MEMS-tunable dielectric metasurface lens (1712.06548v2)

Abstract: Varifocal lenses, conventionally implemented by changing the axial distance between multiple optical elements, have a wide range of applications in imaging and optical beam scanning. The use of conventional bulky refractive elements makes these varifocal lenses large, slow, and limits their tunability. Metasurfaces, a new category of lithographically defined diffractive devices, enable thin and lightweight optical elements with precisely engineered phase profiles. Here, we demonstrate tunable metasurface doublets, based on microelectromechanical systems (MEMS), with more than 60 diopters (about 4%) change in the optical power upon a 1-um movement of one metasurface, and a scanning frequency that can potentially reach a few kHz. They can also be integrated with a third metasurface to make compact microscopes (~1 mm thick) with a large corrected field of view (~500 um or 40 degrees) and fast axial scanning for 3D imaging. This paves the way towards MEMS-integrated metasurfaces as a platform for tunable and reconfigurable optics.

• The paper demonstrates a MEMS-tunable metasurface lens that achieves over a 60 diopter tuning range using a novel doublet design.
• The design uses electrostatic actuation to adjust the separation of metasurfaces, yielding 180-diopter power variation and near-diffraction-limited imaging across a 40° field of view.
• The integration of MEMS with metasurfaces offers rapid adjustments up to 4kHz, paving the way for compact, agile optical systems in advanced imaging applications.

Insights into MEMS-Tunable Dielectric Metasurface Lenses

The paper "MEMS-tunable dielectric metasurface lens" explores the design and applications of metasurfaces integrated with microelectromechanical systems (MEMS) to overcome limitations associated with traditional varifocal lenses. Metasurfaces, due to their thin and lightweight nature, allow for precise phase profile engineering and provide a promising avenue towards the development of tunable and reconfigurable optics.

Overview of Design and Innovation

The authors present a novel concept of a tunable metasurface doublet, comprising a pair of metasurface lenses with adjustable focal lengths, using MEMS technology. The system introduces significant improvements in optical power tuning capabilities, with the prototype demonstrating a remarkable tuning range of over 60 diopters, closely matched with simulation predictions. The design utilizes two metasurfaces—one fixed on a glass substrate and another on a silicon nitride membrane whose separation distance can be mechanically altered. The electrostatic actuation allows for precise variances in focal length by altering the membrane's position by mere micrometers. This proposed method achieves an optical power change corresponding to more than 4% and supports rapid scanning frequencies potentially reaching kilohertz levels, vastly superior to traditional varifocal lenses constrained by mechanical limitations.

Experimental and Numerical Results

Significant research outcomes include the achievement of an over 180-diopter optical power variation with an applied voltage of 80 volts while maintaining absolute focusing efficiencies of above 40%. The dynamic response of these metasurfaces shows a bandwidth of up to 230 Hz, which is promising for applications requiring quick focal adjustments. Further, it is suggested that under lower pressure conditions, the device frequency response could extend to 4 kHz.

Contrary to the conventional varifocal lens, this MEMS-tunable metasurface is optimally poised for integration into compact microsystems. Experimentally, a series of metasurface devices demonstrated nuanced focal tuning, and electrical adjustments were successful in resolving line spaces as fine as 3.5 micrometers. The simulated imaging test showcased exceptional diffraction-limited behaviour across a 40-degree field of view.

Practical Implications and Future Directions

This work has pertinent implications for the field of optical engineering, particularly in the construction of ultra-compact imaging systems such as endoscopes, fiber-tip microscopes, and potentially in-depth scanning lidar systems. The reduction in device size and ability to electronically control focal adjustments bodes well for the development of future precision optical systems. Notably, with evolving microfabrication techniques, larger aperture lenses could be realized, further broadening application areas without sacrificing speed or functionality.

In terms of theoretical advancements, the paper encourages further exploration of chromatic aberration corrections and broadband adaptability to expand operational wavelengths. The integration of dispersive metasurfaces could offer solutions to these challenges, augmenting the device's viability across a spectrum of photonic applications.

Conclusion

The MEMS-tunable dielectric metasurface lens presented in this paper highlights a significant step forward in the miniaturization and flexibility of optical systems. The innovation lies in leveraging MEMS technology for focal length adjustment while maintaining high optical efficiency and speed. The integration of metasurfaces with MEMS delineates a transformative direction for developing agile, high-performance optical devices in both industrial and academic settings. Future research could explore enhanced fabrication techniques and applications in other fields such as augmented and virtual reality (AR/VR) technologies.