Ching-Wei Chen

Optical properties and potential applications of ε-GaSe at terahertz frequencies

Ordinary and extraordinary refractive indices of an ɛ-GaSe crystal at terahertz (THz) frequencies are experimentally determined in this study. Fitting experimental data by THz time-domain spectroscopy (THz-TDS) and a Fourier transform infrared spectrometer (FTIR), we proposed revised complex dielectric functions and Sellmeier equations of GaSe for both ordinary and extraordinary waves from 0.2 to 100 THz. Phonon vibrational modes and overtones in the THz frequency range are examined in detail. The high magnitude of the figure of merit (FOM~103 at 1 THz), the large birefringence (Δn~0.76 at 1 THz). and the low absorption coefficient (α~0.2 cm−1 at 1 THz) of GaSe are also identified. Potential applications to practical photonic devices such as phase shifters at THz frequencies are proposed.

Frequency-Dependent Complex Conductivities and Dielectric Responses of Indium Tin Oxide Thin Films From the Visible to the Far-Infrared

Transparent and conducting indium tin oxide (ITO) thin films form an integral part for various optoelectronic devices. In this paper, we report the frequency-dependent complex conductivities and dielectric responses of several sputtered ITO thin films with thicknesses in the range of 189-962 nm by using terahertz time domain spectroscopy (THz-TDS), optical reflectance spectroscopy, and electrical measurements. The plasma frequencies are verified to be from 1590 to 1930 rad·THz, while the scattering times are in the range 6-7 fs based on the Drude free-electron model. The mobilities of the above ITO thin films are calculated to be 32.7-34.2 cm2 V-1 s-1, whereas the carrier concentrations lie in the range 2.79-4.10× 1020 cm-3. The electrical properties derived from the THz-TDS technique agree well with those determined by Hall measurement. Parameters for the complex dielectric function suitable for ITO in the range 0.2-2 and 4-450 THz are also determined.

Generation properties of coherent infrared radiation in the optical absorption region of GaSe crystal

We report a study of the effect of optical absorption on generation of coherent infrared radiation from mid-IR to THz region from GaSe crystal. The infrared-active modes of epsilon-GaSe crystal at 236 cm(-1) and 214 cm(-1) were found to be responsible for the observed optical dispersion and infrared absorption edge. Based upon phase matching characteristics of GaSe for difference-frequency generation (DFG), new Sellmeier equations of GaSe were proposed. The output THz power variation with wavelength can be properly explained with a decrease of parametric gain and the spectral profile of absorption coefficient of GaSe. The adverse effect of infrared absorption on (DFG) process can partially be compensated by doping GaSe crystal with erbium ions.

Erbium doped GaSe crystal for mid-IR applications

We reported a type-I difference-frequency generator (DFG), based on erbium doped GaSe (Er:GaSe) crystals as a coherent infrared source tunable from 2.4 mum to 28 mum. The two mixing beams used for the DFG are a tunable near infrared output (1.1-1.8 mum) from an optical parametric amplifier (OPA) and the fundamental beam of a picosecond Nd:YAG laser at 1.064 mum. The system can produce a maximum output pulse energy of 5 microJ at wavelength of 3.5 microm, corresponding to a photon conversion efficiency of 8% at a pump intensity of 1.7 GW/cm(2). The nonlinear coefficient (d(eff)) of 0.5 atom % erbium doped GaSe crystal was found to be 55.3 pm/V or 24 % higher than that of a pure GaSe crystal. The improvement of d(eff) is attributed to the substitutive and interstitial doping of Er ion in GaSe unit cell. The optical properties of GaSe influenced by the erbium doping are also presented.

Nonvolatile memory with switching interfacial polar structures of nano Si-in-mesoporous silica

We show an artificially engineered electret with Si nanocrystals embedded in mesoporous silica for nonvolatile memory. We attribute the polarization to from polar layers lying at the interfaces between one-side bonded Si nanocrystals and mesoporous silica matrix. Under external field, the Si nanocrystals could be displaced in the porechannels causing displaced charge distributions and therefore a field-controllable electric polarization. Nonvolatile memory is demonstrated with a metal-oxide-semiconductor field-effect transistor.

Pulse retrieval from interferometric autocorrelation measurement by use of the population-split genetic algorithm.

The population-split genetic algorithm (PSGA) was successfully applied to retrieve femtosecond optical fields from interferometric autocorrelation traces. PSGA strikes a balance between diversity and the size of population in the genetic algorithm. As a result, PSGA is less likely prematurely converging to sub-optimal solutions. Theoretical and experimental studies indicate that the PSGA can yield more accurate results in shorter time compared with conventional genetic algorithm and the iterative method. compared with conventional genetic algorithm and iterative method.

Generation and spectral manipulation of coherent terahertz radiation with two-stage optical rectification

We propose and experimentally demonstrate the generation of single-cycle terahertz radiation with two-stage optical rectification in GaSe crystals. By adjusting the time delay between the pump pulses employed to excite the two stages, the terahertz radiation from the second GaSe crystal can constructively superpose with the terahertz field injected from the first stage. The high mutual coherence between the two terahertz radiation fields is ensured with the coherent optical rectification process and can be further used to synthesize a desired spectral profile of coherent THz radiation. The technique is also potentially useful for generating high-power single-cycle terahertz pulses, usually limited by the pulse walk-off effect of the nonlinear optical crystal used.

Intense picosecond infrared pulses tunable from 2.4, μ m to 38, μ m for nonlinear optics applications

We report an intense picosecond infrared light source on microjoule energy level, widely tunable in the 2.4 - 38 μ m with pulse durations ∼5ps by DFG in GaSe. The maximum photon conversion efficiency is greater than 43%.

Population-split Genetic Algorithm for phase retrieval of ultrafast laser pulses

We report a new population split genetic algorithm for phase retrieval of femtosecond pulses from interferometric autocorrelation traces. After 200 GA generations, pulse width, phase, and chirp could be determined with a convergence error of 6×10−3.

The application of three-intensity measurement technique in ellipsometry

A three-intensity technique is applied in ellipsometry for measuring the ellipsometric parameters of a polymer thin film surface. In addition to the thickness of polymer film, the regions of pure substrate, intermediate and polymer are studied separately.