A method for bidirectional tuning of wavelength in organic micro-lasers

Abstract

The application discloses a method for tuning the wavelength of an organic micro-laser in two directions, and belongs to the field of laser technology and photonics. The method for tuning the wavelength of the organic micro-laser by using the fluorescence resonance energy transfer (FRET) effect comprises the following steps: selecting a whispering gallery mode (WGM) micro-cavity as a resonant cavity for laser emission; uniformly doping a fluorescent substance in the WGM micro-cavity, so as to serve as a donor or acceptor for FRET; selecting another fluorescent substance as another donor or acceptor for FRET; and tightly combining the two substances by electrostatic adsorption. The concentration of the acceptor or donor molecules adsorbed on the surface of the micro-cavity is adjusted, the energy transfer efficiency from the donor to the acceptor is changed, and then the emission wavelength of the organic micro-laser is controlled in two directions.

Description

Technical Field

[0001] The invention relates to a technique for tuning the laser emission wavelength of an organic microlaser through fluorescence resonance energy transfer between two fluorescent substances, which belongs to the fields of laser technology and photonics. Background Technology

[0002] When light propagates along the inner wall of a geometrically symmetric boundary, continuous total internal reflection occurs, confining the beam to the annular boundary. This optical wave mode is called a Whispering Gallery Mode (WGM). If the optical path length of the beam traveling around the geometric boundary is an integer multiple of the wavelength, interference enhancement, or resonance, occurs. The annular structure used to confine the light field is called a Whispering Gallery Mode optical microcavity. Miniature lasers fabricated using these WGM microcavities possess advantages such as extremely high quality factor, extremely small mode volume, extremely low excitation threshold, and extremely strong evanescent field, showing broad application prospects in physical sensors, biological / chemical sensors, and the fabrication of miniature lasers.

[0003] Fluorescence resonance energy transfer (FRET) refers to the process by which electrons in a donor fluorophore, after being excited, transition to an excited state through radiative absorption. FRET occurs when the following conditions are met: the distance between the donor and acceptor molecules is less than 10 nm (which can be extended to 100-200 nm with the presence of a microcavity); there is significant overlap between the absorption and emission spectra of the donor and acceptor molecules; the angle between the emitted dipole of the donor and the absorbed dipole of the acceptor molecules must be within a range other than 90°; and the fluorescence quantum yield of the donor and the molar absorptivity of the acceptor must be sufficiently high. In this case, excited electrons from the donor fluorophore will transfer to the acceptor fluorophore through non-radiative energy transfer, causing electrons in the acceptor molecule to transition to an excited state. Because the energy transfer from the donor fluorophore to the acceptor involves energy loss from the donor and energy gain from the acceptor, the emission wavelength of the donor fluorescent molecule tends to exhibit a blue shift, while the emission wavelength of the acceptor fluorescent molecule tends to exhibit a red shift. FRET has relatively mature applications in biochemical analysis, immunoassay, and cell detection. The FRET effect has broad application prospects in the fabrication of tunable lasers, allowing for the modulation of laser emission wavelength.

[0004] Organic microlasers are lasers capable of transmitting strongly coherent optical signals in the (sub)wavelength range, and they hold broad application prospects in on-chip information processing, high-throughput sensing, and optical chips. Organic molecular materials are a promising platform for constructing high-performance miniature lasers, primarily because they possess a large active cross-section for high-gain emission and flexible assembly structures with high-quality microcavities during abundant excited-state processes. In recent years, increasing efforts have been devoted to developing such low-threshold, multicolor output, broadband tunable, and easily integrated organic microlasers.

[0005] Tunable organic microlasers are lasers whose output wavelength can be continuously changed within a certain range, showing broad development prospects in tunable narrow-linewidth emission and femtosecond laser emission. Tunable organic microlasers have greatly promoted the rapid development of research fields such as photochemistry, spectroscopy, and integrated optics, and have driven the creation and discovery of new research areas such as laser isotope separation, ultrashort pulse laser science, and tunable laser medicine. Summary of the Invention

[0006] This invention relates to an organic microlaser that uses a WGM microcavity as the resonant cavity of an organic microlaser. Inside the microcavity, a fluorescent material A is uniformly doped as a FRET donor (acceptor), and a fluorescent material B is used as a FRET acceptor (donor). By changing the concentration of the fluorescent material B adsorbed on the laser surface, the FRET efficiency can be changed, thereby controlling the blue-shift and red-shift range of the laser emission wavelength of the microcavity, thus fabricating a tunable wavelength organic microlaser.

[0007] The specific technical details are as follows:

[0008] 1. Selection of Organic Microlasers

[0009] The resonant cavity of the organic microlaser used in this invention is a microcavity capable of generating whispering-gallery modes, possessing a rotationally symmetric geometry, such as microsphere cavities, microdisk cavities, microring cavities, and other cavity shapes. The size is at the micrometer level, and the microcavity material is organic. The gain medium of the organic microlaser is a fluorescent material A uniformly doped within it, and fluorescent material A also functions as a FRET donor (acceptor). Optical pumping is used, with the pump wavelength near the excitation wavelength of the fluorescent material acting as the FRET donor, and the excitation wavelength having no effect on the acceptor; the emission spectra of the donor and acceptor must be completely separated.

[0010] 2. Selection of fluorescent substances A and B

[0011] Fluorescent materials A and B can be fluorescent dyes, quantum dots, rare-earth complexes, upconversion luminescent materials, etc., but are not limited to these materials. Fluorescent material A is doped into the interior of the microcavity, and fluorescent material B is prepared into solutions with different concentration gradients at saturation concentration (referring to the solution concentration required for fluorescent material B to completely and uniformly cover the surface of the microcavity; different fluorescent materials correspond to different saturation concentrations). Through methods such as covalent coupling or electrostatic adsorption, the distance between fluorescent materials A and B is made less than 100 nm, and the absorption spectrum of the fluorescent material acting as the acceptor should have a large degree of overlap with the emission spectrum of the fluorescent material acting as the donor, to ensure that energy is transferred from the donor to the acceptor in the form of FRET with high energy transfer efficiency.

[0012] 3. Method for fabricating bidirectional tunable wavelength organic microlasers

[0013] 2 μL of a whispering-gallery microcavity doped with fluorescent substance A at a specific mass-to-volume ratio was mixed with 14 μL of a series of concentration gradient solutions of fluorescent substance B at saturation concentration. The mixed suspension was then placed on a vortex mixer and vortexed thoroughly for 5 seconds to ensure uniform dispersion of the microcavity in the fluorescent substance B solution. Simultaneously, the vortexing process allowed for uniform adsorption of fluorescent substance B onto the surface of the microcavity. A total suspension volume of 16 μL was transferred to a coverslip and sealed. The sealed sample was then placed in a container... Figure 1 Excitation is performed in the optical path diagram shown. Figure 1 The optical path consists of: (1) pulsed laser (wavelength matched with donor excitation wavelength) (2) continuous attenuator (3) dichroic mirror (4) 50× objective lens (5) sample stage (6) total reflection mirror (7) filter (8) semi-transparent and semi-reflective mirror (9) focusing lens (10) CCD camera (11) focusing lens (12) grating spectrometer (13) computer.

[0014] Adjusting the optical path, when fluorescent material A in the microcavity acts as a donor, the pump light spot is focused at an off-center position inside the whispering-gallery microcavity doped with fluorescent material A. The fluorescence emitted after the pump light excites fluorescent material A resonates within the microcavity, simultaneously transferring energy to fluorescent material B via the FRET effect. (If fluorescent material A acts as the FRET acceptor, the light spot needs to be positioned 100-200 nm outside the microcavity, individually exciting fluorescent material B to emit fluorescence, which then couples into the microcavity to excite fluorescent material A). Adjusting the pump light power ensures the microcavity emits a high-quality single-peak laser. Because the emission (absorption) spectra of fluorescent material A and the absorption (emission) spectra of fluorescent material B overlap significantly, the distance between fluorescent materials A and B can be less than 200 nm through electrostatic adsorption or covalent coupling, thus enabling effective FRET between the donor and acceptor. When fluorescent material A is excited by pump light or fluorescent material B, the light beam is confined to the annular boundary of the microcavity. When the optical path of light of a specific frequency traveling around the geometric boundary of the whispering galvanic microcavity is an integer multiple of the wavelength, resonance will occur and it will be detected by the spectrometer in the optical path.

[0015] Next, by changing the concentration of fluorescent substance B, the FRET efficiency between fluorescent substances A and B is altered, thus adjusting the blue (red) shift range of the microcavity emitted laser. When fluorescent substance B acts as an acceptor, the lower its concentration, the less energy is transferred from fluorescent substance A to fluorescent substance B, resulting in a smaller blue shift in the emission wavelength of the whispering-gallery microcavity laser. Conversely, the higher the concentration of fluorescent substance B, the greater the blue shift. When fluorescent substance B acts as a donor, a whispering-gallery microcavity doped with fluorescent substance A is suspended in fluorescent substance B at different concentrations. After excitation of fluorescent substance B, the energy of the excited-state photons is transferred to fluorescent substance A via FRET, resulting in a red shift in the emitted laser. By recording the microcavity laser emission spectrum and plotting a frequency shift concentration standard curve, and fitting it using the Hill growth model, the tuning limit of the emission wavelength of the whispering-gallery microcavity laser with respect to fluorescent substance B (based on the half-maximum linewidth of the whispering-gallery microcavity) can be calculated. By selecting fluorescent substances A and B with different excitation and emission wavelengths, the emission range of the microcavity laser can be broadened, theoretically covering the entire wavelength band.

[0016] To prove that the blue (red) shift of the emission wavelength of the microcavity laser is due to the FRET effect, the change in fluorescence lifetime of the fluorescent material A doped on the microcavity surface before and after the FRET effect can be measured by time-correlated single-photon counting (TCSPC).

[0017] Tunable wavelength lasers fabricated using the FRET effect have advantages such as low threshold, large tuning range, low cost, easy fabrication, and small size, and can be widely used in fields such as on-chip optical communication, integrated optics, and medical imaging.

[0018] Compared with the prior art, the present invention has the following advantages:

[0019] 1. Using fluorescently doped whispering-gallery microcavities as organic miniature microlasers offers advantages such as small size, high Q value, and small mode volume. Therefore, the emitted laser exhibits a low threshold, narrow half-maximum linewidth, and can achieve high-quality single-peak laser output. Furthermore, the fabrication process is simple and cost-effective.

[0020] 2. By using the FRET effect to modulate the emission wavelength of organic microlasers, not only can the spectral broadening of the tunable wavelength of the laser be achieved by selecting donor-acceptor pairs with high FRET efficiency, but also by doping different fluorescent materials into the whispering-gallery microcavity to change the composition of the donor-acceptor pairs, the wavelength tuning range of different bands of the laser can be expanded. Theoretically, full-band wavelength laser tuning can be achieved.

[0021] 3. Excellent biocompatibility. The whispering-gallery microcavities, made from organic molecular materials, exhibit excellent biocompatibility. Furthermore, their small size allows them to be embedded within cells via endocytosis, making them promising for applications in single-cell analysis and cell detection. Attached Figure Description

[0022] Figure 1 Experimental optical path diagram

[0023] Figure 2 DG emission spectrum and CD absorption spectrum

[0024] Figure 3 DG microcavity zeta potential

[0025] Figure 4 Zeta potential of carbon quantum dot solution

[0026] Figure 5 Frequency shift-concentration fitting curve

[0027] Figure 6 DG microcavity laser emission spectra in different carbon quantum dot solutions Detailed Implementation

[0028] The present invention will be further illustrated below with reference to experimental examples, but the present invention is not limited to the following experimental examples.

[0029] Experimental Case: Modulation of Laser Emission Wavelength in Polystyrene Whispering-Gallery Microcavities Doped with Dargon Green (DG) Fluorescent Dye Using Carbon Quantum Dot (CD) Solutions of Different Concentrations

[0030] This experiment uses the mass ratio (w) DG / w PS2% DG microcavities were embedded in carbon quantum dot solutions of different concentrations. The DG microcavities were excited using a 473nm laser (depending on the absorption wavelength of the DG fluorescent material), and their emission spectra were collected.

[0031] The specific experimental steps are as follows:

[0032] (1) A polystyrene whispering-gallery microcavity with a mass ratio of 2% doped DG fluorescent material was selected as the gain medium and resonant cavity of the organic microlaser. DG served as the donor for FRET (Fluorescent Emission Transmission), and carbon quantum dot (CD) solution was selected as the acceptor. This was chosen because the emission spectra of DG and the absorption spectra of CD overlap significantly. Figure 2 As shown. Furthermore, the DG microcavity is negatively charged, with a Zeta potential as shown. Figure 3 As shown, the CD solution is positively charged, and the Zeta potential is as follows: Figure 4 As shown, after the DG microcavity is embedded in the CD solution, the two are connected by electrostatic adsorption, with a distance of less than 200 nm.

[0033] (2) Prepare carbon quantum dot solutions of different concentrations (0 mg / ml, 0.002 mg / ml, 0.004 mg / ml, 0.008 mg / ml, 0.02 mg / ml, 0.03 mg / ml, 0.04 mg / ml, 0.1 mg / ml, 0.2 mg / ml, 1 mg / ml, 1.5 mg / ml, 2 mg / ml, 2.5 mg / ml). Mix 2 μL of DG microcavities and 14 μL of carbon quantum dot solutions of different concentrations in an Eppendorf tube. Place the Eppendorf tube on a vortex mixer and shake it thoroughly for 5 seconds to ensure that the DG microcavities are uniformly dispersed in the carbon quantum dot solutions. Transfer the suspension in the Eppendorf tube to a coverslip, and slowly cover it with another coverslip from one side to avoid air bubbles affecting observation. This process transforms the suspension from a droplet shape on the coverslip to a uniformly spread liquid surface on the coverslip. Use nail polish to seal the edges of both coverslips to prevent water molecules in the carbon quantum dot solution from evaporating into the air and causing a change in refractive index that could affect the experimental results. Place the sealed sample in a container... Figure 1 Excitation is performed in the optical path diagram.

[0034] (3) Adjust the optical path so that the 473nm laser spot is focused at the off-center position on the side of the DG microcavity. Excite the DG with pump light, causing the fluorescence emitted by the DG to resonate within the cavity, satisfying the resonance condition. The emitted laser light is collected by the spectrometer shown in the optical path diagram. Based on the collected spectra in different carbon quantum dot solutions, it can be seen that the emission peak of the DG microcavity blue-shifts from 540nm in the 0mg / ml CD solution to 520nm in the 2.50mg / ml CD solution. The laser emission spectra of the DG microcavity in different carbon quantum dot solutions are as follows: Figure 6As shown. By changing the concentration of carbon quantum dots adsorbed in the DG microcavity, a wide spectral tuning range of 20 nm can be achieved for laser emission wavelength. The frequency shift-concentration curve was fitted using the Hill growth model, based on... Figure 4 Theoretical calculations based on the fitted equations shown indicate that for every 19.8 pg / ml increase in carbon quantum dot solution concentration, the laser emission wavelength of DG microspheres blue shifts by 0.28 nm, which is the full width at half maximum (FWHM) linewidth of the laser emission spectrum of DG microspheres detectable by the spectrometer when FRET does not occur.

[0035] The fluorescence lifetime of DG before FRET with CD was measured to be 3.8 ns and after FRET with CD using time-correlated single-photon counting (TCSPC). Theoretical calculations showed that the FRET efficiency between DG and CD was 50%.

[0036] Next, a solution of carbon nanoparticles (CNPs) with the same particle size and electrical properties as the quantum dots used in the previous experiment, but which neither absorbs nor emits light, was used to verify the effect of concentration changes on laser wavelength emission. Different concentrations of CNPs solutions (0.04 mg / ml, 0.1 mg / ml, 0.5 mg / ml, 2 mg / ml) were prepared, and samples were prepared using the same method. The microcavity was then excited. It was found that even with a large change in the CNPs solution concentration gradient (large change in refractive index), the laser emission wavelength of the whispering-gallery microcavity only showed a slight redshift, unlike the broad blueshift observed with FRET modulation. This experiment demonstrates that the tunable wavelength organic microlaser fabricated in this invention primarily exhibits a wavelength shift due to the FRET effect rather than refractive index. Therefore, the FRET effect can serve as a highly efficient means of controlling the laser emission wavelength of microlasers.

Claims

1. A method for bidirectionally tuning the wavelength of an organic microlaser, characterized in that: The emission wavelength of a laser is tuned using the FRET effect between two fluorescent materials, A and B. A WGM microcavity is selected as the resonant cavity of the organic microlaser, and fluorescent material A is uniformly doped inside it as the donor or acceptor for FRET, while another fluorescent material B is selected as the acceptor or donor for FRET. The microcavity doped with fluorescent material A is embedded in solutions of fluorescent material B with different concentrations. Pump light with a wavelength close to that of the donor is used to excite the microcavity. When fluorescent material A in the microcavity is excited as a donor, the excited photons in the microcavity form a resonant emission laser and transfer part of the energy of the excited photons to fluorescent material B, resulting in a blue shift in the laser emission wavelength. When fluorescent material B is excited as a donor, the energy of the excited photons is transferred to fluorescent material A through the FRET effect, causing it to form a resonant emission laser with a red shift in wavelength. By changing the concentration of fluorescent material B, the FRET efficiency is changed, thereby tuning the emission wavelength of the laser.

2. The method for bidirectionally tuning the wavelength of an organic microlaser according to claim 1, characterized in that: The FRET effect between two fluorescent materials involves two directions of energy transfer: one is that fluorescent material A in the microcavity acts as the FRET donor, with energy propagating from inside the cavity to outside, resulting in a blue shift in the laser emission wavelength; the other is that fluorescent material A in the microcavity acts as the acceptor, with energy propagating from outside the cavity to inside, resulting in a red shift in the laser emission wavelength.

3. The method for bidirectionally tuning the wavelength of an organic microlaser according to claim 1, characterized in that: The organic microlaser is micrometer-sized. The resonant cavity is made of organic molecular material in a rotationally symmetric geometric structure. Fluorescent material is doped inside as a gain medium. When excited, light that meets the resonance condition resonates in the cavity in the whispering-gallery mode (WGM), and finally laser output is generated.

4. The method for bidirectionally tuning the wavelength of an organic microlaser according to claim 1, characterized in that: The selection of fluorescent substances A and B must meet the requirements for FRET generation, namely, the emission spectrum of the donor fluorescent substance and the absorption spectrum of the acceptor fluorescent substance overlap by more than 70%; the distance between them is less than 100 nm in the presence of a microcavity; and the angle between the donor emission dipole and the acceptor absorption dipole is not 90°.

5. The method for bidirectionally tuning the wavelength of an organic microlaser according to claim 1, characterized in that: The distance between fluorescent substance A and fluorescent substance B is achieved through electrostatic adsorption or covalent coupling.

6. The method for bidirectionally tuning the wavelength of an organic microlaser according to claim 1, characterized in that... The wavelength of laser emission from the microlaser can be controlled by selecting different types of fluorescent material A; the blue or red shift of laser emission within the absorption or emission wavelength range of fluorescent material A can be adjusted by regulating the concentration of fluorescent material B on the microcavity surface.

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