Microlens for optical devices

2023: Scientists advance in making lenses of the future

Physicists from MIPT and Lebedev Institute of Physics presented an updated diagram for the production of microscopic devices in the form of a combination of aspherical microlens and an array of microlenses obtained by two-photon lithography. The obtained results have a wide range of applications in the production of complex optical devices, optimized micro-lenses for high-precision measurement of wave front curvature and the manufacture of refractive X-ray lenses. The work is published in the journal Physics of Wave Phenomenena. This was announced on November 21, 2023 by representatives of the Moscow Institute of Physics and Technology.

As reported, the development of technologies in astrophotonics, nanophotonics, optoelectronics and other fields required an increase in the quality of the resulting images. This led to the next generation of microoptic devices, namely microlens arrays. Microlens arrays (also called microlens arrays or lens arrays) are used to increase the optical fill factor. These lens systems serve to focus and concentrate light on the photodiode surface instead of allowing it to fall on the non-current sensitive areas of the device.

Due to their functionality, small size, light weight of the microlens matrix, they have gained great fame and applicability. Nevertheless, the production of such facilities remains a difficult task. There are many production methods, but each has disadvantages. For example, focused ion beam etching and electron beam etching are comparatively complex and expensive, microelectroerosive technology requires special preparation of components to achieve optimal product quality, and direct-recorded thermolithography and UV laser photolithography require accurate control of heat and other external parameters.

Aspherical microlenses and microlens arrays allow optimizing the efficiency of various optical devices. However, the production of such products is technologically difficult. The problem arises due to the impossibility of making microlenses of an arbitrary profile in an area of ​ ​ characteristic dimensions of several tens of micrometers using traditional technologies, such as single-point diamond milling and thermal melting. In our work, we verified the combination of aspherical microlens and an array of microlenses made by us by direct laser recording with two-photon polymerization. This structure was designed and optimized by us using computer modeling methods. explained Alexey Vitukhnovsky, head of the laboratory of 3D printing technologies for functional microstructures of MIPT

To create an array of microlenses, scientists used multiphoton lithography technology (also known as direct laser lithography, or DLW). The authors explained the decision by the comparative ease of implementation and the cheapness of the technology. Lithography in micro- and nanoelectronics is the formation in a special sensitive layer (resistance) applied to the surface of the substrate of a relief pattern repeating the topology of the microcircuit, followed by the transfer of this pattern to the substrates. The principal difference between multiphoton lithography is the use of two-photon absorption to change the solubility of the resist, which makes it possible to achieve clarity of the obtained pattern.

As a result of mathematical modeling using the Zemax program, scientists found the most optimal lens parameters: for array lenses, the radius of curvature R = 5.6 microns, focal length f = 10.9 microns, numerical aperture NA = 0.5, aperture 5.5 microns. The simulated system included a light source, an aspherical microlens, an array of microlens, and a multicore optical fiber (with seven cores). A special radius of curvature was chosen for the aspherical lens, which made it possible to optimize optical characteristics and correct spherical aberrations. The researchers made an aspherical lens of parabolic shape with a radius of curvature R = 24 μm, focal length f = 46.7 μm, numerical aperture NA = 0.43 and aperture 40.2 μm. The distance between the aspherical lens and the microlens was optimized to maximize the signal of the simulated system.

𝓲

Physics of Wave Phenomena

In the course of layer-by-layer manufacture of lenses, a large number of layers of small thickness were used, which made it possible to reduce roughness and thereby increase their optical qualities. A 780 nm laser was used for lithography.

Images for structure analysis were obtained using a confocal microscope. To obtain an image in the photoinitiator molecules included in the lens material, luminescence was excited by continuous exposure to an argon laser with a wavelength of 458 nm. The measurement step during scanning is 0.05 μm, which is equal to the height of the layers from which the lenses are made, which made it possible to accurately compare the results of measurements and numerical modeling. The data obtained showed that the result is consistent with the results of numerical modeling.

𝓲

Physics of Wave Phenomena

The development will be applicable in areas where wavefront sensors are used. Such sensors make it possible to measure the curvature of the wavefront and transmit data to processing devices, which allows you to change the shape or position of lenses or mirrors. This is used in adaptive optics, in particular in astronomy, to compensate for atmospheric turbulence and weather phenomena during observation with. They are Lands also used in production and research in laser devices, optics, space astronomy, in the production of contact and intraocular lenses, including optical elements for, mobile phones microscopes and camera lenses.

The work was carried out with the support of the Russian Science Foundation, project No. 22-79-10153.