Ultraviolet photonic crystal laser

We fabricated two dimensional photonic crystal structures in zinc oxide films with focused ion beam etching. Lasing is realized in the near ultraviolet frequency at room temperature under optical pumping. From the measurement of lasing frequency and spatial profile of the lasing modes, as well as the photonic band structure calculation, we conclude that lasing occurs in the strongly localized defect modes near the edges of photonic band gap. These defect modes originate from the structure disorder unintentionally introduced during the fabrication process.Comment: 4 pages, 4 figure

Interpretable and Generalizable Person Re-Identification with Query-Adaptive Convolution and Temporal Lifting

For person re-identification, existing deep networks often focus on representation learning. However, without transfer learning, the learned model is fixed as is, which is not adaptable for handling various unseen scenarios. In this paper, beyond representation learning, we consider how to formulate person image matching directly in deep feature maps. We treat image matching as finding local correspondences in feature maps, and construct query-adaptive convolution kernels on the fly to achieve local matching. In this way, the matching process and results are interpretable, and this explicit matching is more generalizable than representation features to unseen scenarios, such as unknown misalignments, pose or viewpoint changes. To facilitate end-to-end training of this architecture, we further build a class memory module to cache feature maps of the most recent samples of each class, so as to compute image matching losses for metric learning. Through direct cross-dataset evaluation, the proposed Query-Adaptive Convolution (QAConv) method gains large improvements over popular learning methods (about 10%+ mAP), and achieves comparable results to many transfer learning methods. Besides, a model-free temporal cooccurrence based score weighting method called TLift is proposed, which improves the performance to a further extent, achieving state-of-the-art results in cross-dataset person re-identification. Code is available at https://github.com/ShengcaiLiao/QAConv.Comment: This is the ECCV 2020 version, including the appendi

Single-Shot Visualization Of Evolving Laser- Or Beam-Driven Plasma Wakefield Accelerators

We introduce Frequency-Domain Tomography (FDT) for visualizing sub-ps evolution of light-speed refractive index structures in a single shot. As a prototype demonstration, we produce single-shot tomographic movies of self-focusing, filamenting laser pulses propagating in a transparent Kerr medium. We then discuss how to adapt FDT to visualize evolving laser-or beam-driven plasma wakefields of current interest to the advanced accelerator community. For short (L similar to 1 cm), dense (n(e) similar to 10(19) cm(-3)) plasmas, the key challenge is broadening probe bandwidth sufficiently to resolve plasma-wavelength-size structures. For long (L similar to 10 to 100 cm), tenuous (n(e) similar to 10(17) cm(-3)) plasmas, probe diffraction from the evolving wake becomes the key challenge. We propose and analyze solutions to these challenges.Physic

Toward a Broadband Astro-comb: Effects of Nonlinear Spectral Broadening in Optical Fibers

We propose and analyze a new approach to generate a broadband astro-comb by spectral broadening of a narrowband astro-comb inside a highly nonlinear optical fiber. Numerical modeling shows that cascaded four-wave-mixing dramatically degrades the input comb's side-mode suppression and causes side-mode amplitude asymmetry. These two detrimental effects can systematically shift the center-of-gravity of astro-comb spectral lines as measured by an astrophysical spectrograph with resolution \approx100,000; and thus lead to wavelength calibration inaccuracy and instability. Our simulations indicate that this performance penalty, as a result of nonlinear spectral broadening, can be compensated by using a filtering cavity configured for double-pass. As an explicit example, we present a design based on an Yb-fiber source comb (with 1 GHz repetition rate) that is filtered by double-passing through a low finesse cavity (finesse = 208), and subsequent spectrally broadened in a 2-cm, SF6-glass photonic crystal fiber. Spanning more than 300 nm with 16 GHz line spacing, the resulting astro-comb is predicted to provide 1 cm/s (~10 kHz) radial velocity calibration accuracy for an astrophysical spectrograph. Such extreme performance will be necessary for the search for and characterization of Earth-like extra-solar planets, and in direct measurements of the change of the rate of cosmological expansion.Comment: 9 pages, 6 figure

Raman spectroscopic determination of the length, strength, compressibility, Debye temperature, elasticity, and force constant of the C-C bond in graphene

From the perspective of bond relaxation and vibration, we have reconciled the Raman shifts of graphene under the stimuli of the number-of-layer, uni-axial-strain, pressure, and temperature in terms of the response of the length and strength of the representative bond of the entire specimen to the applied stimuli. Theoretical unification of the measurements clarifies that: (i) the opposite trends of Raman shifts due to number-of-layer reduction indicate that the G-peak shift is dominated by the vibration of a pair of atoms while the D- and the 2D-peak shifts involves z-neighbor of a specific atom; (ii) the tensile strain-induced phonon softening and phonon-band splitting arise from the asymmetric response of the C3v bond geometry to the C2v uni-axial bond elongation; (iii) the thermal-softening of the phonons originates from bond expansion and weakening; and (iv) the pressure- stiffening of the phonons results from bond compression and work hardening. Reproduction of the measurements has led to quantitative information about the referential frequencies from which the Raman frequencies shift, the length, energy, force constant, Debye temperature, compressibility, elastic modulus of the C-C bond in graphene, which is of instrumental importance to the understanding of the unusual behavior of graphene

Electron Delocalization in Gate-Tunable Gapless Silicene

The application of a perpendicular electric field can drive silicene into a gapless state, characterized by two nearly fully spin-polarized Dirac cones owing to both relatively large spin-orbital interactions and inversion symmetry breaking. Here we argue that since inter-valley scattering from non-magnetic impurities is highly suppressed by time reversal symmetry, the physics should be effectively single-Dirac-cone like. Through numerical calculations, we demonstrate that there is no significant backscattering from a single impurity that is non-magnetic and unit-cell uniform, indicating a stable delocalized state. This conjecture is then further confirmed from a scaling of conductance for disordered systems using the same type of impurities.Comment: 6 pages, 3 figures, published versio

Statefinder Parameters for Five-Dimensional Cosmology

We study the statefinder parameter in the five-dimensional big bounce model, and apply it to differentiate the attractor solutions of quintessence and phantom field. It is found that the evolving trajectories of these two attractor solutions in the statefinder parameters plane are quite different, and that are different from the statefinder trajectories of other dark energy models.Comment: 8 pages, 12 figures. accepted by MPL

The Design and Numerical Study of a 2MWh Molten Salt Thermocline Tank

AbstractThe two tank molten salt thermal storage system is widely used in the commercialized solar thermal power plant. However, the thermocline storage system with a low-cost filler material is a more economically feasible option. In this study, a transient two-dimensional and two-temperature model is developed to investigate the heat transfer and fluid dynamics in a molten salt thermocline thermal storage system. After model validation, the effects of inlet flow boundary condition and storage medium properties including fluid and solid materials on the thermal performance of thermocline storage system are investigated. The results show that thermoclne thickness increases slowest with solar salt as heat transfer fluid (HTF) and Cofalit® as solid material in the thermocline tank. Any non-uniformity in the inlet velocity flow would only enhance mixing and widen the thermocline appreciably, which contributes to the loss of thermodynamic availability of stored energy. The thermocline thickness increases with the non-uniformity of the inlet velocity boundary condition. So smaller non-uniformity of inlet flow is better in non-uniform flow though it may causes larger fluctuations in average outlet temperature. Smaller inlet mass flow rate is better for the thermocline storage tank, while it also causes smaller discharging power. With the chosen basic design parameters such as fluid and solid materials, the size of a 2MWh thermocline tank is determined by a simple one-dimensional design method. Tank with larger H/D ratio has higher discharge efficiency. It helps to figure out the thermal stratification mechanism of a storage tank and thereby to determine optimum design and operating conditions