4H-SiC metalenses: suppress the thermal drift effect of high-power laser irradiation

 Recently, the research group of Qiu Min of the Future Industry Research Center of Westlake University and the School of Engineering has successfully developed a new type of silicon carbide photonic device, which can effectively reduce the thermal drift problem in high-power laser processing. The team used semiconductor technology to fabricate large-aperture, high-precision 4H-SiC metalenses, which benchmarked high-performance commercial objective lenses and achieved diffraction-limited focusing. After a long period of high-power laser irradiation, the device performance remains stable and is almost unaffected by heat absorption. This achievement represents a major breakthrough in high-power laser systems, opening up new horizons for their application and efficiency improvement. The research results were published in the international journal Advanced Materials under the title of "4H-SiC Metalens: Mitigating Thermal Drift Effect in High-Power Laser Irradiation".

Background:

In laser processing, precise beam focusing is crucial. However, due to the low thermal conductivity of traditional objective lens materials, it is difficult to dissipate heat in a timely and effective manner under high-power laser irradiation, resulting in deformation or melting of the lens due to thermal stress, resulting in focus drift, decreased optical performance, and even irreversible damage. This thermal drift problem not only affects machining accuracy, but also limits production efficiency and equipment reliability. While cooling units can be used to alleviate heat dissipation issues, they increase the size, weight, and cost of the system, reducing the integration and suitability of the device. Therefore, there is an urgent need for a new type of optical device that can suppress thermal drift while maintaining high optical performance and compact size in high-power laser processing.
As a third-generation semiconductor material, silicon carbide (SiC) has great potential in the fields of high-power electronics, high-temperature and high-frequency devices, optoelectronics and optics due to its excellent characteristics such as wide bandgap, high thermal conductivity, low loss in the visible to near-infrared band, and excellent mechanical hardness. With more than 20 years of accumulation of micro-nano fabrication technology, Min Qiu's research group has developed a large-area, high-aspect-ratio nanostructure processing technology for 4H-SiC materials that is compatible with mass production. Based on the extensive processing capabilities of this process, the team designed a large-aperture 4H-SiC metalens with reference to the optical specifications of high-performance commercial objectives. In the end, the research team successfully realized a high-performance metalens device that works stably and durably under harsh conditions, meeting the industry's strict requirements for transmission focusing devices in high-power laser processing and promoting the development of related industries.
Research Highlights:
In this study, Min Qiu's group designed and fabricated a homogeneous 4H-SiC metalens, which achieved the optical performance of comparable commercial objective lenses and successfully mitigated the thermal drift effect under high-power laser irradiation (as shown in Figure 1). The 4H-SiC material is selected for its high refractive index, low loss in the visible to near-infrared spectral range, excellent mechanical hardness, chemical resistance, and high thermal conductivity. The optical test results show that the 4H-SiC metalens has comparable optical performance to commercial objective lenses. In high-power laser irradiation tests, which simulate long-term continuous processing under harsh operating conditions, 4H-SiC metalenses demonstrate stable performance while freeing themselves from the dependence on complex cooling systems, opening up new applications for SiC photonics.

Figure 1. Schematic diagram of the thermal drift effect of a 4H-SiC metalens (left) and a conventional objective (right)

 This 4H-SiC metalens is benchmarked against a high-performance commercial objective (Mitutoyo 378-822-5) and is designed for a numerical aperture (NA) of 0.5 and a focal length of 1 cm. It is worth noting that the aperture width of 4H-SiC metalenses is 1.15 cm, which exceeds the beam size typically produced by high-power lasers, and has a wide range of adaptability. To achieve both design and fabrication, isotropic nanopillars are used as supercells (as shown in Figure 2a) with a height of H = 1 μm to provide dynamic phase in the form of truncated waveguides. The period between adjacent supercells is P = 0.6 μm, at which diffraction-limited focusing can be achieved. Since the birefringence of 4H-SiC causes a slight phase difference between the x- and y-polarization incidences, the research team optimized each supercell by minimizing the figure of merit. The result is 8 sizes of supercells (Figure 2b-d), each of which achieves phase modulation at a wavelength of 1.060 μm and has a high transmittance greater than 0.85 that is not sensitive to polarization.

                     Figure 2. Optical response of a 4H-SiC metasurface unit

The fabrication of 4H-SiC metalenses uses a series of semiconductor processing processes, such as electron beam lithography, physical vapor deposition, and inductively coupled plasma etching. Fully packed high aspect ratio nanopillars were processed on a substrate surface of 1.15×1.15 cm². As shown in Figure 3a-e, the structure period was 600 nm, the fill factor was 0.3 to 0.78, and the structure height was 1.009 μm, measured by scanning electron microscopy and atomic force microscopy. The results of the sample characterization demonstrate the superiority of the processing process. This large-area, high-precision, high-aspect ratio metasurface fabrication method can be applied to similar devices to achieve mass production.
The optical properties of the 4H-SiC metalenses were tested using a self-built transmission microscope (shown in Figure 3f). The system directs parallel lasers with a wavelength of 1030 nm perpendicularly onto a 4H-SiC metalens for CCD imaging via a coaxial microscope system. Imaging of the focal plane and focal field was obtained by performing a step scan test within ±35 μm of the focal plane (as shown in Figure 3G-h). Data analysis shows that the focal field at a focal length of 1 cm shows a smooth Gaussian distribution. The intensity distribution in the focal plane test shows excellent focusing performance (Fig. 3i-j), with a full width of 2.9 μm at half height. Based on the test results, the focusing efficiency of the 4H-SiC metalens is calculated to be 96.31%. The incident and exit surfaces of the 4H-SiC metalens were measured using an optical power meter, and the transmittance of the device was measured to be 0.71. Based on these optical test results, 4H-SiC metalenses exhibit optical specifications comparable to commercial objective lenses and are able to achieve the same processing capabilities in laser processing systems.

Figure 3. Morphology characterization and optical testing of 4H-SiC metalenses

To simulate the harsh high-power continuous processing conditions in laser processing, the same optical path is used in the thermal drift test as in the optical test, but the light source is replaced with a 15 W 1030 nm laser. The changes in device temperature, focal plane, and cutting effect of 4H-SiC metalenses and commercial objectives were tested for 1 hour of continuous operation, respectively. The change in device surface temperature measured by the thermal imaging camera is shown in Figure 4a-b. After 60 minutes of high-power laser irradiation, the device temperature of the 4H-SiC metalens increased by only 3.2°C, and the temperature change was only 6% of that of the objective (54.0°C). Compared to conventional objectives, 4H-SiC metalenses reach a stable temperature after about 10 minutes of operation without additional cooling components, with less temperature variation and lower operating temperatures. This superior thermal management demonstrates the effectiveness of 4H-SiC metalenses under demanding conditions.
To account for changes in the optical performance of the device, the focal plane shift of the device over a 1-hour period was recorded using a CCD (as shown in Figure 4c-d). The test results showed that the focus of the 4H-SiC metalens did not shift significantly, while the focus of the commercial objective showed a significant shift after 30 minutes, and eventually the CCD could not be imaged due to excessive offset. The half-height full-width and center coordinates of the focal point were obtained by image processing, and the intra-plane displacement data were obtained by comparing the focal coordinates with the initial position. After 1 hour of continuous irradiation with a high-power laser, the offset distance of the device along the optical axis is obtained by moving the Z-axis stage back to the focal plane. The focal plane shift of the commercial objective is 213 μm, while the focal plane shift of the 4H-SiC metalens is only 13 μm, indicating excellent optical stability and consistency during continuous irradiation of high-power lasers.
The same optical path was used for laser cutting experiments, and the influence of thermal drift on the processing effect was compared in the actual laser cutting process. In the experiment, 4H-SiC wafers, which are extremely difficult to process, were selected as the material to be cut. Calibrate the cutting optical path by stepping the scan test, and cut in the x-direction every 10 minutes after calibration, and record the change in cutting effect over 1 hour. The cleavage topography of the cut wafer cross-section was characterized using an optical microscope (as shown in Figure 4e-f). The results show that the laser cutting performance of the 4H-SiC metalens remains stable after 60 minutes of operation, while the focus of the commercial objective is significantly shifted to the inside of the substrate after 30 minutes. Data analysis found that the change in cutting depth of the 4H-SiC metalens after 1 hour of operation was only 11.4% of the change in the commercial objective. The experimental results verify the test of focal plane offset, which shows the better device stability of 4H-SiC metalenses in practical industrial applications.
       

Figure 4. Thermal drift test of 4H-SiC metalens under high-power laser irradiation

Summary and outlook
In this study, we propose a 4H-SiC metalens that can alleviate the problem of thermal drift in high-power laser processing. The experimental results show that the 4H-SiC metalens achieves excellent thermal stability and optical performance due to its excellent thermal conductivity. The metalens is based on the optical index of high-performance commercial objective lenses, and realizes efficient focusing that is not sensitive to polarization based on nanopillar supercells. Through the semiconductor processing technology compatible with mass production, the fabrication problem of large-aperture 4H-SiC metalenses has been successfully solved. Experiments show that the metalens achieves diffraction-limited focusing at the design focal length, and exhibits excellent stability under continuous irradiation of high-power lasers, with a very small focus shift, which is far better than commercial objectives. In laser cutting applications, the topography of the cut using this metalens changes less. These results highlight the superior performance of 4H-SiC metalenses compared to conventional objectives, which often require complex cooling systems to achieve similar levels of stability. Looking ahead, with further research and optimization, 4H-SiC metalenses are expected to be widely used in high-power laser systems, promoting the development of related fields. With its compact design and superior optical and thermal performance, this new generation of metasurface devices can be used in applications such as augmented reality, aerospace, and laser processing to effectively solve key thermal management problems in today's industry.