Liquid Crystal Laser Research Articles

Liquid-crystal lasers

Wide-band tunability, large coherence-area, and in some cases multidirectional emission have made liquid-crystal lasers an attractive light source for applications like miniature medical diagnostic tool and large-area holographic laser displays. This article discusses the scientific origins of the technology of liquid-crystal lasers and reviews the current cutting-edge research.

Development of Laser Dyes to Realize Low Threshold in Dye‐Doped Cholesteric Liquid Crystal Lasers

Development of Laser Dyes to Realize Low Threshold in Dye‐Doped Cholesteric Liquid Crystal Lasers

Polarization-tunable chiral nematic liquid crystal lasing

A chiral nematic liquid crystal laser with arbitrarily controllable polarization was demonstrated by combining polymeric cholesteric and nematic liquid crystal films. The lasing polarization states can be continuously varied by applying voltages from 1.25 to 2 V. Right-handed circular polarization is switched to left-handed circularly polarization through −45° linearly polarized states. The trajectories of the Stokes parameters on the Poincaré sphere are used to analyze the variation in lasing polarization. The change in polarization is simulated and the simulation agrees closely with the experimental data.

High efficiency cholesteric liquid crystal lasers with an external stable resonator

An amplified cholesteric liquid crystal (CLC) laser performance is demonstrated by utilizing a binary-dye mixture (with 62 wt% DCM and 38 wt% PM597) as the active medium and an external stable resonator. The measured results show that the laser efficiency is enhanced as compared to the highest efficiency of each individual dye. Furthermore, using such an active CLC in an external stable resonator leads to a approximately 92X improved efficiency over the single CLC laser. In this instance, the binary-dye doped CLC simultaneously functions as laser oscillator and amplifier.

Polydimethylsiloxane-enclosed liquid crystal lasers for lab-on-chip applications

We demonstrate the operation of a self-organized cholesteric liquid crystal laser confined between optically clear and elastic polydimethylsiloxane (PDMS) substrates. The formation of a planar helical texture in the cholesteric was supported by microsctructuring of PDMS layer surface, using the nanoembossing technique with glass substrates coated with conventional alignment layers as a template. The potential of combining miniature cholesteric laser sources and microfluidic devices for lab-on-chip applications is discussed.

Circularly polarized high-efficiency cholesteric liquid crystal lasers with a tunable nematic phase retarder

A high efficiency dye-doped cholesteric liquid crystal (CLC) is demonstrated by optimizing the dye concentration and using an electrically tunable nematic liquid crystal (NLC) phase retarder. The state of polarization of laser emission in CLC lasers, contrary to our expectations, due to the refractive index mismatch at the boundaries is not exactly circular. A double-cell structure including a CLC laser and an adjustable NLC phase retarder with a mirror reflector on one of the inner surfaces not only purifies the polarization state of the laser output but also improves the laser efficiency by 6.7X, over the single-direction dye-doped CLC laser.

Comparison of the performance of photonic band-edge liquid crystal lasers using different dyes as the gain medium

The primary concern of this work is to study the emission characteristics of a series of chiral nematic liquid crystal lasers doped with different laser dyes (DCM, pyrromethene 580, and pyrromethene 597) at varying concentrations by weight (0.5–2 wt %) when optically pumped at 532 nm. Long-wavelength photonic band-edge laser emission is characterized in terms of threshold energy and slope efficiency. At every dye concentration investigated, the pyrromethene 597-doped lasers exhibit the highest slope efficiency (ranging from 15% to 32%) and the DCM-doped lasers the lowest (ranging from 5% to 13%). Similarly, the threshold was found to be, in general, higher for the DCM-doped laser samples in comparison to the pyrromethene-doped laser samples. These results are then compared with the spectral properties, quantum efficiencies and, where possible, fluorescence lifetimes of the dyes dispersed in a common nematic host. In accordance with the low thresholds and high slope efficiencies, the results show that the molar extinction coefficients and quantum efficiencies are considerably larger for the pyrromethene dyes in comparison to DCM, when dispersed in the liquid crystal host.

Laser Properties of Cholesteric Liquid Crystals with In-Plane Helix Alignment

We describe a cholesteric liquid crystal (ChLC) laser with an in-plane helix alignment where the ChLC helix lies in the in-plane direction of the cell and report an improvement in the threshold characteristics. The in-plane helix alignment of ChLCs was formed by applying an in-plane electric field in a cell with homeotropic surface treatment while cooling the sample from the isotropic phase. The energy threshold was 0.3 mJ/cm2, which was found to be less than 1/10 of conventional planarly-aligned ChLC lasers. The threshold gain was measured using variable stripe length method and was determined as 1 cm-1, which is approximately 1/100 of conventional planar devices with a cell-gap of 10 μm. The improvement of the energy threshold is attributed to small threshold gain attained in these devices.

Removing the discontinuous shifts in emission wavelength of a chiral nematic liquid crystal laser

The laser wavelength of a band-edge liquid crystal laser is known to vary in a stepwise fashion with temperature. This report describes a method for removing the discontinuous shifts in the laser wavelength by doping a nematic sample with two different chiral dopants which exhibit opposing dependences of the natural pitch on temperature. On cooling from the isotropic phase to room temperature the total shift of the laser wavelength is 15 nm with no discontinuities. This is in contrast to that observed for samples consisting of only one chiral dopant whereby the wavelength shift was as large as 75 nm for the same temperature range.

Study of polarization properties of light emitted from a-plane InGaN/GaN quantum well-based light emitting diodes

This paper discusses the optical characteristics of a nonpolar a-plane InGaN/GaN quantum well with different indium compositions, quantum well widths, and injection carrier densities. The self-consistent Poisson and 6×6 k⋅p Schrödinger solver has been applied to study the band structures in nonpolar a-plane InGaN-based quantum well light emitting diodes (LEDs). We find that the larger indium composition and smaller well width make the energy separation of |Y⟩-like state to |Z⟩-like state larger, and as a result enhance the polarization ratio of light. However, the polarization ratio decreases as the carrier injection increases, which might be a drawback for high power applications. We have studied the optimization condition for designing the a-plane InGaN quantum well LED for applications, such as liquid crystal display backlight modules and lasers, which would be useful information for device designs.

Chiral luminescent compounds as a perspective for cholesteric liquid crystal lasers

We present the synthesis and characterization of fluorene based materials for mirror less laser applications. Mirror less lasing is obtained from a cholesteric liquid crystals mixture, that acts as a resonator, doped with a photoluminescent dye, that acts as an active medium. We propose to use fluorene based compounds to combine two necessary properties to achieve laser emission: chirality and luminescence. Two different compounds, a trimer and an oligomer are synthesized. The trimer and the oligomer are both prepared with chiral moieties and, in addition, they are liquid crystalline compatible. More, oligofluorenes are good blue emitters and the emission quantum yield is around 0.8 in the violet–blue range. These fluorene based compounds are used in cholesteric liquid crystals mixtures as luminescent chiral dopants and a fine tuning of the laser emission is obtained. The dependence of lasing efficiency on concentration and the handedness of chiral luminescent dopants are investigated.

Wide band gap materials as a new tuning strategy for dye doped cholesteric liquid crystals laser

A new tuning strategy for mirror-less liquid crystals laser is presented. A three layer cell is prepared with two cholesteric layers sandwiching a layer containing an isotropic mixture of a photoluminescent dye. One of the chiral layers contains a wide band gap material while the second layer consists of a series of small band gap materials. Through the combination of these two layers, a set of mirrors that can selectively reflect different wavelengths is obtained. A different laser wavelength is emitted from different regions of the cell under the pumping beam irradiation.

Polychromatic liquid crystal laser arrays towards display applications

Band-edge liquid crystal lasers are of interest for a number of applications including laser projection displays. Herein, we demonstrate simultaneous red-green-blue lasing from a single liquid crystal sample by creating a two-dimensional laser array fabricated from dye-doped chiral nematic liquid crystals. By forming a pitch gradient across the cell, and optically pumping the sample using a lenslet array, a polychromatic laser array can be observed consisting simultaneously of red-green-blue colors. Specifically, the two-dimensional polychromatic array could be used to produce a laser-based display, with low speckle and wide color gamut, whereby no complex fabrication procedure is required to generate the individual 'pixels'.

Scattering and amplification of light in a layer of a nematic liquid crystal

The role of light scattering in liquid crystal lasers can be of two kinds. It can induce undesirable losses increasing the lasing threshold or it can promote the appearance of feedback necessary for lasing. The light amplification spectra of the liquid crystal in the isotropic and light-scattering nematic phases have been measured. For comparison, at a close pump power, the amplification spectra have been measured on a TiO2 nanoparticle suspension in an isotropic solvent, where the mean free path of photons is the same as that in the liquid crystal. The amplifications in two systems are significantly different, because the shapes of their scattering patterns are strongly different.

Two-dimensional liquid crystal laser array

A two-dimensional liquid crystal (LC) laser array has been demonstrated by photopumping a single LC sample using a lenslet array consisting of plano-convex microlenses. A 5 x 5 array of LC lasers (displaying evidence of mutual coherence) spaced by 1 mm inactive regions has been generated, which could be combined to yield a single monomode output and allows an almost 50-fold increase in energy density in comparison to a single-focus LC cavity. Furthermore, we have demonstrated how the individual and recombined emission spectra vary with different sample topologies and how polydomain samples can be used to generate a multiwavelength laser emission.

Emission characteristics of a homologous series of bimesogenic liquid-crystal lasers

In this study we have fabricated eight different liquid-crystal lasers using the same gain medium but different homologues from the bimesogenic series alpha-(2',4-difluorobiphenyl-4'-yloxy)-omega-(4-cyanobiphenyl-4'-yloxy)alkanes, whereby the number of methylene units in the spacer chain varied from n=5 to n=12. To quantify the performance of these lasers, the threshold energy and the slope efficiency were extracted from the input-output characteristics of each laser. A clear odd-even effect was observed when both the excitation threshold and the slope efficiency were plotted as a function of the number of methylene units in the spacer chain. In all cases, the bimesogen lasers for which n is even exhibit lower threshold energies and higher slope efficiencies than those for which n is odd. These results are then interpreted in terms of the macroscopic physical properties of the liquid-crystalline compounds. In accordance with a previous study [S. M. Morris, A. D. Ford, M. N. Pivnenko, O. Hadeler, and H. J. Coles, Phys. Rev. E. 74, 061709 (2006)], a combination of a large birefringence and high order parameters are found, in the most part, to correlate with low-threshold energy and high slope efficiency. This indicates that the threshold and slope efficiency are dominated by the host macroscopic properties as opposed to intermolecular interactions between the dye and the liquid crystal. However, certain differences in the slope efficiency could not be explained by the birefringence and order parameter values alone. Instead, we find that the slope efficiency is further increased by increasing the elastic constants of the liquid-crystal host so as to decrease the scattering losses incurred by local distortions in the director field under high-energy optical excitation.

Patterned liquid‐crystal laser film for multi‐dimensional multi‐color emissive film technology

Abstract— A two‐dimensional array consisting of dye‐doped reflection‐mode holographic‐polymer‐dispersed liquid crystal (H‐PDLC) lasers with alternating pitch lengths is presented. These post structures each reflect at a narrow bandwidth of light. The addition of laser dye to the H‐PDLC system allows for the generation of laser emission at the edge of the reflection band, or photonic band gap. In patterning these H‐PDLC post structures, a narrow‐linewidth patterned emissive color film is realized. The potential of such films and their implication in the display industry is discussed. In creating a three‐color array, an active emissive color film could replace the backlight and color filter components within the display. Such a patterned system would possess a wide color gamut, through spatial color synthesis, formed by narrow‐linewidth lasing structures with well‐defined wavelengths of emission.

P‐208L: Late‐News Poster: Liquid Crystal Lasers for Projection Displays

AbstractThe liquid crystal laser presents itself as an interesting tool for projection displays, acting as the illumination source. Liquid crystal lasers are fast becoming an interesting area of research within the liquid crystal community. In utilizing integrating these systems with current liquid crystal display (LCD) projection systems, novel designs can be envisioned.

Reversible Tuning of Lasing in Cholesteric Liquid Crystals Controlled by Light‐Emitting Diodes

The lasing wavelength of dye-doped cholesteric liquid-crystal lasers is optically tuned by the reversible photoisomerization of the azo- or azoxy- nematic components of the cholesteric mixture. The figure demonstrates that the laser can be tuned over a range of almost 70 nm. The nematic components are easily and reversibly isomerized by illumination from light-emitting diodes at 405 and 466 nm.

Correlations between the performance characteristics of a liquid crystal laser and the macroscopic material properties

In order to understand how the performance of a liquid-crystal laser depends on the physical properties of the low molar mass nematic host, we have studied the energy threshold and slope efficiency of ten optically pumped liquid-crystal lasers based on different hosts. Specifically, this leads to a variation in the birefringence, the orientational order parameter, and the order parameter of the transition dipole moment of the dye. It is found that low threshold energies and high slope efficiencies correlate with high order parameters and large birefringences. To a first approximation this can be understood by considering analytical expressions for the threshold and slope efficiency, which are derived from the space-independent rate equations for a two-level system, in terms of the macroscopic liquid crystal properties.