Xueru Zhang

A theoretical and experimental study on optical limiting in platinum nanoparticles

Optical limiting performance of PVP-stabilized platinum nanoparticles in methanol is investigated with 8 ns pulses at 532 nm. The experimental results show that the platinum nanoparticles possess strong optical limiting effect. We propose a theoretical model based on Mie extinction theory to describe nonlinear optical limiting (OL) behavior of metal nanoparticles. The theoretical analysis and experimental results show that the main mechanism for OL of platinum nanoparticles can be attributed to interband transition of platinum during the excitation of nanosecond pulses.

Effect of Li+ ions on enhancement of near-infrared upconversion emission in Y2O3:Tm3+/Yb3+ nanocrystals

Near-infrared (NIR) to NIR upconversion emission is investigated in Tm3+/Yb3+/Li+ triply doped Y2O3 nanocrystals. Li+ ions doped in Y2O3:Tm3+/Yb3+ nanocrystals can greatly enhance the NIR upconversion emission intensity of Tm3+ ions. The abnormal shift of the (222) diffraction peak position determined from x-ray diffraction measurements is discussed, by introducing Li+ ions in the Y2O3:Tm3+/Yb3+ nanocrystals. The cause of the enhancement is the modification of the local symmetry induced by the Li+ ions, which increases the intra-4f transitions of Tm3+ ion. Li+ ions doped in Y2O3:Tm3+/Yb3+ nanocrystals also can reduce the OH groups, dissociate the Yb3+ and Tm3+ ion clusters, and create the oxygen vacancies, which are the other reasons for enhancing the upconversion emission intensities. This material may be promising for in vitro and in vivo bioimaging probes.

Intensity threshold in the conversion from reverse saturable absorption to saturable absorption and its application in optical limiting

A theoretical steady-state rate-equation analysis is presented for investigating the critical conditions for the conversion from reverse saturable absorption to saturable absorption. The intensity threshold is defined and seen as an important factor in the interpretation of the reverse saturable absorption (RSA) to saturable absorption (SA) conversion. A comparison between steady-state and dynamic results is made and the application of intensity threshold in optical limiting is proposed.

Discrete plasmonic Talbot effect in subwavelength metal waveguide arrays

Discrete plasmonic Talbot effect in the subwavelength metal waveguide arrays (SMWAS) is theoretically analyzed and numerically simulated. Based on the finite-difference time-domain technique, we discuss the influence of the structural parameters on the Talbot distance. By carefully choosing the geometry parameters, the Talbot distance decreases to about one third of the incident wavelength. The numerical simulation results agree with the theory of the discrete Talbot effect in the SMWAS.

Optical temperature sensing in β-NaLuF4:Yb3+/Er3+/Tm3+ based on thermal, quasi-thermal and non-thermal coupling levels

Three methods for optical temperature sensing are investigated in the NaLuF4:Yb3+/Er3+/Tm3+ phosphor, which is prepared by a hydrothermal method. The temperature-dependent luminescence is investigated under 980 nm excitation. Utilizing fluorescence intensity ratio (FIR) technique, the temperature sensing behaviors are studied in the range of 300–600 K based on thermal coupling levels (2H11/2 and 4S3/2 of Er3+), “quasi-thermal coupling levels” (4F7/2 of Er3+ and 3F2 of Tm3+) and non-thermal coupling levels (1D2 and 1G4 of Tm3+), respectively. The maximum sensitivity is 604 × 10−4 K−1 at 300 K, which is based on non-thermal coupling levels of Tm3+. Meanwhile, the temperature dependent emission colors are discussed, which are changed from bluish green to light blue with the temperature rising. These significant results, high sensitivity and the temperature dependent multicolor emissions, indicate that the NaLuF4:Yb3+/Er3+/Tm3+ phosphor is robust for optical temperature sensing.

Z-scan of excited-state nonlinear materials with reverse saturable absorption

Abstract The nonlinear refraction property of excited-state nonlinear optical materials was studied based on rate-equation theory. The effects of the ratios of excited-state absorption cross section and refrangibility to those of ground state upon their nonlinear refraction properties were analyzed. The nonlinear optical properties of ( m -MeC 6 H 4 ) 5 C 60 H-toluene solution were investigated by using Z -scan method with 8 ns 532 nm laser pulses. The solution was proved to possess strong nonlinear absorption and weak nonlinear refraction.

Excited-state nonlinearity measurements of ZnPcBr 4 /DMSO

Excited-state absorptive and refractive nonlinearities of a ZnPcBr4/DMSO solution are studied using a modified picosecond time-resolved pump-probe system based on a 4f nonlinear imaging technique with phase object. With the help of additional nanosecond laser Z-scan measurements, the parameters associated with the five-level model used to interpret the experimental results are uniquely determined. Unusually large absorption cross sections for the first singlet excited state and for the lowest triplet excited state as well as a rather short intersystem crossing time are found in this system, suggesting ZnPcBr4 (a phthalocyanine) to be an effective optical limiter. The intensity-dependent refractive index of neat dimethyl sulfoxide (DMSO) and the excited-state contribution to the nonlinear refractive index of the solution are also determined.

Time-resolved pump-probe system based on a nonlinear imaging technique with phase object

A nonlinear imaging technique with phase object, which can deduce nonlinear absorption and refraction coefficients by single laser-shot exposure, is expanded to a time-resolved pump-probe system by introducing a pump beam with a variable temporal delay. This new system, in which both degenerate and nondegenerate pump and probe beams in any polarization states can be used, can simultaneously measure dynamic nonlinear absorption and refraction conveniently. In addition, the sensitivity of this new pump-probe system is more than twice that of the Z-scan-based system. The semiconductor ZnSe is used to demonstrate this system.

Fractional Gabor transform

A fractional Gabor transform (FRGT) is proposed. This new transform is a generalization of the conventional Gabor transform (GT) based on the Fourier transform to the windowed fractional Fourier transform (FRFT). The FRGT provides analyses of signals in both the real space and the FRFT frequency domain simultaneously. The space-FRFT frequency pattern can be rotated as the fractional order changes. The FRGT has an additional freedom, compared with the conventional GT, i.e., the transform order. The FRGT may offer a useful tool for guiding optimal filter design in the FRFT domain in signal processing.

Analysis of focal-shift effect in planar metallic nanoslit lenses.

A theoretical analysis based on scalar diffraction theory about the recently reported focal-shift phenomena in planar metallic nanoslit lenses is presented. Under Fresnel approximation, an axial intensity formula is obtained, which is used to analyze the focal performance in the far field zone of lens. The relative focal shift is totally dependent on the Fresnel number only. The influences of the lens size, preset focal length and incident wavelength can be attributed to the change of Fresnel number. The total phase difference of the lens is approximately equal to the Fresnel number multiplied by π. Numerical simulations performed using finite-difference time-domain (FDTD) and near-far field transformation method are in agreement with the theoretical analysis. Using the theoretical formula assisted by simple numerical method, we provide predictions on the focal shift for the previous literatures.