JP4894214B2 - High efficiency laser oscillator - Google Patents

Description

  The present invention relates to a laser generator that does not require an external resonator. Specifically, the present invention relates to a microlaser generator that can easily perform laser oscillation by using a chiral liquid crystal. More specifically, the present invention relates to a laser oscillation device with extremely low excitation light energy by optically exciting a chiral liquid crystal to which a fluorescent material is added with circularly polarized light.

  Current injection type organic semiconductor lasers are attracting attention as next-generation photonic devices. To achieve this, an optical resonator structure that feeds back the phase and propagation direction is indispensable. Prior to the construction of a current injection type organic laser device, research examples on photoexcited laser oscillation using a light emitting material such as a π-electron conjugated polymer or a polymer added with a dye have been reported (Non-Patent Documents 1 and 2). ).

  That is, various distributions such as DFB (Distributed Feedback) (Non-Patent Document 3), DBR (Distributed Bragg Reflector) (Non-Patent Documents 4 and 5), Microdisk (Non-Patent Document 6) and Micro Ring (Non-Patent Document 7). A feedback resonator structure has been proposed so far, 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 cannot be easily obtained.

  On the other hand, when a chiral liquid crystal (cholesteric liquid crystal) is sandwiched between rubbed substrates, it forms a planar alignment in which the molecular helical axis is aligned perpendicularly to the substrate in a self-organized manner, along the helical axis of the liquid crystal molecules. Since the refractive index periodically changes, light interference occurs, and certain light satisfying the black reflection condition is reflected to form a kind of resonator. Recently, it has been found that when a chiral liquid crystal added with a dye is excited with pulsed light, laser oscillation occurs at the reflection band edge of the liquid crystal (Non-Patent Documents 8 and 9).

A. Dodabalapur, E .; A. Chandross, M.M. Berggren, and R.A. E. Sluster, Science, 277, 1787 (1997). M.M. D. McGehee and A.M. J. et al. Heeger, Adv. Mater. , 12, 1655 (2000). H. Kogelnik and C.I. V. Shank, Appl. Phys. Lett. , 18, 152 (1971). I. P. Kaminov, H.M. P. Weber, and E.M. A. Chandross, Appl. Phys. Lett. , 18, 497 (1971). N. Tessler, G.M. J. et al. Denton, and R.D. H. Friend, Nature, 382, 695 (1996. M.M. Kuwata-Gonokami, R.A. H. Jordan, A.M. Dodabalapur, H.M. E. Katz, M.M. Schilling, R.M. E. Slasher, and S.M. Ozawa, Opt. Lett. , 20, 2093 (1995). S. V. Frolov, Z. V. Vardeny, and K.C. Yoshino, Appl. Phys. Lett. , 72, 1802 (1998). V. I. Kopp, B.H. Fan, H.C. K. M.M. Vithana, A.R. Z. Genack, Opt. Lett. , 23, 1707 (1998). Shuichi Furukai and three others, Optnews vol. 138, no. 6 (The topic of light; electric field control of laser oscillation by chiral liquid crystal molecules), (2003)

  However, previous studies on laser oscillation using chiral liquid crystals have not fully elucidated the relationship between the laser oscillation behavior and the polarization characteristics of the excitation light, and the relationship has not always been clear. The research proposed so far, including Non-Patent Document 8 and Non-Patent Document 9 reported by the research group of the present inventors, is based on the fact that excitation light and laser oscillation behavior for chiral liquid crystal molecules are based on excitation light by linearly polarized light. It was done.

  In view of this, the present inventors have intensively studied to further clarify the relationship between the laser oscillation behavior and the polarization characteristics of the excitation light. As a result, when a chiral liquid crystal (cholesteric liquid crystal) added with a light-emitting material is photoexcited with circularly polarized light, it emits light having a half-value width of 1 nm or less above a certain threshold, that is, laser light having a single wavelength. The threshold of the photoexcitation energy depends strongly on the circular polarization of the excitation light, and it is extremely low when excited by circular polarization in the opposite direction to the chirality of the chiral liquid crystal. Obtained important findings. The present invention has been made based on these findings, and the configuration of the invention is as follows.

(1) A laser oscillation apparatus comprising: an excitation light source; a polarizer capable of circularly polarizing light from the excitation light source; and an optical device capable of oscillating a laser upon incidence of the circularly polarized light. Is formed by adding a luminescent material to a chiral liquid crystal that is a mixture of a liquid crystal material having no optically active site and a compound having an optically active site, and the luminescent material is a luminescent organic dye, Either a polymer light emitting material or an inorganic light emitting semiconductor fine particle, the reflection spectrum of the chiral liquid crystal overlaps the emission spectrum of the light emitting material, and the circularly polarized light is a reverse, rate greater than zero degrees which is the substrate normal direction, by Rukoto is photoexcited by circularly polarized light is incident at an incident angle of 45 degrees or less with respect to the substrate normal direction, the optical device is efficiently Wherein the chromatography is can oscillate, the laser oscillation apparatus.
(2) The chiral liquid crystal is a nematic liquid crystal material having no optically active site represented by (Chemical Formula 4) (where n and m are 1 to 5), and the chiral agent is The laser oscillation device according to (1), characterized in that it is a compound (i) or (ii) of the enantiomer compound represented by (Chemical Formula 5).




(3) The laser oscillation device according to (1) , wherein the luminescent organic dye contains a compound represented by (Chem. 6).

Here, chiral liquid crystals that can be used in the present invention are cholesteric (chiral nematic) liquid crystals having a helical structure of molecules, ferroelectric, antiferroelectric (chiral smectic) liquid crystals, blue phase liquid crystals, and twist grain boundary liquid crystals. is there. For example, when a cholesteric liquid crystal is sandwiched between rubbed substrates, the molecular helical axes are aligned perpendicularly to the substrate in a self-organized manner, and parallel layers are aligned in parallel at a helical pitch interval to obtain planar alignment (granularity). Gen organization). Since the refractive index periodically fluctuates along the helical axis of the liquid crystal molecules, when the helical pitch is about the wavelength, light interference occurs and specific light that satisfies the Bragg reflection condition is selectively reflected. In other words, a chiral liquid crystal forms a spiral structure that acts as a kind of resonator, and oscillates a single wavelength laser beam. Such chiral liquid crystals are known per se, and are described in technical literature including, for example, cholesterol derivatives, compounds having optically active sites, and mixtures thereof (Non-Patent Document 10, Non-Patent Document 11, Non-patent document 12). As a selection condition for the chiral liquid crystal and the light emitting material, it is necessary that the selective reflection band of the chiral liquid crystal overlaps the light emitting band of the light emitting material. When the reflection band of the chiral liquid crystal and the emission band of the light emitting material are overlapped and excited, a resonance state of light emission is induced, and an effect that can oscillate a single wavelength laser can be obtained.

C. J. et al. Booth, “Handbook of Liquid Crystals, Vol. 2A”, D.C. Demus, J.M. Goodby, G. W. Gray, H.M. -W. Spiss, V. Vill, eds. , Wiley-VCH, Wienheim, (1998), pp. 303-334. Liquid Crystal Handbook, Liquid Crystal Handbook Editorial Committee, Maruzen Co., Ltd., Tokyo, (2000), pp. 280-312. Fundamentals and Applications of Liquid Crystals, Shoichi Matsumoto, Ryoko Kakuda, Industrial Research Society, Tokyo, (1996), pp. 149-312.

  The chiral agent used in the present invention is a compound that induces a cholesteric liquid crystal. For example, a cholesteric liquid crystal can be adjusted by adding a chiral agent to an achiral (no optically active site) nematic liquid crystal. it can. As the chiral agent, those disclosed in JP-A No. 2002-180051, JP-A No. 2002-179682, JP-A No. 2002-179670 may be used. Examples of the chiral agent also include a photoreactive chiral agent. A photoreactive chiral agent has a chiral site and a photoreactive site that changes its structure by light irradiation. For example, the helical twisting power (HTP: Helical Twisting Power) of the chiral liquid crystal depends on the amount of irradiation light (irradiation time). It is a compound that changes greatly. In order to increase the helical structure inducing force by light irradiation, a material having a large degree of structural change by light irradiation is preferable. In addition, when the structure of the photoreactive chiral agent has one or more polymerizable bonding residues, the heat resistance of the chiral liquid crystal phase is improved.

  Examples of the light emitting material that can be used in the present invention include a light emitting organic dye, a polymer light emitting material, an inorganic light emitting semiconductor material, and the like, and any of them can be used without any particular limitation. In particular, as described above, it is preferable that a preferable material has an emission band in the ultraviolet, visible, and near-infrared wavelength band and overlaps with a reflection band of chiral liquid crystal.

  Among them, as the luminescent organic dye, known fluorescent dyes used for dye lasers can be used. Examples of such a dye include DCM (4-dicyanomethylene-2-methyl-6- (p-dimethylaminostyryl) -4H-pyran), pyromethane, coumarin, rhodamine, oxazine, pyridine, fluorescein, kitten red, And derivatives thereof. For these dyes, commercially available materials can be used.

  Furthermore, as the light emitting material, a polymer material can be used. Examples of the polymer light emitting material include a polymer to which a fluorescent dye is added and a π electron conjugated polymer, and many of them are known in various literatures, and any of them can be used. For example, polystyrene, polymethacrylate, polyacrylate, polyimide, polyamide, polysilicon, cellulose, DNA and derivatives thereof to which the fluorescent dye is added as a fluorescent dye-added polymer, and polypyrrole, polythiophene, -P-phenylene, poly-p-phenylene vinylene, polydialkylfluorene, and derivatives thereof.

Also, inorganic luminescent materials are well known in various literatures and can be used in the present invention. Inorganic light-emitting materials are generally durable and are particularly preferable as the light-emitting materials used in the present invention without being altered by long-term use. Examples include zinc oxide, zinc selenide, zinc sulfide, cadmium selenide, cadmium sulfide, lead sulfide, lead selenide, gallium nitride, silicon, gold, silver, etc. Or rod-like substance.

  These light-emitting materials are used in an appropriate amount, but in the case of organic dyes, they can usually be mixed in the liquid crystal material by about 0.3 to 1.0% by weight.

  In addition to the above-mentioned chiral liquid crystal material, chiral agent, and luminescent organic dye, the laser oscillation device of the present invention includes a polymerizable monomer, a polymerization initiator, a binder resin, a solvent, a surfactant, and a polymerization prohibition as necessary. Other components such as an agent, a thickener, a dye, a pigment, 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, when applying a liquid coating type liquid crystal composition to form a liquid crystal in a layered state, the alignment state at the air interface on the surface of the liquid crystal layer can be controlled sterically by the action of the surface activity, and a highly selective reflection wavelength can be obtained. Obtainable.

  Until now, most laser oscillations using chiral liquid crystals have been reported to be excited by linearly polarized light, and few studies have been conducted using circularly polarized light excitation as in the present invention. According to the present invention, it has been clarified that controlling the circular polarization state of the excitation light is extremely lower than the threshold value of the optical excitation energy required for conventional laser oscillation, and its significance is extremely large.

  That is, according to the present invention, it is possible to induce laser oscillation by continuous light excitation with respect to an organic chiral liquid crystal. By appropriately selecting the circular polarization state of the excitation light, the luminescent chiral liquid crystal This makes it possible to induce laser oscillation efficiently.

  Furthermore, according to the present invention, the laser oscillation device of the present invention is not easily affected by an external environment such as heat, so that the laser oscillation operation can be performed even in a poor environment. Since no special external resonator is required, miniaturization is easy and integration with other optical elements and combination are possible. Of course, in addition to being freed from maintenance and inspection such as fine adjustment of external resonator mirrors for laser oscillation, the production process is simple, and a number of functions and effects are expected, such as enabling power saving and space saving design. , Which has advantages.

  Hereinafter, the present invention will be described in detail with reference to examples and drawings. However, these examples are for specifically explaining the present invention and are not intended to limit the present invention.

  FIG. 1 shows the chemical structural formula of each material including chiral liquid crystal used in designing a laser oscillation optical device used in the examples of the present invention, of which (a) is chiral liquid crystal (Dainippon Ink) The structural formula of the company name DON-103) is shown in the text (Chemical Formula 1) and (Chemical Formula 4). (B) shows a structural formula of a chiral agent (trade name R-811 or S-811 manufactured by Dainippon Ink Co., Ltd.) and is represented by (Chemical Formula 2) and (Chemical Formula 5) in the text. Moreover, (c) shows the chemical structural formula of an organic fluorescent material (Nile Red: Tokyo Chemical Industry Co., Ltd.), and is represented by (Chemical Formula 3) and (Chemical Formula 6) in the text.

FIG. 2 shows an optical measurement experimental apparatus composed of various optical instruments used in the examples of the present invention. The laser oscillation optical device (hereinafter referred to as an optical device) manufactured in the examples of the present invention is shown in FIG. Among these, 9 is shown. FIG. 3 shows a circularly polarized light transmission spectrum (a) and a circular dichroism spectrum (b) of the optical device 9 designed in the embodiment of the present invention. Further, FIG. 4 shows a reflection spectrum (a) and a circularly polarized laser emission spectrum (b) of the optical device 9. The inset in FIG. 4B shows a laser emission spectrum in which the wavelength range from 602 to 608 nm is expanded. Furthermore, FIG. 5 shows (a) the elliptical angle (F _Ex ) of the excitation light is −45 ° (●: equivalent to left circularly polarized light), 0 ° (▲: equivalent to linearly polarized light), + 45 ° (black square ■ (Hereinafter referred to as black square): equivalent to right circularly polarized light), each intensity change of light emission of the optical device with respect to the energy of the Nd: YAG laser light, and (b) excitation light of photoexcitation energy required for laser oscillation The dependence of the elliptical angle (F _Ex ) is shown respectively.

[Liquid Crystal Cell = Production of Optical Device]
As the liquid crystal material, a compound having the structural formula shown in FIG. 1A (DON-103 manufactured by Dainippon Ink Co., Ltd., trade name) was used in this example. Further, as the chiral agent, a compound having a structural formula shown in FIG. 1B (R-811 manufactured by Dainippon Ink Co., Ltd.) was used. In this example, 23 wt.% R-811 was added to DON-103 to prepare a chiral liquid crystal phase exhibiting selective reflection in the visible region and having a right molecular helix. The helical twisting power (β) of R-811 against DON-103 was 11.0 (μm wt%) ⁻¹ . A known fluorescent material can be used as the fluorescent material. In this embodiment, Nile red (Nile red) having the structural formula shown in FIG.
Red) was mixed at a ratio of 0.4 weight percent with respect to the previously prepared chiral liquid crystal to obtain a fluorescent dye-containing chiral liquid crystal. However, the liquid crystal, chiral agent, and fluorescent material are examples, and the present invention is not limited to these materials.
The liquid crystal cell was produced using two glass substrates. A 1 weight percent aqueous solution of polyvinyl alcohol (PVA) was spin-coated on a glass substrate to promote a rubbing treatment in a uniaxial direction. Two glass substrates subjected to rubbing treatment were sandwiched between 7.5 μm polyimide spacers to prepare empty cells, and the previously prepared chiral liquid crystal mixture was enclosed. The fluorescent dye-added chiral liquid crystal cell thus obtained was used as an optical device.

[Measurement method of luminescence obtained from optical devices]
The measurement of luminescence obtained from the optical device produced as described above was examined by the optical measurement apparatus 1 shown in FIG. That is, as shown in FIG. 2, the second harmonic of the Nd: YAG laser 2 is used as the light excitation laser. From this light excitation laser, the output is 2000 nJ or less, the pulse width is 3 nsec, the pulse frequency is 5 Hz, and the wavelength. A laser beam of 532 nm was emitted. Next, the intensity of the laser light emitted from the laser for photoexcitation is adjusted by the dichroic mirror 3 and the light intensity adjusting element (manufactured by Sorlab, ND filter 4, λ / 2 wavelength plate 5, Grand laser linear polarizer 6). The optical device 9 was introduced through the λ / 4 wavelength plate 7 and the convex lens 8. The circular polarization state of the excitation light was controlled by adjusting the angle between the fast axis (or slow axis) of the λ / 4 wavelength plate and the polarization axis of the linear polarizer. The laser beam from the laser for photoexcitation was condensed at about 200 μm on the optical device 9 by the convex lens 8 (f = 80 mm) and made incident at 45 degrees with respect to the substrate normal of the optical device. And the light emission of the normal direction of the optical device 9 was condensed on the spectroscope 10 (Ocean Optics company make, USB2000 or HR4000) through the two convex lenses, and the emission spectrum was measured. FIG. 2 shows the structure based on the arrangement of the optical measuring devices set in this measurement experiment. The results measured by this optical measuring device are shown in FIGS. 3A, 3B, 4A, and 4B, respectively. As a result, it has been clarified that the optical device 9 set in the example shows a circularly polarized light transmission spectrum shown in FIG. 3A and a circular dichroism spectrum shown in FIG. Moreover, it was clarified that the optical device 9 shows the reflection spectrum shown in FIG. 4A and the circularly polarized laser emission spectrum shown in FIG. The inset in FIG. 4B is a laser emission spectrum in which the wavelength range from 602 to 608 nm is expanded. According to this, it became clear that the optical device set in the example oscillates laser light having a single wavelength characteristic at 605 nm.

[Dependence of optical excitation energy required for laser oscillation of optical device on circular polarization]
Next, the circular polarization dependence of the optical device 9 was examined closely. That is, the elliptical angle of the excitation light (Φ
_The change in light emission intensity of the optical device with respect to the energy of the Nd: YAG laser beam when _EX ) is −45 °, 0 °, and + 45 ° was examined in detail. As a result, a result as shown in FIG. 5A was obtained. In either case, when the optical excitation energy is increased, the emission intensity increases several thousand times and the spectral line width sharply decreases from 70 nm to 0.8 nm. However, it should be noted here that optical excitation required for laser oscillation is important. It was found that the energy (threshold value) strongly depends on the circular polarization state of the excitation light. The threshold value is 35 nJ / pulse when the elliptical angle (Φ _EX ) is −45 ° (●: equivalent to left circularly polarized light), 125 nJ / pulse when + 0 ° (▲: equivalent to linearly polarized light), + 45 ° (+ 45 °) Black square: equivalent to right-handed circularly polarized light) and 700 nJ / pulse. FIG. 5B shows the dependence of the excitation energy required for laser oscillation on the elliptical angle (Φ _EX ) of the excitation light. It was found that the photoexcitation energy required for laser oscillation can be controlled by changing F _Ex from + 45 ° to −45 °. When the elliptical angle (Φ _EX ) is −45 °, that is, when the light is excited by left circularly polarized light, the threshold value (35 nJ / pulse) corresponds to 50 kW / cm ² when converted to peak power. Compared to reports on laser oscillation using, it was about an order of magnitude lower. The results shown above are obtained when a right spiral chiral liquid crystal is used. If a left spiral chiral liquid crystal prepared by addition of S-811 which is an enantiomer of R-811 shown in FIG. Efficient laser oscillation with polarized light excitation. Therefore, considering the molecular helical direction of the chiral liquid crystal and the circular polarization direction of the excitation light, if the optical excitation is performed with an appropriate circular polarization, a distributed feedback (DFB) effect can be realized with high efficiency, and a laser is emitted from the chiral liquid crystal at the reflection band edge. It was revealed that it was possible to oscillate.

  The present invention is an optical device that oscillates laser with high efficiency by adding a luminescent material to chiral liquid crystal and photoexciting with circularly polarized light in the direction opposite to the molecular hand, and the threshold of photoexcitation energy is the excitation light. Depending on the circular polarization of the liquid crystal, it was found that the excitation energy shows a minimum when circularly polarized light was excited in the direction opposite to the molecular helix of the liquid crystal. The present invention can realize high-efficiency laser oscillation, and can be applied to a novel single minute light source, an optical amplifier, a low-threshold laser oscillation device, and a high-luminance display in an optical integrated circuit. It is expected to be used greatly in various fields.

The figure which shows each structural formula of the chiral liquid crystal material (a), the chiral agent (b), and organic luminescent material which were used in the Example of this invention. The figure which shows the optical measurement experiment apparatus used in the Example. Circular polarization transmission spectrum (a) and circular dichroism spectrum (b) of the optical device 1. The figure which shows the reflection spectrum (a) and circularly polarized laser emission spectrum (b) of the optical device 1. FIG. The inset in (b) shows a laser emission spectrum in which the wavelength range from 602 to 608 nm is expanded. (A) The elliptical angle (F _Ex ) of the excitation light is −45 ° (●: equivalent to left circularly polarized light), 0 ° (▲: equivalent to linearly polarized light), + 45 ° (black square: equivalent to right circularly polarized light) The figure which showed the intensity change of the light emission of the optical device with respect to the energy of Nd: YAG laser light at the time, and (b) dependence of the elliptical angle ( _FEx ) of the excitation light of the optical excitation energy required for laser oscillation, respectively.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical measuring device used when evaluating laser oscillation of liquid crystal cell 2 Excitation light source 3 Dichroic mirror 4 ND filter 5 λ / 2 wavelength plate 6 Linear polarizer 7 λ / 4 wavelength plate 8 Convex lens 9 Optical device 10 Fiber spectrometer

Claims (3)

A laser oscillation device comprising: an excitation light source; a polarizer capable of circularly polarizing light from the excitation light source; and an optical device capable of laser oscillation upon incidence of the circularly polarized light,
The optical device comprises a luminescent material added to a chiral liquid crystal that is a mixture of a liquid crystal material having no optically active site and a compound having an optically active site ,
The light emitting material is any one of a light emitting organic dye, a polymer light emitting material, or inorganic light emitting semiconductor fine particles,
The reflection spectrum of the chiral liquid crystal overlaps the emission spectrum of the luminescent material;
The circularly polarized light is in a direction opposite to the chirality of the chiral liquid crystal ;
A substrate normal direction 0 degrees or more, by Rukoto is photoexcited by circularly polarized light is incident at an incident angle of 45 degrees or less with respect to the substrate normal direction, said optical device is capable of laser oscillation with high efficiency A feature of a laser oscillation device . The chiral liquid crystal is a nematic liquid crystal material having no optically active site represented by (Chemical Formula 1) (where n and m are 1 to 5)), and the chiral agent is (Chemical Formula). 2. The laser oscillation device according to claim 1, wherein the laser oscillation device is any one of (i) and (ii) of the enantiomer compound represented by 2).





2. The laser oscillation device according to claim 1 , wherein the luminescent organic dye contains a compound represented by (Chemical Formula 3).