Cholesteric Liquid Crystal Laser Research Articles

Enhanced cone-shaped lasing from cholesteric liquid crystals.

Strong conical lasing in the violet-blue range of the spectrum from dye doped cholesteric liquid crystals is obtained. In contrast to the previously reported cases, the cone laser emission is sharply increased at the local maximums of the luminescence peaks of the dyes. Besides that, for obtaining the cone lasing strong focusing of the pumping beam is not required. Extremely low pump density is achieved for the cholesteric liquid crystal lasers. It is important that the cone lasing can be observed without simultaneous manifestation of any other laser emissions.

Optically guided spatially modulated Laser generation doped with rhodamine 6G dye in the polymer (PVS) layer

The article is a continuation of a previous work in which optically controlled spatially modulated laser generation by means of a cholesteric liquid crystal laser layer was studied. In this work, a layer of polyvinyl alcohol doped with rhodamine 6G dye is used. The mentioned has been studied Laser radiation characteristics of the structure.

From Single to Multi Mode Lasing: The role of materials revealed in optical simulations

ABSTRACT The comparative study of thin film Cholesteric Liquid Crystal (CLC) lasers made from different materials and optically pumped by an external solid state laser reveals a striking dependence of lasing behaviour (ranging from single mode at the edge of the selective reflection band to multimode lasing) on the morphology and microstructure of CLC films. The materials studied belong to two groups: low molar mass liquid crystals and polymers. It is shown that the orientation of individual chiral domains and fluctuations in helical pitch contribute significantly to the type of lasing displayed by the material. Different ways of preparing CLC cells that lead to predominantly one type of lasing are discussed. The importance of variations of the helical pitch and domain orientation in producing single and multimode lasing is justified by optical simulations (4x4 matrix method) of lasing in multilayered samples.

Adjustment of electronic and emissive properties of indolocarbazoles for non-doped OLEDs and cholesteric liquid crystal lasers

Derivatives of indolo[3,2-b]carbazole containing differently substituted triphenyl ethenyl moieties were synthesized and characterized as multifunctional materials for optoelectronic devices. A structure-properties relationship of the compounds was studied. Alteration of the substituents of triphenyl ethenyl moiety (H-, Ph-, CN-) afforded to modify thermal, photophysical and charge-transporting properties of the compounds. The synthesized compounds demonstrated high thermal stability with the 5% weight loss temperatures reaching 453 °C. Indolo[3,2-b]carbazoles mono- and disubstituted with tetraphenyl ethenyl groups exhibited polymorphism with two melting peaks at 211/216 °C and 236/245 °C, respectively. Modification of the substitution pattern resulted in the changes of emission of the solid films from blue centered at 471 nm to yellow peaking at 583 nm. Addition of the tetraphenylethylene and triphenyl acrylonitrile fragments resulted in aggregation induced emission (AIE) or aggregation enhanced emission (AEE). The solutions of AIE/AEE-active compounds exhibited low photoluminescence quantum yields which did not reach 1%, while the solid films demonstrated the highly boosted values of 23–35%. Hole mobilities of ca. 10−3 cm2/V × s were observed for tri(tetra)phenyl ethenyl substituted indolo[3,2-b]carbazoles compounds. The compounds containing acrylonitrile moieties exhibited bipolar charge transport with drift mobilities of ca. 10−5 cm2/V × s. The synthesized compounds showed promising electroluminescent properties as fluorescent emitters. OLEDs containing these emitters showed maximum external fluorescent quantum efficiency up to 3.0% and maximum brightness up to 18 800 cd/m2. They also demonstrated lasing properties. Using the most promising compound dissolved in cholesteric liquid crystal, laser beam at 515 nm with full width at half maximum of ca. 3.5 nm was generated.

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.

Lasing stability of quasi-CW driven cholesteric liquid crystal lasers

Lasing stability in dye-doped cholesteric liquid crystal (ChLC) lasers largely depends on repetition frequency of the pumping laser, and therefore stabilization of output power under quasi-continuous-wave operation with high repetition frequency greater than 1 kHz is a tough challenge. In this study, a significant improvement of lasing stability in dye-doped ChLC lasers is obtained by lowering dye concentration, increasing cell gap, and moving LC cells with a piezo stage. Considering the balance between lasing stability and slope efficiency, the optimal conditions regarding cell gap and dye concentration in a ChLC laser doped with Pyrromethene 597 are determined to be 50 μm and 0.3 wt%, respectively. Under severe conditions of high repetition frequencies of 1 kHz and 10 kHz, the optimized ChLC laser demonstrates durability retaining 100% and more than 72% against the initial output power for 30 min, respectively.

High-quality tuning of cholesteric liquid crystal lasers based on polymer composite system

Electrically fine tuning is demonstrated in a cholesteric liquid crystal (ChLC) laser based on the polymer composite system. Photo-polymerization in the mixture of LC monomer and LC hardly causes phase separation due to miscibility, forming a microscopically well-mixed polymer-LC composite structure like gel. Light scattering in the polymer-LC composite system decreases with increasing the polymer component, and the composite of equal parts of the polymer and LC exhibits high transparency in electric modulation of the selective reflection band, resulting in high-quality tuning of lasing wavelength in ChLC laser. In tuning operation, the ChLC laser retains a high slope efficiency greater than 19%, a low threshold smaller than 3 nJ/pulse, and a spectral linewidth narrower than 0.13 nm, while the change in lasing wavelength is fully continuous and reaches 13 nm.

Development of highly fluorescent distyryl dyes for liquid crystal lasers

We developed new reddish fluorescent distyryl dyes, showing strong fluorescence around 600 nm, for optoelectronic devices. They have high fluorescent quantum efficiencies over 0.8, with large Stokes’ shifts exceed 60 nm in 1,4-dioxane. These features of new dyes are desirable for optoelectronic applications such as electroluminescent and dye-laser devices. We fabricated distributed feedback cholesteric liquid crystal lasers using the dyes and observed laser oscillation at a low excitation power of ∼22 nJ/pulse. Laser emission characteristics of the dyes are partly different from expectation from steady-state dichroic measurements. The origin of the discrepancy is discussed considering the relaxation process of the dyes.

Small footprint cholesteric liquid crystal laser.

We demonstrate a small optical bench footprint laser assembly based on the small pulsed Nd:YAG laser head SSY-1 for pumping cholesteric liquid crystal (CLC) lasers and illustrate its performance using low molecular weight CLC samples doped with the fluorescent dye PM597. A low lasing threshold, narrow laser line, and far-field interference patterns of the CLC laser were observed using the SSY-1-based laser assembly as the pump. The emission characteristics of the CLC laser are similar to those observed with comparable CLC materials pumped by an order of magnitude physically larger and many orders of magnitude more expensive commercial Nd:YAG laser systems. The small footprint CLC laser demonstrated in this work provides an opportunity for significant size and cost reduction of CLC lasers and fostering their practical applications.

Effect of combined laser dyes on the efficiency of cholesteric liquid crystal lasers

A high efficiency band-edge cholesteric liquid crystal (CLC) laser comprising an optimal binary-dye mixture (OBD) with 62 wt% DCM and 38 wt% PM597 as the active medium is scrutinized. The measurements indicate that both the fluorescence spectrum width and the order parameter of OBD in the host of nematic liquid crystal (BL009) enhance compared with those for each individual dye. Furthermore, at the fluorescence peak, the optical efficiency of the PM597-doped CLC laser is ~ 1.5× higher than that of DCM, and the laser emission energy for OBD-doped CLC between the wavelengths 595 nm and 613 nm is even higher than that of PM597-doped CLC, so that at the maximum fluorescence of OBD, lambda approx 605;{text{nm}}, the improvement is over 20%.

Cone-Shaped Emissions in Cholesteric Liquid Crystal Lasers: The Role of Anomalous Scattering in Photonic Structures

The out-of-normal emission in cholesteric liquid-crystal lasers is studied experimental and theoretically. Apart from the well-known dominant laser in the direction of the helix axis, three other types of radiation, with cone-shaped spatial patterns, are identified and characterized. The physical mechanisms responsible for the different emissions are clarified. One of the radiations is a weak colorful lasing emission whose wavelength changes continuously depending on the propagation direction. The wavelengths of the other two radiations take place at the long-wavelength and short-wavelength edges of the photonic bandgap. These emissions are attributed to an anomalous scattering phenomenon that gives rise to energy transfer from the main laser beam to some specific directions where the amount of final photonic states is high. An expression for the scattering cross section, reminiscent of Fermi’s golden rule for spontaneous emission in photonic structures, is proposed. Some other phenomena independent of th...

Wavelength-Tunable and Highly Stable Perovskite-Quantum-Dot-Doped Lasers with Liquid Crystal Lasing Cavities.

This study applies a low-cost solvothermal method to synthesize all-inorganic (lead-free cesium tin halide) perovskite quantum dots (AIPQDs) and to fabricate AIPQD-doped lasers with cholesteric liquid crystal (CLC) lasing cavities. The lasers present highly qualified lasing features of low threshold (150 nJ/pulse) and narrow line width (0.20 nm) that are attributed to the conjunction of the suppression of photoluminescence (PL) loss caused by the quantum confinement of AIPQDs and the amplification of PL caused by the band-edge effect of the CLC-distributed feedback resonator. In addition, the lasers possess highly flexible lasing-wavelength tuning features and a long-term stability under storage at room temperature and under high humidity given the protective role of CLC. These advantages are difficult to confer to typical light-emitting perovskite devices. Given these merits, the AIPQD-doped CLC laser device has considerable potential applications in optoelectronic and photonic devices, including lighting, displays, and lasers.

Narrowing the oscillation spectra of a cholesteric liquid crystal laser

The possibility of a single-mode laser generation on the basis of cholesteric liquid crystals (CLC) at the creation of a defect in a spiral structure is studied. To create a defect in CLC on a viscous 3-component mixture of esters of cholesterol, we use the orthogonal orientation of directors on focusing substrates with an additional turn of one of the substrates. As a result, the small ’’dip of transparency’’, which was predicted by the theory of a photon crystal at the infringement of its spiral structure, arose in the selective reflection (SR) band. The appearance of such a dip in the SR-band was just possible at its spectral shift from the dye absorption band about 100 nm. The formation of this defect in the structure was accompanied by narrowing the oscillation spectrum to the only longitudinal mode in a wide range of the above-threshold pumping. The nature of the phenomenon is discussed.

Spectral and Spatial Features of Radiation Emitted by a Cholesteric Liquid-Crystal Laser

Spectral and spatial characteristics of radiation emitted by a laser operating on the Bragg structure arising in cholesteric liquid crystals (CLCs) have been studied, as well as their variations with a change of the planar CLC orientation. A defect in the helical structure of the CLC formed by a ternary mixture of cholesterol viscous esters is revealed at the mutually orthogonal orientations of the CLC director at the substrates. This defect manifests itself as a local dip in the selective reflection band, which agrees with the behavior of the defect mode in the photonic crystal. Such a defect in the helical structure stimulates the selection of longitudinal modes with the indices N = ±1, so that the single-mode lasing regime is realized. A spatial ring structure in the laser radiation is found to arise, when higher longitudinal modes are generated.

Upgrading the Performance of Cholesteric Liquid Crystal Lasers: Improvement Margins and Limitations.

The topic of cholesteric-liquid-crystal lasers is a rapidly expanding research area in the field of soft-matter photonics. The increasing interest in this field is due to the high versatility that these lasers may possibly present and the prospects of giving rise to new miniaturized devices. However, further improvements in their operation capabilities are still required for potential applications. In this paper, we critically analyze the main strategies proposed up to now to optimize their performance. We show theoretically and experimentally that possible innovations in the device structure cannot produce lasers with threshold energies below a certain limit. This limit is determined by the light scattering and absorption losses inside the liquid crystal. Even assuming the case of samples free of defects and perfectly non-absorbing, an intrinsic light scattering, typical of mesogens, still remains. Numerical estimates of the thresholds indicate that these lasers could hardly be driven by compact light sources such as current electroluminescent or light-emitting diodes. Since the improvement possibilities regarding cell architecture seem to be exhausted, the advance must come from the use of new dye molecules. These molecules should show enhanced emission cross-sections and be efficiently integrable within the mesogenic solvent. In addition, the fluorescent systems must present very small quantum yields to triplet states if continuous-wave lasing is sought. In this respect, quantum dots are an alternative to explore for further investigations.

An Ambipolar BODIPY Derivative for a White Exciplex OLED and Cholesteric Liquid Crystal Laser toward Multifunctional Devices.

A new interface engineering method is demonstrated for the preparation of an efficient white organic light-emitting diode (WOLED) by embedding an ultrathin layer of the novel ambipolar red emissive compound 4,4-difluoro-2,6-di(4-hexylthiopen-2-yl)-1,3,5,7,8-pentamethyl-4-bora-3a,4a-diaza-s-indacene (bThBODIPY) in the exciplex formation region. The compound shows a hole and electron mobility of 3.3 × 10-4 and 2 × 10-4 cm2 V-1 s-1, respectively, at electric fields higher than 5.3 × 105 V cm-1. The resulting WOLED exhibited a maximum luminance of 6579 cd m-2 with CIE 1931 color coordinates (0.39; 0.35). The bThBODIPY dye is also demonstrated to be an effective laser dye for a cholesteric liquid crystal (ChLC) laser. New construction of the ChLC laser, by which a flat capillary with an optically isotropic dye solution is sandwiched between two dye-free ChLC cells, provides photonic lasing at a wavelength well matched with that of a dye-doped planar ChLC cell.

Electrically tunable laser based on oblique heliconical cholesteric liquid crystal

A cholesteric liquid crystal (CLC) formed by chiral molecules represents a self-assembled one-dimensionally periodic helical structure with pitch [Formula: see text] in the submicrometer and micrometer range. Because of the spatial periodicity of the dielectric permittivity, a CLC doped with a fluorescent dye and pumped optically is capable of mirrorless lasing. An attractive feature of a CLC laser is that the pitch [Formula: see text] and thus the wavelength of lasing [Formula: see text] can be tuned, for example, by chemical composition. However, the most desired mode to tune the laser, by an electric field, has so far been elusive. Here we present the realization of an electrically tunable laser with [Formula: see text] spanning an extraordinarily broad range (>100 nm) of the visible spectrum. The effect is achieved by using an electric-field-induced oblique helicoidal (OH) state in which the molecules form an acute angle with the helicoidal axis rather than align perpendicularly to it as in a field-free CLC. The principal advantage of the electrically controlled CLCOH laser is that the electric field is applied parallel to the helical axis and thus changes the pitch but preserves the single-harmonic structure. The preserved single-harmonic structure ensures efficiency of lasing in the entire tunable range of emission. The broad tuning range of CLCOH lasers, coupled with their microscopic size and narrow line widths, may enable new applications in areas such as diagnostics, sensing, microscopy, displays, and holography.

Dye-doped CLC-lasers: Interdependence of spectral and angular characteristics and boundary conditions of the active layer

ABSTRACTWe have studied the spectral and angular emission characteristics of a cholesteric liquid crystal (СLС) laser under specific conditions of its planar orientation. It is discovered that the orthogonal orientation of directors on substrates greatly improves the lasing spectrum and the angular divergence. A shift of the lasing spectrum and an increase in the lasing threshold under such conditions can testify to the presence of a defect in such structure, though no dip in the spectrum is registered. The origin of the ring structure of a laser beam is studied, and it is established that this structure is caused by the excitation of the lasing on higher axial modes.

Role of the sample thickness on the performance of cholesteric liquid crystal lasers: Experimental, numerical, and analytical results

We have studied the performance characteristics of a dye-doped cholesteric liquid crystal (CLC) laser as a function of the sample thickness. The study has been carried out both from the experimental and theoretical points of view. The theoretical model is based on the kinetic equations for the population of the excited states of the dye and for the power of light generated within the laser cavity. From the equations, the threshold pump radiation energy Eth and the slope efficiency η are numerically calculated. Eth is rather insensitive to thickness changes, except for small thicknesses. In comparison, η shows a much more pronounced variation, exhibiting a maximum that determines the sample thickness for optimum laser performance. The predictions are in good accordance with the experimental results. Approximate analytical expressions for Eth and η as a function of the physical characteristics of the CLC laser are also proposed. These expressions present an excellent agreement with the numerical calculations. Finally, we comment on the general features of CLC layer and dye that lead to the best laser performance.

Kinetic behavior of light emission in cholesteric liquid crystal lasers: An experimental study.

Some dynamical aspects of fluorescence and lasing have been studied in a dye-doped cholesteric liquid crystal by measuring the response of the material to nanosecond optical pumping. It has been found that as the pumping energy is increased the fluorescence pulse duration decreases, reaching a minimum at the lasing threshold. Above the threshold the temporal profiles are irregular and consist of a set of narrow pulses whose measured duration is limited by the detector risetime (1 ns). The results are interpreted in terms of a recently proposed model [JETP, 118, 822 (2014)] that makes use of rate equations to account for the laser generation in cholesteric liquid crystals. The prediction of such equations for an experimental configuration appropriate for fluorescence lifetime measurements is analyzed.