolution points by increasing the amplifier lengths.

Absorption induced by activated magnesium (Mg) in a p-type layer contributes considerable optical internal loss in GaN-based laser diodes (LDs). An LD structure with a distributed polarization doping (DPD) p-cladding layer (CL) without intentional Mg doping was designed and fabricated. The influence of the anti-waveguide structure on optical confinement was studied by optical simulation. The threshold current density, slope efficiency of LDs with DPD p-CL, and Mg-doped CL, respectively, were compared. It was found that LDs with DPD p-CL showed lower threshold current density but reduced slope efficiency, which were caused by decreasing internal loss and hole injection, respectively.

We report a long-term frequency-stabilized optical frequency comb at 530–1100 nm based on a turnkey Ti:sapphire mode-locked laser. With the help of a digital controller, turnkey operation is realized for the Ti:sapphire mode-locked laser. Under optimized design of the laser cavity, the laser can be mode-locked over a month, limited by the observation time. The combination of a fast piezo and a slow one inside the Ti:sapphire mode-locked laser allows us to adjust the cavity length with moderate bandwidth and tuning range, enabling robust locking of the repetition rate (fr) to a hydrogen maser. By combining a fast analog feedback to pump current and a slow digital feedback to an intracavity wedge and the pump power of the Ti:sapphire mode-locked laser, the carrier envelope offset frequency (fceo) of the comb is stabilized. We extend the continuous frequency-stabilized time of the Ti:sapphire optical frequency comb to five days. The residual jitters of fr and fceo are 0.08 mHz and 2.5 mHz at 1 s averaging time, respectively, satisfying many applications demanding accuracy and short operation time for optical frequency combs.

Different laminated structures of TiO2/SiO2 composite film were prepared via atomic layer deposition (ALD) on alumina substrates. The effect of the annealing temperature in the air on the surface morphologies, crystal structures, binding energies, and ingredient content of these films was investigated using X-ray diffraction, field emission scanning electron microscopy, and X-ray photoelectron spectroscopy. Results showed that the binding energy of Ti and Si increased with decrease of the Ti content, and the TiO2/SiO2 nanolaminated films exhibited a complex bonding structure. As the annealing temperature increased, the thickness of the nanolaminated films decreased, and the density and surface roughness increased. An increase in the crystallization temperature was proportional to the SiO2 content in TiO2/SiO2 composite film. The annealing temperature and thin thickness strongly affected the phase structure of the ALD TiO2 thin film. To be specific, the TiO2 thin film transformed into an anatase phase from an amorphous phase after an increase in the annealing temperature from 400°C to 550°C, and the TiO2 film exhibited an anatase phase until the annealing temperature reached 850°C, owing to its extremely small thickness. The annealing process caused the Al ions in the substrate to diffuse into the films and bond with O.

Optically pumped magnetometers (OPMs) have developed rapidly in the bio-magnetic measurement field, which requires lasers with stable frequency and intensity for high sensitivity. Herein we stabilize a vertical-cavity surface-emitting laser (VCSEL) without any additional setup except for the parts of an OPM. The linewidth of the absorption spectrum as a frequency reference is broadened to 40 GHz owing to pressure broadening. To enhance performance, the VCSEL injection current and temperature are tuned simultaneously using a closed-loop control system. The experiments reveal that the VCSEL frequency stability achieves 2×10-7 at an average time of 1 s, and the intensity noise is 1×10-6 V/Hz1/2 at 1–100 Hz. This approach is useful for suppressing OPM noise without additional sensor probe parts.

In this work, we used femtosecond laser double-pulse trains to produce laser-induced periodic surface structures (LIPSS) on 304 stainless steel. Surprisingly, a novel type of periodic structure was discovered, which, to the best of our knowledge, is the first in literature. We surmised that the cause for this novel LIPSS was related to the weak energy coupling of subpulses when the intrapulse delay was longer than the thermal relaxation time of stainless steel. Furthermore, we found that the fluence combination and arrival sequence of subpulses in a double-pulse train also influenced LIPSS morphology.

Multifrequency superscattering is a phenomenon in which the scattering cross section from a subwavelength object simultaneously exceeds the single-channel limit at multiple frequency regimes. Here, we achieve simultaneously, within a graphene-coated subwavelength structure, multifrequency superscattering and superscattering shaping with different engineered scattering patterns. It is shown that multimode degenerate resonances at multiple frequency regimes appearing in a graphene composite structure due to the peculiar dispersion can be employed to resonantly overlap electric and magnetic multipoles of various orders, and, as a result, effective multifrequency superscattering with different engineered angular patterns can be obtained. Moreover, the phenomena of multifrequency superscattering have a high tolerance to material losses and some structural variations. Our work should anticipate extensive applications ranging from emission enhancing, energy harvesting, and antenna design with improved sensitivity and accuracy due to multifrequency operation.

We investigate the strong coupling from 5,5’,6,6’-tetrachloro-1,1’-diethyl-3,3’-di(4-sulfobutyl)-benzimidazolocarbocyanine (TDBC) molecules near pure nano-triangular Ag prisms or Ag@Au hollow nanoshells. When TDBC molecules are deposited on pure Ag nanoprisms or Ag@Au hollow nanoshells with the plasmonic resonance peak, matching very closely with the absorption band of TDBC J-aggregates, obvious Rabi splitting can be observed due to the strong coupling regime. Meanwhile, the photoluminescence intensity decreased with the increasing of the temperature, verifying the decreasing plasmon–exciton coupling interaction in the higher temperature. Our experimental results are coincident with the simulation results calculated by finite-difference time-domain method.

Transparent paper is a kind of promising and environmentally friendly material. In this study, we show that transparent paper can be fabricated in an ultra-fast and low-cost way. This low-cost top-down method only takes three steps of cell separation, lignin removal, and cold pressing to obtain a high-quality transparent paper. The fabrication time is further reduced, and the resulted transparent paper shows high transparency up to 90.3%. The application as a substrate material for transparent and flexible electronic devices is demonstrated by emulating the printed circuit on the prepared transparent paper. This top-down method will greatly promote the market-oriented applications of transparent paper as an environment friendly material.

We investigate the dynamics of a system that consists of ultra-cold three-level atoms interacting with radiation fields. We derive the analytical expressions for the population dynamics of the system, particularly, in the presence and absence of nonlinear collisions by considering the rotating wave approximation (RWA). We also reanalyze the dynamics of the system beyond RWA and obtain the state vector of the system to study and compare the time behavior of population inversion. Our results show that the system undergoes two pure quantum phenomena, i.e., the collapse–revival and macroscopic quantum self-trapping due to nonlinear collisional interactions. The occurrence of such phenomena strongly depends on the number of atoms in the system and also the ratio of interaction strengths in the considered system. Finally, we show that the result of the perturbed time evolution operator up to the second-order is in agreement with the numerical solution of the Schrödinger equation. The results presented in the paper may be useful for the design of devices that produce a coherent beam of bosonic atoms known as an atom laser.

Remarkable achievements have been witnessed in free-space quantum key distribution (QKD), which acts as an available approach to extend the transmission range of quantum communications. The feasibility of transmitting qubits through the free-space channel with the aid of moving platforms like satellites, aircraft, unmanned aerial vehicles (UAVs) has been verified. In view of the limited working time and resource consumption of the satellite-based QKD and the last-mile challenges of connecting satellite nodes with terrestrial networks, the airborne QKD is expected to provide flexible and relay links for the large-scale integrated network. This paper reviews the recent significant progress of QKD based on aircraft or UAVs, highlights their critical techniques, and prospects the future of airborne quantum communications.

In this Letter, we use electromagnetic simulations to systematically investigate the influence of a thin dielectric layer on the local electric field and molecular spectroscopy in the plasmonic junction. It is found that both the intensity and spatial confinement of the electric field and molecular spectroscopy can be significantly enhanced by applying a dielectric layer with large dielectric constant. We also discuss the optimal dielectric layer thickness to obtain the largest quantum efficiency of a dipole emitter. These results may be instructive for further studies in molecular spectroscopy and optoelectronics in plasmonic junctions.

We calculate the time-energy distribution (TED) and ionization time distribution (ITD) of photoelectrons emitted by a double-extreme-ultraviolet (XUV) pulse and a two-color XUV-IR pulse using the Wigner distribution-like function based on the strong field approximation. For a double-XUV pulse, besides two identical broad distributions generated by two XUV pulses, many interference fringes resulting from the interference between electrons generated, respectively, by two pulses appear in the TED. After adding an IR field, the TED intuitively exhibits the effect of the IR field on the electron dynamics. The ITDs during two XUV pulses are no longer the same and show the different changes for the different two-color fields, the origin of which is attributed to the change of the electric field induced by the IR field. Our analysis shows that the emission time of electrons ionized during two XUV pulses mainly depends on the electric field of the combined XUV pulse and IR pulse.

High-harmonic generation in metasurfaces, driven by strong laser fields, has been widely studied. Compared to plasma, all-dielectric nanoscale metasurfaces possess larger nonlinearity response and higher damage threshold. Additionally, it can strongly localize the driven field, greatly enhancing its peak amplitude. In this work, we adopt a Fano resonant micro-nano structure with transmission peaks at different wavelengths and explore its nonlinear response by both single and two-color pump fields. Compared to the high-order harmonics induced by the first resonant wavelength, the intensity of the high-harmonic radiation results is enhanced by one order of magnitude, when the metasurface is driven by various resonant and non-resonant wavelength combinations of a two-color field.

The strong coupling between vibrational modes of molecules and surface plasmon resonance (SPR) modes in graphene makes them an ideal platform for biosensor techniques. In this paper, a new optical biosensor for molecule detection based on silver metallic nanoparticles (MNPs) and graphene/gold MNPs in a terahertz frequency range is achieved. It is established that the nonlinear electrical properties of graphene can play a major role in realizing a biosensor for molecule detection. The performance parameters of the proposed device are reported with respect to the chemical potential μ of graphene, noting that the sensitivity of our device passes from 255 nm/RIU (nanometers/refractive index unit) for μ=1.21 eV to 2753 nm/RIU for μ=0.21 eV. Finally, this structure exhibits an optical sensing region that can be adjusted to meet the requirements of optical detection.