RU2341856C2 - Method of obtaining laser generation and laser element, controlled by electric field - Google Patents

Abstract

FIELD: laser technology.

SUBSTANCE: between two luminescent layers of "КЖК", a retarder controlled by an electric field is put. Feedback and consequently intensity of generation is determined by matching the phase between natural waves of the laser generation. Generation in the given configuration of the laser element occurs at the edge of the band gap, where there are different generation modes, distinguished by wavelength, and as a rule pumping threshold. All these modes have different polarisations. If there is a small change of phase delay between the "КЖК" layers, conditions for generation can be more favourable first for one mode, and then for the other, i.e. there can be swapping of intensity from one generation mode to another. That way change in phase delay, controlled by the electric field, leads to change in intensity and/or wavelength of the laser radiation.

EFFECT: smooth control of generation of laser radiation.

7 cl, 6 dwg

Description

The invention relates to laser technology, namely to liquid crystal dye lasers. In this case, chiral liquid crystals are used as a laser medium with distributed feedback. The invention can be used to create laser elements controlled by an electric field.

Chiral liquid crystals (CLCs) have a helicoidal structure, the pitch of which P _{0 is} comparable with the wavelength of light λ [1]. By chiral liquid crystals we mean actually liquid crystals, consisting of cholesterol derivatives, or nemato-cholesteric mixtures, consisting of a mixture of a nematic liquid crystal (NLC) and cholesteric liquid crystal (CLC), as well as chiral nematics, consisting of NLC and optically active compounds not necessarily having a liquid crystal phase. Due to the spatial modulation of the direction of the local orientation of the molecules (director or local optical axis), the light propagating along the axis of the helicoid diffracts on the periodic structure. According to Bragg’s law in a certain spectral range P ₀ n _⊥ <λ <P ₀ n _|| , where n _⊥ and n _|| are the main refractive indices of CLC, the light of one of the circular polarizations is completely reflected from the structure, i.e. its transmission is blocked. In other words, CLC is a one-dimensional photonic crystal with a forbidden energy band (stop band) for the propagation of photons of a certain polarization. This makes it possible to use FLC as a working medium in distributed feedback lasers. The wavelength of laser radiation is determined by the position of one of the edges of the stop band of the CLC, which should overlap with the luminescence spectrum of the dye.

Due to the lability of the liquid crystal structure, the helix pitch and, therefore, the radiation spectrum can be controlled by external influences of various nature, such as temperature [3], mechanical stress [4], light [5]. This opens up the possibility of creating a whole gamut of new miniature lasers, in particular, for integrated optics, communications, and optical computers.

Of greatest interest is the control of laser generation using an electric field. This is due to the fact that liquid crystal devices are controlled by low voltages and are characterized by low power consumption, which ensures good compatibility with integrated microelectronics.

A known method of producing laser radiation in a mirrorless element on a cholesteric liquid crystal with a luminescent dye dissolved in it using optical pumping [2]. CLC with dissolved dye is placed between two glass plates processed to achieve a homogeneous orientation of the CLC. The result is a defect-free, non-scattering, planar-oriented texture, with the axis of the helicoid directed perpendicular to the surfaces of the substrates. Such a texture selectively reflects one of the circularly polarized components of the incident light in the wavelength range that covers the luminescence spectrum of the dye. Another component of light with the opposite sign of circular polarization passes through the layer without distortion. Pumping is performed using a pulsed laser, the wavelength of which is within the spectral range of absorption of the dye. As a result, laser radiation arises at the edge of the CLC selective reflection zone, directed perpendicularly to the surfaces of the bounding glasses in opposite directions. Unfortunately, smooth control of the step of the cholesteric structure using an electric field in such a structure cannot be realized. Although it follows from thermodynamic consideration that each field strength corresponds to a certain spiral pitch [6], the topological restrictions imposed by the bounding walls do not allow this situation to be realized [7, 8]. Therefore, in laser experiments, usually only the repayment of the generation due to the violation of the spiral structure is observed [9]. In this case, the restoration of the disturbed structure after the field is turned off takes a long time (several hours).

The aim of the invention is to provide a method and device that allows you to smoothly control the generation of laser radiation on chiral liquid crystals using electric voltage.

This goal is achieved by the fact that instead of one layer of CLC, two layers of CLC are used, and a thin phase plate controlled by an electric field is inserted between them. In this case, the feedback in the laser element is controlled by an electric field applied to the phase plate due to a change in the phase shift between the natural waves of laser generation. The fact is that distributed feedback is present in each layer of QLC, but generation can occur only if the phases between the eigenwaves in the entire structure are consistent. The most favorable conditions for generation is a phase delay equal to zero or a multiple of 2π. In this case, the generation reaches its maximum value. If the phase delay is not equal to zero and not a multiple of 2π, then the lasing intensity decreases, and for some values of the phase delay lasing cannot occur. For example, when the phase delay is a multiple of π, the photon emitted in one layer of CLC, when it enters the other layer, CLC changes its circular polarization to polarization of opposite sign. Thus, it falls into the forbidden zone and cannot propagate; accordingly, generation does not occur.

If the phase delay changes in the region of zero values or multiples of 2π, then the complete suppression of generation may not be achieved. However, since the conditions for generation will be different for different generation modes at the edge of the GLC band gap, the intensity can be pumped from one generation mode to another. The fact is that generation, in this configuration of the laser element, occurs only at the edge of the forbidden zone, where the movement of photons of a certain polarization in both directions is allowed. Therefore, various lasing modes arise at the edge of the band gap, which differ in wavelength and, as a rule, in the pump threshold. In addition, these modes also differ in polarization, that is, they do not have circular circular polarization. With a small change in the phase delay between the QLC layers, the lasing conditions may be more favorable first for one mode, then for another.

The essence of the invention is illustrated in figures 1-6 and is illustrated by examples 1-2.

Figure 1 depicts a block diagram of a laser element and a method for producing laser radiation with a controlled radiation intensity and wavelength using a phase plate with a phase controlled by an electric field. 1, 2 - layers of a chiral liquid crystal doped with a luminescent dye; 3 - phase plate controlled by an electric field.

Figure 2 depicts a block diagram of a laser element on low molecular fatty acids with NLC as an electric field-controlled phase plate. 1, 1 '- layers of KZhK with a luminescent dye; 2, 2 ', 3, 3' - glass plates; 4, 4 ', 5, 5', - orienting layers; 6, 6 ', 8, - gaskets to provide the required layer thickness; 7 - layer of NLC; 9, 9 '- transparent conductive coatings, for example ITO; 10, 10 '- orientation layers for planar or homeotropic orientation of NLC.

Figure 3 depicts the absorption and luminescence spectra of the dyes PM-567 and Oxazin-17, the transmission curve of the fatty acids for Oxazin-17, as well as the stop band of the fatty acids for PM-567. Upper picture: D - absorption spectrum of RM-567, L - luminescence spectrum of RM-567, NI - spectral region of the stop band. Bottom figure: D is the absorption spectrum of Oxazin-17, L is the luminescence spectrum of Oxazin-17, T is the transmission curve of CLC.

Figure 4 depicts the generation spectra of the laser element on the dye Oxazin-17 for voltages 0 V and 5.2 V (main figure) and the dependence of the intensity of generation I _E on voltage (insert).

Figure 5 depicts the dependence of the phase delay on voltage (at a wavelength of 633 nm) for two planar NLC layers with a thickness of 5 and 10.4 μm.

6 depicts the generation spectra of a laser element on dyes PM-567 at various voltages on the NLC layer (0.2 V, 1.8 V, 8 V).

The essence of the method for controlling the intensity and wavelength of laser radiation is shown in Fig. 1 and consists in the fact that instead of one layer, two layers of CLC are taken (1, 2), and a phase plate 3 controlled by the electric field is installed between them. Thus, a change in the phase delay leads to a change in the intensity and, or wavelength of the laser radiation.

Figure 2 shows a block diagram of a laser element with two low molecular weight layers of fatty acids and a layer of fatty acids as a controlled phase plate. The laser element consists of two layers of fatty acids, doped with a luminescent dye (1, 1 '), bounded by glasses 2, 3 and 2' 3 ', on the inner surfaces of which are oriented coatings of polyimide 4, 5 and 4', 5 ', which are rubbed in one direction to create a uniform planar KZhK texture. The gap between glasses 2 and 3, as well as 2 'and 3' is set by gaskets 6 and 6 '. Between the glasses 2 and 2 'is placed a layer of NLC (7). The gap between glasses 2 and 2 'is defined by gaskets 8. On the outer surfaces of glasses 2 and 2', which bound the NLC layer, transparent, conductive coatings of In _x Sn _1-x O ₂ (9, 9 ') are applied. Orienting layers of polyimide are applied on top of the conductive layers 9 and 9 ', which are rubbed to create a homogeneous orientation of the NLC or chromium stearyl chloride to create a homeotropic orientation of the NLC (10, 10'). An electrical voltage is applied to the transparent electrodes 9, 9 '.

Example 1

As FFA indicated in FIG. 2 (1,1 '), a nematic mixture was used consisting of phenyl- and diphenyl-cyclohexanes compounds doped with an optically active additive (OAD) - levorotatory 4, 4'-di-leucine carboxybiphenyl with a concentration of 20 % (helix pitch P ₀ = 384 nm). Accordingly, the Bragg stop band occupies the region of 615 nm>λ> 580 nm, Fig. 3 (lower part, curve T). Oxazin-17 (manufactured by “NIOPIK”) with a concentration of 0.15% was taken as a dye added to the fatty acid mixture. The optical density and luminescence spectra of this dye for a 28-μm-thick QLC layer are shown in the same figure (curves D and L, respectively). A mixture of cyclohexylcarboxylic acids and phenylcyclohexanes with positive dielectric anisotropy (ε _// = 4.25, ε _⊥ = 2.75) and low optical anisotropy (n _{a =} n _|| -n _⊥ ; n _|| = 1,507 and n _⊥ = 1.451 at λ = 633 nm). The thickness of the NLC layer with an accuracy of ± 0.5 μm was 10 μm.

When this element is excited by a laser pulse at a wavelength of 532 nm, laser radiation arises directed perpendicular to the glass surface in both directions. The characteristic spectra of laser emission intensity (I _E ) at two voltage values applied to the NLC are shown in the main graph of Fig. 4, and the dependence of the maximum values of I _E on voltage is shown in the inset to Fig. 4. All curves are given at the same pump pulse energy, E = 52 μJ / pulse. The generation spectrum consists of one broadened line (Δλ = 3 nm at half maximum) with a maximum at a wavelength of λ _max = 617 nm, which does not depend on either the pump or the applied voltage. At the same time, the intensity of the emission depends on the voltage very much. At voltages U <3 V and U> 8 V, the generation intensity is maximum, and within the interval 3 V <U <8 V, it is almost completely suppressed. This dependence strictly correlates with the behavior of the phase delay of the NLC layer ΔФ (U), see the curve for d _N = 10.4 μm in Fig. 5. The generation maxima correspond to either ΔФ≈360 ° or ΔФ≈0 °, when the NLC acquires an almost homeotropic orientation and the birefringence disappears in a strong field. On the contrary, the suppression of generation at U≈5 V corresponds to a phase delay ΔФ≈180 °, when the NLC turns into a half-wave phase plate (d _N n _a = λ / 2).

Example 2

The same nematic mixture with the same OAD as in Example 1 was used as FFA, but the concentration of the latter was 20.6% (helix pitch P ₀ = 377 nm). PM567 (manufactured by Exciton) with a concentration of 0.47% was used as a dye. The spectra of optical density (D) and luminescence (L) of this dye for a layer thickness of 30 μm QLC are shown in the upper part of Fig. 3. The selective reflection band (stop band) of this mixture is between the vertical dotted lines, as shown in FIG. The location of the maximum of the dye luminescence spectrum to the left of the stop band provided a relatively rare possibility of controlling generation at the short-wave edge of the stop band. Here, we used the same mixture as the NLC for controlling the phase delay as in Example 1. The thickness of the NLC layer with an accuracy of 0.5 μm was 5 μm. Its phase delay depending on the voltage is shown in figure 5 (curve 5).

Laser generation in this cell has its own characteristics. First, the generation lines are observed at the short-wavelength edge of the stop band, where the luminescence intensity, which correlates with the light gain, is maximum. Secondly, here we observe the pumping of the emission intensity with a change in the voltage on the NLC layer, Fig.6. Before the reorientation of the NLC director (curve for U _rms = 0.2 V), the main generation line is located at λ = 564 nm with a satellite at 567.5 nm. With increasing voltage and decreasing phase delay (see the curve for d _N = 5 μm in FIG. 5), the entire lasing intensity goes into a band centered at λ = 567.5 nm.

Information sources

1. V.A. Belyakov, A.S. Sonin, Optics of cholesteric liquid crystals, Moscow, Science (1982).

2. L. S. Goldberg, J. M. Schnur, US Patent 3,771,065, class. 331 / 95.5 L, 11/06/1973.

3. K. Funamoto, M. Ozaki, and K. Yoshino Jpn. J. Appl. Phys. 42, L1523 (2003).

4. H. Finkelmann, S. T. Kim, A. Mucoz, P. Palffy-Muhoray, and B. Taheri, Adv. Mater. 13, 1069 (2001).

5. A. Chanishvili, G. Chilaya, G. Petriashvili, R. Barberi, R. Bartolino, G. Cipparrone, A. Mazzulla, R. Gimenes, L. Oriol, and M. Pinol, Appl. Phys. Lett. 86, 051107 (2005).

6. De Gennes, Sol. State Commun. 6, 163 (1968).

7. S.P. Palto, ZhETF, 121, issue 2, 309-319 (2002).

8. S.P. Palto and L. M. Blinov, J. Soc. Elect. Mat. Eng. 14, 115 (2005).

9. G. Strangi, V. Barna, R. Caputo, A. de Luca, C. Versace, N. Scaramuzza, C. Umeton, R. Bartolino, G. Price, Phys. Rev. Lett. 94. 063903 (2005).

Claims (6)

1. A method of obtaining controlled laser generation in a liquid crystal element, comprising exciting the liquid crystal element and controlling the intensity and wavelength of the laser radiation, characterized in that two or more layers of a chiral liquid crystal (QLC) are used as the liquid crystal element, between which at least at least one phase plate, and the intensity and wavelength of the laser radiation are controlled by an electric field applied to at least one phase plate. 2. A laser element based on a homogeneously oriented layer of a luminescent chiral liquid crystal, the luminescence spectrum of which covers the selective reflection zone of CLC, characterized in that two layers of luminescent CLC are used to control the intensity and wavelength of the laser radiation using an electric field, between which there is a phase plate controlled by an electric field. 3. The laser element according to claim 2, characterized in that a low molecular weight chiral liquid crystal is used as a chiral liquid crystal. 4. The laser element according to claim 2, characterized in that the polymer chiral liquid crystal is used as a chiral liquid crystal. 5. The laser element according to claim 2, characterized in that between the two layers of CLC there is a homogeneously oriented layer of a nematic liquid crystal (NLC) with positive dielectric anisotropy, controlled by an electric field using transparent electrodes deposited on the boundary layer of the NLC surface. 6. The laser element according to claim 2, characterized in that between the two layers of CLC there is a homeotropically oriented layer (NLC) with negative dielectric anisotropy, controlled by an electric field using transparent electrodes deposited on the surface bounding the NLC layer.

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