Refractive index sensing chip of integrated BIC metasurface laser

Abstract

The invention discloses a refractive index sensing chip of an integrated BIC metasurface laser, which integrates a vertical cavity surface emitting laser (VCSEL) and a novel ball road pattern metasurface designed based on a BIC theory, and designs a high-sensitivity refractive index sensor by using a continuous domain bound state theory. The ball road pattern structure is prepared through electron beam lithography and a gold evaporation-stripping process, has uniform nanoscale patterns, and can realize high refractive index sensitivity. The chip can be used in the field of biosensing. By optimizing structural parameters and a preparation process, the optical sensing chip with high integration level and high sensitivity is realized, and a new solution is provided for unmarked biological detection.

Description

Technical Field

[0001] The present invention relates to the field of optical sensing technology, and particularly to a laser refractive index sensing chip integrated with a nanostructured spherical grating metasurface, which is suitable for label-free optical sensing of high-sensitivity biomolecules. Background Art

[0002] Existing refractive index sensors have limitations in terms of sensitivity and anti-interference ability, and it is difficult to meet the requirements of high-precision biological detection. In addition, traditional refractive index sensing technologies usually rely on complex optical systems, which is not conducive to the miniaturization and high integration of devices. Compared with traditional optical sensors, metasurface sensors have the advantages of ultra-high sensitivity, high Q factor, chip-level integration, multi-parameter detection, and low cost, and are widely applicable to portable high-precision detection. A metasurface is composed of sub-wavelength structural units (such as nanoantennas, micro-rings, hole arrays, etc.), which can precisely control the characteristics of light waves such as amplitude, phase, and polarization. However, the design of traditional metasurfaces is limited by a relatively low Q factor, which affects the sensing sensitivity. To improve the sensing performance, the present invention introduces the BIC mode, which is a non-radiative mode with an infinite Q factor. Through symmetry-breaking design, a quasi-BIC mode can be excited in the metasurface, thereby achieving a high Q factor and a high-sensitivity spectral response.

[0003] The present invention optimizes the spherical grating structure of the metasurface based on the BIC theory, enabling the sensor to have excellent refractive index sensitivity in the visible wavelength range. The spherical grating metasurface structure draws on the spherical grating pattern of ancient Chinese silk culture, and its geometric design of multiple intersecting circles can enhance the electromagnetic coupling effect and improve the response of the resonance peak to changes in the environmental refractive index. Its main adjustable parameters include the radius r of the circle and the minimum unit spacing d. Fewer structural optimization parameters help reduce the computational complexity and facilitate the scanning of refractive index parameters to obtain the optimal structure. In terms of light source selection, the present invention uses a VCSEL as the laser light source. Compared with LEDs and LDs, VCSELs have the advantages of small size, circular output light spot, single longitudinal mode output, low threshold current, wide operating temperature range, etc., and are easy to achieve large-area array integration and are highly compatible with the metasurface structure. For the integrated fabrication process of the metasurface and the VCSEL, high-precision exposure of nanostructures is completed by bombarding photoresist with high-energy electrons, providing technical support for the fabrication of high-performance refractive index sensing chips. Summary of the Invention

[0004] The present invention proposes a refractive index sensing chip based on a nanostructured spherical grating integrated with a BIC metasurface laser, aiming to achieve high sensitivity, miniaturization, and characteristics suitable for biomolecule detection. To ensure the uniformity of the nanostructure, electron beam lithography and gold evaporation and lift-off processes are used. By combining the BIC theory with the spherical grating structure, an ultra-high Q factor and a narrow linewidth spectral response are achieved.

[0005] Existing refractive index sensors have deficiencies in sensitivity, anti-interference ability and system integration, and are difficult to meet the needs of high-precision, portable detection. The purpose of the present invention is to provide a refractive index sensor chip based on a BIC metasurface, which uses a spherical pattern nanostructure to design a metasurface and is integrated into the light-emitting surface of a vertical cavity surface emitting laser to achieve high Q factor resonance, ultra-high sensitivity detection and miniaturized integration, and is suitable for point-of-care testing (POCT) and on-site rapid analysis. In order to achieve the purpose of the present invention, the technical solution adopted by the present invention includes:

[0006] The present invention designs a metasurface based on the "ball pattern brocade" pattern in ancient Chinese silk culture, ingeniously integrating traditional aesthetics with modern optics to construct an asymmetric nanostructure with high electromagnetic coupling capability. The structure is composed of three circles arranged in a cluster, breaking the axial symmetry of the traditional metasurface, thereby stimulating a quasi-bound state with a high quality factor, significantly enhancing the local electric field, and achieving a highly sensitive response to tiny changes in the refractive index. Compared with traditional low-Q-factor metasurface structures that are difficult to meet high-performance sensing requirements, the present invention obtains a resonance peak with a narrow linewidth and high Q factor on the metasurface by introducing the BIC mode and its symmetry breaking mechanism, which greatly improves the refractive index sensing performance.

[0007] In order to achieve the optimal design of structural parameters, the present invention uses the finite difference time domain method (FDTD) for electromagnetic simulation, adjusts key parameters such as the circle radius r and the minimum unit spacing d, and optimizes the resonance wavelength and sensitivity. The simulation results show that in the visible light band of 600-800nm, as the refractive index of the covering medium increases from 1.0 to 1.4, the resonance wavelength shows a significant red shift, showing excellent optical response characteristics, verifying the potential of this structure in high-sensitivity sensing applications.

[0008] In terms of nanostructure preparation, in order to ensure the consistency of the pattern and the repeatability of the process, the present invention uses a high-precision electron beam lithography (EBL) process to complete the metasurface processing. The specific process includes:

[0009] S1: Spin coating ZEP520 photoresist on silicon substrate;

[0010] S2: Etching ball pattern using electron beam exposure system;

[0011] S3: Perform gold evaporation-stripping process to deposit a nano-gold layer on the surface of the structure.

[0012] In order to obtain a nano-metal metasurface with complete structure, clear interface and high stability. The above process ensures the consistency of structure in large-area preparation and provides strong support for the mass production of high-performance sensor chips.

[0013] The laser light source uses a VCSEL, and the metasurface is directly integrated on the light-emitting surface of the VCSEL to form a compact integrated sensing chip. Compared with traditional laser diodes (LDs) and light-emitting diodes (LEDs), VCSELs have the advantages of single longitudinal mode output, low threshold current, circular beam, high power density, etc. In terms of light source preparation, a vertical cavity surface emitting laser with a high reflectivity distributed Bragg reflector structure is used as the chip light source. The upper part of the laser contains 30.5 pairs of p-type DBRs, and the lower part contains 28 pairs of n-type DBRs. The materials are all made of alternating Al 0.9 Ga 0.1 As / Al 0.12 Ga 0.88 As structure; an Al 0.98 Ga 0.02 As oxide layer with a thickness of 30 nm is integrated above the active region. A 300-nm SiO2 mask layer is deposited by PECVD technology, and a large mesa structure with a diameter of 50 μm and a height of 5 μm is defined by UV lithography and ICP-RIE processes. The SiO2 mask is etched away by BOE solution to prevent it from affecting the subsequent wet oxidation process. A 500-nm SiO2 passivation layer is grown by PECVD to insulate and protect the sidewalls of the device. At the same time, BCB with a thickness of 2 μm is spin-coated for surface planarization. After thermal curing, the top BCB of the mesa is etched by RIE to complete the electrode opening. Finally, Ti / Au top ohmic P contacts and AuGeNi / Au bottom ohmic N contacts are prepared through double-sided lithography and rapid thermal annealing steps to complete the construction of the VCSEL device. The prepared chips are tested by PIV characteristic curves, and the qualified VCSELs are selected as the light sources for subsequent experiments.

[0014] To achieve specific biological detection functions, the surface of the ball grid array structure is further biofunctionalized. First, 11-mercaptoundecanoic acid (11-MUA) is self-assembled on the surface of the structure to form an ordered monolayer; then EDC and S–NHS are used to activate the carboxyl groups, which react with the amino groups on the Alzheimer's disease biomarker Aβ 42 antibody to form amide bonds to achieve antibody conjugation; after the reaction is completed, BSA is used to block the unreacted carboxyl groups to improve the sensing selectivity and reduce non-specific adsorption. The entire functionalization process can be stored at a low temperature of 4 °C, which is suitable for long-term and stable use. By detecting the wavelength shift (red shift or blue shift) of the metasurface resonance peak, highly sensitive measurement of the refractive index is achieved.

[0015] The functionalized BIC ball grid array structure is integrated onto the light-emitting surface of the VCSEL by bonding to construct a highly integrated light source–sensing integrated platform. At the same time, combined with the design of a PDMS microfluidic channel and a stainless steel sampling tube, automatic guidance, transmission, and elution of biological samples are realized, which is convenient for efficient and repeatable biomolecule detection in a liquid environment.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] Ultra-high sensitivity: High refractive index sensitivity is achieved through the collaborative design of ball-pattern nanostructure and BIC theory.

[0018] Miniaturization and integration: The metasurface is directly integrated into the light-emitting surface of the VCSEL to form a compact chip. This eliminates the need for a complex external optical system, significantly reduces the size and cost of the detection equipment, and is suitable for portable instant detection.

[0019] High Q factor and narrow linewidth: The quasi-BIC mode designed based on symmetry breaking has a high Q factor and a narrow resonance linewidth. It can accurately distinguish tiny refractive index changes and has strong resistance to environmental interference.

[0020] Diverse application scenarios: It can detect refractive index changes (such as sucrose solution concentration gradient) and can also detect specific molecules through antibody-antigen binding. One core has multiple uses and strong scalability. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Schematic diagram of the integrated BIC metasurface laser structure.

[0022] Figure 2 Schematic diagram of the microfluidic design integrating the BIC metasurface laser.

[0023] Figure 3 This is the design diagram of the ball texture structure.

[0024] Figure 4 is the refractive index sensitivity of the spherical texture structure. DETAILED DESCRIPTION

[0025] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0026] Example 1: Preparation and integration of VCSEL laser

[0027] This embodiment provides a method for preparing a vertical cavity surface emitting laser, which focuses on achieving accurate etching of large table structures, electrical insulation treatment of the electrical injection area, and high-quality deposition of electrodes to ensure its subsequent integration capability with the BIC metasurface. The specific process steps are as follows:

[0028] First, select an epitaxially grown laser wafer as the starting material. The wafer sequentially includes from top to bottom: 30.5 pairs of p-type top distributed Bragg reflector (DBR) pairs composed of alternating Al 0.9 Ga 0.1 As and Al 0.12 Ga 0.88 As, an active region, and 28 pairs of n-type bottom DBR pairs composed of the same materials. Deposit a 30-nm-thick Al 0.98 Ga 0.02 As oxide layer above the top DBR to form a current confinement structure.

[0029] Subsequently, deposit a 300-nm-thick SiO2 layer on the wafer surface by plasma-enhanced chemical vapor deposition (PECVD) as a large mesa mask for subsequent etching. Define the pattern by standard ultraviolet lithography technology and combine it with inductively coupled plasma reactive ion etching (ICP-RIE) process to etch out a circular mesa structure with a diameter of 50 μm and a height of about 5 μm.

[0030] After the etching is completed, use buffered oxide etchant (BOE, with a ratio of HF:NH3F:H2O = 1:2:3) to completely remove the SiO2 mask layer. This step must be completed before wet oxidation to avoid the difficulty of removing SiO2 after high temperature, which will affect the electrical injection performance of the subsequent device.

[0031] Then, use PECVD again to deposit a 500-nm-thick SiO2 layer on the structure surface to form a passivation layer on the sidewalls of the device, achieving good insulation protection for the electrical injection area.

[0032] To improve the surface morphology of the device, use benzocyclobutene (BCB) for surface planarization. The specific operation is as follows: spin-coat a 2-μm-thick BCB on the SiO2 passivation layer at a speed of 3000 revolutions per minute, and then perform thermal curing treatment on a hot plate by slowly heating from room temperature (25 °C) to 250 °C.

[0033] After the BCB is cured, use reactive ion etching (RIE) technology to remove the remaining BCB area on the top of the mesa, and further remove the remaining SiO2 passivation layer on the mesa by chemical wet etching.

[0034] Finally, define the electrode pattern by double-sided lithography, combine metal deposition and lift-off processes to form a top Ti / Au circular p-type ohmic contact and a bottom AuGeNi / Au n-type ohmic contact. All metal contacts are subjected to rapid thermal annealing at 350 °C for 35 seconds to improve the ohmic contact quality and ensure the stability of the electrical injection efficiency and laser performance.

[0035] Example 2: Design and Fabrication of Nanosphere Circuit Pattern Structure

[0036] The present invention provides a technical solution: The spherical pattern structure of the "Cluster Six Filled Flower Ball Pattern Brocade" type uses three intersecting circles to form a group and split the smallest unit to manufacture an electric field coupling to achieve a nanostructure with high refractive index sensitivity, which can create more electromagnetic couplings and avoid an axisymmetric structure.

[0037] As Figure 3 shown, the calculation of the optical properties of this structure is realized by the finite-difference time-domain method. The Maxwell equations are discretized into difference equations, and the behavior of electromagnetic waves is simulated by solving these equations in the discrete space and time. To calculate the near electric field, an infinite array of plasmonic nanospherical pattern structures is considered. The unit cell consists of two spherical pattern structures, and the plasmonic nanospherical pattern structure is mainly composed of two parameters: the circle forming the smallest unit and the minimum unit spacing. The optimal parameters can be obtained by adjusting the radius r of the circle and the minimum unit spacing d through FDTD simulation software.

[0038] The refractive index solution response spectrum characterization of the nano-plasmonic structure can test the performance of its sensing application. Under the optimal parameter conditions, different refractive index media are used to replace the air medium. As Figure 4 shown, media with different refractive indices of 1.0, 1.2, 1.3, and 1.4 are sequentially scanned for parameters on the surface of the spherical pattern structure. As the refractive index increases, the resonance peaks generated by the plasmonic nanospherical pattern structure shift to the red end in sequence. The overall resonance peaks are in the visible wavelength range of 600 - 800 nm, showing high-sensitivity optical properties to the refractive index.

[0039] Electron beam lithography technology uses high-energy electron beams to locally expose photosensitive materials, and then through process steps such as chemical etching or evaporation, the unexposed parts are removed, and finally the required pattern is formed. With the help of EBL and gold evaporation-lift-off process, a nanostructure with a uniform spherical pattern structure is realized. 200 nm of ZEP520 glue is spin-coated on a silicon wafer. After electron beam exposure, the glue in the parts of ZEP520 that need to be etched will be dissolved by the developer. Then, 200 nm of Cr-Au is evaporated on the chip, and the remaining photoresist is removed, and the designed electron beam lithography structure can be left on the substrate. The size of the prepared plasmonic nanospherical pattern structure is consistent with the designed size, the surface is flat, and the plasmonic nanospherical pattern structures prepared in a large area are uniform and defect-free.

[0040] Example 3: Functional modification of the plasmonic nanospherical pattern structure

[0041] The realization of biofunctionalization is mainly achieved through the dehydration condensation of self-assembled 11-MUA to couple antibodies. The steps of biofunctionalization include:

[0042] S1. Use 11-MUA for surface self-assembly;

[0043] S2. Couple antibodies to the sensing layer;

[0044] S3. Block unbound antibodies with BSA;

[0045] S4. Capture the target through specific binding of antigen and antibody.

[0046] During this process, 11-MUA (1 mM, ethanol solution) forms an ordered self-assembled monolayer on the surface of gold nanoparticles at 4 °C after 12 h of reaction. Subsequently, the carboxyl group of 11-MUA is activated by mixing equal volumes of EDC (0.4 mM, ethanol solution) and S–NHS (0.1 mM, PBS solution) for 30 min. The activated carboxyl group can bind to the Alzheimer's Aβ 42 antibody protein (10 μg / mL) to form stable amide bonds, and the reaction time is 2 h.

[0047] However, some carboxyl groups may remain unlinked, which will affect the capture of antigens. Therefore, within 2 h, BSA (5%, PBS solution) can fill these unlinked positions. The chip is rinsed with PBS after each step, and the functionalized chip is stored at a low temperature of 4 °C for a long time.

[0048] The embodiments disclosed in the present invention are preferred embodiments, but not limited thereto. Those of ordinary skill in the art can easily understand the spirit of the present invention based on the above embodiments and make different extensions and changes. As long as they do not depart from the spirit of the present invention, they are within the protection scope of the present invention.

Claims

1. An integrated refractive index sensing chip with a BIC metasurface laser, characterized in that include: Vertical Cavity Surface Emitting Laser (VCSEL), used to provide a stable vertically emitting laser light source; A spherical pattern metasurface integrated on the light emitting surface of the vertical cavity surface emitting laser (VCSEL); The spherical pattern metasurface is designed based on a quasi-bound state, and achieves high Q factor narrowband resonance through symmetry breaking, which is used to respond to external refractive index changes with high sensitivity. The microfluidic module matched with the ball-patterned metasurface includes a PDMS microfluidic channel and inlet and outlet stainless steel capillaries, which facilitates the uniform flow of liquid samples on the chip surface and biofunctionalization modification, thereby realizing diversified sensing detection; As the core light source device of the refractive index sensor chip, the vertical cavity surface emitting laser VCSEL stably generates narrow line width laser output inside the refractive index sensor chip; by optimizing the intracavity reflector DBR structure and electrical injection method, efficient light field output is achieved and stably coupled to the sensing structure above; the light output surface of the vertical cavity surface emitting laser VCSEL is integrated with a ball-patterned metasurface based on the "cluster six-filled flower ball-patterned brocade" pattern design; when the laser is vertically emitted from the vertical cavity surface emitting laser VCSEL and passes through the ball-patterned metasurface, a light field enhancement area is formed on its surface; the resonance wave of the light field enhancement area The metasurface is extremely sensitive to the refractive index of the environment it contacts, thus achieving a highly sensitive response to tiny changes in the external refractive index; the microfluidic module is integrated above the entire sensing structure by bonding, including a polydimethylsiloxane (PDMS) microfluidic channel and inlet and outlet stainless steel capillaries; the microfluidic module is used to guide the liquid sample to be evenly distributed on the surface of the spherical pattern structure to ensure the uniformity and detection efficiency of the biological functional modification; the target molecules in the liquid sample specifically bind to the recognition molecules on the functionalized metasurface, changing the local refractive index, thereby causing a measurable red shift in the resonance wavelength of the metasurface, thereby achieving accurate biosensing detection.

2. The refractive index sensing chip of the integrated BIC metasurface laser according to claim 1, characterized in that, The vertical cavity surface emitting laser (VCSEL) comprises, from top to bottom, a P-surface electrode (1), phenylcyclobutene (2), a P-surface distributed Bragg reflector (3), an oxidized hole (4), an N-surface distributed Bragg reflector (5), a gallium arsenide substrate (6), an N-surface electrode (7) and a BIC metasurface (8).

3. The refractive index sensing chip of the integrated BIC metasurface laser according to claim 1, characterized in that, The ball pattern structure is evolved from the "cluster six filled flower ball pattern brocade" pattern; the ball pattern pattern is first extracted from the "cluster six filled flower ball pattern brocade" to obtain a pattern of multiple intersecting circles, and in order to construct electromagnetic resonance enhancement, the intersecting circle pattern is split into a group of three intersecting circles; the intersecting circle pattern breaks the structural symmetry, thereby exciting the quasi-bound state resonance mode; The non-axisymmetric minimum unit composed of three equal-sized intersecting circles forms a periodic arrangement to form a spherical pattern structure, and its structural parameters include: the radius r of a single circle and the minimum unit spacing d; r and d are adjustable to adjust the resonance wavelength and field enhancement intensity, so as to achieve the best refractive index sensitivity; First, the spherical path pattern structure is based on asymmetric three - circle units arranged periodically, with significant electromagnetic local enhancement effects and high - Q - factor narrow - band resonance characteristics. By combining optical simulation tools to model and optimize this structure, it can achieve sensitive responses to different refractive index changes, and is suitable for detecting extremely small refractive index perturbations. In terms of chip integration, the spherical path metasurface is directly constructed on the light - emitting surface of the VCSEL. After the laser light is emitted vertically through this structure, it is effectively modulated, thus exciting a high - intensity local resonance electric field on the surface of the structure. Once the refractive index of the surface environment changes, the resonance wavelength of the structure will shift accordingly, realizing highly sensitive optical sensing.

4. The refractive index sensing chip integrated with the BIC metasurface laser according to claim 1, characterized in that The microfluidic module includes: A microfluidic chamber (9) made of PDMS, which is used to enclose the vertical - cavity surface - emitting laser (VCSEL) to form a microfluidic chamber for incubating biological samples; Metal injection and sampling stainless - steel capillary tubes (10). The metal injection and sampling stainless - steel capillary tubes (10) are connected to the outside and the vertical - cavity surface - emitting laser (VCSEL) through the microfluidic chamber (9), and are used to connect with an external sample injection system to realize the processing of biological samples in a closed environment.

5. The refractive index sensing chip integrated with the BIC metasurface laser according to claim 1, characterized in that The ball path pattern structure is used for the specific recognition and detection of disease-related biomarkers through surface functionalization modification, including the Aβ antibody targeting detection related to Alzheimer's disease, and has high sensitivity, high selectivity, and reusability. 42 ​ 6. The refractive index sensing chip of the integrated BIC metasurface laser according to claim 3, characterized in that The BIC metasurface of the spherical path pattern is realized by electron - beam lithography (EBL) and gold evaporation - lift - off process. The thickness of the photoresist used is 50 - 300 nm to ensure pattern resolution and topography uniformity; the thickness of the gold layer is 50 - 300 nm to achieve the best electromagnetic field coupling and functionalized surface combination effect.

7. The refractive index sensing chip of the integrated BIC metasurface laser according to claim 3, characterized in that, The size range of the smallest building unit of the spherical path pattern structure is as follows: the radius r of the circle is 10 - 300 nm, and the minimum unit spacing d is 50 - 500 nm. Through simulation and experimental adjustment, resonance enhancement and sensitivity maximization within the target wavelength band are achieved.

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