Reflection properties of metallic photonic crystals

Received: 10 November 1997/Accepted: 16 November 1997     

Cross-phase modulation in one-dimensional photonic crystals: applications to all-optical devices

We present a numerical study of a finite photonic band gap structure with a χ⁽³⁾ nonlinearity that couples two input pump beams at frequencies ω₁ and ω₂. We show that in this configuration a variety of all-optical devices can be obtained: an optical transistor, a double switch, and a dynamical switch.     

Sub-terahertz on-off switch based on a two-dimensional photonic crystal infiltrated by liquid crystals

A sub-terahertz switch is realized by infiltration of a two-dimensional photonic crystal (PC) with the liquid crystal 5CB. On-off switching is based on a shift of the bandgap of the PC by applying an external electric field which rotates the 5CB molecules. We confirm theoretically and experimentally that rotating the optical axis of the 5CB molecules considerably affects the transmission of the electromagnetic waves of TM polarization in the stop band. The effect can be used for on-off switching of the electromagnetic waves in the sub-terahertz range. Experimentally we demonstrate an extinction ratio of 13.3 dB at 91 GHz.     

Near-field optics and control of photonic crystals

We discuss recent progress and the exciting potential of scanning probe microscopy methods for the characterization and control of photonic crystals. We demonstrate that scanning near-field optical microscopy can be used to characterize the performance of photonic crystal device components on the sub-wavelength scale. In addition, we propose scanning probe techniques for realizing local, low-loss tuning of photonic crystal resonances, based on the frequency shifts that high-index nanoscopic probes can induce. Finally, we discuss prospects for on-demand spontaneous emission control. We demonstrate theoretically that photonic crystal membranes induce large variations in spontaneous emission rate over length scales of 50 nm that can be probed by single light sources, or nanoscopic ensembles of light sources attached to the end of a scanning probe.     

High repetition rate planar laser induced fluorescence of OH in a turbulent non-premixed flame

Received: 1 February 1999 / Revised version: 17 February 1999     

Impact of the cell geometry on the spectral properties of photonic crystal structures

Photonic-crystal-based devices are expected to perform many of the functionalities of standard integrated optics devices used in optical communication systems. Their reliability and the reliability of fabrication technologies in achieving acceptable geometrical tolerance are still to be demonstrated. In this work an analysis of the effects of the cell geometry is presented, showing how small variations can cause hundreds of GHz shifts in the spectral response. A wide-band, finite-element time-domain approach and a frequency-domain formulation have been used for this kind of analysis. The ability of the finite-element method in coping with any kind of geometry has been successfully exploited to investigate the effect of the cell shape. Received: 16 May 2001 / Revised version: 8 August 2001 / Published online: 30 October 2001     

Mode splitting in an optical slab waveguide filled with nematic liquid crystals

Mode splitting was observed when a He-Ne laser beam was reflected through a prism-coupled liquid-crystal slab waveguide under an applied electric field. The splitting manifests itself as the imposed voltage reaches a critical level, and diminishes when the voltage increases above a critical high value. If the applied voltage increases even further, mode splitting vanishes, attributing to the result that almost all the directors of the liquid crystal turn upright to the surface. The multi-layer matrix simulation can satisfactorily account for this phenomenon by exploiting the property of the anisotropic optical birefringence of a liquid crystal under applied voltages. Received: 2 December 2002 / Revised version: 24 January 2003 / Published online: 23 May 2003 RID="*" ID="*"Corresponding author. Fax: +886/3572-3052, E-mail: jtlue@phys.nthu.edu.tw     

Transient gain in photonic crystals

Using the optical response formula for photonic crystals and the Bloch equations, we analyze coherent emission in arbitrary two-dimensional photonic crystals. The transient emission depends not only on the photon density of the states, but also on the intensity and duration of the pump laser pulses. A transient gain exists if the pump pulse area is a certain value and the transition frequency is tuned to the band edge. The sharp structure of the density of states near the band edge leads to a dramatic enhancement of the transient emission intensity and an abrupt change in the oscillation of the emission field. PACS 42.70.Qs; 32.80.-t; 42.25.Bs     

Extended plane-wave-based transfer-matrix approach to simulating dispersive photonic crystals

It has been difficult to compute the band structures and transmission spectra for photonic crystals (PCs) with dispersive components included in the periodic units. Here we show that by using an extended plane-wave-based transfer-matrix method, we are able to formulate the problem for computing optical properties of dispersive PCs, including magnetic and left-handed PCs. This approach is very general, since it can treat PCs with arbitrary Bravais lattice composed of materials with arbitrary dielectric permittivities and magnetic permeabilities. Combined with the supercell method, this method can further simulate defective PCs such as PC-based waveguides and microcavities.     

Spectroscopy of YVO4:Ho3+ crystals

Crystals of YVO₄:Ho were grown by the Czochralski method. The structure of the low-energy levels of Ho³⁺ has been estimated experimentally. Intensity parameters, radiative lifetimes, and branching ratios have been calculated in the framework of the Judd–Ofelt theory. The influence of temperature on luminescence lifetimes has been investigated. The calculated nonradiative transition rate was compared with the energy-gap law. The potential laser gain for the ⁵ I ₇–⁵ I ₈ transition has been estimated. Received: 27 June 2001 / Final version: 10 December 2001 / Published online: 7 February 2002     

Flexible all-plastic mirrors for the THz range

We present a simple and cheap approach to fabricate large-area stop-band filters and mirrors for the THz range. This approach extends the well-known concept of dielectric mirrors to the far infrared. We use alternating layers of different polymer materials with a typical thickness of several tens of micrometers to build a flexible all-plastic mirror. The structures are characterized by THz time-domain spectroscopy. The experimental results are in good agreement with transfer-matrix simulations. Received: 28 September 2001 / Accepted: 4 October 2001 / Published online: 20 December 2001     

Temporal characterization of ultra short laser pulses based on multiple harmonic generation on a gold surface

Received: 29 July 1996     

Signal-to-noise enhancement techniques for quantum cascade absorption spectrometers employing optimal filtering and other approaches

Optical feedback to the laser source in tunable diode laser spectroscopy (TDLS) is known to create intensity modulation noise due to elatoning and optical feedback (i.e. multiplicative technical noise) that usually limits spectral signal-to-noise (S/N). The large technical noise often limits absorption spectroscopy to noise floors 100-fold greater than the Poisson shot noise limit due to fluctuations in the laser intensity. The high output powers generated from quantum cascade (QC) lasers, along with their high gain, makes these injection laser systems especially susceptible to technical noise. In this article we discuss a method of using optimal filtering to reduce technical noise. We have observed S/N enhancements ranging from ∼20% to a factor of ∼50. The degree to which optimal filtering enhances S/N depends on the similarity between the Fourier components of the technical noise and those of the signal, with lower S/N enhancements observed for more similar Fourier decompositions of the signal and technical noise. We also examine the linearity of optimal filtered spectra in both time and intensity. This was accomplished by creating a synthetic spectrum for the species being studied (CH₄, N₂O, CO₂ and H₂O in ambient air) utilizing line positions and linewidths with an assumed Voigt profile from a commercial database (HITRAN). Agreement better than 0.036% in wavenumber and 1.64% in intensity (up to a 260-fold intensity ratio employed) was observed. Our results suggest that rapid ex post facto digital optimal filtering can be used to enhance S/N for routine trace gas detection. Received: 1 April 2002 / Revised version: 7 May 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-509/376-6066, E-mail: robert.disselkamp@pnl.gov     

Characterization and optimization of a laser-scanning microscope in the femtosecond regime

Received: 26 August 1997/Revised version: 2 February 1998     

Towards practical quantum cryptography

Quantum cryptography bases the security of quantum key exchange on the laws of quantum physics and is likely to become the first application employing quantum effects for communication. Here we present performance tests of a new design based on polarization encoding of attenuated, coherent light pulses. Our measurements show that this compact setup can achieve an effective key-bit rate in the kHz range with low alignment requirements and thus offers the tools for fast and user-friendly quantum key exchange. Received: 27 July 1999 / Revised version: 3 September 1999 / Published online: 10 November 1999     

Laser conditioning and nonlinear absorption of LaF3/MgF2 dielectric multilayers at 193 nm

A sensitive and simple pulsed surface thermal lens (TL) technique is used in situ to investigate the laser conditioning and to measure the nonlinear absorption of LaF₃/MgF₂ dielectric multilayers deposited on CaF₂ substrates at 193 nm. Due to the high single-shot sensitivity of the surface TL technique, the laser conditioning can be monitored from the first shot of irradiation on a shot-by-shot basis. The LaF₃/MgF₂ multilayers show a very strong conditioning effect. The ratio of the absorption before and after the laser irradiation is in the range 4–8 for a highly reflective (LH)²⁰ LaF₃/MgF₂ multilayer, and 3–4 for (1L3H)⁷ and (3L1H)⁷ multilayers. In comparison, a (LH)²⁰ LaF₃/AlF₃ multilayer shows only a weak conditioning effect, with an absorption ratio of approximately 1.4. Our experimental results suggest that the strong conditioning effect of the LaF₃/MgF₂ multilayer is due to the absorption conditioning of the LaF₃ layers. However, the MgF₂ layers are shown to be responsible for the considerable increase in LaF₃ absorption as well as the ability to condition the absorption. The fluoride multilayers present non-negligible nonlinear absorption and the two-photon absorption coefficient of the multilayers is estimated to be approximately 5×10^-7 cm/W. Received: 11 December 2000 / Accepted: 20 February 2001 / Published online: 25 July 2001     

Hankel transform of focus wave modes

The Hankel transform generally used to analyse the transverse behaviour of Gaussian beams also supplies new classes of scalar focus wave modes. Received: 4 January 1999 / Revised version: 4 March 1999 / Published online: 2 June 1999     

Investigation of electrooptic modulation from organic-inorganic crystals

Received: 28 September 1998     

Narrow transmission bands of quasi-1D comb-like photonic waveguides containing negative index materials

Using the interface response theory, we investigate the band structure and transmission of quasi-1D comb-like photonic waveguides with side branches composed of negative index materials. Numerical results exhibit the existence of discrete modes in band structure. These discrete modes are corresponding to narrow transmission bands which separated by large forbidden band in the transmission spectrum. Meanwhile it is shown that the narrow transmission bands become narrower with the increase of the number of side branches. The above properties are still maintained when the negative index materials are dispersive and lossy.     

Light–induced charge transport processes in photorefractive barium titanate crystals doped with iron

3 ) are annealed in a hydrogen atmosphere at various temperatures. After these reducing treatments, absorption, light–induced absorption changes, two–beam coupling direction, photo electron paramagnetic resonance (photo EPR), dark and photoconductivity as well as bulk photovoltaic current density are investigated. The samples are electron conductive and the charge transport is governed by only one level, which is identified by photo EPR as Fe²⁺/Fe³⁺. The photoconductivity exceeds the dark conductivity for intensities above 1 kWm^-2. A relation between the absorption constant and the Fe²⁺ concentration is derived. From the known charge transport parameters the advantageous photorefractive properties of optimized reduced BaTiO₃:Fe are deduced; possible response times in the millisecond range at an intensity of 10 kWm^-2 are estimated. Received: 22 January 1997/Accepted: 23 January 1997