CN112198146A

CN112198146A - Up-conversion optical microcavity and application thereof

Info

Publication number: CN112198146A
Application number: CN202011058504.0A
Authority: CN
Inventors: 舒学文; 韩孟孟; 朱泽策
Original assignee: Huazhong University of Science and Technology
Current assignee: Huazhong University of Science and Technology
Priority date: 2020-09-30
Filing date: 2020-09-30
Publication date: 2021-01-08
Anticipated expiration: 2040-09-30
Also published as: CN112198146B

Abstract

The invention belongs to the technical field of optical micro-cavities, and particularly relates to an up-conversion optical micro-cavity and application thereof. The up-conversion optical microcavity comprises a cavity body, wherein the cavity body is prepared from a cavity body medium, the cavity body medium comprises a photosensitizer and an annihilation agent, the lowest excited state energy level of the photosensitizer is higher than the lowest triplet state energy level of the annihilation agent, and the annihilation agent can realize up-conversion through triplet state-triplet state annihilation. The invention makes full use of microcavity effect to make the exciting light generate total internal reflection on the interface of high and low refractive index media to form a whispering gallery mode, can fully increase the interaction between the exciting light and the up-conversion material, improve the utilization rate of the exciting light, provide enough singlet excited state photons for the ISC process of TTA up-conversion, and further effectively increase the quantum yield of up-conversion.

Description

Up-conversion optical microcavity and application thereof

Technical Field

The invention belongs to the technical field of optical micro-cavities, and particularly relates to an up-conversion optical micro-cavity and application thereof.

Background

TTA (triplet-triplet Annihilation) up-conversion is an anti-Stokes process performed by an energy donor (photosensitizer) and an energy acceptor (annihilator), and the light emission mechanism is the arrival of a singlet photosensitizer, which absorbs low-energy photons, through interstitial crossing (ISC)To its triplet state, and then energy is transferred to the annihilator by triplet-triplet energy transfer to make it transition to the triplet excited state; two triplet annihilators collide with each other, one transitioning to a singlet excited state and one relaxing to a ground state; the annihilator in the singlet excited state radiates fluorescence of a short wavelength while relaxing to the ground state. TTA systems can be at lower power densities than two-photon absorption and lanthanide step-absorption upconversion ((R))<100mW/cm²) Quantum yields higher than 10% are achieved. Therefore, TTA up-conversion has great application potential and research significance in the fields of solar energy utilization, photocatalysis, biological imaging and the like.

Currently, technologists design and synthesize new sensitizers by optimizing and modifying molecular structures and energy levels to improve the upconversion fluorescence efficiency. However, both of these studies neglect the effect of the optical microcavity on the up-converted fluorescence. The optical microcavity can limit photons in a very small space for a long time, greatly enhances the interaction between light and substances, and becomes an important platform for fundamental photophysics and photonics research. In the prior art, lanthanide series up-conversion materials are doped into micro-column or micro-sphere cavities, so that the utilization rate of exciting light can be increased, and the stimulated radiation efficiency can be improved (reference nat. nanotech.2018,13, 572-minus 577; ACS Nano 2017,11, 843-minus 849; ACS Photonics 2017,4, 1539-minus 1543 and the like). Therefore, a method of increasing the efficiency of excitation light utilization by using an optical microcavity, thereby increasing the upconversion efficiency, has attracted much attention from researchers.

Disclosure of Invention

Aiming at the improvement requirement of the prior art, the invention provides an up-conversion optical microcavity, and aims to provide a method for increasing the utilization rate of exciting light by arranging cavity structures with refractive index difference, so as to increase the up-conversion efficiency. The detailed technical scheme of the invention is as follows.

The up-conversion optical microcavity comprises a cavity body, wherein the cavity body is prepared from a cavity body medium, the cavity body medium comprises a photosensitizer and an annihilation agent, the lowest excited state energy level of the photosensitizer is higher than the lowest triplet state energy level of the annihilation agent, and the annihilation agent can realize up-conversion through triplet state-triplet state annihilation.

Preferably, the cavity medium comprises one of polymer and gel, and the cavity has a definite shape, and the definite shape is one of a circular ring, a cylinder, an ellipsoid, a hemisphere and a polygonal prism. The cavity has a definite shape, namely, the cavity is in a non-flowing state and can be stably formed in the air.

Preferably, the cavity medium is prepared by ultraviolet lithography or laser micromachining.

Preferably, the optical fiber further comprises an external cavity, the external cavity is made of an external cavity medium, the refractive index of the cavity medium is larger than that of the external cavity medium, a cavity is arranged in the external cavity, and the external cavity seals the cavity in the cavity.

Preferably, the external cavity medium is one of quartz, borosilicate glass and organic silicate glass, the external cavity is prepared by laser micromachining or chemical etching, and the cavity medium comprises one of polymer, gel and solvent. The chamber, when solvent, obviously does not have a definite shape and needs to be confined inside a sealed space by an external chamber.

Preferably, the photosensitizer has a structural formula of one of the following structural formulas:

preferably, the photosensitizer is one of the photosensitizers (1), (2), (3) and (4). Wherein (1) is abbreviated as PtOEP, (2) is abbreviated as BTZ-DMAC, and (3) is abbreviated as 2PF₂And (4) Pdph for short₄TBP。

Preferably, the structural formula of the annihilator is one of the following structural formulas:

preferably, the molar ratio of the photosensitizer to the annihilator is 1: (0.2-200).

The invention also protects the application of the optical microcavity in solar cells or photocatalytic organic synthesis.

The invention has the following beneficial effects:

(1) the invention makes full use of microcavity effect to make the exciting light generate total internal reflection on the interface of high and low refractive index media to form a whispering gallery mode, and the exciting light is transmitted in the cavity in the form of energy ring, compared with the exciting light which passes through the up-conversion material in the cuvette once, the interaction between the exciting light and the up-conversion material can be increased sufficiently, the utilization rate of the exciting light is improved, enough singlet excited state photons are provided for the ISC process of TTA up-conversion, and the quantum yield of the up-conversion is increased effectively.

(2) The cavity structure of the invention can be in a determined shape or an uncertain shape, is not limited by the physical state of the external cavity medium, can be selected from quartz, polymer and even gas low-refractive-index medium as the external cavity according to different requirements, and fully improves the integration and utilization efficiency of the up-conversion microcavity and other equipment.

(3) The invention utilizes the micro-processing technologies such as laser processing, chemical corrosion, ultraviolet lithography and the like to prepare the optical microcavity, effectively avoids the problem that the fluid structure and the flow rate are difficult to control in the micro-fluidic technology, and the quartz and polymer external cavity medium is easy to package and store.

(4) The invention can realize the high-efficiency up-conversion of near infrared light to visible light and visible light to ultraviolet light through the TTA process, and the performance of the invention ensures that the invention has huge application value and potential in the fields of solar cells, photocatalytic organic synthesis and the like, and has wide market prospect.

Drawings

Fig. 1 is a schematic structural diagram of embodiment 1 of the present invention.

Fig. 2 is a cross-sectional view corresponding to fig. 1 and an optical path diagram of excitation light.

FIG. 3 is a schematic structural diagram of a TTA upconversion optical microcavity fluorescence test system.

FIG. 4 is a geometrical optical path diagram of excitation light in a microcavity of a 200 μm quartz tube, wherein (a) in FIG. 4 is an optical path diagram of the excitation light tangentially incident to the inner wall of the cavity; fig. 4 (b) is an enlarged view of fig. 4 (a) within a dashed line frame.

FIG. 5 is a graph of fluorescence spectrum of upconversion microcavity as a function of power density, wherein (a) in FIG. 5 is a graph of fluorescence spectrum of upconversion in TTA inside microcavity of 200 μm inside diameter quartz tube in example 1; FIG. 5 (b) is a graph of TTA upconversion fluorescence spectrum in a 500 μm internal diameter polyethylene glycol diacrylate gel microcavity in example 2; FIG. 5 (c) is a graph showing the upconversion fluorescence spectrum on TTA in the microcavity of a 260 μm inner diameter glass tube in comparative example 1.

FIG. 6 is a TTA up-converted blue fluorescence image of an optical microcavity. FIG. 6 (a) is a TTA up-converted blue fluorescence image of the microcavity of a 200 μm inner diameter quartz tube of example 1; FIG. 6 (b) is a TTA up-conversion blue fluorescence image of the 500 μm ID PEG-diacrylate gel microcavity of example 2; FIG. 6 (c) is a TTA up-converted blue fluorescence image of the 260 μm inner diameter glass tube microcavity of comparative example 1.

FIG. 7 is a graph of TTA up-conversion quantum yield of an optical microcavity as a function of excitation power density. FIG. 7 (a) is a graph of TTA up-conversion quantum yield as a function of excitation power density for a 200 μm inner diameter quartz tube microcavity in example 1; FIG. 7 (b) is a plot of TTA up-conversion quantum yield as a function of excitation power density for a 500 μm ID PEG-diacrylate gel microcavity in example 2; FIG. 7 (c) is a plot of TTA up-conversion quantum yield as a function of excitation power density for the 260 μm ID glass tube microcavity of comparative example 1.

The same reference numbers will be used throughout the drawings to refer to the same or like elements or structures, wherein: the device comprises a cavity 101, an external cavity 102, ultraviolet curing glue 103, a CW laser 201, a TTA up-conversion optical microcavity 202, an amplifying objective 203, a half-mirror 204, a CCD camera 205, a focusing lens 206 and a spectrum analyzer 207.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

Example 1

An upconversion optical microcavity, as shown in fig. 1-2, includes a cavity 101 and an external cavity 102, where the external cavity 102 encloses the cavity 101. The medium of chamber 101 comprised an equal volume of a mixed TTA upconverting toluene solution of 1mM BTZ-DMAC (photosensitizer (2)) and 5mM DPA (annihilator (9)), with a 1:5 molar ratio of BTZ-DMAC to DPA and a refractive index of 1.49. The matrix of the chamber 101 in this example is toluene, which is not of a definite shape and requires the outer chamber 102 to be sealed.

The external cavity 102 is made of quartz with a refractive index of 1.46, and the external cavity 102 is a cylindrical quartz capillary tube with an inner diameter of 200 μm and a length of 35mm, and is manufactured by laser micromachining. The tube is filled with the TTA upconversion solution using the capillary principle. Two ends of the capillary tube are packaged by self-made ultraviolet curing adhesive 103 so as to block the contact between TTA solution in the tube and air, and further prevent the quenching of triplet excitons by oxygen in the air.

Example 2

This embodiment is different from embodiment 1 in that an outer chamber is not included. The matrix of the cavity 101 is a polyethylene glycol diacrylate gel having a defined shape that is stable in air, as described below.

The cavity medium 101 is a TTA upconversion polyethylene glycol diacrylate gel comprising 0.2mM PtOEP (sensitizer (1)) and 20mM DPA (annihilator (9)) with a refractive index of 1.47. The cavity 101 is a cylinder with an inner diameter of 500 micrometers and a length of 1mm, which is manufactured by a femtosecond laser processing technology. The cavity 101 is surrounded by air, which has a refractive index of 1.0.

Comparative example 1

This embodiment is different from embodiment 1 in that the refractive index of the external cavity 102 is larger than the refractive index of the cavity 101, as described below.

Based on example 1, as shown in FIG. 1, the external cavity medium 102 was replaced with a cylindrical glass capillary having an inner diameter of 260 μm, the refractive index of the glass capillary was 1.51, and the cavity 101 was kept constant. The two sides of the glass capillary tube are kept packaged based on the self-made ultraviolet curing glue 103.

Test examples

1. Up-conversion quantum yield testing

The present invention utilizes the TTA upconversion fluorescence detection system shown in fig. 3 for a test upconversion quantum yield test. And the focused excitation light is quasi-up to a TTA up-conversion microcavity fixed on a two-dimensional translation stage. By means of the objective lens and the CCD camera, the position of the microcavity is carefully adjusted, when the incident angle of the exciting light outside the cavity is 16.86 degrees, the exciting light tangentially enters the inner wall of the cylindrical capillary tube, as shown in FIG. 4, after multiple total reflections of the exciting light on the inner wall of the microcavity, the exciting light is transmitted along the inner wall of the cavity in the form of an energy ring, so that the echo wall microcavity is formed, and the utilization rate of the exciting light is improved. In addition, although part of the excitation light finally escapes from the microcavity after multiple reflections in the cavity, the interaction field of the excitation light and sensitizer molecules is increased, and the absorption efficiency of the excitation light is further enhanced.

The results of the upconversion quantum yields for the various examples are set forth in table 1.

TABLE 1 table of parameters and test results of the examples of the present invention

2. Fluorescence spectroscopy test

FIG. 5 is a TTA upconversion fluorescence spectrum in an optical microcavity, wherein (a) in FIG. 5 is a TTA upconversion fluorescence spectrum in a microcavity of a 200 μm inner diameter quartz tube in example 1; FIG. 5 (b) is a graph of TTA upconversion fluorescence spectrum in a 500 μm internal diameter polyethylene glycol diacrylate gel microcavity in example 2; FIG. 5 (c) is a graph showing the upconversion fluorescence spectrum on TTA in the microcavity of a 260 μm inner diameter glass tube in comparative example 1. The spectral intensities of all three examples gradually increased with increasing excitation power density. From the comparison of example 1 and comparative example 1, it can be seen that the same sensitizer and annihilator molecules match the anti-stokes shifts of both examples, both at 89 nm. However, since the external cavity medium of comparative example 1 is glass with a refractive index of 1.51, and does not support the full emission of excitation light at the inner wall of the microcavity, the spectral intensity is lower than that of example 1 at the same pumping power density. From the comparison of example 2 and comparative example 1, both achieve an anti-stokes shift of 89nm with the same annihilating agent (DPA), but different sensitizers result in down-converted fluorescence at different wavelengths, 652nm and 645nm, respectively. Comparing example 1 with example 2, it can be seen that the photon diffusion speed in the cavity gel medium of example 2 is relatively slow, which limits the electron transfer during the TTA upconversion process, so that the upconversion blue fluorescence intensity is lower than that of example 1 under the same excitation power density.

3. And testing blue fluorescence images.

FIG. 6 is a TTA up-converted blue fluorescence image of the microcavity after passing through a 400-500nm filter, collected by a CCD camera, wherein (a) in FIG. 6 is the TTA up-converted blue fluorescence image of the microcavity of the 200 μm inner diameter quartz tube in example 1; FIG. 6 (b) is a TTA up-conversion blue fluorescence image of the 500 μm ID PEG-diacrylate gel microcavity of example 2; FIG. 6 (c) is a TTA up-converted blue fluorescence image of the 260 μm inner diameter glass tube microcavity of comparative example 1. It can be known from the

embodiments

1 and 2 that the fluorescence images of the two have clear linear upper and lower boundaries, that is, the interface between the cavity medium and the external cavity environment has uniform intensity distribution, which fully indicates that the up-conversion fluorescence uniformly exists on the microcavity interface, thereby proving that the excitation light is fully emitted on the inner wall of the cavity to form the optical whispering gallery mode. The exciting light of the whispering gallery mode is transmitted on the inner wall of the cavity in the form of an energy ring, so that the utilization rate of TTA solution to the exciting light is increased, and the up-conversion quantum yield can be effectively improved. Unlike examples 1 and 2, the up-converted blue fluorescence image of comparative example 1 has neither sharp linear upper and lower boundaries nor uniform intensity distribution in the radial direction. This is because: the refractive index (1.51) of the external cavity is greater than the refractive index (1.46) of the toluene solution of the cavity TTA, and the total reflection condition is not satisfied, so that the excitation light does not form an energy ring in a whispering gallery mode in the glass capillary, and the up-conversion blue fluorescence is not uniformly generated on the inner wall of the cavity.

4. And testing the relation of the up-conversion quantum yield with the change of the excitation power density.

FIG. 7 is a plot of upconversion quantum yield as a function of increasing excitation power density for an optical microcavity, where (a) in FIG. 7 is a plot of the upconversion quantum yield as a function of increasing excitation power density for a TTA microcavity for a 200 μm inner diameter quartz tube of example 1; FIG. 7 (b) is a plot of TTA up-conversion quantum yield as a function of excitation power density for a 500 μm ID PEG-diacrylate gel microcavity in example 2; FIG. 7 (c) is a plot of TTA up-conversion quantum yield as a function of excitation power density for the 260 μm ID glass tube microcavity of comparative example 1. The highest quantum yields for examples 1, 2 and comparative example 1 were 27.7%, 21.4% and 16.1%, respectively. Wherein, 27.7 percent is the highest quantum yield of the organic sensitizer based on the thermal delay fluorescent molecules at present, and is improved by nearly 15 times compared with the 1.9 percent quantum yield of BTZ-DMAC measured in a cuvette (J.Mater.chem.C 2017,5, 12674-12677). Comparing example 1, example 2 and comparative example 1, it can be seen that the quantum yield of comparative example 1 is less than the values of examples 1 and 2 because the refractive index of the outer cavity glass medium (1.51) is greater than the refractive index of the inner cavity TTA toluene solution (1.49), the total reflection condition is not satisfied, and the WGM microcavity effect does not work. However, comparative example 1 still gave a quantum yield of more than 1.9% in the cuvette because: although the excitation light does not form a full emission in the glass capillary cavity, multiple reflections by the inner walls of the cavity before escaping the cavity can still promote its interaction with the TTA up-conversion solution, increasing the singlet excitons of the sensitizer during ISC. Comparing example 2 with example 1, the quantum yield is less than that of example 1. Firstly, although the metal complex PtOEP is selected in the embodiment 2, the mobility of electrons in the gel medium as the cavity is far smaller than the rate in the solution, and the efficiency of each process in the TTA up-conversion process is seriously reduced; secondly, because the outside of the inner cavity of the embodiment is air, oxygen in the embodiment quenches part of triplet sensitized excitons, and the up-conversion quantum yield is reduced.

It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims

1. The up-conversion optical microcavity is characterized by comprising a cavity, wherein the cavity is prepared from a cavity medium, the cavity medium comprises a matrix photosensitizer and an annihilator, the lowest excited state energy level of the photosensitizer is higher than that of the annihilator, and the annihilator can realize up-conversion through triplet-triplet annihilation.

2. The optical microcavity of claim 1, wherein the cavity has a defined shape that is one of a ring, a cylinder, an ellipsoid, a hemisphere, and a polygonal prism.

3. The optical microcavity of claim 2, wherein the cavity medium comprises one of a polymer and a gel, and the cavity medium is fabricated by uv lithography or laser micromachining.

4. The optical microcavity of claim 1, further comprising an external cavity, the refractive index of the cavity being greater than the refractive index of the external cavity, the external cavity having a cavity therein, the external cavity sealing the cavity in the cavity.

5. The optical microcavity of claim 4, wherein the external cavity is made of an external cavity medium, the external cavity medium is one of quartz, borosilicate glass and organic silicate glass, the external cavity is made by laser micromachining or chemical etching, and the cavity medium comprises one of polymer, gel and solvent.

6. The optical microcavity of any one of claims 1-5, wherein the photosensitizer has a structural formula that is one of the following structural formulae:

7. the optical microcavity of claim 6, wherein the photosensitizer is one of (1), (2), (3), (4).

8. The optical microcavity of claim 6, wherein the annihilating agent has a structural formula of one of the following:

9. the optical microcavity of claim 8, wherein the molar ratio of the photosensitizer to the annihilator is 1 (0.2-200).

10. Use of an optical microcavity according to any one of claims 1-9 in a solar cell or photocatalytic organic synthesis.