Cholesteric Mirrors Research Articles

Luminescent Solar Concentrators: A Review of Nanoengineering Opportunities for Reducing Surface Losses

Luminescent solar concentrators are a lesser-known solar power technology that have the potential to make their way into the constellation of building integrated photovoltaics products. A transparent plastic sheet with solar cells on its edges incorporating luminescent species acts as a wave guide to concentrate light entering through its exposed surface to smaller area photovoltaics. Although they have many advantages, the technology's low efficiency constitutes a major barrier to potential commercialization. While there are several factors contributing to this issue, the low efficiency is largely attributed to the escape cone losses through the light-receiving, large area surfaces. Advances in nanoengineering have provided new, promising ways to reduce these losses. Spectrally selective mirrors, cholesteric mirrors, photonic crystals, asymmetric light transmission interfaces, and quantum dots and dyes with anisotropic emission are all possible solutions that can be engineered at the nanoscale to reduce surface losses and improve overall performance of luminescent solar concentrators. Continued research and improvements in these fields could enable luminescent solar concentrators to become staple products for green buildings.

Liquid-crystal-based resonant cavities as a strategy to design low-threshold electrically-tunable lasers

ABSTRACT We present an experimental study of an electrically tunable laser based on liquid-crystal elements and analyse theoretically its main working principles. The laser structure includes two polymer-stabilised cholesteric-liquid-crystal slabs that act as polarisation-sensitive reflectors, and a dye-doped nematic layer sandwiched between them. The wavelength of lasing can be changed by modifying the optical retardation of the nematic layer by means of a low-amplitude square wave. For all wavelengths the laser light is circularly polarised with opposite handedness to that of the cholesteric mirrors. Good laser performance has been achieved, with low thresholds and quasi-continuous tuning ranges. The laser threshold varies with the wavelength, and was found to be highly dependent on the spatial location of the indium-tin-oxide (ITO) electrodes, inside or outside the laser resonator. A theoretical account for this effect is given using simulations based on the Berreman 4 × 4 matrix method.

Spot profile and beam divergence in cholesteric liquid crystal lasers

ABSTRACT The spot shapes of cholesteric liquid crystal lasers have been studied experimental and theoretically. A broad variety of profiles has been found depending on the oscillator architecture, thickness, and lasing conditions. Simple cells, lasing at the short-wavelength edge of the gap, give rise to simple spots, whereas if the laser wavelength is at the long-wavelength edge the spots are surrounded by rings. On the other hand, complex cells, which incorporate passive cholesteric mirrors connected with glass substrates, produce spots with many sharp concentric rings. In all cases we have demonstrated that the different intensity profiles can be explained as due to anomalous light scattering of the main laser propagating along the helical axis towards oblique directions. For simple cells, the beam divergence is essentially determined also by scattering, showing a decreasing tendency with the sample thickness. Complex cells have a much smaller beam divergence, and its value is limited by diffraction.

Optical properties of selective diffraction from Bragg-Berry cholesteric liquid crystal deflectors

The operational bandwidth of a cholesteric liquid crystal deflector based on the Bragg-Berry effect is analyzed using the two-dimensional finite-difference time-domain method. Despite its similarity in structure with a conventional cholesteric mirror under oblique incidence, the bandwidths of selective reflection and selective diffraction are different. The selective reflection wavelength from the cholesteric mirror has a cosine dependence on the Bragg angle, while that of selective diffraction from the cholesteric deflector has a cosine-squared dependence on the slant angle. We also propose equations that approximate the selective diffraction bandwidth of the deflector. The equations can be used to find the helical pitch required to achieve a deflector with a specified deflection angle and operational wavelength, thereby facilitating the development of cholesteric liquid crustal based diffractive optical elements.

Liquid Crystals Under Two Extremes: (1) High-Power Laser Irradiation, and (2) Single-Photon Level

This paper describes some of my results on liquid crystal investigations under unconventional, incident light powers: (1) under high-power laser irradiation both in free space and inside laser resonators, and (2) in single-photon source applications for quantum information technology. Several effects under high-power, nanosecond laser irradiation are outlined: athermal helical pitch dilation and unwinding of cholesteric mirrors, showing the limits for using them in laser physics; some pitfalls in measurements of thermal-density refractive nonlinearity and the first observation of thermal lens effects in liquid crystals under several nanosecond, low-pulse-repetition rate (2–10 Hz) laser irradiation in the presence of two-photon absorption; feedback-free kaleidoscope of patterns (hexagons, stripes, etc.) in dye-doped liquid crystals. At the single-photon level, definite linear and circular polarizations of single (antibunched) photons for quantum communications were obtained using single-emitter fluorescence in planar-aligned nematic and cholesteric hosts. Circular polarized cholesteric microcavity resonances were also observed under cw-excitation. In addition, using near-field optical microscopy and AFM, 2D-hexagonal arrays made of cholesteric oligomers were investigated. With progress of this technology, similar arrays can be used for fluorescence control of single emitters.

Effect on the output of a luminescent solar concentrator on application of organic wavelength-selective mirrors

To reduce surface loss in luminescent solar concentrators (LSCs), we systematically apply organic wavelength-selective mirrors, chiral nematic (cholesteric) liquid crystals, onto the LSCs with an air gap and determine their effect on waveguide output. The highest output is achieved using a scattering background and cholesteric mirror with a reflection band significantly redshifted (approximately 150 nm) from the emission peak of the fluorescent dye. The use of an air gap results in light bending away from the waveguide surface normal and, consequently, a redshift of the cholesteric mirrors is required. Up to 35% more dye-emitted light energy exits the waveguide edge after application of the cholesteric, and an increase in absolute edge power of 12% was found for a waveguide using a separate scatterer.

Electrically Tuneable Cholesteric Mirror

Chiral nematic liquid crystals which possess a large negative dielectric anisotropy (Δε < 0), exhibit a larger dielectric constant parallel to the helical axis than perpendicular to it. In a liquid crystal cell with homeotropic boundary conditions, the direction of the helical axis is parallel to the cell surfaces and the sample adopts a pseudo focal conic texture. On the application of a suitable AC field, the director is rotated, such that the helix direction is now perpendicular to the surface and the sample adopts a Grandjean texture. The wavelength of reflected light in this “field On” state is a function of temperature and the magnitude of the applied field. In this paper we show that highly twisted chiral nematic liquid crystals, which possess a large negative dielectric anisotropy are potentially useful as optical modulators and in electrically tuneable mirrors.

Behaviour of nonlinear liquid-crystal mirrors, made of a nonabsorbing cholesteric, in the cavity of an Nd:YAG laser operating in the cw regime and at a high pulse repetition frequency

Nonlinear bleaching of output mirrors, made of a nonabsorbing cholesteric, in an Nd:YAG laser was investigated. In the cw regime there was no difference between the output powers of lasers with dielectric and cholesteric mirrors. In the pulse-periodic regime (pulse duration 500 ns, repetition frequency 4.5 kHz) the lasing disappeared after 0.5–5.0 min because of a strong reduction in the reflection coefficient of the output mirror bleached by laser pulses. The observed effect was independent of the average power density and this made it possible to attribute it to an increase in the pitch of the cholesteric helix in the optical wave field.

Cholesteric Liquid Crystal Mirrors for Pulsed Solid-State Lasers

Abstract Cholesteric liquid crystals have been investigated as mirrors in an optical resonator for pulsed solid-state lasers. The out coupling efficiency has been optimized by tuning the reflectivity of the Cholesteric mirror with temperature. Furthermore, the polarizing properties of the helical Bragg-reflection have been used in an electro-optical Q-switched laser instead of the conventionally used combination of dielectric mirror plus polarizer. Pulses of 10 ns duration with peak intensities up to 1 GW/cm2 have been obtained. It is further shown that light-induced changes in reflectivity lead to a passive cavity-dumping and improved transverse beam profile, which may be also of interest for other photonics applications.