In the world of optics, Microlens Arrays (MLAs) have consistently played an indispensable role. With their high resolution, large depth of field, and compact structure, they are widely used in imaging systems, optical communications, sensing and detection, among other fields.

However, traditional microlens arrays have a significant limitation—fixed focal lengths, which cannot adapt to dynamically changing imaging requirements. With the rapid development of smart materials, micro-nano fabrication technologies, and optoelectronic control methods, tunable microlens arrays have emerged as the core components of next-generation intelligent optical systems. They not only overcome the limitations of fixed focal lengths but also achieve major breakthroughs in imaging flexibility, system integration, and functional diversity.

I. What is a Tunable Microlens Array

A microlens array is an optical element composed of tens of thousands of micron-scale lenses arranged in a specific pattern. Its initial design inspiration came from the compound eye structure of insects. This type of structure can capture multi-angle light simultaneously, enabling wide-angle imaging and depth-of-field extension. However, the limitation of fixed focal lengths becomes particularly prominent in complex, dynamic environments. Tunable microlens arrays introduce external control mechanisms, allowing real-time adjustment of the focal length, curvature, and even refractive index of the microlenses to adapt to different imaging conditions and functional requirements.

II. Tuning Methods for Microlenses

Currently, tuning methods are primarily divided into three categories: changing the physical shape of the microlens, adjusting the refractive index of the material, and phase modulation based on metasurfaces. Each method has its own characteristics, is suitable for different application scenarios, and drives the evolution of microlens technology from "static" to "dynamic," and from "passive" to "intelligent."

Shape Tuning: Making Microlenses "Dynamic"

Electrowetting-on-Dielectric (EWOD): Voltage-Controlled Curvature

Electrowetting is a technology based on controlling droplet behavior. When voltage is applied to a conductive substrate, the contact angle between the droplet and the substrate changes, leading to a change in droplet curvature and thus focal length adjustment. This method offers advantages such as fast response speed (up to millisecond level), large tuning range, and good reversibility.

For example, researchers have used inkjet printing technology to precisely deposit droplet microlenses on planar electrodes, with each lens corresponding to an independent electrode, achieving independent control of individual lenses. This approach avoids crosstalk between lenses and improves the system's control accuracy. Furthermore, electrowetting technology can also be used to manipulate the merging, splitting, and movement of microfluidic droplets, providing new implementation pathways for microfluidic optics and Lab-on-a-chip systems.

Nevertheless, electrowetting technology also faces challenges such as dielectric layer stability, contact angle saturation, and long-term reliability. Future research will focus on optimizing electrode structures and developing new dielectric materials to enhance overall performance.

Force-Induced Deformation: Pneumatic and Hydraulic Actuation

Using external mechanical forces (such as air pressure or hydraulic pressure) to deform flexible materials is another common tuning method. Commonly used materials include elastomers like polydimethylsiloxane (PDMS) and hydrogels. This method is particularly suitable for constructing biomimetic compound eye structures, capable of achieving large field-of-view, varifocal imaging.

For instance, one study integrated a PDMS microlens array with microfluidic channels. By adjusting the amount of liquid injected, the originally planar array transformed into a hemispherical structure, achieving a field of view up to 180°, with focal length continuously adjustable from 3.03 mm to infinity. This varifocal compound eye structure combines the advantages of human single-lens and insect compound eyes, showing broad prospects in fields like robotic vision and panoramic surveillance.

Another method involves using negative pressure to deform a membrane, thereby changing its curvature. This approach is simple in structure and relatively fast in response but requires maintaining a stable negative pressure environment, posing higher demands on sealing and control systems.

Stimuli-Responsive Materials: The Magic of Smart Materials

Smart materials can undergo controlled deformation under external stimuli (such as pH, temperature, electric field, light, etc.), providing rich means for dynamic tuning of microlenses.

For example, pH-responsive protein hydrogels swell or shrink in different acid-base environments, thereby changing lens curvature. Researchers used femtosecond laser direct writing to fabricate protein microlens arrays, achieving focal length tuning in biocompatible environments, offering new possibilities for endoscopy and bio-imaging.

Polyvinyl chloride (PVC) gel exhibits rapid deformation under an electric field, which can be used to construct electrically tunable microlenses. By designing multi-electrode driving structures, even independent multi-directional focus control can be achieved, simulating the accommodation mechanism of the human eye's crystalline lens, significantly improving imaging quality and adaptability.

Thermally responsive materials (like glycerol) expand upon heating and can also be used to construct thermally actuated microlenses. Combined with photothermal conversion materials (such as graphene, indium tin oxide), remote focal control via infrared light can be achieved, suitable for non-contact surface measurement in special environments. Although shape tuning methods are flexible and diverse, they also share common issues such as increased aberrations during deformation, material fatigue, and response uniformity, which remain key research focuses and challenges.

Refractive Index Tuning: Constant Morphology, Changing the Core

Unlike shape tuning, refractive index tuning does not alter the physical structure of the microlens but achieves focal length changes by adjusting the optical properties of the material. This method typically results in smaller aberrations and higher precision, making it suitable for high-resolution imaging systems.

Optofluidic Microlenses: Fusion of Microfluidics and Optics

Optofluidic technology integrates microfluidic channels and microlenses on the same chip. By injecting liquids with different refractive indices (such as deionized water, ethanol, sucrose solutions, etc.), the effective refractive index of the system is changed, thereby achieving focal length adjustment. This method avoids instability and wear caused by mechanical movement and is particularly suitable for biomedical fields like cell imaging and pathological detection.

For example, researchers used femtosecond laser-assisted etching to fabricate 3D optofluidic chips with embedded microlenses in glass. By switching liquids in the channels, focal length tuning in the range of 50–2500 µm was achieved. Another study printed droplet microlenses within narrow microfluidic channels, achieving a threefold focal length change by adjusting liquid proportions, successfully used for observing cell flow processes.

Challenges for optofluidic microlenses include the stability of liquid materials, the complexity of channel design, and environmental sensitivity. With future advancements in microfluidic technology, such systems are expected to find wide application in portable medical devices and labs-on-a-chip.

Liquid Crystal Microlenses: The Polarization Art of Electric Field Control

Liquid crystal materials possess optical anisotropy; the orientation of their molecules changes under applied electric fields or temperature variations, leading to changes in refractive index distribution. Utilizing this characteristic, voltage-tunable liquid crystal microlenses can be constructed.

These lenses are not only focal-tunable but also polarization-sensitive, enabling functionalities like multi-focus and dynamic aberration correction. For instance, by pre-setting the alignment patterns of liquid crystal molecules, bifocal or multifocal lenses can be fabricated; by adding chiral nanoparticles, circularly polarized light response can be achieved, expanding the tuning dimensions.

The fabrication processes for liquid crystal microlenses are also increasingly diverse, including advanced techniques like two-photon polymerization, nanoimprinting, and photo-alignment, further enhancing device integration and functional diversity. However, the response speed, driving voltage, and optical efficiency of liquid crystal materials still require optimization.

Metalens Tuning: Revolution in Planar Optics

A metalens is a two-dimensional optical element based on a metasurface, which precisely modulates light wavefronts through arrays of subwavelength structures. Compared to traditional lenses, metalenses offer advantages like small thickness, light weight, easy integration, and can achieve advanced functions such as broadband achromaticity and polarization control.

Tuning of metalenses is typically achieved by changing the geometry, arrangement, or material phase state of the meta-atoms. For example, combining a metalens with a liquid crystal layer allows voltage control of polarization state for dynamic adjustment of focal length and focusing efficiency; using the optical property changes of phase-change materials (like GST, VO₂) enables non-volatile tuning; employing MEMS technology to displace the metalens can also achieve large-range focal length adjustment.

Stretchable metalenses represent another innovative direction. By fabricating metasurfaces on flexible substrates, the structural period changes during stretching, enabling simultaneous control of focal length and chromatic aberration. Such devices offer new possibilities for applications like wearable optics and flexible displays. Although metalens technology is still in the laboratory stage, its advantages in integration, lightweight design, and multifunctionality make it an important development direction for future optical systems.

III. Application Prospects: From Laboratory to Life

Tunable microlens arrays show immense application potential in multiple fields:

Imaging and Detection: In light field cameras, tunable microlenses enable depth-of-field extension and real-time focusing, improving 3D reconstruction accuracy; in medical endoscopes, varifocal lenses can adapt to imaging needs at different tissue depths, enhancing diagnostic accuracy.

Display and Interaction: In AR/VR devices, tunable microlenses support 2D/3D mode switching, alleviating visual fatigue; in naked-eye 3D displays, microlens arrays provide multi-view images, enhancing immersion.

Artificial Intelligence and Robotics: Used for tasks like object tracking, scene perception, and depth estimation in machine vision, improving the environmental adaptability of autonomous systems.

Biological and Chemical Sensing: Combined with microfluidic technology, used for cell analysis, protein detection, environmental monitoring, etc., enabling high-throughput, high-sensitivity sensing functions.

IV. Challenges and Future Directions

Although significant progress has been made in tunable microlens array technology, numerous challenges remain:

1. Manufacturing Processes: High-precision, high-volume, low-cost manufacturing technologies are not yet mature, especially integration on flexible substrates or complex curved surfaces remains difficult.

2. Response Performance: Response speed, stability, and durability need further improvement to meet the demands of real-time applications.

3. System Integration: Efficiently integrating tuning elements with driving circuits and control systems is key to achieving commercial applications.

4. Multi-functional Integration: Future exploration is needed for multi-physical field cooperative tuning (e.g., electro-optical-mechanical coupling) to achieve more complex optical functions.

In the future, with the development of new materials (such as 2D materials, ferroelectric polymers), new processes (such as nano 3D printing, heterogeneous integration), and intelligent algorithms (such as machine learning-optimized design), tunable microlens arrays will further evolve towards high performance, low cost, and multi-functionality, becoming core components of intelligent optoelectronic systems.

Specialist in high-precision measuring instruments—white light interferometer