The effect of temperature on laser wavelength from a dye-doped cholesteric liquid crystal (CLC) is studied.It is found that the laser wavelength is red shifted with the temperature rise.For a thin dye-doped CLC sample,the laser wavelength abruptly jumps at a certain temperature and then slightly decreases with the temperature rise within a certain temperature range.When the sample is thick enough,i.e.the sample thickness is more than 25 μm,the laser wavelength almost increases linearly with the temperature rise.The physical mechanism of this phenomenon is analyzed,theoretical simulation is performed,and the simulation result is consistent with the experimental result.The property that the laser wavelength increases linearly with the temperature rise can be used for temperature sensors.

为了提高掺杂染料的胆甾相液晶的激光效率,通过采用电击摩擦处理来改善染料掺杂胆甾相液晶内部分子的排列,结果表明对于不同厚度的胆甾相液晶样品,其内部分子排列的改善都能提高其输出激光效率,且样品越厚,经过电击摩擦后激光效率提高的倍数越大.这是因为在没有经过电击摩擦处理前,胆甾相液晶分子因不能理想排列而形成缺陷,造成光散射,降低激光效率.且样品越厚,其内部缺陷越多,光散射引起的损耗也越大,对激光效率的降低越严重.经过处理后,内部分子排列得到大幅改善,损耗极大降低,激光效率被提高,且电击摩擦对厚的胆甾相液晶样品内部分子排列的改善更显著,因此经过电击摩擦后,厚的样品的激光效率提高的更大.

The authors measured the absorption and the fluorescence spectra of the quantum dots CdSe/ZnS with 4 nm in size at different concentration with the use of the UV-Vis absorption spectroscopy and fluorescence spectrometer. The effect of quantum dots CdSe/ZnS's concentration on its fluorescence was especially studied and its physical mechanism was analyzed. It was observed that the optimal concentration of the quantum dots CdSe/ZnS for fluorescence is 2 micromole x L(-1). When the quantum dot's concentration is over 2 micromol x L(-1), the fluorescence is decreased with the increase in the concentration. While the quantum dot's concentration is less than 2 micromol x L(-1), the fluorescence is decreased with the decrease in the concentration. There are two main reasons: (1) fluorescence quenching and 2) the competition between absorption and fluorescence. When the quantum dot's concentration is over 2 micromol x L(-1), the distance between quantum dots is so close that the fluorescence quenching is induced. The closer the distance between quantum dots is, the more serious the fluorescence quenching is induced. Also, in this case, the absorption is so large that some of the quantum dots can not be excited because the incident light can not pass through the whole sample. As a result, the fluorescence is decreased with the increase in the quantum dot's concentration. As the quantum dot's concentration is below 2 micromol x L(-1), the distance between quantum dots is far enough that no more fluorescence quenching is induced. In this case, the fluorescence is determined by the particle number per unit volume. More particle number per unit volume produces more fluorescence. Therefore, the fluorescence is decreased with the decrease in the quantum dot's concentration.

Right- and left-handed cholesteric liquid crystals (CLCs) with the same reflection band at the same temperature have been successfully fabricated by doping oversaturated chiral dopant S811 (left-handed) or R811 (right-handed) in the liquid crystal host BL006. The reflection band of both the right- and left-handed CLCs is blue-shifted with an increase in temperature. On the basis of this property, a polarization-independent tunable optical filter with variable bandwidth was demonstrated by combining left- and right-handed CLCs as a unit and configuring two CLC units in the reflection mode, in which the incident light is first reflected by one pair of CLCs and then reflected by the other one. The central wavelength can be tuned from 573 to 500 nm and the bandwidth can be varied from 10 to 70 nm.

Because a lot of defects will be formed in thick cholesteric liquid crystals (CLC), which destroys its photonic bandgap structure and dramatically reduces the transmissivity, its applications in photonics have been dramatically limited. In order to overcome this problem, we developed a method to improve the alignment of the liquid crystal (LC) molecules in the CLC sample. The procedure is as follows: firstly, high enough AC voltage with 1 kHz is applied to swithch on the CLC sample. After switching on and off the sample at this voltage for several times, massage is done on the CLC surface for a while. The results show that after massage, the defects inside the CLC sample can be dramatically reduced, therefore the planar structure of CLC is greatly improved, and the transmission is drastically increased. It indicates that this method can be used for fabricating thicker CLC samples. Above all, it provides a useful fabrication basis for the CLC′s applications in photonics.

A widely tunable optical filter was designed and demonstrated by configuring two spatially distributed cholesteric liquid crystals (LCs) in a reflection mode, in which the incident light was first reflected by one cholesteric LC and then by the other. The spatially distributed cholesteric LCs were fabricated by injecting four cholesteric LC mixtures with different chiral concentration into the same LC cell one by one, in which the spectral position of the reflection band is spatially changed from blue to red color from one side to the other side. Using this method, a tunable optical filter with the ability of tuning the central wavelength from similar to 480 to similar to 613 nm and FWHM from similar to 10 to similar to 80 nm was successfully demonstrated. (C) 2013 Society of Photo-Optical Instrumentation Engineers (SPIE)

An optical tunable filter with variable bandwidth has been demonstrated using two cholesteric liquid crystals. The incident light was first reflected by the first cholesteric liquid crystal and then by the second one. By rotating the two cholesteric liquid crystals simultaneously, the central wavelength can be tuned. By fixing one of the cholesteric liquid crystals and rotating the other one, the bandwidth of the tunable filter can be varied. The central wavelength of the tunable optical filter can be tuned from 513.4 nm to 576.8 nm and the bandwidth is varied from 10 nm to 80 nm. This property will allow it to be widely used in many fields, including optical communications and multispectral and hyperspectral imaging systems.

A widely tunable optical filter with variable bandwidth has been demonstrated based on the thermal effect on cholesteric liquid crystals. The central wavelength can be widely tuned from 826 to 517 nm and the bandwidth can be varied from 10 to 70 nm. It will have potential applications in many fields, especially in telecommunications and multi-spectral/hyper-spectral imaging systems.

Cholesteric liquid crystal lasers have been extensively studied but barely studied in our country.The periodic helical structure and selective reflection property of cholesteric liquid crystal were introduced.The principle and research progress of cholesteric liquid crystal based lasers and its potential applications were reviewed to provide some reference for the specialists in our country.

An optical filter with tunable wavelength and bandwidth has been demonstrated using two cholesteric liquid crystals configured in a reflection mode, in which the incident light is first reflected by one cholesteric liquid crystal and then by the other one. The central wavelength can be tuned from 527 nm to 574 nm and the bandwidth can be changed from 10 nm to 80 nm. It has potential applications in many fields, especially in optical communications and multispectral and hyperspectral imaging systems.

We have investigated the optical properties of the polymer-stabilized cholesteric liquid crystal (CLC). It was observed that the reflectance was decreased and the transmittance was increased with the increase of the applied voltage. Based on this property, a polarization independent two-way variable optical attenuator (VOA) has been demonstrated by sandwiching a lambda/2 film between two left handed polymer-stabilized CLC films. Different from a conventional VOA, the VOA based on our developed polymer-stabilized CLC can continuously change the optical intensity in both the reflection and transmission directions by applying voltage on it. This unique property will allow it to be widely used in many applications, such as optical communications.

A multi-wavelength laser is demonstrated using a dye-doped cholesteric polymer film whose reflection bandwidth is broadened with several oscillations. Due to the abrupt change of the density of state between oscillation peak and valley, each oscillation functions as a photonic band gap for generating a laser wavelength under the excitation of a pumping laser. As a result, a multiple wavelength laser is generated. Results indicate that the dye-doped cholesteric liquid crystal polymer film is a good candidate for fabricating broadband lasers such as white light lasers. Potential applications include experimental testing of laser materials, identification markers, information displays, and inertial confinement laser fusion.

Low threshold laser action of dye-doped cholesteric liquid crystals (CLCs) is demonstrated using an input circularly polarized light whose handedness is the same as the cholesteric helix of the sample at the high-energy band edge of the reflection band. The mechanism originates from the dramatic increase of the photon density of state at the band edges. We also demonstrate an enhanced laser action of a CLC in a dielectric multilayer cavity. In such a device configuration, the band-edge excitation at high-energy band edge improves the lasing performance not only for the same handedness circularly polarized pump beam as the cholesteric helix but also for the opposite one. It stems from the polarization independence of the dielectric multilayers.

In this paper, we demonstrated a new dye-doped cholesteric liquid crystal (CLC) photonic band edge laser with emission enhanced by an external cholesteric resonator. As one-dimensional photonic crystal, the 5-&mgr;m dye-doped cholesteric liquid crystal cell generates circularly polarized laser emission at its photonic band edge. When sandwiched between two 5-&mgr;m cholesteric liquid crystal mirrors whose reflection band reflects the laser emission from the central dye-doped CLC laser, the emission can be enhanced by ~800X. In experiment, a second-harmonic Q-switched Nd-YAG pulsed laser is used to pump the CLC laser assembly at normal incidence. The detected laser emission is elliptically polarized and is still located at the band edge wavelength of the central CLC cell. The beam divergence is decreased by ~10X due to an increased cavity length. Theoretical analysis using 4x4 transfer matrix and scattering matrix has shown that the circular resonator produces transmission peaks based on Fabry-Perot effect inside reflection band and, moreover, the transmission peak at the band edge of central CLC can be well-preserved. Both experiment results and simulation results are present in good agreement.

A scattering-free broadband (approximately 120 nm bandwidth) circular polarizer is demonstrated by stacking three chiral polymer films with different pitch lengths. Using 4 x 4 matrix method, we have theoretically simulated the transmission spectra of each chiral polymer film and the three stacked films. Simulation results agree well with experiment. A broadband circular polarizer with bandwidth ranging from 400 to 736 nm can be achieved by stacking 8 such chiral polymer films together. Simulation results indicate that if a high birefringence (Deltan approximately 0.35) polymer film is employed the number of films can be reduced to three. Potential applications of these circular polarizers for liquid crystal displays, optical communications, and optical remote sensors are discussed.

We have obtained a dye-doped chiral photonic crystal (PC) film with reflection band gap much wider than its original band gap without dye dopants by using multiple-step fabrication processes. Moreover, the dye- doped chiral PC films using Our multiple-step fabrication processes exhibit many oscillations within the broadened reflection band gap. The abrupt change of the optical density of state (DOS) around the oscillations provides the possibility of generating laser emission when the dye-doped chiral PC film is pumped by a pulsed laser with wavelength in the absorption region of the laser dye. Based on this property, we demonstrated random lasers which exhibit different multiple-mode laser wavelength at different spatial positions. Different from the random lasers induced by the scattering mechanism, the random lasers from the dye-doped cholesteric polymer film exhibit Gaussian-like beam shape and specific propagation orientation which is normal to the cholesteric planar surface. It is foreseeable that a high efficiency and high power broadband laser can be generated using cholesteric polymer films.

We demonstrate an enhanced circularly polarized laser emission whose handedness is opposite to the cholesteric helix by the stacked cholesteric polymeric films. The dye-doped right-handed cholesteric polymer film is sandwiched between a mirror and a cholesteric polymer reflector. Due to the stimulated amplification and light recycling effects, the original laser emission from the middle active layer is not only dramatically enhanced but also converted to a left-handed circularly polarized emission with high purity. Moreover, a single 15 mum dye-doped cholesteric film is found to lase more efficiently when the top side faces the pump source than when the bottom side does. This phenomenon is attributed to the band gap broadening of the bottom side.

A low threshold and high efficiency laser based on dye-doped cholesteric liquid crystals (CLCs) is demonstrated using an input excitation with the same handedness of circular polarization as the helical structure of the sample at the shorter wavelength band edge of the reflection band. The responsible mechanism originates from the dramatic increase of the optical density of state (DOS) at the band edges. The calculated DOS of the CLC system confirms the authors’ experimental results.

In this paper, a cholesteric liquid crystal (CLC) and cholesteric monomer mixture are used for broadband circular polarizer, instead of pure cholesteric polymer. In this case, the birefringence and, thus, the bandwidth of each film are both increased. More importantly, unlike the solid CLC polymeric films, diffusion process can be induced in this CLC and CLC polymer composite film so that broadband reflection is achieved as a result of pitch gradient.

We designed a broadband quarter wave plate in the visible range using a twisted nematic liquid crystal film sandwiched between two compensation films. The quarter wave plate exhibits much wider bandwidth than the commercial product, which is composed of a half wave plate and a quarter wave plate.