Tsong-Ru Tsai

Nanoimprint of gratings on a bulk metallic glass

The authors demonstrate that optical gratings with 600 and 1500nm periods on a Pd40Ni40P20 bulk metallic glass (BMG) can be faithfully imprinted in air from Si dies. Results of scanning electron microscopy, atomic force microscopy, and optical diffraction analysis show the fine line feature of ∼150nm. The gratings have smooth and uniform surface profiles with comparable optical properties as the original Si dies. The BMG gratings can be further used to imprint the second-generation replicas on polymethylmethacrylate. Thereby, BMG is a suitable material not only for imprinting nanostructured parts such as gratings, but also as a good die material for nanoimprints.

Terahertz response of GaN thin films

The indices of refraction, extinction constants and complex conductivities of the GaN film for frequencies ranging from 0.2 to 2.5 THz are obtained using THz time-domain spectroscopy. The results correspond well with the Kohlrausch stretched exponential model. Using the Kohlrausch model fit not only provides the mobility of the free carriers in the GaN film, but also estimates the relaxation time distribution function and average relaxation time.

Imaging layer number and stacking order through formulating Raman fingerprints obtained from hexagonal single crystals of few layer graphene

Quantitative mapping of layer number and stacking order for CVD-grown graphene layers is realized by formulating Raman fingerprints obtained on two stepwise stacked graphene single-crystal domains with AB Bernal and turbostratic stacking (with ~30°interlayer rotation), respectively. The integrated peak area ratio of the G band to the Si band, A(G)/A(Si), is proven to be a good fingerprint for layer number determination, while the area ratio of the 2D and G bands, A(2D)/A(G), is shown to differentiate effectively between the two different stacking orders. The two fingerprints are well formulated and resolve, quantitatively, the layer number and stacking type of various graphene domains that used to rely on tedious transmission electron microscopy for structural analysis. The approach is also noticeable in easy discrimination of the turbostratic graphene region (~30° rotation), the structure of which resembles the well known high-mobility graphene R30/R2(±) fault pairs found on the vacuum-annealed C-face SiC and suggests an electron mobility reaching 14,700 cm(3) V(-1) s(-1). The methodology may shed light on monitoring and control of high-quality graphene growth, and thereby facilitate future mass production of potential high-speed graphene applications.

Observation of optical second harmonic generation from suspended single-layer and bi-layer graphene

We have experimentally investigated the optical second harmonic generation (SHG) on suspended single-layer and bi-layer graphene sheets. By shining normally incident 800-nm light with polarization along the sample planes, the SHG intensities of single-layer and bi-layer graphene are found to be comparable to the one of polar GaAs with large second order susceptibility, which is unexpected because both have the centrosymmetric property. Our experimental results reveal that the strong SHG is not due to the defects breaking the symmetry. Instead, we suggest that the SHG signals result from the long-range curvature fluctuations on the suspended single-layer and bi-layer graphene sheets.

Estimating Young?s modulus of graphene with Raman scattering enhanced by micrometer tip

We demonstrate that the Raman intensities of G and 2D bands of a suspended graphene can be enhanced using a gold tip with an apex size of 2.3 μm. The enhancement decays with the tip-graphene distance exponentially and remains detectable at a distance of 1.5 μm. Raman mappings show that the enhanced area is comparable to the apex size. Application of a bias voltage to the tip can attract the graphene so that Raman signals are intensified. The exponential enhancement-distance relationship enables the measurement of the graphene deformation, and the Youngs modulus of graphene is estimated to be 1.48 TPa.

Terahertz responses of high-resistivity YBCO thin films

Abstract We used pulse terahertz radiation to study the complex conductivity of high-resistivity YBCO thin films. The complex conductivity of YBCO superconducting thin films in the 50–300 GHz range was obtained. The temperature dependence of λ 2 (0)/ λ 2 ( T ) of such films exhibits a T 1 behavior which is closed to the prediction of the d-wave theory in the clean limit. However, its zero-temperature value is larger than that of typical high-quality single crystal YBCO. The temperature dependence of the real part of the conductivity below T c exhibits a frequency-dependent broad peak. Using the two-fluid Drude model for the superconducting state, the temperature dependence of the quasiparticle scattering rate is observed. The experimental result indicated that the quasiparticle scattering rate is higher than that reported on very-high-quality YBCO single crystals in the millimeter wave region.

Characterization of nonlinear absorption of InN epitaxial films with femtosecond pulsed transmission Z-scan measurements

The femtosecond pulsed Z-scan measurements of the resonant nonlinear optical absorption of the InN epitaxial films in the range of 720–790nm were reported. The absorption saturation behavior was found to gradually decrease with increasing photon energy. The nonlinear optical absorption cross sections of the InN films were estimated and the values are found to be in good agreement with the calculations based on the band-filling model. These results are relevant for the future development of nonlinear optical devices based on InN.

Direct patterning of zinc oxide with control of reflected color through nano-oxidation using an atomic force microscope

Atomic force microscope oxidation on Zn creating amorphous ZnO (a-ZnO) with the a-ZnO showing multiple colors under white light at different oxidation voltages was successfully demonstrated. Simulation of reflected colors at different thicknesses of a-ZnO was also conducted. The presented technique can not only be applied to near diffraction limit multilevel optical data storage, but also makes it possible to represent the color spectra observed in nature at near diffraction limits. It can also be used for device fabrication in situations exploiting the semiconductor nature of ZnO.

Terahertz optical constants of ytterbium-doped yttrium aluminum garnet crystals

Terahertz time-domain spectroscopy has been employed to measure the optical constants of ytterbium-doped yttrium aluminum garnet (YbxY1−x)3Al5O12 (Yb:YAG) crystals for nominal x values of 0.0, 0.1, 0.2, 0.5, 0.8, and 1.0 in the frequency range from 0.2to1.8THz. The real refractive indices for Yb:YAG crystals increase linearly with the concentration of Yb3+. The experimental results can be fitted by the Sellmeier equation. The results imply that the phonon modes of the Yb:YAG crystals shift to lower frequencies with substitution of Yb for Y. The extinction coefficients of the Yb:YAG crystals are smaller than 0.05.

Spectral dependence of time-resolved photoreflectance of InN epitaxial films

Femtosecond pulses at wavelengths ranging from 750to900nm (1.38–1.65eV) were used in the excitation and probing of ultrafast carrier dynamics in InN epitaxial films. Experimental results show that the hot electron relaxation rate increases with increasing electron energy, which is measured as E0.53. This observation agrees with the prediction of electron-electron scattering relaxation mechanism. In addition, the electron-hole recombination rates are independent of the electron energy and have values of ∼7×109Hz. We attribute this result to the Auger recombination in InN being insensitive to temperature.