Volume 97, Issue 4, 15 February 2005
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS
Cell parameter determination of a twisted-nematic liquid crystal display by single-wavelength polarimetry97(2005); http://dx.doi.org/10.1063/1.1846142View Description Hide Description
Optical transmittance of a 90-twisted nematic liquid crystal cell inserted between crossed or parallel polarizers is neither sensitive to the molecular twist sense nor to the exchange between the extraordinary and the ordinary birefringence axis at the input face of the cell. In this article, the equivalence between a twisted-nematic liquid crystal cell and the combination of a retardation wave plate and a polarization rotator is fully exploited to understand the physical origin of these ambiguities. We determine, in a simple way, cell parameters by means of standard cell transmittance measurements when both crossed and parallel polarizers are rotated simultaneously. We show that this procedure, which is subjected to the aforementioned ambiguities, can be completed by additional polarimetric measurements. In particular, we demonstrate that the twist angle, twist sense, retardation, and the rubbing direction can be univocally determined by experimental measurement of the Stokes parameters of the light transmitted through the cell at one wavelength. Laboratory results corresponding to a commercial liquid crystal displaypanel are carried out.
Spatial homogeneity of optically switched semiconductor photonic crystals and of bulk semiconductors97(2005); http://dx.doi.org/10.1063/1.1846949View Description Hide Description
In this paper we discuss free carrier generation by pulsed laser fields as a mechanism to switch the optical properties of semiconductorphotonic crystals and bulk semiconductors on an ultrafast time scale. Requirements are set for the switching magnitude, the time-scale, the induced absorption, as well as the spatial homogeneity, in particular for silicon at . Using a nonlinear absorption model, we calculate carrier depth profiles and define a homogeneity length . Homogeneity length contours are visualized in a plane spanned by the linear and two-photon absorption coefficients. Such a generalized homogeneity plot allows us to find optimum switching conditions at pump frequencies near . We discuss the effect of scattering in photonic crystals on the homogeneity. We experimentally demonstrate a 10% refractive index switch in bulk silicon within with a lateral homogeneity of more than . Our results are relevant for switching of modulators in the absence of photonic crystals.
97(2005); http://dx.doi.org/10.1063/1.1835538View Description Hide Description
Luminance and quantum yields of organic light emitting diodes(OLEDs) are generally calculated by using a hypothetic monochromatic emission even though the actual external emission is lower than the internal emission because of internal reflection, and is polychromatic. For organic materials, we present the effects of the total internal reflection on quantum yields. Calculations for the luminance and the efficiency of an ideal pseudomonochromatic source are compared with those for a real polychromatic source. While we demonstrate that the calculation of the yield is practically unchanged whatever hypothesis is used, we do show, however, that by assuming that the source is pseudomonochromatic, the luminances of green and blue OLEDs are overvalued or undervalued by a factor of 4, respectively.
97(2005); http://dx.doi.org/10.1063/1.1849824View Description Hide Description
An in-line optical-fiber modulator that uses magnetic fluid as a cladding layer has been developed to control transmission loss generated from external magnetic fields. In this work, we investigate the instant variation in the transmission loss of an optical-fiber modulator after an external magnetic field has been turned on (referred to as the charging operation) or turned off (referred to as the discharging operation). Research showed that when the external field was turned on∕off, the transmission loss of the optical-fiber modulator did not change until after . Successively, the transmission loss varied exponentially. Thus, two kinds of characteristic time were observed to describe quantitatively the instant variation in the transmission loss: retarding time and response time. It was found that both the retarding time in a charging∕discharging operation increased with the length of the magnetic-fluid cladding over a critical level, whereas the response time in a charging∕discharging operation was cladding length independent.