Wen-Yuan Zhou

Polarization-dependent optical absorption of graphene under total internal reflection

It is shown that graphene exhibits strong polarization-dependent optical absorption under total internal reflection. Compared with universal absorbance of 2.3%, larger absorption was observed in monolayer, bilayer, and few-layer graphenes for transverse electric (TE) wave under total internal reflection. Our result indicates that reflectance ratio of transverse magnetic wave to TE waves can easily provide the information of number of graphene layers. Furthermore, the enhanced light-graphene coupling in a wide spectral range will be great potential in many applications such as photodetector, photovoltaics, and optical sensor.

Nonlinear optical and optical limiting properties of graphene oxide–Fe3O4 hybrid material

The nonlinear optical (NLO) and optical limiting properties of a graphene oxide hybrid material coordinated with Fe3O4 nanoparticles (GO–Fe3O4) were studied by using the Z-scan technique at 532 nm in the nanosecond and picosecond regimes. Results show that GO–Fe3O4 exhibits enhanced NLO and optical limiting properties in comparison with the pristine GO in the nanosecond regime. Compared with fullerene (C60) in toluene at different concentrations, GO–Fe3O4 exhibits a weaker optical limiting effect than C60 at high concentration, but shows a stronger optical limiting effect than C60 at low concentration in the high input fluence region.

Analysis of optical trapping and propulsion of Rayleigh particles using Airy beam

The radiation forces and trajectories of Rayleigh dielectric particles induced by one-dimensional Airy beam were numerically analyzed. Results show that the Airy beam drags particles into the optical intensity peaks, and guides particles vertically along parabolic trajectories. Viscosity of surrounding medium significantly affects the trajectories. Random Brownian force affects the trajectories. Meanwhile, trapping potential depths and minimum trapping particle radii in different potential wells were also discussed. The trapping stability could be improved by increasing either the input peak intensity or the particle radius.

Broadband all-optical modulation using a graphene-covered-microfiber

We demonstrate all-optical modulation based on ultrafast saturable absorption in graphene-covered-microfiber. By covering the microfiber surface with polydimethylsiloxane supported graphene film along the fiber length, a greatly enhanced interaction between the propagating light and the graphene can be obtained via the strong evanescent field of the microfiber. The strong light–graphene interaction results in high-speed, broadband all-optical modulation with maximum modulation depths of 5 dB and 13 dB for single-layer and bi-layer graphene, respectively. Such a graphene all-optical modulator is easy to fabricate, is compatible with optical fiber systems and has high potential in photonics applications such as all-optical switching and all-optical communications.

Study on Z-scan characteristics for a large nonlinear phase shift

Using the Gaussian decomposition (GD) method, we studied the characteristics of a Z scan for a thin nonlinear medium with a large nonlinear phase shift induced by a pulsed laser. It has been verified that the GD method is still valid for analyses of Z-scan measurements with a large nonlinear phase shift and is better than some others, i.e., Fresnel-Kirchhoff diffraction and the aberration-free approximation model. By comparing the peak-to-valley configuration of Z-scan curves for a large nonlinear phase shift induced by a pulsed laser with that by a cw laser, we found that some peak-to-valley features of Z-scan curves appear as the aperture size or the light intensity increases in the case of a large nonlinear phase shift. Meanwhile, we carried out the Z-scan experiments of pure CS2 by a picosecond pulsed laser to verify the theoretical calculations in the case of a large nonlinear phase shift. The experimental results agree well with the theoretical calculations.

Unique characteristics of a selective-filling photonic crystal fiber Sagnac interferometer and its application as high sensitivity sensor.

We demonstrate a Sagnac interferometer (SI) based on a selective-filling photonic crystal fiber (SF-PCF), which is achieved by infiltrating a liquid with higher refractive index than background silica into two adjacent air holes of the innermost layer. The SF-PCF guides light by both index-guiding and bandgap-guiding. The modal birefringence of the SF-PCF is decidedly dependent on wavelength, and the modal group birefringence has zero value at a certain wavelength. We also theoretically and experimentally investigate in detail the transmission and temperature characteristics of the SI. Results reveal that the temperature sensitivity of the interference spectrum is also acutely dependent on wavelength and temperature, and an ultrahigh even theoretically infinite sensitivity can be achieved at a certain temperature by choosing proper fiber length. An ultrahigh sensitivity with -26.0 nm/°C (63,882 nm/RIU) at 50.0 °C is experimentally achieved.

Measurement of the complex refractive index of tissue-mimicking phantoms and biotissue by extended differential total reflection method.

Refractive index of biotissue is a useful optical parameter in the biomedical field. An extended differential total reflection method is introduced to determine the complex refractive index. The real part is directly determined by differential of the reflectance curve, and the imaginary part is obtained from nonlinear fitting. The method is verified by a series of tissue-mimicking phantoms, porcine muscle and porcine adipose.

Enhanced light transmission through a single subwavelength aperture in layered films consisting of metal and dielectric

The light transmission through a single subwavelength aperture surrounded by periodic grooves in layered films consisting of Ag and transparent dielectric is analyzed numerically by finite difference time domain (FDTD) method. Results show that the transmission through the aperture in the composite films is strongly enhanced by the modulation of grooves on the transparent dielectric. Two models are employed to explain the mechanisms of transmission enhancement.

Polarization dependence of Z-scan measurement: theory and experiment

Here we report on an extension of common Z-scan method to arbitrary polarized incidence light for measurements of anisotropic third-order nonlinear susceptibility in isotropic medium. The normalized transmittance formulas of closed-aperture Z-scan are obtained for linearly, elliptically and circularly polarized incidence beam. The theoretical analysis is examined experimentally by studying third-order nonlinear susceptibility of CS2 liquid. Results show that the elliptically polarized light Z-scan method can be used to measure simultaneously the two third-order nonlinear susceptibility components chi(3)(xyyx) and chi(3)(xxyy). Furthermore, the elliptically polarized light Z-scan measurements of large nonlinear phase shift are also analyzed theoretically and experimentally.

Nonlinear ellipse rotation modified Z-scan measurements of third-order nonlinear susceptibility tensor

We present a method that combines the Z-scan technique with nonlinear ellipse rotation (NER) to measure third-order nonlinear susceptibility components. The experimental details are demonstrated, and a comprehensive theoretical analysis is given. The validity of this method is verified by the measurements of the nonlinear susceptibility tensor of a well-characterized liquid, CS2.