阅读说明：本技术 一种基于飞秒激光直写的光学模拟平台制备方法 (Femtosecond laser direct writing-based optical simulation platform preparation method ) 是由 侯智善 曹宇 蔡燕 丁潇川 薛伟 陈瑞溢 朱小伟 于 2021-10-09 设计创作，主要内容包括：本发明提供了一种基于飞秒激光直写的光学模拟平台制备方法,包括：样品片准备、加工台调平；之后启动加工程序,使得飞秒激光焦点位于光刻胶/硅的交界面而且在激光焦点保持不动的同时,样品片按照设计结构实现长程连续移动,最终在样品片上完成整个波导阵列的扫描；然后再经未交联光刻胶去除、旋涂波导阵列包层等步骤,既得到微米级别聚合物基波导阵列。该制备方法利用飞秒激光双光子聚合高精度点-线-面加工能力实现微米级别聚合物基波导阵列的快速制备。由于飞秒激光无掩模、真三维的加工特点,实现了聚合物波导阵列任意的高度、宽度、截面的精确定制,进而利用波导截面双折射梯度实现相关光学模拟实验。(The invention provides a femtosecond laser direct writing-based optical simulation platform preparation method, which comprises the following steps: preparing a sample sheet and leveling a processing table; then starting a processing program to enable the femtosecond laser focus to be positioned at the interface of the photoresist/silicon and to realize long-range continuous movement of the sample piece according to the design structure while the laser focus is kept still, and finally completing the scanning of the whole waveguide array on the sample piece; and then removing the uncrosslinked photoresist, spin-coating a waveguide array cladding and the like to obtain the micron-level polymer fundamental waveguide array. The preparation method utilizes femtosecond laser two-photon polymerization high-precision point-line-surface processing capability to realize the rapid preparation of the micron-level polymer-based waveguide array. Due to the maskless and true three-dimensional processing characteristics of the femtosecond laser, the accurate customization of the random height, width and section of the polymer waveguide array is realized, and then the relevant optical simulation experiment is realized by utilizing the birefringence gradient of the waveguide section.)

1. A femtosecond laser direct writing-based optical simulation platform preparation method is characterized by comprising the following steps:

step (1): sample piece preparation

Firstly, cutting an SOI wafer into a rectangle as a substrate according to a dissociation plane, sequentially wiping the substrate with absorbent cotton soaked with acetone and ethanol, then flushing the substrate with deionized water, and drying the substrate with nitrogen for later use; then, spin-coating the polymer photoresist solution to a substrate at the rotation speed of 500-4000r/min for 20-60s, and finally, placing the spin-coated wafer on a hot table for pre-baking at 65 ℃ for 5min and 90-120 ℃ for 15-30min to obtain a sample wafer to be processed with the film thickness of 2-20 microns;

step (2): machining table leveling

Firstly, fixing a sample piece to be processed on a sample table with an adjusting device; then, opening a laser shutter, and adjusting the height of the sample stage to enable the appearance of a light spot on the sample to be observed in the CCD; moving the position of a sample, and adjusting a knob on the sample table to ensure that a laser focus is always focused on a photoresist/substrate interface without relative movement and the appearance of a light spot is kept unchanged when the sample table translates on an X-Y surface; at this time, the processing table is leveled;

and (3): femtosecond laser direct writing scanning of waveguide array

Starting a processing program to enable the femtosecond laser focus to be positioned at the interface of photoresist/silicon and to realize long-range continuous movement of the sample piece according to the designed structure while the laser focus is kept still, and finally completing the scanning of the whole waveguide array on the sample piece;

and (4): uncrosslinked photoresist removal

Firstly, placing a scanned sample piece on a flat hot table for post-baking, wherein the temperature is 5min at 65 ℃, and the temperature is 10-30min at 90-120 ℃; then, after the sample is cooled to room temperature, the sample is placed in a developing solution for soaking and developing, and the time is 30-120 s; then respectively washing with ethanol and deionized water, and drying by ear washing balls;

and (5): spin-coated waveguide array cladding

Firstly, coating a cladding polymer on a sample by spin coating at a rotation speed of 200-; then placing the sample on a flat plate hot table for pre-drying at 65 ℃ for 5min and at 90-120 ℃ for 15-30 min; finally, the whole sample piece is placed under a large-field ultraviolet lamp for exposure; then placing the exposed sample piece on a flat plate hot bench for post-baking at 90-120 ℃ for 10-30 min; thus obtaining the micron-scale polymer fundamental waveguide array.

2. The method for preparing an optical simulation platform based on femtosecond laser direct writing according to claim 1, wherein the substrate in the step (1) is an oxide film with 2-5 μm,<100>a tangential silicon wafer; the silicon wafer is cut into 10x2cm pieces according to the dissociation plane in advance²And (5) standby.

3. The method for preparing an optical simulation platform based on femtosecond laser direct writing according to claim 1, wherein the polymer photoresist solution in the step (1) is an Epocore photoresist.

4. The femtosecond laser direct writing-based optical simulation platform preparation method according to claim 1, wherein the adjusting device in the step (2) is a modified adjustable mirror frame with two knobs; the fixing mode is that the silicon chip is flatly pasted on a cover glass by using a double-sided adhesive tape, and then the cover glass is downwards arranged on a mirror bracket with a clamp; the moving platform is a combined three-axis ABL1000 linear motor driven air bearing platform with a stroke of 50mm x50mm x50 mm.

5. The femtosecond laser direct-writing-based optical simulation platform preparation method according to claim 1, wherein the interface position in the step (3) is positioned by using an overexposure method: firstly, adjusting the angle of an attenuation sheet in a light path to ensure that the power reaching a sample sheet is 40 mw; then, controlling the Z-axis movement to make the distance between the sample and the objective lens smaller than the focal length of the objective lens, namely the laser focus is in the silicon chip; then, slowly raising the height of the Z axis, wherein the laser focus position is not moved, and the interface is slowly close to the focus; the position where the CCD is observed to explode in the rising process is the interface of the photoresist/silicon wafer, namely the processing starting position.

6. The method for preparing an optical simulation platform based on femtosecond laser direct writing according to claim 1, wherein the wavelength range of the femtosecond laser continuous light source in the step (3) is 780-810 nm; the laser scanning power is 17-25 mw; the point-by-point scanning speed of the waveguide is 0.05-0.5 mm/s; the processing objective lens is a 20x-100x lens. The waveguide scanning mode is that the waveguide is divided into a plurality of longitudinal planes, X-Y in-plane scanning is completed when Z is 0, then X-Y in-plane scanning is completed when Z is 100nm, and the rest is done in sequence, and finally the whole scanning of the waveguide is realized.

7. The femtosecond laser direct writing-based optical simulation platform preparation method according to claim 1, wherein the uncrosslinked photoresist removal method in the step (4) is as follows: firstly, placing a scanned sample piece on a flat hot table for post-baking at 65 ℃ for 5min and at 90-120 ℃ for 5-15min, wherein strong acid generated in a laser scanned area can promote the photoresist monomer to realize intramolecular and intermolecular crosslinking; then, after the sample is cooled to room temperature, the sample is placed in an acetone solution for soaking and developing, and the time is 30-120 s; then, respectively washing with ethanol and deionized water to respectively remove residual acetone and ethanol, and lightly drying by ear washing balls; finally, the extra material outside the unscanned waveguide is dissolved and removed by the acetone solution, and the scanned area forms a dense grid due to molecular cross-linking and is not dissolved in the acetone solution.

8. The method as claimed in claim 1, wherein the UV exposure power in step (5) is 100-300W high power exposure, and the exposure time is 30-120 s.

Technical Field

The invention belongs to the field of optical simulation platforms, and particularly relates to preparation of micro-optical devices such as large-area waveguide arrays and directional coupler structures.

Background

In physics, the optical lattice provides a very effective observation platform for researching optics, quantum optics, condensed state phenomena and the like. For example, bloch oscillation, inspired by the solution of schrodinger equation, deduces that the wave function of moving electrons in a periodic potential field is an amplitude-modulated plane wave, and the amplitude-modulated factor (bloch wave packet) has the same periodicity as the lattice potential field, which has not been verified experimentally, thus causing a controversy for more than 60 years. Since bloch oscillation generates a condition that the scattering time is longer than the bloch period, the natural lattice cannot satisfy this condition, which was observed in the semiconductor superlattice until 1991. Later, it was found that it is easier to simulate this phenomenon on optical systems. Advantages of the optical simulation study compared to quantum systems include: (1) directly carrying out visual research on a typical ultrafast phenomenon in space; (2) photons can be guided by simple geometric bending or twisting to simulate coherent laser material interaction structures.

The implementation of optical bloch oscillations generally involves the construction of a periodic structure with a linearly varying refractive index profile. This is analogous to the electrostatic field acting on electrons in an atomic crystal. There are several methods of inducing refractive index gradients in waveguide arrays: firstly, heat sources with different temperatures are placed on two sides of an array to form a transverse temperature gradient field, and refractive index gradient is induced through a thermo-optic effect; in addition, the effective index of refraction of each waveguide can be controlled by applying a lateral voltage to the array; finally, the conformal transformation produced by the bending of the waveguide array causes the light to accelerate to form a gradient. However, the existing optical simulation platform is basically a silicon-based waveguide array prepared based on an SOI platform, and the implementation of the height gradient of the waveguide is limited by the natural flatness of the optical simulation platform, so that important information unit carriers and polarization information in the optical simulation are ignored.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides a method for preparing an optical simulation platform based on femtosecond laser direct writing. The preparation method utilizes the femtosecond laser two-photon polymerization high-precision point-line surface processing capability to realize the rapid preparation of the micron-level polymer-based waveguide array. Due to the maskless and true three-dimensional processing characteristics of the femtosecond laser, the accurate customization of the random height, width and section of the polymer waveguide array is realized, and then the relevant optical simulation experiment is realized by utilizing the birefringence gradient of the waveguide section.

The invention is realized by the following technical scheme:

a femtosecond laser direct writing-based optical simulation platform preparation method comprises the following steps:

step (1): sample piece preparation

Firstly, cutting an SOI wafer into a rectangle as a substrate according to a dissociation plane, sequentially wiping the substrate with absorbent cotton soaked with acetone and ethanol, then flushing the substrate with deionized water, and drying the substrate with nitrogen for later use; then, spin-coating the polymer photoresist solution to a substrate at the rotation speed of 500-4000r/min for 20-60s, and finally, placing the spin-coated wafer on a hot table for pre-baking at 65 ℃ for 5min and 90-120 ℃ for 15-30min to obtain a sample wafer to be processed with the film thickness of 2-20 microns;

step (2): machining table leveling

Firstly, fixing a sample piece to be processed on a sample table with an adjusting device; then, opening a laser shutter, and adjusting the height of the sample stage to enable the appearance of a light spot on the sample to be observed in the CCD; moving the position of a sample, and adjusting a knob on the sample table to ensure that a laser focus is always focused on a photoresist/substrate interface without relative movement and the appearance of a light spot is kept unchanged when the sample table translates on an X-Y surface; at this time, the processing table is leveled;

and (3): femtosecond laser direct writing scanning of waveguide array

Starting a processing program to enable the femtosecond laser focus to be positioned at the interface of photoresist/silicon and to realize long-range continuous movement of the sample piece according to the designed structure while the laser focus is kept still, and finally completing the scanning of the whole waveguide array on the sample piece;

and (4): uncrosslinked photoresist removal

Firstly, placing a scanned sample piece on a flat hot table for post-baking, wherein the temperature is 5min at 65 ℃, and the temperature is 10-30min at 90-120 ℃; then, after the sample is cooled to room temperature, the sample is placed in a developing solution for soaking and developing, and the time is 30-120 s; then respectively washing with ethanol and deionized water, and drying by ear washing balls;

and (5): spin-coated waveguide array cladding

Firstly, coating a cladding polymer on a sample by spin coating at a rotation speed of 200-; then placing the sample on a flat plate hot table for pre-drying at 65 ℃ for 5min and at 90-120 ℃ for 15-30 min; finally, the whole sample piece is placed under a large-field ultraviolet lamp for exposure; then placing the exposed sample piece on a flat plate hot bench for post-baking at 90-120 ℃ for 10-30 min; thus obtaining the micron-scale polymer fundamental waveguide array.

Further, the substrate in the step (1) is an oxide film with 2-5 μm,<100>a tangential silicon wafer; the silicon wafer is cut into 10x2cm pieces according to the dissociation plane in advance²And (5) standby.

Further, the polymer photoresist solution in the step (1) is an Epocore photoresist.

Further, the adjusting device in the step (2) is a modified adjustable spectacle frame with two knobs; the fixing mode is that the silicon chip is flatly pasted on a cover glass by using a double-sided adhesive tape, and then the cover glass is downwards arranged on a mirror bracket with a clamp; the moving platform is a combined three-axis ABL1000 linear motor driven air bearing platform with a stroke of 50mm x50mm x50 mm.

Further, the interface position in the step (3) is positioned by using an overexposure method: firstly, adjusting the angle of an attenuation sheet in a light path to ensure that the power reaching a sample sheet is 40 mw; then, controlling the Z-axis movement to make the distance between the sample and the objective lens smaller than the focal length of the objective lens, namely the laser focus is in the silicon chip; then, slowly raising the height of the Z axis, wherein the laser focus position is not moved, and the interface is slowly close to the focus; the position where the CCD is observed to explode in the rising process is the interface of the photoresist/silicon wafer, namely the processing starting position.

Further, the wavelength range of the femtosecond laser continuous light source in the step (3) is 780-810 nm; the laser scanning power is 17-25 mw; the point-by-point scanning speed of the waveguide is 0.05-0.5 mm/s; the processing objective lens is a 20x-100x lens. The waveguide scanning mode is that the waveguide is divided into a plurality of longitudinal planes, X-Y in-plane scanning is completed when Z is 0, then X-Y in-plane scanning is completed when Z is 100nm, and the rest is done in sequence, and finally the whole scanning of the waveguide is realized.

Further, the method for removing the uncrosslinked photoresist in the step (4) comprises the following steps: firstly, placing a scanned sample piece on a flat hot table for post-baking at 65 ℃ for 5min and at 90-120 ℃ for 5-15min, wherein strong acid generated in a laser scanned area can promote the photoresist monomer to realize intramolecular and intermolecular crosslinking; then, after the sample is cooled to room temperature, the sample is placed in an acetone solution for soaking and developing, and the time is 30-120 s; then, respectively washing with ethanol and deionized water to respectively remove residual acetone and ethanol, and lightly drying by ear washing balls; finally, the extra material outside the unscanned waveguide is dissolved and removed by the acetone solution, and the scanned area forms a dense grid due to molecular cross-linking and is not dissolved in the acetone solution.

Further, the ultraviolet exposure power in the step (5) is 100-.

Compared with the prior art, the invention has the following advantages:

(1) the femtosecond laser two-photon polymerization technology without a mask is used for preparing the waveguide, and the waveguide section with complex appearance can be realized due to the processing capability of the femtosecond laser two-photon polymerization technology for penetrating through the interior of the material and extremely high processing resolution. Particularly, longitudinal thickness control and laser direct writing can realize the preparation of special-shaped waveguides such as circular waveguides, rhombic waveguides and the like which are difficult to realize by the traditional photoetching method.

(2) The optical simulation platform based on the invention has the capability of precisely customizing the longitudinal thickness, so that an important parameter of height gradient is introduced for the waveguide array, another control degree of freedom is provided for the optical simulation platform, the applicability and the functionality of the simulation platform are greatly expanded, and if the height can influence the birefringence of the waveguide section, the polarization information in the waveguide is further controlled.

(3) In terms of expansibility, the material is wide in adaptability and can customize most of polymer material systems based on a processing means of femtosecond laser two-photon polymerization direct writing; in addition, the polymer material can realize the modulation of the refractive index by changing the component proportion; different processing powers of the laser can influence the polymerization density of the polymer grid, and the refractive index distribution of the device is further finely adjusted. In conclusion, the invention has excellent expansibility, and can adapt to various simulation requirements by flexibly changing parameters such as refractive indexes of the core layer and the cladding layer, thickness, width and the like.

Drawings

FIG. 1 is an experimental flow chart of the method for manufacturing an optical simulation platform based on femtosecond laser direct writing according to the present invention;

FIG. 2 is a schematic diagram of the preparation of the femtosecond laser direct writing-based optical simulation platform preparation method according to the present invention;

(FIG. 2(a) is a schematic diagram of spin-coating an EpoCore photoresist; FIG. 2(b) is a schematic diagram of laser scanning; FIG. 2(c) is a schematic diagram of developing; FIG. 2(d) is a schematic diagram of spin-coating an EpoClad photoresist);

FIG. 3 is a scanning electron microscope photograph of a waveguide array based on the femtosecond laser direct writing optical simulation platform manufacturing method of the present invention;

FIG. 4 is an optical test experimental result of the femtosecond laser direct writing-based optical simulation platform preparation method of the present invention;

Detailed Description

Example 1

The method for preparing the optical simulation platform based on femtosecond laser direct writing comprises the following steps of, by using the femtosecond laser direct writing polarization-dependent Bloch oscillation optical waveguide array, as shown in figure 1:

step (1) sample piece preparation

First, a film having an oxide film of 5 μm is taken<100>The tangential silicon slice is cut into 10x2cm according to the tangential direction by a glass cutter²Taking out absorbent cotton soaked with acetone and ethanol by using tweezers to wipe a silicon wafer as a substrate, washing the silicon wafer by using deionized water, and drying the silicon wafer by using nitrogen for later use; then, taking out 2ml of solution from the commercial EpoCore photoresist by using a rubber head dropper, dripping the solution on a silicon wafer, and then placing the wafer on a rotary table for spin coating, wherein the rotating speed is 3000r/min, and the time is 30 s; finally, the spin-coated wafer was placed on a hot stage for pre-baking at 65 ℃ for 5min and at 95 ℃ for 120min, respectively, to obtain a sample wafer to be processed having a film thickness of about 5 μm, as shown in fig. 2 (a).

Step (2) leveling of the processing table

Firstly, adsorbing a sample piece subjected to pre-baking on a processing table; then, opening a laser optical gate, and adjusting the height of the sample stage by controlling the moving stage to enable the appearance of a light spot on the sample to be observed in the CCD; then, controlling the moving platform to move along the X-axis direction, and adjusting a knob of the sample platform until the laser focus is always focused on the interface of the photoresist/the substrate without relative movement and the appearance of a light spot is kept unchanged during movement; and then, controlling the moving platform to move along the Y-axis direction, and adjusting a knob on the other side of the sample platform until the laser focus is always focused on the photoresist/substrate interface without relative movement and the appearance of a light spot is unchanged during movement. The processing table is now leveled.

Femtosecond laser direct writing scanning of waveguide array in step (3)

Firstly, controlling the power output of laser to ensure that the power reaching a sample wafer is 18 mw; then, the computer is used for controlling the mobile platform to move along the Z axis, so that the sample piece is tightly attached to the objective lens but is not contacted with the objective lens; then, slowly raising the height of the Z axis to enable the interface to slowly approach the focus; the photoresist is observed to be gasified when rising to a certain height through the CCD; determining the position as a photoresist/substrate interface, namely a processing starting surface; and controlling the X axis and the Y axis to move to one corner of the wafer, starting a processing program, keeping the laser focus still, and realizing long-range continuous movement of the sample wafer according to the designed structure, thereby finally completing the scanning of the whole waveguide array on the sample wafer, as shown in fig. 2 (b). The femtosecond laser wavelength is 810nm, the repetition frequency is 120Mhz, the processing power is 18mw, the scanning speed is 1.0mm/s, the used processing objective lens is a 40x lens, the height of the waveguide array is 2-4 μm gradual change, the width is 4 μm, and the length is 8 cm.

Step (4) removing the uncrosslinked photoresist

Firstly, placing a scanned sample piece on a flat hot table for post-baking, wherein the temperature is 5min at 65 ℃, and the temperature is 10-30min at 90-120 ℃; then, the flat plate heating table is closed, after the sample is cooled to room temperature, a culture dish is taken and poured with 20ml of acetone, the sample is placed in an acetone solution to be soaked for 30s and then taken out, then the sample is respectively washed with ethanol and deionized water, and the sample is lightly dried by an ear washing ball, as shown in fig. 2 (c).

Step (5) spin coating the waveguide array cladding

Firstly, 5ml of coating polymer EpoClad is taken to be spin-coated on a sample sheet, the rotating speed is 2000r/min, and the spin-coating time is 30 s; then placing the sample on a flat plate hot table for pre-drying at 65 ℃ for 5min and at 90-120 ℃ for 15-30 min; finally, the whole sample piece is placed under a 300W ultraviolet lamp for exposure for 30s, and the distance between the sample piece and the bulb is 0.3 m; then placing the exposed sample piece on a flat plate hot bench for post-baking at 95 ℃ for 15 min; the post-baking development is performed again to uniformly crosslink the cladding material to form a medium with uniform refractive index. As shown in fig. 2(d), a micron-scale polymer fundamental waveguide array was obtained.

In order to prove that the waveguide array is not adhered in the processing, the incomplete processing does not occur; and the waveguide array was optically tested in order to prove that the fabricated devices were highly satisfactory. Firstly, splitting two ends of a prepared sample into pieces according to a dissociation surface by using a glass cutter, and placing two ends of a waveguide at the edges of the sample pieces; then, a 10-time objective lens is used for coupling test laser 810nm infrared laser into one end of the waveguide, the laser power is 200mw, and the input light polarization is in TM and TE modes respectively; shooting a dark field photo at the top of the device by using a CCD, moving the position of the CCD, and traversing the length range of the waveguide; and carrying out data processing on the pictures to obtain a continuous laser transmission path. Fig. 3 is a scanning electron microscope photograph of the prepared waveguide array, fig. 4 is an optical test experiment result of a sample, and an analysis experiment result shows that the device successfully realizes optical oscillation and the oscillation periods of the device for different input light polarizations are different, which indicates that the device realizes the expected height gradient modulation.

It will be obvious to those skilled in the art that the present invention may be varied in many ways, and that such variations are not to be regarded as a departure from the scope of the invention. All such modifications as would be obvious to one skilled in the art are intended to be included within the scope of this claim.