CN101867144A - Wavelength-tunable liquid crystal random laser device - Google Patents

Abstract

The invention discloses a wavelength-tunable liquid crystal random laser device. The wavelength-tunable liquid crystal random laser device comprises a pump light source, flat-plate photonic crystal liquid crystal micro resonant cavity, wherein the flat-plate photonic crystal liquid crystal micro resonant cavity is connected with the pump light source through a transmission optical fiber, and comprises a fiber core and a plurality of round air holes; the fiber core is arranged at the center of the flat-plate photonic crystal liquid crystal micro resonant cavity and is used for filling liquid crystal scattering media; the round air holes are arranged at the periphery of the fiber core; and the liquid crystal scattering media filled in the fiber core are nematic liquid crystals doped with a laser gain medium and scattering particles. The invention has the advantages that the wavelength is tunable, the width of generated laser emission spectral bands is narrow, and the quality factor which can reach above 10000.

Description

The liquid crystal random laser device of wavelengthtunable

Technical field

The invention belongs to laser technology field, specifically be meant a kind of liquid crystal random laser device of wavelengthtunable of wavelength-tunable.

Background technology

Since the nearly more than ten years, research field at random medium, people have repeatedly found high-intensity stimulated radiation, especially in the last few years, people such as the Cao of Northwestern Univ USA observe the stimulated radiation phenomenon in ZnO semiconductor random medium, and studied its optical property, think that the localization of laser phenomenon in the random medium and photon is closely related, and explained the characteristics of luminescence of random medium preferably with the annular chamber theory, this in highly unordered medium in stimulated radiation and the conventional laser stimulated radiation have differently significantly, people call accidental laser to the laser that utilizes this stimulated radiation to constitute.Because this laser has special physical mechanism, and numerous areas such as, nano photoelectronic devices integrated at photon, optical sensing, plane demonstration, the generation of special wave band laser, laser anti-counterfeit, medical imaging have broad application prospects.

The Random Laser radiation is under the excitation of pumping light, light is repeatedly formed closed-loop path at random after the scattering in the high density scattering object that adds the laser gain material, light path has changed the integral multiple of optical wavelength, because the multiple scattering between the scattering particles has prolonged the path of photon in gain media, thereby it is formed to obtain bigger gain.

By retrieval, concentrate on substantially on the ZnO medium at accidental laser at present, the accidental laser that is applied to for other media does not appear in the newspapers.

Summary of the invention

The objective of the invention is in order to overcome the shortcoming and defect that prior art exists, is scattering medium and the liquid crystal random laser device of realizing the wavelengthtunable of wavelength-tunable with the liquid crystal and provide a kind of.

For achieving the above object, technical scheme of the present invention is to include pump light source, it is characterized in that: also include the plate photonic crystal liquid crystal micro-resonant cavity that links to each other with Transmission Fibers with pump light source, described plate photonic crystal liquid crystal micro-resonant cavity includes the fibre core of the filling liquid crystal scattering medium at the center of being arranged at, and a plurality of round airport that is arranged at the fibre core periphery, the scattering liquid crystal medium of filling in the described fibre core is the nematic liquid crystal that is doped with gain medium and scattering particles.

The two ends up and down that further setting is described plate photonic crystal liquid crystal micro-resonant cavity are provided with electrode.

The periodic structure that is described round airport for forming on quartz substrate and by delta arrangement further is set.

Further being provided with is that described scattering particles are selected silicon ball particulate for use, and described gain medium is selected laser dye DCM and R6G for use.

The invention has the advantages that: the laser emission band width of generation, the quality factor height can reach more than 10000; Utilize liquid crystal to temperature and electric field change sensitivity,,, can realize the arrowband liquid crystal random laser output of the wavelengthtunable of temperature and electric field modulation again as the micro-resonant cavity of modulated laser both as the scattering medium that produces Random Laser.Simultaneously, because plate photonic crystal quality factor height, the laser volume can be very little, and application is wide.

Below in conjunction with specification drawings and specific embodiments the present invention is done further introduction.

Description of drawings

Fig. 1 structure principle chart of the present invention;

The plate photonic crystal liquid crystal of Fig. 2 micro-resonant cavity structural representation;

The plate photonic crystal liquid crystal of Fig. 3 micro-resonant cavity sectional view;

Fig. 4 is filled with the schematic diagram of the fibre core of nematic liquid crystal scattering medium.

Embodiment

Below by embodiment the present invention is carried out concrete description; only be used for the present invention is further specified; can not be interpreted as the qualification to protection range of the present invention, the technician in this field can make some nonessential improvement and adjustment to the present invention according to the content of foregoing invention.

Referring to the specific embodiment of the present invention shown in Fig. 1-4, include pump light source 1, this pump light source can adopt typical Nd:YAG laser, also include the plate photonic crystal liquid crystal micro-resonant cavity 2 that links to each other with Transmission Fibers 3 with pump light source, described plate photonic crystal liquid crystal micro-resonant cavity 2 includes the fibre core 21 of the filling nematic liquid crystal scattering medium at the center of being arranged at, and a plurality of round airport 22 that is arranged at fibre core 21 peripheries, be doped with gain medium 212 and scattering particles 213 in the nematic liquid crystal scattering medium 211 of filling in the described fibre core 21.The two ends up and down of the described plate photonic crystal liquid crystal micro-resonant cavity 2 of present embodiment are provided with electrode 23.In addition, described round airport 22 is on quartz substrate 24 and presses the periodic structure that delta arrangement forms that promptly per three round airports 22 are arranged by triangle and formed the periodic arrangement structure.

The present invention with nematic liquid crystal as scattering medium, gain medium (laser dye) is entrained in the liquid crystal material, and gain media is the absorptive pumping energy under the pump light excitation, has prolonged the gain length of radiation photon, reduce scattering loss, the laser emission behavior can occur.Liquid crystal has the optical characteristics of partial order, can reduce the threshold value of accidental laser, obtains the liquid crystal random laser emission of high Q value and narrow linewidth.Select for use Si ball particulate as scattering particles, laser dye DCM and R6G, in the nematic liquid crystal mixture that different liquid crystal monomers constitute, the Si ball and the laser dye of doping different proportion form the liquid crystal random medium.

In addition, it also is feasible also being filled with other liquid crystal materials different with nematic liquid crystal scattering medium 211 refractive indexes of filling in the fibre core 21 in the described round airport, and this measure can also realize recruiting of optical maser wavelength.The method of its filling liquid crystal adopts infusion process, soaks into liquid crystal random medium solution in the fibre core at center of the present invention, the airport, forms dissimilar plate photonic crystal liquid crystal microcavitys.

The application, the gain medium that mixes in liquid crystal can prolong the gain length of radiation photon, has reduced scattering loss, strengthened and pump light between coupling, the relevant distributed feed-back of photon is provided, reduced the threshold value of laser emission; Patent adopts plate photonic crystal defect micro-resonant cavity, nematic liquid crystal with doping laser dye and scattering particles is filled photonic crystal defect, forms the liquid crystal microcavity of light wave three-dimensional constraining, the laser emission band width of generation, the quality factor height can reach more than 10000; Utilize liquid crystal to temperature and electric field change sensitivity,,, can realize the adjustable arrowband liquid crystal random laser output of temperature and electric field modulation again as the micro-resonant cavity of modulated laser both as the scattering medium that produces Random Laser.Simultaneously, because plate photonic crystal quality factor height, the laser volume can be very little.

Provide mentality of designing of the present invention and theoretical principle below so that further set forth characteristics of the present invention.

Mentality of designing of the present invention is that plate photonic crystal has forbidden photon band, the slab photonic crystal that contains defective can form the slab photonic crystal micro-resonant cavity, light wave can be limited in the photonic crystal defect cavity, have less mode volume and bigger quality factor.If liquid crystal is filled in the plate photonic crystal defect cavity, can form the liquid crystal micro-resonant cavity, change the resonance wavelength that temperature and electric field can change the liquid crystal microcavity.The liquid crystal of doping laser dye and scattering particles is incorporated in the plate photonic crystal defect, resonance wavelength is among the photon band gap, periodic structure has on every side been carried out the reflection feedback to light, the light of the overwhelming majority strictly is limited within the point defect of microcavity, the edge of photon band gap just in time is in the spontaneous radiation spectral position of gain media, more counter-rotating particle is then arranged at the photon band gap edge, form liquid crystal random laser emission, such strong constraint can cause laser to produce the very big decline of threshold value, the pattern of liquid crystal Laser emission and wavelength can utilize the change of temperature or electric field controls liquid-crystal refractive-index to modulate, and realize the adjustability of liquid crystal random Laser emission.Laser dye is observed the Random Laser emission in containing the nematic liquid crystal solution of inorganic scattering object, in this system, because the anisotropic character of liquid crystal solution has strong scattering process to light, help Laser emission.When improving the liquid crystal solution temperature, laser threshold can increase even laser disappears, and optical maser wavelength also can change thereupon, thereby utilizes this method may modulated laser.In the laser dye gain media, add the fine particle scattering object and cause strong light scattering, increase exciting light random walk path, thereby surpassed gain length, finally realized Laser emission, this method has realized separating of gain media and scattering object, helps laser design.

Theoretical principle of the present invention:

1, the temperature of nematic liquid crystal and field effect

Fill nematic liquid crystal in the air column of fibre core, set up rectangular coordinate system, x and y axle are positioned at the optical fiber cross section, the z axle along fiber axis to, promptly axial along air column.The orientation of liquid crystal molecule in the optical fiber airport depends primarily on diameter capillaceous, boundary condition and extraneous parameter (as temperature, electric field etc.).When temperature or electric field change, the molecules align characteristic of liquid crystal will change, and respective indices of refraction will change, thereby the optical transmission property that causes liquid crystal to fill photonic crystal fiber changes, and shows as the temperature modulation photonic crystal fiber.When only considering influence of temperature variation, the orientation of liquid crystal molecule generally is the direction of principal axis along optical fiber, n _eAnd n ₀Be respectively the refractive index of non-ordinary light of liquid crystal and ordinary light, they are functions of temperature T and light wavelength lambda:

$n_0\left(T\right)=A-\mathrm{BT}-\frac{{\left(\mathrm{\&Delta;n}\right)}_0}{3}{(1-\frac{T}{T_C})}^{\mathrm{\&beta;}}(T<T_C)---\left(1\right)$

$n_e\left(T\right)=A-\mathrm{BT}+\frac{{2\left(\mathrm{\&Delta;n}\right)}_0}{3}{(1-\frac{T}{T_C})}^{\mathrm{\&beta;}}(T<T_C)---\left(2\right)$

$n_i\left(\mathrm{\&lambda;}\right)=A_i+\frac{B_i}{{\mathrm{\&lambda;}}^2}+\frac{C_i}{{\mathrm{\&lambda;}}^4}(i=0,e)(T<T_C)---\left(3\right)$

n(T，λ)＝A-BT(T≥T _C) (4)

(Δ wherein _n) ₀The birefringence of liquid crystal when being temperature T=0K, T _CBe the phase point temperature of liquid crystal nematic phase-each homogeny phase, A, B are approximately constant, β and Cauchy coefficient A _i, B _i, C _iIt is the function of temperature.These parameters all depend on liquid crystal material, can obtain by the refractive index empirical curve of match liquid crystal.

Liquid crystal is as a kind of special photoelectric material, anisotropy with crystal, can produce birefringence, Bragg reflection, diffraction and rotation effect as crystal, also can produce hot light, electric light or magneto optical effect under outer field action, its molecularly oriented is orderly but the position is unordered.The optical axis direction of liquid crystal molecule can change with the difference of external condition temperature, shows various different optical characteristics.Because part liquid crystalline phase molecularly oriented and arrangement have the periodic structure of self-organizing, if in this liquid crystal, mix gain medium, can prolong the gain length of radiation photon, reduced scattering loss, strengthened and pump light between coupling, the relevant distributed feed-back of photon is provided, has helped the laser emission of low Fujian value and narrow linewidth.During variations in temperature, variation has taken place in the diffusion constant of liquid crystal sample, and variation has also taken place its radiation characteristic; Liquid crystal was become isotropic critical temperature by nematic phase near, diffusion constant reached maximum, and the laser emission threshold value just in time appears near this critical temperature; When temperature was lower than this threshold temperature, diffusion constant reduced gradually, laser emission occurred and its intensity obviously increases, and bandwidth obviously reduces; The gain saturation phenomenon appears when temperature is lower than threshold temperature at a lower temperature, this moment, diffusion constant was almost constant, and this temperature just in time is liquid crystal phase transition is the critical temperature of crystal phase, the physical mechanism that has shown the liquid crystal random laser emission that temperature is adjustable: the liquid crystal as scattering medium has different molecularly oriented and refraction index profile under different temperatures, present different liquid crystalline phases, variations in temperature has changed the scattering constant of scattering medium, thereby has changed the radiation characteristic at random that activates liquid crystal.

The Liquid Crystal Molecules Alignment direction changed when liquid crystal was applied electric field, and will the electrically conerolled birefringence phenomenon take place in liquid crystal according to the light of certain polarization direction incident is electro optic effect.Utilize the electro-optical characteristic of liquid crystal, thereby the arrangement control liquid crystal of adjusting the orientation of liquid crystal molecule or layer of liquid crystal molecule by the variation of extra electric field periodically dielectric structure reaches the purpose of control laser emission characteristic, and this is the basic functional principle of the adjustable liquid crystal laser of electric field.

For wavelength be λ, perpendicular to the incident light of liquid crystal layer, the ordinary light n in liquid crystal ₀With non-ordinary light n _eBetween the phasic difference that produces be:

δ＝2πLΔn/λ (5)

L is the thickness of liquid crystal layer in the formula, and Δ n is the equivalent birefringence of liquid crystal, and it mainly depends on voltage V and temperature T etc.When two polarizers of liquid crystal cell were vertically placed, the transmitance of incident light was:

$t={\sin }^2\frac{\mathrm{\&delta;}}{2}---\left(6\right)$

For acting on voltage certain on the liquid crystal, transmitance maximum in the time of can determining corresponding a certain wavelength by (5) and (6) formula; And when change in voltage, the wavelength of maximum transmission correspondence also changes thereupon, thereby realizes the filter function of voltage tuning optical wavelength.The equivalent birefringence of liquid crystal is:

$\mathrm{\&Delta;n}\left(V\right)=\frac{1}{L}{\&Integral;}_0^{L}n\left(\mathrm{\&theta;}\right)\mathrm{dz}-n_0---\left(7\right)$

Wherein $n\left(\mathrm{\&theta;}\right)=\frac{n_0{n}_e}{\sqrt{{n_0}^2{\mathrm{<figure-callout\; id="2"\; label="cos"\; filenames="HSA00000122148600011.png"\; state="\{\{state\}\}">cos</figure-callout>}}^2\mathrm{\&theta;}+{{\mathrm{<figure-callout\; id="2"\; label="n\; e"\; filenames="HSA00000122148600011.png"\; state="\{\{state\}\}">n</figure-callout>}}_e}^2{\sin }^2\mathrm{\&theta;}}}---\left(8\right)$

The pass of the tiltangle of liquid crystal director n and position z is:

$\frac{\mathrm{d\&theta;}}{\mathrm{dz}}=-\frac{{\mathrm{\&epsiv;}}_0({\mathrm{\&epsiv;}}_{\&perp;}+\mathrm{\&Delta;\&epsiv;}{\sin }^2\mathrm{\&theta;})V}{d\sqrt{{\mathrm{\&epsiv;}}_0{\mathrm{\&epsiv;}}_{\&perp;}{k}_{11}}}\sqrt{\frac{C+{(1+\mathrm{\&gamma;}{\sin }^2\mathrm{\&theta;})}^{-1}}{1+k{\sin }^2\mathrm{\&theta;}}}---\left(9\right)$

γ=Δ ε/ε wherein _⊥, ε _⊥With Δ ε be the dielectric constant and the dielectric anisotropy of vertical liquid crystal molecular long axis, k=(k ₃₃-k ₁₁)/k ₁₁, k ₁₁And k ₃₃Be the exhibition song and the bend elastic constant of liquid crystal, C is an integral constant.

2, plate photonic crystal liquid crystal micro-resonant cavity

The most important characteristic of slab photonic crystal energy band diagram also is the basic element that it is different from common photonic crystal, is exactly light cone.In a homogeneous background zone, light cone is that simple wave vector is divided by refractive index.Concerning the background of one-period, light cone is exactly the low-energy zone of corresponding two-dimentional system.The above part of light cone line is a tunnelling ray, i.e. a continuity zone that comprises the possible frequency (leakage mode) of institute in the background.Any attitude that is arranged under the light cone all can not be coupled with the attitude of background, therefore, discontinuous band is a guided mode below the light cone, these attitudes are the lossless propagation of energy in plate level, but be exponential damping in the background area, this restriction and total reflection machine-processed similar, it be since in dull and stereotyped the effective refractive index of guided mode than height in background area (air).When guided mode arrived the light cone edge, small disturbance just can make it to be coupled in the background to go, and no longer is limited in the flat board though amplitude is very little.Delta arrangement air column structure has very large forbidden band, is a kind of important structure in the design of forbidden band.The dielectric constant that increases central defect forms defective, can increase the effective dielectric constant of photonic crystal, causes the portion of air band to enter the forbidden band and forms imperfect tape at high frequency region.And along with the increase of central defect dielectric constant, imperfect tape moves to low frequency range gradually, and defective mould centre wavelength just moves to the long wave direction gradually.Quality factor increases gradually along with the increase of central defect dielectric constant.Slab-thickness to a certain degree determines whether there is band gap in the slab photonic crystal guided mode.Along with the increase of medium rod structure and airport structure slab photonic crystal thickness, its forbidden band edge frequency constantly descends, and energy gap increases afterwards earlier and reduces, and has maximum.The existence of slab photonic crystal optimum thickness is readily appreciated that: if flat board is too thick, the longitudinal mode of high-order only need spend very little energy and just can be established, and the energy of these patterns is higher than the lowest-order mould a little, has stoped the formation of band gap; If flat board is too thin, a perturbation on the background dielectric constant can only be provided, guided mode will exist, but their can be near the light cone edges and can only very faintly be directed, and any band gap all is very small.

Plate photonic crystal is the limited 2 D photon crystal of thickness, has forbidden photon band, introduces defective in photonic crystal, and the photon that matches with the defect state frequency can be formed the resonant cavity of high-quality-factor by the position of local in defective.Design chamber thickness has identical magnitude with wavelength, forms micro-resonant cavity.The model of plate photon crystal micro resonance cavity is formed by one section hollow photon crystal optical fiber, the photonic crystal center is the circle airport, the periphery is the periodic structure that all nicely rounded airport forms by delta arrangement on quartz substrate, in the airport of center or covering, can fill nematic liquid crystal, the identical magnitude of intercepted length with optical wavelength, the plate photonic crystal liquid crystal micro-resonant cavity of formation.Cross section and structural representation are as shown in Fig. 2 is.

3, scattering liquid crystal medium

Liquid crystal is a kind of anisotropic photoelectric material that has, and produces hot light, electric light or magneto optical effect under outer field action.Liquid crystal can undergo phase transition under different temperatures or electric field, and the variation of molecules align causes the change of its refraction index profile, and birefringent phenomenon takes place., as scattering medium gain medium (laser dye) is entrained in the liquid crystal material with liquid crystal, gain media is the absorptive pumping energy under the pump light excitation, has prolonged the gain length of radiation photon, has reduced scattering loss, the laser emission behavior can occur.Liquid crystal has the optical characteristics of partial order, can reduce the threshold value of accidental laser, obtains the liquid crystal random laser emission of high Q value and narrow linewidth.Select for use Si ball particulate as scattering particles, laser dye DCM and R6G, in the nematic liquid crystal mixture that different liquid crystal monomers constitute, the Si ball and the laser dye of doping different proportion form the liquid crystal random medium.Adopt infusion process in photonic crystal center airport, covering airport and all airports, to soak into liquid crystal random medium solution, form dissimilar plate photonic crystal liquid crystal microcavitys.

Claims (4)

1. the liquid crystal random laser device of a wavelengthtunable, include pump light source, it is characterized in that: also include the plate photonic crystal liquid crystal micro-resonant cavity that links to each other with Transmission Fibers with pump light source, described plate photonic crystal liquid crystal micro-resonant cavity includes the fibre core of the filling liquid crystal scattering medium at the center of being arranged at, and a plurality of round airport that is arranged at the fibre core periphery, the scattering liquid crystal medium of filling in the described fibre core is the nematic liquid crystal that is doped with gain medium and scattering particles.

2. the liquid crystal random laser device of a kind of wavelengthtunable according to claim 1, it is characterized in that: the two ends up and down of described plate photonic crystal liquid crystal micro-resonant cavity are provided with electrode.

3. the liquid crystal random laser device of a kind of wavelengthtunable according to claim 1 and 2 is characterized in that: the periodic structure of described round airport for also forming by delta arrangement on quartz substrate.

4. the liquid crystal random laser device of a kind of wavelengthtunable according to claim 3, it is characterized in that: described scattering particles are selected silicon ball particulate for use, and described gain medium is selected laser dye DCM and R6G for use.

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