TUNABLE OPTICAL MICROCAVITY FOR MODULATION AND GENERATION OF SPECIFIC RADIATION

Abstract

The present invention relates to a tuneable optical microcavity, characterised in that it comprises electrodes (12) on substrates (11), wherein the electrodes are comprised in the structure of dielectric or metal mirrors (13), or each of the electrodes has at least one dielectric or metal minor (13) on it, or the electrodes are semitransparent metal minors (13), wherein the mirrors are preferably located at a separation being a multiple of ½ lambda, where lambda is the central wavelength of the cavity mode, the cavity between the mirrors being filled with material (15) that changes the effective refractive index under the influence of external fields, preferably such as electric, magnetic field, thermal and mechanical stress.

Claims

1. An optical microcavity characterised in that it comprises electrodes (12) on substrates (11), wherein the electrodes are comprised (included) in the structure of dielectric or metal mirrors (13), or each of the electrodes has at least one dielectric or metal mirror (13) on it, or the electrodes are semitransparent metal mirrors (13), wherein the mirrors are preferably located at a separation being a multiple of ½ lambda, where lambda is the central wavelength of the cavity mode, the cavity between the mirrors being filled with material (15) that changes the effective refractive index under the influence of external fields, preferably such as electric, magnetic field, thermal and mechanical stress.

2. The optical microcavity according to claim 1 characterised in that the electrodes (12) are transparent for electromagnetic wave, preferably in the visible VIS and/or infrared IR and/or medium wavelength infrared MWIR ranges.

3. The optical microcavity according to claim 1, characterised in that the electrodes (12) are made of such material as indium tin oxide, conductive polymer, metal, or a combination thereof.

4. The optical microcavity according to claim 1, characterised in that the mirrors (13) are Bragg reflectors composed of multiple alternating layers of dielectrics with different refractive indices, and the optical thickness of the layers is ¼ lambda.

5. The optical microcavity according to claim 1, characterised in that the electrodes (12) included in the structure of metal or dielectric mirrors (13) or the dielectric or metal mirrors (13) are located at a separation from ½ lambda to 20 lambda, lambda being the central wavelength of the cavity mode.

6. The optical microcavity according to claim 1, characterised in that the material (15) is a liquid crystalline material in the isotropic phase, or in the nematic phase, or the cholesteric phase, or the blue phase, or the smectic phase, particularly in the SmC* and SmC*A phases, or a material exhibiting a Kerr or an analogous effect, as it is the case for mesogenic materials in the isotropic phase, or a polymeric composite material comprising a liquid crystal, and/or a luminophor, and/or a dye, /or nanoparticles, /or proteins.

7. The optical microcavity according to claim 1, characterised in that the substrate (11) can be transparent or non-transparent.

8. The optical microcavity according to claim 1, characterised in that it has the form of a flat-parallel cell.

9. The optical microcavity according to claim 1, comprising inside an electromagnetic wave emitter, preferably on the surface of one or two mirrors (13), or dissolved or suspended in a material (15) that fills the cell and changes the refractive index under the influence of physical fields, wherein the emitter emits an electromagnetic wave that matches the central wavelength of the cavity mode.

10. The optical microcavity according to claim 9, wherein the emitter of the electromagnetic wave is preferably selected from such as MoSe.sub.2, CdSe, WSe.sub.2, luminescent perovskite, nanodiamond, dye, luminophore or proteins.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] FIG. 1 shows the structure of a tuneable optical microcavity for modulation and generation of specific radiation. 11—optical substrate, preferably made of quartz glass, 12—typical position of the electrode, preferably transparent, preferably made of tin indium oxide, 13—semitransparent mirrors, preferably Bragg reflectors, 14—layer aligning the liquid crystalline material, preferably polyimide or polyamide layer.

[0033] FIG. 2 shows a simplified schematic diagram of a measurement system used for investigation of the optical microcavity filled with material that changes the effective refractive index under the influence of physical fields according to the present invention.

[0034] FIG. 3 shows the dispersion of the real part of electric permittivity of a 1999C liquid crystalline material. The squares correspond to the perpendicular component ε.sub.⊥(f), the diamonds correspond to the parallel component ε.sub.∥(f) of the real part of the dielectric constant as a function of frequency of the alternating electric field acting on a 1999C (DFNLC) dual frequency liquid crystalline material, measured at 21° C., frequency at the intersection point f.sub.C˜11 kHz.

[0035] FIG. 4 shows the energy position of the cavity mode for two perpendicular light polarizations and the Q-factor of the cavity as a function of voltage applied to the electrodes for a material with planar texture (HG).

[0036] FIG. 5 shows the dispersion dependence (the energy dependence on the light propagation angle for light coming out of the cavity) for a material with planar texture (HG).

[0037] FIG. 6 shows the angular distribution in x and y directions of the degree of sigma plus (positive values) and sigma minus (negative values) circular polarization for light transmitted through the microcavity for a material with planar texture (HG) directed at the angle of 45 degrees with respect to the TE and TM polarization axes.

[0038] FIG. 7 shows the energy position of the cavity mode for two perpendicular light polarizations and the Q-factor of the cavity as a function of voltage applied to the electrodes for a liquid crystalline material with homeotropic texture (HG).

[0039] FIG. 8 shows the dispersion dependence (the energy dependence on the light propagation angle for light coming out of the cavity) for a liquid crystal material with homeotropic texture (HG).

[0040] FIG. 9 shows a simulated distribution of electric field of 810 nm wavelength inside the microcavity filled with a liquid crystal with an emitter in the form of MoSe.sub.2 monolayers deposited on the surface of one of the mirrors (in the 0 nm position).

[0041] FIG. 10 shows the emission intensity from a liquid crystalline microcavity with MoSe.sub.2 monolayers as a function of voltage applied to electrodes of the structure.

[0042] FIG. 11 shows the luminescence spectrum from a +/−5 deg range from a cavity filled with a liquid crystal with HT texture, additionally doped with CdSe quantum dots for five different angles of rotation of the liquid crystal molecules from the growth axis: 0; 20; 30 and 40 degrees. The microcavity consists of two Bragg reflectors composed of 5 pairs of SiO.sub.2/TiO.sub.2 oxide layers each.

[0043] FIG. 12 shows the dependence of the refractive index n of CN liquid as a function of squared voltage U2 applied to the optical wedge filled with this liquid. The refractive index was determined based on the deviation of the direction of the laser beam propagation on a wedge filled with CN liquid. [Ewa Uliszewska, Diploma thesis “Badanie zjawiska odchylenia biegu promienia laserowego z wykorzystaniem efektu Kerra w klinie optycznym” (“A study on laser beam deflection using the Kerr effect in an optical wedge”) Military University of Technology, Faculty of New Technologies and Chemistry (WTC), 2015].

DETAILED DESCRIPTION OF THE INVENTION

[0044] FIG. 1 shows schematically the structure of an optical microcavity filled with a liquid crystal in the nematic phase (15). The mirrors (13) are dielectric Bragg reflectors fabricated for the lambda wavelength. For specific implementation, the energy of the lambda wave is shown in FIG. 4. A liquid crystal (15) with planar texture (HG), with the director (and thus the direction of the optical axis) parallel to the cavity plane was used. Under the influence of the voltage applied to transparent electrodes (12), preferably made of indium tin oxide (ITO), the liquid crystal director, here with a positive anisotropy of dielectric permittivity (15) rotates, striving for a direction perpendicular to the plane of the cavity. Thus, the splitting of the TE and TM cavity modes decreases in line with the applied voltage, as shown in FIG. 4 illustrating the position of the mode minimum, and in FIG. 5 illustrating the full dispersion dependence (energy-angle) for the energy of light coming out of the microcavity. FIG. 6 shows the angular dependence of the degree of circular polarization for light coming out of the microcavity.

[0045] According to another embodiment, a liquid crystal (15) with homeotropic texture (HT), with the director (and thus the direction of optical axis) perpendicular to the cavity plane, is used in an optical microcavity schematically illustrated in FIG. 1, filled with the liquid crystal (15), wherein mirrors (13) are dielectric Bragg reflectors fabricated for the lambda wavelength. When voltage is applied to transparent electrodes (12), preferably fabricated from indium tin oxide (ITO), the director of the liquid crystal (15) rotates striving towards the direction lying in the cavity plane. Thus, the splitting of the TE and TM cavity modes increases due to the applied voltage, as shown in FIG. 7 illustrating the position of the mode minimum, and FIG. 8 illustrating the full energy-angle dispersion dependence for the energy of light coming out of the cavity.

[0046] In yet another embodiment of an optical microcavity illustrated in FIG. 1, filled with a liquid crystal (15), the mirrors (13) are semitransparent metallic mirrors that at the same time can be used as electrodes (12).

EMBODIMENTS

Example 1

[0047] A microcavity was assembled based on two quartz glass substrates (QP). In the illustrated example a JG3 quartz glass with dimensions 12.7 mm×25 mm and initial thickness 5 mm was used. The glass was mechanically polished for an optical flatness better than lamda/14 (the central wavelength lambda=633 nm). The final thickness of the substrates was about 4 mm. The flat-parallelism of the substrate surfaces was better than 2 arcsec. The substrates were ultrasonically washed and placed in a working chamber of a vacuum system for deposition of metallic and dielectric layers. A transparent electrode in the form of a 30 nm thick indium tin oxide (ITO) layer with refractive index n.sub.ITO=1.890 (as measured for a wavelength Δ=632.8 nm) and specific resistance 100 Ohm/sq was deposited on a quartz substrate by vacuum evaporation in a controlled low-pressure oxygen atmosphere. The ITO deposition process was carried out at substrate temperature 200° C., at oxygen pressure of 7×10.sup.−5 mbar and with assistance of a XIAD ion source used to clean and degas the substrate surface, on which ITO was deposited and the ITO layer structure was formed.

[0048] A stack of dielectric layers was deposited on the surface of a transparent ITO electrode to form a so called Bragg reflector (DBR). In a specific embodiment, the reflector was composed of 6 pairs of TiO.sub.2 layers of a high refractive index for an electromagnetic wave (n.sub.TiO2=2.436 for lambda=0.6328 μm), deposited at 5×10.sup.−5 mbar oxygen pressure at temperature 250° C., and a SiO.sub.2 layer of a low refractive index (n.sub.SiO2=1.456 for lambda=0.6328 μm), deposited at 5×10.sup.−5 mbar oxygen, at temperature 200° C. During the deposition process of all layers a XIAD ion source was used for whipping the deposited layers with argon ions. The electrodes and DBR reflectors were finally shaped using suitably formed masks covering/uncovering areas of quartz substrates.

[0049] So prepared substrates were transferred to a spin coater where the DBR surface was spin coated with a PI solution (SE1211 polyimide from Nissan Chemicals, refractive index for lambda=633 nm, n.sub.PI=1.54). Then, the polyimide layer was dried (at 70° C.) and polymerized at elevated temperature (about 180° C.). A so prepared PI layer was rubbed with a specialised device with a roller covered with dedicated technical textile. An about 60 nm thick polyimide layer was finally obtained.

[0050] The PI layer ensures an almost normal to surface (homeotropic—HT) orientation of the director of the liquid crystalline structure (i.e., normal to surface orientation of the optical axis of the liquid crystalline medium that fills the cavity), and thus a normal to surface orientation of the optical axis of the liquid crystalline medium which is not exposed to an electric or magnetic field.

[0051] The QP substrates prepared with the ITO electrode, the DBR mirror and the PI orienting layer were subsequently assembled with the DBR mirrors facing each other so as to obtain a flat-parallel cell for a liquid crystalline material between the surfaces with ITO, DBR and PI (FIG. 1). To ensure identical separation between the QP surfaces, a few micrometer thick (thermosetting) adhesive line was applied on edges of one of the substrates using a fluid dispenser. Before the adhesive was applied, glass microroller spacers with a nominal diameter 0.9 μm were added to the adhesive. The assembled cell was subsequently compressed so as to have the QP surfaces settled on the glass rollers thus forming a cell with flat-parallel boundary walls. The flat-parallelism of the cell was controlled during the compression process by observation of possible interference fringes (a D line from sodium lamp was used for cell illumination). After suitable cell parameters were obtained during the compression process (thickness, flat-parallelism), the adhesive was cured (by warming up the entire cell up to the polymerization temperature of the adhesive).

[0052] In the line formed by the adhesive with spacers, a small inlet was left for filling the cell with a liquid crystalline material. To fill the cell with a liquid crystalline material, the cell was placed in an oven that allowed to obtain a low vacuum. The cell was laid so that the cell to be filled was placed in the vicinity of a special technological sponge, saturated with a liquid crystalline material. Then, the oven was evacuated and heated up to a temperature by 20° C. exceeding the temperature of the liquid crystalline material transition to the isotropic phase. After the cell was held at the aforementioned temperature and the low vacuum was maintained for a suitable time, the cell was moved so that the inlet for filling the cell was brought into contact with the liquid crystalline material filling the sponge. After the liquid crystalline material was drawn by the capillary action into the cell, the oven was slowly cooled down and filled with air.

[0053] The inlet for filling was cleaned and sealed with a special technological adhesive to prevent the material from flowing out from the cell and to prevent the contact of the liquid crystalline material with air.

[0054] Using an ultrasonic soldering iron and a special alloy with a low melting point, power supply cables were soldered to the ITO electrodes. The cables and cell joints were strengthened with special adhesives.

[0055] In the presented embodiment of microcavity, a liquid crystalline material labelled 1999C (marked herein as DFNLC), developed and produced at the Military University of Technology, Warsaw, Poland, was used. The material used is a dual frequency material, because in a wide temperature range, including room temperature, it shows a change in value, and even in sign, of the anisotropy of the electric permittivity as a function of frequency of the electric field affecting this material. This DFNLC property allows to induce a torque directing the DFNLC director to a position parallel to the direction of the electric field induced in the medium, when the electric field frequency is lower than the cross-over frequency, or a torque directing the director to a position perpendicular to the electric field, when the field frequency is higher than the cross-over frequency. The cross-over frequency is a frequency at which the anisotropy of electric permittivity is equal to zero, see FIG. 3. The cross-over frequency for the DFNLC working mixture is about 10 kHz.

[0056] The microcavity can be fabricated using various types of substrates, transparent or non-transparent (back side of the cell for a cell operating in the reflection mode). The DBR mirrors can be replaced with other types of mirrors, or the effect of the wave reflection at the interface between the media can be replaced by suitable selection of the refractive index of the medium filling the cell and the refractive index of the cell wall.

[0057] The DFNLC liquid crystal can be replaced with a typical nematic material, smectic material, isotropic liquid that changes its refractive index under the influence of temperature (preferably a mesogenic material at the temperature of the isotropic phase) or electric field, a composite material (preferably a polymeric-liquid crystalline, preferably comprising a material exhibiting the Kerr or an analogous effect, e.g., nanomaterial, e.g., graphene oxide, or graphene oxide decorated with a functional material) that changes its refractive index under the influence of physical fields.

TABLE-US-00001 TABLE 1 Selected material data for DFNLC liquid crystalline material. Material data for a DFNLC (1999C) liquid crystalline material Crystallization temperature T.sub.cr (° C.) <−20 Isotropization temperature T.sub.Iso (° C.) 146.4 Refractive index anisotropy Δn (589 nm) at 23° C. 0.33 Dielectric constant anisotropy for low (f << f.sub.c) frequency of the 3.19 external electric field Δ∈.sub.low (1 kHz) at 23° C. Dielectric constant anisotropy for high (f >> f.sub.c) frequency of the −2.91 external electric field Δ∈.sub.high (1 MHz) at 23° C. Cross-over frequency f.sub.c at 23° C. (kHz) 8.35 Cross-over frequency f.sub.c at 50° C. (kHz) 79.4

Example 2

[0058] A microcavity was assembled as in Example 1, wherein one Bragg reflector was coated with an emissive material in the form of a single MoSe.sub.2 layer, from both sides covered with a hBN (hexagonal boron nitride) layer with thickness 80 nm. The thicknesses of the hBN layers as well as the SiO.sub.2 and TiO.sub.2 layers were selected so that the MoSe.sub.2 emitter was located at the maximum of the electric field strength of a standing electromagnetic wave in the cavity, as shown in FIG. 9, which illustrates a simulated (with the transfer matrix method) distribution of electric field of 810 nm wavelength inside the microcavity filled with a liquid crystal with an emitter in the form of MoSe.sub.2 monolayers deposited on the surface of one of the mirrors (in the 0 nm position). The cavity thickness was 2.5 lambda (1255 nm for the central wavelength 810 nm and an average refractive index of liquid crystal 1.55). MoSe.sub.2 was excited with 523 nm light. When voltage was applied to microcavity electrodes, the MoSe.sub.2 emission line changed its energy in the range 780-810 nm, as shown in FIG. 10 illustrating the emission intensity from a liquid crystalline microcavity with MoSe.sub.2 monolayers as a function of voltage applied to electrodes of the structure.

Example 3

[0059] A microcavity was assembled as in Example 1 for the wavelength 635 nm, cavity thickness 1220 nm, filled with HT liquid crystal, wherein the liquid crystalline material was additionally doped with a material emitting light as a result of optical excitation. In this case the emitter was CdSe 3.3 nm quantum dots (Lumidot™ 640, from Sigma Aldrich) excited with 532 nm light. When voltage was applied to the microcavity electrodes and upon light excitation, the emission line changed its energy in the range 619-648 nm, as shown in FIG. 11 illustrating the luminescence spectrum from a +/−5 deg range for five different angles of rotation of molecules from the growth axis: 0; 20; 30 and 40 degrees. The microcavity with the central wavelength 635 nm consisted of two Bragg reflectors composed of 5 SiO.sub.2/TiO.sub.2 pairs.

Example 4

[0060] A microcavity was assembled as in Example 1, except that the microcavity was filled with a material exhibiting an analogous effect to the Kerr effect, maintaining the isotropic phase at room temperature, preferably a mesogenic material, for example a nematogen, preferably a liquid referred to as CN, developed and prepared at the Military University of Technology, with the composition as shown in Table 2.

TABLE-US-00002 Compound CN [00001] 60% [00002] 40% Clarification temperature [° C.] 13.7

[0061] Under the influence of the electric field, CN liquid changes the effective refractive index, which leads to the energy shift of the microcavity optical mode in both linear polarizations and thus the change in the angle of propagation of the monochromatic light transmitted by the sample.

[0062] FIG. 12. shows the dependence of the refractive index n for the He—Ne laser wave propagating along the direction of the electric field vector acting on CN liquid. Cavity thickness was 2.5 lambda (1255 nm for the central wavelength 810 nm and an average refractive index of liquid crystal 1.55).