Modeling and design of an optimized patterned electrode liquid crystal microlens array with dielectric slab

To eliminate the occurrence of disclination lines in the hole patterned electrode liquid crystal microlens array (LC MLA), inserting an ultrathin dielectric slab was proved to be an effective method. The thickness of the dielectric slab played an important role in effecting the optical performance of the liquid crystal microlens array device, including the dynamic focal range, focus diameter and symmetry of phase profile. In this paper, we studied the effect of dielectric slab thickness on the optical performance of the liquid crystal microlens array by numerical simulation. It is indicated that the optical performance of the device could be improved by reducing the dielectric slab thickness, assuming that the dielectric slab thickness was larger than the threshold thickness. The dependence of the threshold thickness on some key parameters was investigated and the associated effect on the optical performance by changing these key parameters was also studied. In the end, the approaches to enhance the optical performance, namely the dynamic focal range of the liquid crystal microlens array was proposed and proved to be in effect by numerical simulation results.     

High-Sensitivity Sensor Based on Surface Plasmon Resonance Enhanced Lateral Optical Beam Displacements

We present a new optical sensor based on surface plasmon resonance （SPIt） enhanced lateral optical beam displacements. Compared with the traditional SPIt methods, the new method provides higher sensitivity to the sensor system. Theoretical simulations show that the refractive index （RI） detection sensitivity of the SPR sensor based on the displacement measurement has a strong dependence on the thickness of the metal film. When the optimal thickness of the metal film is selected, the RI resolutlon of the SPIt sensor is predicted to be 2.2 × 10^-7 refractive index units （RIU）. Furthermore, it is found that the incidence angle can be used as a parameter to adjust the operating range of the sensor to different refractive index ranges.     

Improvement of the outcoupling efficiency of an organic light-emitting device by attaching microstructured films

In this paper, we present and analyse the optical characteristics, such as spectral shift, CIE coordinates, viewing angle dependence, luminous current efficiency and luminous power efficiency, of an organic light-emitting device (OLED) with a commercial diffuser film or a brightness-enhancement film (BEF) attached. Compared to a planar green OLED, the luminous current efficiencies of the OLED with an attached diffuser film or BEF increase by 29% and 23%, respectively. The overall luminous power efficiencies are enhanced by 28% and 7%. Compared to the planar green device, we observe blue shifts at different viewing angles when microstructured films are attached, which is the evidence that the waveguiding modes are being extracted. In our planar OLED, the peak wavelength blue shifts and the full width at the half maximum (FWHM) decrease with increasing viewing angles due to the microcavity effect. When the diffuser is attached, the spectral peak has a constant blue shift (6 nm) compared to that of the planar OLED. On the other hand, in the BEF case, the spectral shift depends on the viewing angle (2-12 nm blue shifts from 0 to 80°). This is due to the different operating principles (scattering and redirected light) of the diffuser and BEF. Since the transmittance spectra of both the diffuser film and the BEF are flat over the visible range, it is suitable for lighting applications by using white OLED. When attaching the films on a commercial white OLED, the luminous current efficiencies of the OLED with an attached diffuser film or BEF increase by 34% and 31%, respectively. The overall luminous power efficiencies are enhanced by 42% and 8%.     

Efficiency improvement and spectral shift of an organic light-emitting device with a square-based microlens array

In this paper, we present and analyze the influences of the fill factor and the height ratio of square-based microlens arrays on the optical characteristics of an organic light-emitting device (OLED). These properties include spectral shift, CIE coordinates, viewing-angle dependence, luminous current efficiency and luminous power efficiency. Both the luminous current efficiency and luminous power efficiency of the OLED were found to increase linearly with increasing the fill factor of microlenses. It is also found that the full width at half maximum (FWHM) of the OLED spectra and CIE coordinate decreased linearly with increasing the fill factor of the microlenses. Besides, the efficiency improvement of the OLED increased with the height ratio of attached microlenses. Compared to the OLED, the luminous current efficiency and luminous power efficiency of the device can be enhanced by 42% and 47%, respectively, by attaching the microlens array having a fill factor of 90% and a height ratio of 0.57. We also observed blue shifts at different viewing angles when microlens arrays were attached to the OLED, which is the evidence that the waveguiding modes are being extracted. In our planar OLED, the peak wavelength blue shifted and the FWHM decreased with increasing the viewing angles due to the microcavity effect.     

Back-side-coated chirped mirrors with ultra-smooth broadband dispersion characteristics

We demonstrate a new technique for the design of chirped mirrors with extremely smooth dispersion characteristics over an extended ultra-broadband wavelength range. Our approach suppresses spectral dispersion oscillations, which can lead to unwanted strong spectral modulations and limit the bandwidth of mode-locked laser pulses. Dispersion oscillations are significantly reduced by coating the chirped mirror structure on the back side of a substrate, providing ideal impedance matching between coating and ambient medium. An anti-reflection coating may be added on the front side of the substrate, geometrically separated from the chirped mirror. The chirped mirror structure and the anti-reflection coating are non-interfering and can be independently designed and optimized. The separation of both coating sections provides a much better solution for the impedance-matching problems than previous approaches to chirped mirror design. We show by a theoretical analysis and numerical simulations that minimum dispersion oscillations are achieved if the index of the substrate is identical to the index of one of the coating materials and if double-chirping is used for the chirped mirror structure. Based on this analysis, we design a mirror that supports a bandwidth of 220 THz with group delay dispersion oscillations of about 2 fs² (rms), an order-of magnitude improvement compared to previous designs of similar bandwidth. In a first experimental demonstration of back-side-coated (BASIC) mirrors, we achieve nearly transform-limited and virtually unchirped pulses of 5.8 fs duration from a Kerr-lens mode-locked Ti:sapphire laser. BASIC mirrors are particularly suited for higher-order dispersion compensation schemes. They support the extremely broad spectra of few-cycle pulses and promise to provide clean pulse shapes in this regime. Received: 19 April 2000 / Published online: 6 September 2000     

Loss-tolerant, narrow-band reflector in the UV using a grating-waveguide structure

We theoretically demonstrate the feasibility of using a grating-waveguide structure (GWS) as a loss-tolerant, narrow-band reflector in the UV. We simulate device operation using a Green-function technique. Our numerical simulations indicate that a GWS with a 25-nm grating can be used as a 95% reflector for 157-nm light, even in the presence of intensity absorptions of 50 cm^-1. Received: 23 July 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +49-551/5035-99, E-mail: mbader@llg.gwdg.de     

Generalized Principal-State-of-Polarization Analysis and Matrix Model for Piezoelectric Polarization Controllers

We introduce a generalized concept of principal state of polarization （PSP） to analyse the piezoelectric polarization controller （PPC） and find each PPC unit can be described by a rotation matrix determined by the PSP. Our PPC has three components, each made of a jaw and a piezoelectric actuator with the squeezing direction tilted 0°, 45° and 0°, which are driven by a tunable power supply. We demonstrate that all the polarization rotation angles are linear to driving voltages and the PSP of unit 2 is nearly orthogonal to others which are almost equal. Taking some approximate treatments we obtain the matrix model of our PPC with respect to three driving voltages. The average error of our theoretical model is 1.51°, and the polarization response time is 〈50 μs, which is promising to realize an open-loop control of polarization.     

Theoretical Design of a Wedged Output Coupler Coated by Capacitive Strip-Grating for Optically Pumped Terahertz Lasers

An output coupler for optically pumped terahertz laser consisting of capacitive strip-grating and wedged high- resistivity silicon substrate is designed theoretically and its transmittance performance is also discussed. The etalon effects frequently occurring in previous experiments are effectively suppressed, and thus a fiat and accurate transmittance spectrum is obtained in a narrow wavenumber interval of 2 cm^-1 near a particular center wavenumber. Furthermore the transmittance sensitivity to the slight shift of substrate thickness is also completely eliminated. The wedged output coupler is easy to fabricate and its substrate may be used repeatedly to meet various transmittance requirements.     

Prediction of negative index material lenses based on metallo‐dielectric nanotubes

We propose to assemble negative index materials (NIMs) from dielectric nanotubes with inner and outer surfaces covered by thin metallic films. The focusing properties of flat and concave lenses assembled from metallized titania nanotubes are compared with those of lenses made from nanorods with the refractive index n = –1 by performing numerical calculations using a multiple‐scattering approach. Focusing is proved for both types of lenses, however, the focusing properties of concave lenses are better. The lenses are shown to be tolerant to the introduction of disorder in the arrangement of nanotubes. Moreover, the disorder proves to improve the quality of the focal spot. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Transmission intensity disturbance in a rotating polarizer

Random disturbance was observed in transmission intensity in various rotating prism polarizers when they were used in optical systems. As a result, the transmitted intensity exhibited cyclic significant deviation from the Malus cosine-squared law with rotation of prisms. The disturbance spoils the light quality transmitted through the polarizer thus dramatically depresses the accuracies of measurements when the prim polarizers were used in light path. A rigorous model is presented based on the solid basis of multi-beams interference, and theoretical results show good agreement with measured values and also indicate effective method for reducing the disturbance.     

Fabrication of gradient refractive index ball lenses using the method of combination of ion exchanging and sagging

Based on the Fick’s diffusion equations, the distribution function of refractive index of a gradient refractive index ball lens (GRIN ball lens/GBL) is derived. Lithium containing silicate glass is fabricated and GRIN ball lenses (GBLs) which diameters are from 0.3 mm to 3.0 mm are made by the method of combination of Ion exchanging and sagging in sodium nitrate. Refractive index profiles of these GBLs are measured by interferometer, and the performances such as effective focal length (EFL), back focal length (BFL) and numerical aperture (NA) between GBLs and homogeneous ball lenses (HBLs) are compared. Results show that the distribution of the index of refraction is parabolic curve and its Δn is about 0.0002, the performances of the former are super to the latter.     

Diffractive performance of square Fresnel zone plates

We analyze the optical behavior of square Fresnel zones plates. A theoretical analysis and numerical simulations based on the Rayleigh-Sommerfeld approach have been developed analyzing properties such as the depth of focus and the intensity of the focus in terms of the number of zones. In addition, an experimental verification has been performed using a Spatial Light Modulator to implement the designed square Fresnel zones plates.     

Influence of grooves in the electromagnetic transmission of a periodic metallic grating filter

The light transmittance of metal film with periodic slits and grooves structure has been investigated. It was demonstrated that the grooves significantly affect the transmittance of the metal film. The structure excavating two grooves symmetric to the central slit in a cell of a period grating displays a dent in the transmission spectrum comparing with the structure with only one slit in the cell. Deepening the grooves moves the dents to the longer wavelengths in the transmission spectrum. An analytical equation is also provided to approximately locate the dents. The grooves in the grating filter supply the advantage of removing the needless transmission of certain frequency.     

Theoretical investigation of chirped mirrors in semiconductor lasers

This paper reports a theoretical design of chirped mirrors in 1.3-μm double-section semiconductor lasers to achieve high reflectivity and dispersion compensation over a broad bandwidth. Analytic expressions for reflectivity, group delay and group delay dispersion are derived. We use for the first time chirped air/semiconductor layer pairs as mirrors for higher-order dispersion compensation in semiconductor lasers. Our optimised calculations demonstrate that the broad-band mirrors designed consist of a total of only 12 air/semiconductor layers and achieve a reflectivity higher than 99.8%, a smooth group delay and almost stable dispersion in the laser cavity over a 100-nm bandwidth. Due to a high index contrast of both types of the layers, n _l = 1, n _h~ 3.5, a high-reflectivity bandwidth of > 700 nm is obtained in 1.3-μm semiconductor lasers. We also compare our results with that of a commercial simulation program and show a good agreement between them. As a conclusion, we assume from the theoretical results that air/semiconductor layer pairs with varying thicknesses used at one end of double-section semiconductor lasers can lead to femtosecond optical pulse generation using mode-locking techniques. An erratum to this article can be found at .     

Design of Highly Sensitive Surface Plasmon Resonance Sensors Using Planar Metallic Films Closely Coupled to Nanogratings

We investigate the sensitivity enhancement of surface plasmon resonance （SPR） sensors using planar metallic films closely coupled to nanogratings. The strong coupling between localized surface plasmon resonances （LSPRs） presenting in metallic nanostructures and surface plasmon polaritons （SPPs） propagating at the metallic film surface leads to changes of resonance reflection properties, resulting in enhanced sensitivity of SPR sensors. The effects of thickness of the metallic films, grating period and metal materials on the refractive index sensitivity of the device are investigated. The refractive index sensitivity of nanograting-based SPR sensors is predicted to be about 543 nm/RIU （refractive index unit） using optimized structure parameters. Our study on SPR sensors using planar metallic films closely coupled to nanogratings demonstrates the potential for significant improvement in refractive index sensitivity.     

Photonic nanojet modulation by elliptical microcylinders

The detailed analysis of localized photonic nanojets generated at the shadow side surfaces of dielectric elliptical microcylinder illuminated by a plane wave is reported. We have studied the distribution of the electric energy density within and in the vicinity outside an elliptical microcylinder by using finite-difference time-domain calculation with high resolution. The location of photonic nanojet depends on the radius ratio of the microcylinder. We have further analyzed the effect of the rotation angle of elliptical microcylinder upon the focal length of photonic nanojets. The horizontal and vertical shifts of photonic nanojet can be changed by rotating the rotation angle of elliptical microcylinder. The mechanisms of elliptical microcylinder may supply a new ultra-lens approach to distinguish nanoscale targets such as nanoparticles, optical gratings, and nanoscale surface roughness.     

Generation of thin and hollow beams by the axicon with a large open angle

We propose a method to produce diffraction-free thin and hollow beams. The method is based on Laguerre-Gaussian (LG) beams incident on a large open-angle axicon. We use the vector diffraction integrals and stationary phase method to deduce a simple and analytical formula of the propagating field of the linearly polarized LG beams through an axicon. The numerical results show that the hollow beams of whose diameter is in the order of the wavelength can be obtained by using the axicon with the refractive index n = 2 and the open angle α = 25°. These diffraction-free thin and hollow beams may be very useful to accurately trap and manipulate atoms. However, when the open angle is over large, the conversion efficiency from the LG beam to the diffraction-free hollow beam will decrease obviously.     

Micro-lens arrays generated by UV laser irradiation of doped PMMA

Micro-lenses with well-defined optical parameters are generated on polymethylmethacrylate (PMMA) substrates doped with diphenyltriazene (DPT) by controlled use of a swelling effect generated under conditions of subablative excimer laser illumination. The surface profiles depend on the laser spot size and energy density. A sensitively balanced combination of matrix softening, substrate volume expansion due to photochemical nitrogen release, and surface tension is responsible for the final shape of the lenses. Complete arrays of identical lenses with 15 μm diameters and a focal length of 30 μm are produced by irradiation of (0.25 wt. %) DPT-PMMA with a single laser pulse at a wavelength of 308 nm and a fluence of 3 J/cm². It is shown experimentally and theoretically that appropriate volume expansion is possible without introducing internal light scattering due to the formation of small bubbles. Received: 7 April 1999 / Accepted: 8 April 1999 / Published online: 5 May 1999     

Ultra-Narrow Dual-Channel Filter Based on Symmetrical Sampled Fibre Bragg Grating

A kind of ultra-narrow dual-channel filter is proposed in principle and demonstrated experimentally. This filter is designed by means of two sampled fibre Bragg gratings （SFBGs）, where one is periodic O-π sampling and the other is symmetrical spatial sampling. The former can create two stopbands in the transmission spectra and the latter can produce two ultra-narrow passbands. Our filter has the 3-dB bandwidth of about 1 pm, whose value is two orders of magnitude less than the bandwidth of the traditional SFBG filters. The proposed filter has a merit that the channel spacing remains unchanged when tuning the filter.     

Focal Shift of Paraxial Gaussian Beams in a Left-Handed Material Slab Lens

Focal shift is inevitable in conventional lens systems due to the Fresnel number and angular aperture. In this Letter, we demonstrate that there is no focal shift when a paraxial Gaussian beam passes through a left-handed material slab lens without absorption or gain. However, the effect is exhibited in the presence of absorption or gain, and becomes larger as the absorption or gain increases. When the absorption is equal to the gain, the phenomenon of the focal shift caused by the gain is more obvious. In addition, the field distribution is not affected by the absorption or gain and always remains Gaussian both in internal and external focus planes.