JP2005101464A - Laser oscillation control device - Google Patents

Abstract

An object of the present invention is to apply an external stimulus other than temperature and pressure to a cholesteric liquid crystal to change the alignment state of liquid crystal molecules in the liquid crystal cell, to determine whether laser oscillation is oscillated from the liquid crystal cell, the intensity of the laser beam, and the laser beam. To provide a technology that controls the line width of the laser, and to provide a laser oscillation technology that gives a sharp spectrum with a small spectral line width.
By controlling the intensity of excitation light applied to a liquid crystal cell and the intensity of an external electric field applied to the liquid crystal cell, the presence / absence of laser oscillation oscillated from the liquid crystal cell is changed by changing the alignment state of liquid crystal molecules. Control method of laser oscillation, controlling laser beam intensity and laser beam line width [Selection]

Description

  The present invention relates to a laser oscillation control method. More specifically, the present invention controls the intensity of excitation light applied to the liquid crystal cell and the intensity of the external electric field applied to the liquid crystal cell, thereby changing the alignment state of the liquid crystal molecules and oscillating from the liquid crystal cell. The present invention relates to a laser oscillation control method for controlling presence / absence of laser oscillation, intensity of laser light, and line width of laser light.

  With the development of organic electroluminescent devices and organic solid-state laser devices, the importance of the emission process of organic molecules in the excited state is increasing. Recently, a current injection type organic semiconductor laser has been attracting attention as a next-generation element of an organic electroluminescence device, and in order to achieve this, an optical resonator structure that feeds back a phase and a propagation direction is indispensable. Prior to the construction of current injection type organic laser devices, there have been many studies on photo-excited laser oscillation using light-emitting materials such as π-electron conjugated polymers and dye-added polymers (A. Dodabalapur, EA). Chandross, M. Berggren, and RE Slusher, Science, 277, 1787 (1997), MD McGehee and AJ Heeger, Adv. Mater., 12, 1655 (2000).).

  DFB (Distributed Feedback) (H. Kogelnik and CV Shank, Appl. Phys. Lett., 18, 152 (1971).), DBR (Distributed Bragg Reflector) (IP Kaminov, HP Weber, and EA Chandross, Appl Phys. Lett., 18, 497 (1971), N. Tessler, GJ Denton, and RH Friend, Nature, 382, 695 (1996).), Microdisk (M. Kuwata-Gonokami, RH Jordan, A. Dodabalapur) , HE Katz, M. Schilling, RE Slusher, and S. Ozawa, Opt. Lett., 20, 2093 (1995).) And microrings (SV Frolov, ZV Vardeny, and K. Yoshino, Appl. Phys. Lett. , 72, 1802 (1998).), And various distributed feedback resonator structures have been proposed, and laser oscillation by optical excitation has been confirmed.

  However, in order to produce these optical resonator structures, complicated steps such as a photolithography method are required, and there is a problem that they cannot be obtained easily.

Cholesteric (chiral nematic) liquid crystals exhibit supramolecular helical structures created from chiral molecules. When a cholesteric liquid crystal is sandwiched between rubbed substrates, self-organized planar alignment (Grangen structure) is formed with the molecular helical axis aligned perpendicular to the substrate (H. Coles, "Handbook of Liquid Crystals, Vol 2A ", D. Demus, J. Goodby, GW Gray, H.-W. Spiess, and V. Vill eds., Wiley-VCH, Weinheim (1998), pp. 335-409.). Since the refractive index periodically fluctuates along the helical axis of the liquid crystal molecule, light interference occurs, and specific light satisfying the Bragg reflection condition is selectively reflected. The central wavelength (λ _max ) of the reflection band is determined by the average refractive index (n) and the helical pitch (p) of the liquid crystal as shown in equation (1), and the wavelength band (Δλ) is expressed by equation (2). It can be determined from the helical pitch and birefringence (Δn).

λ _max = np, (1)
Δλ = Δnp, (2)

  Reflected light strongly depends on the palm of the cholesteric liquid crystal, and selectively reflects circularly polarized light in the same direction as the helix of the liquid crystal molecules in the wavelength range satisfying equations (1) and (2), and in the opposite direction to the helix. The circularly polarized light is transmitted. The helical pitch of cholesteric liquid crystals responds to external stimuli such as temperature and pressure, and the wavelength of the reflection band changes accordingly (H. Coles, "Handbook of Liquid Crystals, Vol 2A", D. Demus, J. Goodby, GW Gray, H.-W. Spiess, and V. Vill eds., Wiley-VCH, Weinheim (1998), pp. 335-409.).

  The planar orientation of cholesteric liquid crystals can be regarded as a one-dimensional photonic crystal structure because it spontaneously forms a structure with a periodically modulated dielectric constant (LS Goldberg and JM Schnur (The United States of America as represented by the Secretary of the Navy), US Patent 3771065 (1973). Recently, research on laser oscillation using the self-organized photonic crystal structure of cholesteric liquid crystals has become active (S. Furumi, S. Yokoyama, A. Otomo, and S. Mashiko, Thin Solid Films, 438 / 439, 422 (2003).). A characteristic of laser oscillation using cholesteric liquid crystal is that no special external optical resonator is required.

  Small molecules (H. Finkelmann, ST Kim, A. Munoz, P. Palffy-Muhoray, and B. Taheri, Adv. Mater., 13, 1069 (2001).), Polymer cholesteric liquid crystals (H. Finkelmann, ST Kim , A. Munoz, P. Palffy-Muhoray, and B. Taheri, Adv. Mater., 13, 1069 (2001).), Ferroelectric liquid crystals (M. Ozaki, M. Kasano, D. Ganzke, W. Hasse , and K. Yoshino, Adv. Mater., 14, 306 (2002).), photopolymerizable cholesteric liquid crystals (J. Schmidtke, W. Stille, H. Finkelmann, and ST Kim, Adv. Mater., 14, 746). (2002).), Blue phase liquid crystal (W. Cao, A. Munoz, P. Palffy-Muhoray, and B. Taheri, Nature Materials, 1, 111 (2002).) Laser oscillation has also been confirmed using lyotropic cholesteric liquid crystals (PV Shibaev, K. Tang, AZ Genack, V. Kopp, and MM Green, Macromolecules, 35, 3022 (2002).). In addition, efficient laser oscillation using photoexcited Forster energy transfer has been reported (M. Chambers, M. Fox, M. Grell, and J. Hill, Adv. Funct. Mater., 12, 808 ( 2002).).

As described above, cholesteric liquid crystals have a characteristic of forming a one-dimensional photonic crystal structure in a self-organizing manner while having fluidity, and therefore, a new laser oscillation device that responds to an external stimulus can be expected. So far, pressure has been applied to cholesteric liquid crystals (see Non-Patent Document 1 (H. Finkelmann, ST Kim, A. Munoz, P. Palffy-Muhoray, and B. Taheri, Adv. Mater., 13, 1069 (2001).) ) And temperature (see Non-Patent Document 2 (M. Ozaki, M. Kasano, D. Ganzke, W. Hasse, and K. Yoshino, Adv. Mater., 14, 306 (2002).)). An example has been reported in which laser oscillation is controlled by adding. Therefore, it is expected to develop technology to control laser oscillation by applying external stimuli other than pressure and temperature to cholesteric liquid crystals.
H. Finkelmann, ST Kim, A. Munoz, P. Palffy-Muhoray, and B. Taheri, Adv. Mater., 13, 1069 (2001). M. Ozaki, M. Kasano, D. Ganzke, W. Hasse, and K. Yoshino, Adv. Mater., 14, 306 (2002).

  The present invention applies an external stimulus other than temperature and pressure to a cholesteric liquid crystal to change the alignment state of liquid crystal molecules in the liquid crystal cell, whether laser oscillation is oscillated from the liquid crystal cell, the intensity of laser light, An object is to provide a technique for controlling the line width of light.

  Another object of the present invention is to provide a laser oscillation technique that provides a sharp spectrum with a small spectral line width while achieving the above object.

  (1) In order to solve at least one of the above problems, the laser oscillation control method of the present invention controls the intensity of the excitation light applied to the liquid crystal cell and the intensity of the external electric field applied to the liquid crystal cell. This is a laser oscillation control method in which the alignment state of liquid crystal molecules is changed to control the presence / absence of laser oscillation oscillated from the liquid crystal cell, the intensity of the laser beam, and the line width of the laser beam. That is, by controlling the intensity of the excitation light and the intensity of the external electric field as external stimuli applied to the liquid crystal cell, the state of the liquid crystal can be changed and laser oscillation can be controlled.

  (2) In the laser oscillation control method according to the present invention, preferably, the liquid crystal cell is a liquid crystal cell having two transparent electrode glass substrates facing each other so as to sandwich a cholesteric liquid crystal layer containing a fluorescent dye. This is a control method. Since the fluorescent dye emits light by excitation light, and its photons are localized in the helical structure of the cholesteric liquid crystal, the reflection band of the cholesteric liquid crystal functions as a one-dimensional photonic band gap. The distributed feedback (DFB) effect in the liquid crystal cell enables sharp laser oscillation at the reflection band edge from the cholesteric liquid crystal.

  (3) The laser oscillation control method of the present invention is preferably a laser oscillation control method in which in the cholesteric liquid crystal containing the fluorescent dye, the reflection spectrum of the cholesteric liquid crystal overlaps the emission spectrum of the fluorescent dye.

  (4) The method for controlling laser oscillation according to the present invention is preferably such that the cholesteric liquid crystal layer comprises an achiral (having no chiral moiety) nematic liquid crystal having positive dielectric anisotropy and a chiral agent that induces the cholesteric liquid crystal. It is configured. A fluorescent dye is added to the cholesteric liquid crystal layer so as to be 0.2 to 1.0% by weight. By adjusting the concentration of the chiral agent added to the liquid crystal, the laser oscillation wavelength can be controlled, and the threshold of photoexcitation energy can also be controlled. Further, by controlling the concentration of the fluorescent dye, sharp laser oscillation as described above becomes possible.

前記のキラル剤は、下記構造式（３）で表される化合物を５〜６重量部含み、

前記の蛍光色素は、ＤＣＭ（4-ジシアノメチレン-2-メチル-6-(p-ジメチルアミノスチリル)-4H-ピラン）である。 (5) In the laser oscillation control method of the present invention, preferably, the liquid crystal having positive dielectric anisotropy contains either or both of the compounds represented by the following structural formulas (1) and (2). Including 100 parts by weight,

The chiral agent includes 5 to 6 parts by weight of a compound represented by the following structural formula (3),

The fluorescent dye is DCM (4-dicyanomethylene-2-methyl-6- (p-dimethylaminostyryl) -4H-pyran).

  That is, the present invention uses a cholesteric liquid crystal to which a fluorescent dye is added, performs laser oscillation by photoexcitation, applies an alternating electric field to the cholesteric liquid crystal cell, causes a change in the helical structure of the liquid crystal molecule, and changes the orientation of the liquid crystal molecule. A related method of controlling laser oscillation behavior is provided.

  According to the present invention, laser oscillation generated from a liquid crystal cell by controlling the intensity of excitation light as an external stimulus and the intensity of an applied electric field on the cholesteric liquid crystal to change the alignment state of liquid crystal molecules in the liquid crystal cell. The presence / absence of laser light, the intensity of laser light, the line width of laser light, and the like can be controlled. The intensity of the excitation light and the intensity of the applied electric field can be controlled relatively easily, and the presence / absence of laser oscillation, the intensity of the laser light, and the line width of the laser light controlled thereby are greatly changed. Therefore, the control method of the present invention is effective as a method for controlling laser oscillation.

  According to the present invention, it is possible to obtain laser oscillation that gives a sharp spectrum with a small spectral line width.

  The present invention changes the alignment state of the liquid crystal molecules by controlling the intensity of the excitation light applied to the liquid crystal cell and the intensity of the external electric field applied to the liquid crystal cell, and the presence or absence of laser oscillation generated from the liquid crystal cell. The present invention relates to a laser oscillation control method for controlling the intensity of laser light and the line width of laser light. The liquid crystal cell is a liquid crystal cell having a cholesteric liquid crystal layer containing a fluorescent dye and two ITO transparent electrode glass substrates facing each other so as to sandwich the cholesteric liquid crystal layer containing the fluorescent dye. The cholesteric liquid crystal layer having a fluorescent dye is manufactured from a liquid crystal material containing a cholesteric liquid crystal containing a liquid crystal and a chiral agent that induces the cholesteric liquid crystal and a fluorescent dye added thereto.

(liquid crystal)
As the liquid crystal, a known liquid crystal can be used, and a liquid crystal having positive dielectric anisotropy is particularly preferable. An example of such a liquid crystal is a liquid crystal (RDP-60774 manufactured by Dainippon Ink Co., Ltd.) containing two equimolar amounts of two compounds having the following chemical formula. Dainippon Ink Co., Ltd .: RDP-60774 is a nematic liquid crystal having positive dielectric anisotropy, which is a mixture of compounds having a cyano group at the end, and its physical properties are TNI = 43 ° C, Δn = 0.139, Δε = + 11.3.

(Chiral agent)
The chiral agent is an agent that induces a cholesteric liquid crystal, for example, induces an achiral nematic liquid crystal in the cholesteric liquid crystal. As the chiral agent, those disclosed in JP-A-2002-180051, JP-A-2002-179682, JP-A-2002-179670 and the like may be used. Examples of the chiral agent include photoreactive chiral agents. A photoreactive chiral agent is a compound that has a chiral site and a photoreactive site that undergoes a structural change upon irradiation with light, and greatly changes, for example, the twisting force (HTP) of the helical structure of the liquid crystal depending on the amount of irradiation light. In order to increase the helical structure inducing force by light irradiation, it is preferable that the degree of structural change by light irradiation is large. Furthermore, as the photoreactive chiral agent, those having a solubility parameter SP value close to that of a liquid crystal compound are desirable. In addition, when a structure in which one or more polymerizable bonding groups are introduced into the molecule of the photoreactive chiral agent, the heat resistance of the liquid crystal phase is improved.

Specific examples of the chiral agent include a chiral agent represented by the following formula (Merck Japan, Inc .: R-1011). Merck Japan, Inc .: R-1011 is a chiral agent in which two mesogens are linked by an asymmetric carbon atom.

  The chiral agent is usually added in an amount of 5 to 6 parts by weight with respect to 100 parts by weight of the liquid crystal.

(Fluorescent dye)
As the fluorescent dye, known fluorescent dyes used for lasers can be used. Examples of such fluorescent dyes include DCM (4-dicyanomethylene-2-methyl-6- (p-dimethylaminostyryl) -4H-pyran: 4-dicyanmethylene-2-methyl-6- (p-dimethylaminostyryl). -4H-pyran, Pyrromethene, coumarin, Rhodamine, Oxazine, Pyridine, Fluorescein, Kiton Red, among them Preferred is DCM or pyromethane (eg, pyromethane 597). In addition, what is marketed widely can be used for these pigment | dyes.

  The fluorescent dye is usually adjusted to 0.3 to 0.5% by weight in the liquid crystal material.

  In addition to the above liquid crystal materials, chiral agents, and fluorescent dyes, a polymerizable monomer, a polymerization initiator, a binder resin, a solvent, a surfactant, a polymerization inhibitor, a thickener, a dye, and a pigment, if necessary. Other components such as an ultraviolet absorber and a gelling agent can be included. In the liquid crystal composition of the present invention, it is particularly preferable to use a surfactant in combination. For example, the orientation state at the air interface on the surface of the layer can be controlled sterically, such as when forming a layer by applying a coating liquid crystal composition, and in the case of a cholesteric liquid crystal phase, a selective reflection wavelength with higher color purity can be obtained. Can be obtained.

(ITO transparent electrode glass substrate)
A publicly known ITO transparent electrode glass substrate can be used as the ITO transparent electrode glass substrate, and is not particularly limited.

(Manufacturing method of liquid crystal cell)
Hereinafter, an example of the manufacturing method of the liquid crystal cell of the present invention will be described. A polyvinyl alcohol (PVA) aqueous solution is applied on an ITO transparent electrode glass substrate and rubbed in a uniaxial direction. Two glass ITO transparent electrode substrates subjected to rubbing treatment are sandwiched between polyimide spacers to produce empty cells, and the previously prepared fluorescent dye-containing cholesteric liquid crystal is enclosed. In this way, a liquid crystal cell can be created.

(1.1 Fabrication of materials and liquid crystal cell)
As a liquid crystal material, Dainippon Ink Co., Ltd .: RDP-60774 was used. Moreover, R-1011 of Merck Japan was used as a chiral agent. In this example, by adding 6.0 wt% (about 2 mol%) R-1011 to RDP-60774, a cholesteric liquid crystal phase exhibiting selective reflection in the visible region and having a right molecular helix was prepared. The helical twisting power (β) of the chiral agent against RDP-60774 was 42.1 (μm wt%) ⁻¹ . As the fluorescent dye, DCM was used, and DCM was mixed with the previously prepared cholesteric liquid crystal at a ratio of 0.4 wt% to obtain a fluorescent dye-containing cholesteric liquid crystal.

  The liquid crystal cell was produced using two ITO transparent electrode glass substrates. A 1.0 wt% polyvinyl alcohol (PVA) aqueous solution was spin-coated on an ITO transparent electrode glass substrate to promote rubbing treatment in a uniaxial direction. Two glass ITO transparent electrode substrates subjected to rubbing treatment were sandwiched between 7.5 μm polyimide spacers to prepare empty cells, and the previously prepared fluorescent dye-containing cholesteric liquid crystal was enclosed. A liquid crystal cell was thus prepared.

(1.2 Evaluation of optical characteristics of liquid crystal cell)
FIG. 1 shows an experimental optical system 1 for evaluating laser oscillation of a liquid crystal cell. The second harmonic (New Wave Research; Polaris; wavelength: 532 nm; pulse width: 6 ns) of a Q-switched Nd: YAG laser was used as an excitation light source (pump light source). The laser light was incident on the liquid crystal cell 3 from an oblique direction of 45 ° with respect to the normal line of the liquid crystal cell, and was condensed using a convex lens 4 (f = 100). The condensing diameter on the liquid crystal cell was about 300 μm. Adjust the intensity of the laser beam with a wave plate (not shown), polarizing prism 5, and ND filter (not shown), and use a photodetector 6 (pyroelectric energy analysis system (Coherent; Smart Sensor LM-P-209)) to adjust its intensity. Detected. Light emitted from the liquid crystal cell was collected by two convex lenses (f = 100) and evaluated with a fiber spectrometer (Ocean Optics; USB2000 or Acton Research; Spectra Pro 150). When applying an external electric field to the liquid crystal cell, a 60 Hz AC power source (Yamabishi; Volt slider V-130-5) was used.

(2.1 Emission characteristics of liquid crystal cells)
A cholesteric liquid crystal to which a fluorescent dye (DCM) was added showed a granule structure (planar alignment). By measuring the reflection spectrum, it was confirmed that a light selective reflection band was present between 580 nm and 630 nm of the liquid crystal cell. This indicates that the helical axis of the cholesteric liquid crystal is aligned in a direction perpendicular to the substrate. In addition, the emission spectrum of the xenon lamp by steady-state excitation showed an emission band extending to about 720 nm on the long wavelength side centering on the maximum emission wavelength of 590 nm. Therefore, the reflection band of the cholesteric liquid crystal sufficiently overlaps the emission spectrum of the DCM dye.

  When this cholesteric liquid crystal cell was optically excited with a 0.8 μJ / pulse Nd: YAG pulsed laser, suppression of emission from the reflection band and enhancement of emission at the band edge were observed in the fluorescence spectrum of the DCM dye. This suggests that the photons of dye emission are localized in the helical structure of the cholesteric liquid crystal, and the reflection band of the cholesteric liquid crystal functions as a one-dimensional photonic band gap.

  After that, when the photoexcitation energy was increased to 1.1 μJ / pulse, it suddenly changed to an emission spectrum with a narrow line width. The emission wavelength was 630 nm, which coincided with the long wavelength end of the liquid crystal reflection band.

  Next, the emission intensity and spectral line width of the liquid crystal cell with respect to photoexcitation energy were examined in detail. FIG. 2 is a graph showing changes in emission intensity and spectrum half width with respect to photoexcitation energy. 2A shows the relationship between the photoexcitation energy and the emission intensity, and FIG. 2B shows the relationship between the photoexcitation energy and the half width of the spectrum. In a liquid crystal cell containing 6.0 wt% R-1011, the threshold value of the photoexcitation energy required for laser oscillation was about 1.1 μJ / pulse (see circle in FIG. 2A). Before and after laser oscillation, the emission intensity increased several thousand times, and the spectral line width suddenly decreased from 80 nm to 0.8 nm (see ○ in FIG. 2B). Therefore, it is considered that laser oscillation occurred from the cholesteric liquid crystal at the reflection band edge due to the distribution feedback (DFB) effect in the liquid crystal cell.

  Considering the fact that laser emission is induced at the reflection band edge of cholesteric liquid crystal, liquid crystal with different concentration of R-1011 chiral agent was prepared and the lasing wavelength was examined. FIG. 3 shows the reflection spectrum and the laser emission spectrum. 3A shows a case where the concentration of the chiral agent is 6 wt%, FIG. 3B shows a case where the concentration of the chiral agent is 5.9 wt%, and FIG. 3C shows a case where the concentration of the chiral agent is 5. It is the reflection spectrum and laser emission spectrum in the case of 4 wt%. In FIG. 3, the solid line represents the laser emission spectrum, and the dotted line represents the reflection spectrum. As shown in FIG. 3, when the concentration of the chiral agent was increased, the reflection band (λmax) shifted to the short wavelength side. This wavelength shift was in accordance with equation (3).

λmax = n / βC (3)

Here, n is the average refractive index of the liquid crystal, β is the helical twisting force of R-1011 against RDP-60774, and C is the weight concentration of R-1011. FIG. 3 shows that the laser emission wavelength strongly depends on the position of the reflection band. As shown in FIG. 3A, when a liquid crystal containing 6.0 wt% of a chiral agent (R-1011) was used, one laser emission appeared on the long wavelength side (630 nm) of the reflection band. On the other hand, as shown in Fig. 3 (c), when the chiral agent (R-1011) is 5.4 wt% of the liquid crystal, the reflection band of the cholesteric liquid crystal is 80 nm away from the fluorescence maximum wavelength of the DCM dye. One laser emission was confirmed at the reflection band edge (674 nm) on the short wavelength side. In these two types of liquid crystal cells, only one laser emission could be observed, whereas when a liquid crystal mixed with 5.9 wt% R-1011 was used, the laser emission was as shown in FIG. Appeared at both ends of the reflection bands at 608 nm and 658 nm. This is presumably because the reflection band of the cholesteric liquid crystal completely overlaps the spectrum on the longer wavelength side than the maximum fluorescence wavelength of the DCM dye. Therefore, it can be seen that the laser oscillation wavelength can be controlled by adjusting the concentration of the chiral agent added to the nematic liquid crystal according to the formula (3).

  As can be seen from FIG. 2, the photoexcitation energy required for laser oscillation depends on the addition amount of the chiral agent in the liquid crystal, that is, the position of the reflection band of the cholesteric liquid crystal. The threshold values for photoexcitation were 1.1 μJ / pulse for liquid crystal with 6.0 wt% of R-1011, 0.9 μJ / pulse for liquid crystal with 5.9 wt%, and 1.2 μJ / pulse for liquid crystal with 5.4 wt%. In order for light emission to be amplified in the cholesteric liquid crystal cell and to emit laser light outside the liquid crystal cell, the gain of the laser radiation must be greater than the loss in the cell. Since the fluorescence spectrum and the gain spectrum are similar, a gain can be obtained efficiently when the reflection band of the cholesteric liquid crystal exists in the vicinity of the maximum fluorescence wavelength of the DCM dye. On the other hand, when a reflection band is located at the long wavelength end of the fluorescence spectrum of DCM dye, such as a liquid crystal cell containing 5.4 wt% R-1011, it is relatively optically excited because it is difficult to obtain the gain necessary for laser oscillation. The energy threshold increases. That is, by controlling the addition amount of the chiral agent, not only the laser oscillation wavelength can be adjusted, but also the threshold of photoexcitation energy can be controlled.

(2.2 Polarization characteristics of laser emission)
The polarization characteristics of laser emission from a cholesteric liquid crystal cell were examined using a polarization emission characteristics evaluation system 7 whose basic configuration is shown in FIG. This evaluation system for polarized light emission characteristics includes a ¼ wavelength plate (Forenerrom wavelength plate) 8, a linear polarizer 9, and a photodetector 10. The arrows in the figure indicate the direction of laser light emission. As described above, the cholesteric liquid crystal has a twisted structure in the light selective reflection dispersion relation of the cholesteric liquid crystal. For this reason, degeneracy is eliminated with respect to left and right circularly polarized light that is intrinsic polarization, and a photonic band gap is generated in circularly polarized light having the same palmarity as a molecular helix, but no gap effect occurs in opposite circularly polarized light. Therefore, it can be expected that the laser emission has circular polarization characteristics. From the experimental results shown in FIG. 3 (b), since two laser emissions were confirmed from a cholesteric liquid crystal cell containing 5.9 wt% R-1011, it was decided to evaluate their circular polarization characteristics. As shown in FIG. 4 (a), a Fresnel ROM wave plate with little wavelength dependence for phase conversion is used as a quarter wave plate, and the laser light that has passed through the wave plate is rotated by a linear polarizer. The light emission characteristics were evaluated. The laser emission spectrum is shown in FIG. When the angle (θ) of the linear polarizer was rotated counterclockwise from + 45 °, the intensity of the laser emission increased, and the emission intensity showed the maximum value when θ was + 45 ° ± 180 °. Since θ = + 45 ° corresponds to right-handed circularly polarized light and θ = −45 ° corresponds to left-handed circularly polarized light, the degree of circular polarization (g) of laser emission was calculated from equation (4).

          g = 2 (IL-IR) / (IL + IR) (4)

  IL represents left circularly polarized light emission, and IR represents right circularly polarized light emission. As can be seen from Equation (4), when g = ± 2, this means complete circularly polarized light emission. From the laser emission spectrum of FIG. 4, the g value of laser emission at 608 nm and 658 nm can be calculated as approximately −1.8, and it was found that right circularly polarized light emission with high directivity was obtained. Compared with the previously reported g value of circularly polarized light emission from cholesteric liquid crystals, this g value was large.

  FIG. 5 shows the results of measuring the laser emission intensity while rotating the linear polarizer. The black circle in the figure is the case where R-1011 is used as the chiral agent, and the white circle is the graph in the case where S-1011 which is the enantiomer of R-1011 is used. When a chiral agent of R-1011 is added to a nematic liquid crystal and a cholesteric liquid crystal with a right helix is used, it can be seen that a right circularly polarized laser oscillation occurs. On the other hand, when a similar experiment was performed using S-1011, it was confirmed that left circularly polarized laser light emission was exhibited. Therefore, it can be seen that the helical direction of circularly polarized laser emission using cholesteric liquid crystal depends on the optically active site of only 2 mol% of the chiral agent added to the nematic liquid crystal.

  In other words, since the cholesteric liquid crystal cell is excited by a linearly polarized laser, circularly polarized laser emission is induced, so the reflection band of the cholesteric liquid crystal functions as a “chiral photonic band gap”. .

(2.3 Electric field control of laser oscillation)
The nematic liquid crystal used in this example has a relatively large positive dielectric anisotropy. Therefore, by applying an external electric field, the orientation change of the cholesteric liquid crystal cell occurs, and the laser oscillation control associated therewith can be expected. FIG. 6 is a graph showing the change in laser emission intensity when a 60 Hz AC electric field is applied to the cholesteric liquid crystal cell while photoexcitation at 1.9 μJ / pulse. 6A shows the relationship between the intensity of the applied electric field and the intensity of laser emission, and FIG. 6B shows the reflection spectrum of the liquid crystal cell when the applied electric field is changed. As shown in FIG. 6 (a), laser emission could be confirmed up to an applied voltage of 18 V, but emission suddenly disappeared when the voltage was increased to 21 V. At an applied voltage of 21 V, laser emission could not be induced even when photoexcitation was performed at 5.0 μJ / pulse. It is considered that the disappearance of the laser emission due to the application of the electric field originates from the change of the reflection band edge as shown in FIG.

  In order to examine the state of the liquid crystal when the applied electric field was changed, the liquid crystal cell was observed with a polarizing microscope by applying the electric field. In the state where no electric field was applied, the liquid crystal molecules were in a horizontal orientation and exhibited a granule structure as shown in FIG. When the applied voltage is 21 V, the spiral axis changes to a horizontal random horizontal fan-focal conic structure as shown in Fig. 7 (b). Subsequently, when a voltage of 70 V or higher is applied, Fig. 7 (c) Thus, the birefringence of the liquid crystal molecules disappeared completely. This suggests that the electric field induced cholesteric nematic phase transition occurred and changed to nematic homeotropic orientation. When the electric field was suddenly removed from this homeotropic alignment, it reverted back to the grange structure, and laser emission was reconfirmed as the alignment changed. As described above, we succeeded in reversible control of laser oscillation based on the electric field induced orientation change of cholesteric liquid crystal.

  Since the present invention relates to a laser oscillation control method, it has various uses such as a laser apparatus.

FIG. 1 is a schematic configuration diagram showing a basic structure of an experimental optical system when evaluating laser oscillation of a liquid crystal cell. FIG. 2 is a graph showing changes in emission intensity and spectrum half width with respect to photoexcitation energy. 2A shows the relationship between the photoexcitation energy and the emission intensity, and FIG. 2B shows the relationship between the photoexcitation energy and the half width of the spectrum. FIG. 3 shows the reflection spectrum and the laser emission spectrum. 3A shows a case where the concentration of the chiral agent is 6 wt%, FIG. 3B shows a case where the concentration of the chiral agent is 5.9 wt%, and FIG. 3C shows a case where the concentration of the chiral agent is 5. It is the reflection spectrum and laser emission spectrum in the case of 4 wt%. FIG. 4A is a schematic configuration diagram showing a basic configuration of a measurement system for the polarization characteristics of laser emission from a cholesteric liquid crystal cell. FIG. 4B is a graph evaluating the circularly polarized light emission characteristics of the laser emission spectrum. FIG. 5 is a graph showing the results of measuring the laser emission intensity while rotating the linear polarizer. FIG. 6 is a graph showing the change in laser emission intensity when a 60 Hz AC electric field is applied to the cholesteric liquid crystal cell while photoexcitation at 1.9 μJ / pulse. 6A shows the relationship between the intensity of the applied electric field and the intensity of laser emission, and FIG. 6B shows the reflection spectrum of the liquid crystal cell when the applied electric field is changed. FIG. 7 is a diagram showing the state of the liquid crystal when the applied electric field is changed, and a photograph that changes to a drawing. FIG. 7A shows the case where no electric field is applied, FIG. 7B shows the case where the applied voltage is 21 V, and FIG. 7C shows the case where the applied voltage is 70 V or more.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Experimental optical system at the time of evaluating the laser oscillation of a liquid crystal cell 2 Excitation light source 3 Liquid crystal cell 4 Convex lens 5 Polarizer 6 Photodetector 7 Polarization light emission characteristic evaluation system 8 1/4 wavelength plate (Forenerrom wavelength plate)
9 Linear polarizer 10 Photodetector

Claims (5)

  By controlling the intensity of the excitation light applied to the liquid crystal cell and the intensity of the external electric field applied to the liquid crystal cell, the alignment state of the liquid crystal molecules is changed, the presence or absence of laser oscillation generated from the liquid crystal cell, laser light Control method of laser oscillation, which controls the intensity of laser and the line width of laser light.   2. The laser oscillation control method according to claim 1, wherein the liquid crystal cell is a liquid crystal cell having two transparent electrode glass substrates facing each other so as to sandwich a cholesteric liquid crystal layer containing a fluorescent dye.   The method for controlling laser oscillation according to claim 2, wherein in the cholesteric liquid crystal containing the fluorescent dye, the reflection spectrum of the cholesteric liquid crystal overlaps the emission spectrum of the fluorescent dye.   The cholesteric liquid crystal layer is composed of an achiral nematic liquid crystal having positive dielectric anisotropy and a chiral agent that induces the cholesteric liquid crystal. The method for controlling laser oscillation according to claim 2, wherein a fluorescent dye is added to the cholesteric liquid crystal layer so as to be 0.2 to 1.0% by weight. The liquid crystal having positive dielectric anisotropy contains 100 parts by weight of either or both of the compounds represented by the following structural formulas (1) and (2):

The chiral agent includes 5 to 6 parts by weight of a compound represented by the following structural formula (3),

The fluorescent dye is DCM;
The method for controlling laser oscillation according to claim 3.

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