Lasing In Cholesteric Liquid Crystals Research Articles

Continuous wave mirrorless lasing in cholesteric liquid crystals with a pitch gradient across the cell gap

Despite numerous efforts, continuous wave (CW) lasing in dye doped, one-dimensional (1D) photonic bandgap cholesteric liquid crystal (CLC) structures has not been previously reported, to our knowledge. Here we report on the observation of lasing in such structures under both coherent (laser) and incoherent (LED) CW light excitation. To achieve this effect, we used a 1D-photonic bandgap structure made of a polymer stabilized CLC with a pitch gradient across the cell thickness. A spectral reflectivity profile of such a CLC structure reveals local minima in the area within a photonic stopband and close to it. The realization of lasing pumped by low power CW light sources opens the possibility of all-organic, compact, tunable CW lasers for display and medical applications.

Multiwave out-of-normal band-edge lasing in cholesteric liquid crystals

We have investigated the lasing effect in dye-doped cholesteric liquid crystals, where the photonic stop-band for light propagating along the helical axis coinciding with the layer normal is strongly shifted to longer wavelengths with respect to the luminescence band of the dye. In the absence of overlapping of the photonic and luminescence bands, the well-known lasing effect along the normal at the stop-band edges is forbidden. However, the lasing is still allowed for out-of-normal directions, for which the photonic band is shifted to shorter wavelengths corresponding to the luminescence band. Despite of the out-of-normal light propagation, the positive feedback necessary for the lasing is still available in a case of the monodomain cholesteric layer, when its thickness is much less than the optical pumping aperture. We have observed a significantly increased energy threshold for the out-of-normal lasing modes despite they are characterized by lower threshold gain compared to the modes propagating along the helix axis. The increased energy threshold is explained in terms of continuous distribution of the lasing emission over a large angular sector and energy leakage caused by the leaky lasing into the substrates.

Increasing the lasing efficiency in cholesteric liquid-crystal photonic structures

Two types of lasing in cholesteric liquid crystals (LCs) in the range of luminescence of laser dye molecules have been investigated. The first type belongs to the Bragg modes at the photonic band edge, which propagate along the normal to the LC layer. The second type of lasing is related to the modes leaking into the substrate and propagating at small angles to the LC layer. It is shown that the Bragg lasing efficiency can be significantly increased under wide-aperture optical pumping. The method proposed for increasing the lasing efficiency is based on suppressing the excitation of leaky laser modes using partially absorbing thin films as the coatings for LC-orienting substrates. Both experimental results and the theoretical model of the effect using the numerical simulation data are discussed.

Enhanced laser performance of cholesteric liquid crystals doped with oligofluorene dye

We propose a new highly efficient organic dye, oligofluorene, which has great potential for lasing in cholesteric monomeric and glassy liquid crystal oligomers. We perform a detailed comparative experimental study of the laser characteristics of monomeric cholesteric liquid crystals (CLCs) doped with oligofluorene and a well-studied dye, 4-(dicyanomethylene)-Z-methyl-6-(4-dimethylaminostryl)-4H-pyran (DCM), commonly used for lasing in CLCs. Oligofluorene-doped CLCs yield a total output energy in the transverse single-mode regime five times that of DCM-doped CLCs with superior temporal and spatial stability.

Lasing in Cholesteric Liquid Crystal Oriented by Acoustic Streaming

We have demonstrated lasing in cholesteric liquid crystals (CLCs) of various thicknesses oriented by acoustic streaming. Although alignment control of CLC molecules in a thick cell is difficult, acoustic streaming can change the CLC alignment from a focal conic state to a planar state by molecular flow. The molecular reorientation leads to making a large uniform CLC layer that can function as a laser cavity. Therefore, laser action cannot be observed in the untreated thick cells because of the focal conic state, while a sharp emission appears from the thick cells treated by acoustic streaming. Furthermore, lasing wavelength shifts with a small wavelength interval are realized using a large cavity.

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.

Lasing in Cholesteric Liquid Crystals Doped with Oligothiophene Derivatives

Lasing behavior has been studied in cholesteric liquid crystals (CLCs) doped with three oligothiophene (OT) derivatives such as α,α'-dimethylquaterthiophene (DMQtT) as laser dyes. CLCs spontaneously form a distributed feedback cavity, in which OT molecules are highly oriented parallel to the local director of CLCs with an order parameter of about 0.65, as studied by polarized emission spectra. Lasing occurs at only the low-energy edge of the photonic bandgap above a threshold input power of about 4.6 mJ/cm2/pulse in a DMQtT-doped CLC cell by pulsed (4 ns duration and 10 Hz repetition) excitation. This lasing emission mode selection is attributed to the high order parameter of OT molecules.

Fiber coupled cholesteric liquid crystal laser

Lasing in cholesteric liquid crystals is an exciting new phenomenon. Cholesteric liquid crystals (CLC) are inherently periodic. In their ground state, they form a structure where the optic axis of the uniaxial material twists to form a helix and leads to one dimensional photonic band-gap behavior. If a fluorescent dye with emission matching the reflection band is added to these materials, the distributed feedback properties of the structure can result in laser emission when optically pumped. In this work, we show lasing in a CLC directly coupled to an optical ber. The laser emission of this device was characterized by pumping the input end of the optical fiber with a nanosecond Nd:YAG (532 nm) laser.

Mirrorless Lasing and Energy Transfer in Cholesteric Liquid Crystals Doped with Laser Dyes

Lasing in cholesteric liquid crystal (CLC) films doped with a mixture of laser dyes has been observed under UV excitation with 35ps pulses at 355nm. To probe optical interactions between molecules of different species, the emission and excitation spectra of the constituents have been studied in the 300–700nm range. Two dyes, a Benzofuran and an Oxazine dye were used in the system under study. Energy transfer between the host CLC and the dissolved dyes occurs, in agreement with previous observations. In addition, spectroscopic data indicates that active energy transfer from the host CLC to the Benzofuran dye, and from the Benzofuran dye to the Oxazine dye plays an important role in lasing in these systems. The Benzofuran dye acts as an impedance transformer, facilitating energy transfer from the host CLC to the lasing Oxazine dye.

Ultraviolet lasing in cholesteric liquid crystals

We report the observation of stimulated emission and mirrorless lasing in pure cholesteric liquid crystals. The lasing action is attributed to the combination of the fluorescence and the distributed feedback that are due to the inherent periodic structure of the liquid crystal. If the reflection band matches the intrinsic emission of the cholesteric liquid crystal, the crystal becomes a natural laser material, which will self-lase, without any optical elements or the addition of dyes, under picosecond excitation at 355 nm. Samples have been made to lase at different wavelengths in the near UV by shifting of the edge of the reflection band in the range of 385-405 nm. Typical linewidths observed are of the order of 0.5 nm.

Low Threshold Lasing in Cholesteric Liquid Crystals

Cholesteric liquid crystals, due to their periodic structure and large birefringence, are 1-d photonic band-gap materials. Circularly polarized light of the same handedness as the cholesteric structure cannot propagate in the reflection band. We have studied the effects of this band-gap structure on the fluorescence spectrum of dyes dissolved in the cholesteric liquid crystal, as well as on the fluorescence spectrum of the liquid crystal itself. We have found that emission is enhanced at the band edges, and, above a certain pump threshold, lasing occurs both in dye-doped and in pure liquid crystals. Extremely low lasing thresholds and high efficiencies were observed in dye-doped systems. The results suggest an active energy transfer mechanism between the LC host and the fluorescent dye molecules.

51.3: Lasing in Cholesteric Liquid Crystals

AbstractWe have observed mirrorless lasing in dye‐doped and pure cholesteric liquid crystals. The periodic structure and large birefringence gives rise to photonic band‐gap behavior in these materials. We have studied the effects of this band‐gap structure on the fluorescence spectrum of dyes dissolved in the cholesteric liquid crystal as well as on the emission characteristics of the liquid crystal itself. Enhancement of fluorescence at the band edges, and, above a certain pump threshold, lasing both in dye‐doped and in pure liquid crystals was observed. Extremely low lasing thresholds and high efficiencies were obtained in dye‐doped systems. Experimental results suggest an active energy transfer mechanism between the LC host and the fluorescent dye molecules.