Zongwei Ma

Broadband Black-Phosphorus Photodetectors with High Responsivity

An array of black-phosphorus photodetectors with channel lengths down to 100 nm is fabricated, and temperature-dependent photodetection measurements from 300 K down to 20 K are carried out. The devices show high photoresponse in a broadband spectral range with a record-high photoresponsivity of 4.3 × 10(6) A W(-1) at 300 K for the 100 nm device.

Plasmon resonance enhanced large third-order optical nonlinearity and ultrafast optical response in Au nanobipyramids

The third-order optical nonlinearity and response time of Au nanobipyramids have been investigated by using optical Kerr effect technique. Large third-order nonlinear optical susceptibility (χ(3)) and ultrafast optical response time have been obtained at the longitudinal surface plasmon resonance (LSPR) wavelength. As the excitation laser wavelength varies from non-resonance wavelength of 780 nm to the LSPR wavelength of 825 nm, χ(3) increases from 7.4 × 10−14 to 3.9 × 10−13 esu, the fast component of the response time decreases from 141 ± 23 to 83 ± 8 fs and the ratio of the fast component remains around 48%, while the slow response time decreases from 3200 ± 200 to 2310 ± 158 fs. The large enhancement of χ(3) is due to the large local field enhancement caused by the surface plasmon resonance, and the shortening of the response times are induced by the increased probability of the electron-electron and electron-phonon scatterings in the nanosystem. This significant ultrafast optical property in Au nanobi...

The influence of high magnetic field on electric-dipole emission spectra of Eu3+ in different single crystals

In this work, the effect of pulsed high magnetic field on the integral intensity of the electric-dipole emission spectrum of Eu3+ in YVO4:Eu3+ and GdVO4:Eu3+ single crystals was studied and the results showed magnetic field induced luminescence suppression in YVO4:Eu3+ and luminescence expansion in GdVO4:Eu3+. The single crystals were prepared by the optical floating zone method. The strong effect of the magnetic field on the splitting and emission of 5D0–7F2 and 5D0–7F4 transitions in Eu3+ was investigated. The integral luminescence intensities of Eu3+ are proportional to the magnetic field at higher than 15 T. For GdVO4:Eu3+, the integral intensity increases with the strengthening of magnetic field, and is completely opposite to that for YVO4:Eu3+. In this study, the dependence of luminescence integral intensity on the magnetic field was proposed to be the result of the structural change induced by high magnetic field which changes the symmetry of Eu3+ ions in single crystals.

Magnetic field induced great photoluminescence enhancement in an Er 3+ :YVO 4 single crystal used for high magnetic field calibration

An optical technique is proposed for the accurate calibration of pulsed high magnetic fields utilizing the magnetic field dependent photoluminescence (PL) properties in an Er(3+):YVO(4) single crystal at 80 K. Bright green PL emissions are excited by a 487.5 nm laser line and can be enhanced greatly by a magnetic field at certain field values (B(c)). Since the B(c)s under 10 T are extremely stable for a given sample at a certain temperature, and the FWHM of the enhancement peaks are less than 0.9 T, an Er(3+):YVO(4) single crystal is proven to be a good candidate for pulsed high magnetic field calibration. The detailed processes and numerous advantages of the technique are presented in this work.

Magnetic field induced extraordinary photoluminescence enhancement in Er3+:YVO4 single crystal

A bright green photoluminescence (PL) from 4S3∕2 → 4I15∕2 emission band in Er3+:YVO4 single crystal has been observed with the excitation of an argon laser at 488.0 nm. More than two orders of PL enhancement have been obtained under the effect of magnetic fields, and the enhancement factor f reaches 170 when the applied magnetic field is 7.7 T under the sample temperature of 4.2 K. Unusually, the PL enhancements only happen at some certain magnetic fields (Bcs), and a decrease of sample temperature will lead to the increase of f and decrease of Bc. The results confirm that this PL enhancement originates from the resonance excitation of the electron transitions induced by the cross of the laser energy and the absorption energy modulated by both the magnetic field and temperature. This special PL enhancement in Er3+:YVO4 single crystal can be applied in the calibration of pulsed high magnetic field, detection of material fine energy structures, and modulation of magneto-optical devices.

Near-UV-enhanced broad-band large third-order optical nonlinearity in aluminum nanorod array film with sub-10 nm gaps

Plasmonic nanostructures with sub-10 nm gaps possess intense electric field enhancements, leading to their high reputation for exploring various functional applications at nanoscale. Till now, although large amounts of efforts have been devoted into investigation of such structures, few works were emphased on the nonlinear optical properties in near-ultraviolet (UV) region. Here, by combining sputtering technique and an optimized anodic aluminum oxide (AAO) template growing method, we obtain aluminum (Al) nanorod array film (NRAF) with average rod diameter and gap size of 50 and 7 nm, respectively. The Al-NRAF exhibits large third-order optical nonlinear susceptibility (χ(3)) and high figure of merit (χ(3)/α) over a broad wavelength range from 360 to 900 nm, and reaches their maximums at the shortest measured wavelength. In addition, comparisons with Au-NRAF and Ag-NRAF samples further confirm that Al-NRAF has better nonlinear optical properties in the blue and near-UV wavelength regions. These results indicate that Al nanostructures are promising candidates for nonlinear plasmonic applications at blue and near-UV wavelengths.

Surface plasmon enhanced third-order optical nonlinearity of silver nanocubes

We present the linear and nonlinear optical properties of Ag nanocubes. The surface plasmon resonance absorption peak shows a red shift with increasing cube size. The nonlinear absorption and nonlinear refraction were measured by using Z-scan technique. The Ag nanocubes with the edge size of 60 nm show the largest third-order optical nonlinearity, which is supported by the largest field enhancement factor through the FDTD simulations. In the wavelength range from 720 nm to 920 nm, the one-photon and two-photon figures of merit, W and T satisfy the demand of W > 1 and T < 1, which implies it is a promising candidate for the optical switching devices.

Cooling field and temperature dependence on training effect in NiFe2O4-NiO nanogranular system

The training effect has been systematically studied in exchange coupled NiFe2O4/NiO nanogranular system. Both exchange bias field (HEB) and vertical magnetization shifts (MShift) can be observed after the system field cooled from 350 K to low temperatures, which decrease monotonically through consecutive loop cycles. During this procedure, linear dependence between HEB and MShift is found for this system, revealing the critical role of the pinned uncompensated spins. With the increase of cooling field, the relative change of HEB becomes more pronounced, which shows that the rapid reduction of the pinned uncompensated spins for high cooling field. Moreover, the reduction of HEB becomes weakened with decreasing measured temperatures, which indicated the spin configuration at low temperatures possesses higher dynamic stability. The cooling field and temperature dependence on training effect is discussed in terms of the evolution of the metastable spin configurations at the interfaces and fitted by a recent t...

Origin of the Avalanche-Like Photoluminescence from Metallic Nanowires.

Surface plasmonic systems provide extremely efficient ways to modulate light-matter interaction in photon emission, light harvesting, energy conversion and transferring, etc. Various surface plasmon enhanced luminescent behaviors have been observed and investigated in these systems. But the origin of an avalanche-like photoluminescence, which was firstly reported in 2007 from Au and subsequently from Ag nanowire arrays/monomers, is still not clear. Here we show, based on systematic investigations including the excitation power/time related photoluminescent measurements as well as calculations, that this avalanche-like photoluminescence is in fact a result of surface plasmon assisted thermal radiation. Nearly all of the related observations could be perfectly interpreted with this concept. Our finding is crucial for understanding the surface plasmon mediated thermal and photoemission behaviors in plasmonic structures, which is of great importance in designing functional plasmonic devices.

Hidden local symmetry of Eu3+ in xenotime-like crystals revealed by high magnetic fields

The excellent optical properties of europium-doped crystals in visible and near infrared wavelength regions enable them to have broad applications in optoelectronics, laser crystals and sensing devices. The local site crystal fields can affect the intensities and peak positions of the photo-emission lines strongly, but they are usually difficult to be clarified due to magnetically degenerate 4f electronic levels coupling with the crystal fields. Here, we provide an effective way to explore the hidden local symmetry of the Eu3+ sites in different hosts by taking photoluminescence measurements under pulsed high magnetic fields up to 46 T. The zero-field photoluminescence peaks split further at high magnetic fields when the Zeeman splitting energy is comparable to or larger than that of the crystal field induced zero-field splitting. In particular, a magnetic field induced crossover of the local crystal fields has been observed in the GdVO4:Eu3+ crystal, which resulted from the alignment of Gd3+ magnetic mom...