Highly tunable elastic dielectric metasurface lenses (1604.03597v1)

Abstract: Dielectric metasurfaces are two-dimensional structures composed of nano-scatterers that manipulate phase and polarization of optical waves with subwavelength spatial resolution, enabling ultra-thin components for free-space optics. While high performance devices with various functionalities, including some that are difficult to achieve using conventional optical setups have been shown, most demonstrated components have a fixed functionality. Here we demonstrate highly tunable metasurface devices based on subwavelength thick silicon nano-posts encapsulated in a thin transparent elastic polymer. As proof of concept, we demonstrate a metasurface microlens operating at 915 nm, with focal distance tuning from 600 $\mu$m to 1400 $\mu$m through radial strain, while maintaining a diffraction limited focus and a focusing efficiency above 50$\%$. The demonstrated tunable metasurface concept is highly versatile for developing ultra-slim, multi-functional and tunable optical devices with widespread applications ranging from consumer electronics to medical devices and optical communications.

• The paper demonstrates tunable metasurface lenses using elastic encapsulation, achieving a focal range from 600 µm to over 1400 µm by stretching up to 50%.
• Experimental results confirm diffraction-limited performance with a focusing efficiency of 75% when relaxed and above 50% when stretched, closely matching simulations.
• The study details a robust fabrication process using electron beam lithography and PDMS encapsulation, ensuring mechanical stability over repeated stretch-release cycles.

Overview of Highly Tunable Elastic Dielectric Metasurface Lenses

This paper presents the development and experimental demonstration of highly tunable dielectric metasurface lenses encapsulated in elastic substrates. The research highlights the potential of integrating elasticity into metasurface designs, thereby extending the functionality and adaptability of optical components. This advancement is notable for applications requiring reconfigurable optics, such as portable devices, adaptive optics, and beyond.

The core component of the proposed system is a metasurface composed of amorphous silicon nano-posts arranged in a square lattice and encapsulated in a thin, flexible PDMS membrane. These dielectric metasurfaces manipulate optical phase with subwavelength precision, offering a streamlined alternative to bulky traditional optics without sacrificing efficiency or resolution. The metasurface is inherently versatile due to its ability to independently modulate phase and polarization of transmitted light. By encapsulating these nano-posts in an elastic material, the metasurface can be mechanically tuned, thus modulating its optical properties without altering its fundamental structural design.

Technical Accomplishments

1. Tunability and Functionality: The paper demonstrates a metasurface lens with a tuning range surpassing 130%—from 600 µm to over 1400 µm focal length—achievable by stretching the substrate up to 50%. The operational wavelength is 915 nm, and the lens maintains consistent quality with diffraction-limited performance and high focusing efficiency across the tuning range.
2. Strong Numerical Results: Measured and analytically predicted focal distances aligned well, affirming the tunability of the device. Notably, the device achieved a high focusing efficiency of approximately 75% when relaxed, decreasing to above 50% at stretched states, aligning closely with simulations.
3. Manufacturing Process: The paper outlines the fabrication process, involving the deposition of a germanium sacrificial layer, patterning of the metasurface via electron beam lithography, and successful encapsulation of the nano-posts within PDMS. The release of the metasurface from the substrate without compromising optical performance further indicates the robustness of the manufacturing method.
4. Mechanical Stability: Validation of mechanical resilience is demonstrated through the maintenance of optical performance after over ten cycles of stretching and releasing, indicating reliable repeatability which is essential for practical deployment.

Implications and Future Directions

The development of such tunable metasurface optics potentially revolutionizes the adaptability of photonic devices, paving the way for ultra-thin, multifunctional devices. Key applications lie in consumer electronics where compact, variable-focus lenses offer substantial benefits. Moreover, the integration of this technology with electronic systems, particularly flexible and wearable electronics, could spur advances in wearable sensors or mobile AR/VR optics.

Practically, this work suggests exploring the potential of high-speed electronically tunable elastomers to achieve rapid response times, enhancing device applicability in dynamic environments. The robust design principles laid out in this research provide a foundation for future investigations into integrating metasurfaces into larger, complex systems, leveraging their ultra-thin form factors for space-constrained applications.

In conclusion, this paper contributes valuable insights into the use of elastic encapsulation in metasurfaces, highlighting important advancements in tunable optics. The successful demonstration of a metasurface-based microlens with considerable tunability underscores the versatility and future potential of metasurface technologies in diverse optical systems. Further developments may focus on expanding wavelength tunability, enhancing mechanical durability, or integrating additional optical functionalities to meet the demands of emerging technologies.