High efficiency dielectric metasurfaces at visible wavelengths (1603.02735v1)

Abstract: Metasurfaces are planar optical elements that hold promise for overcoming the limitations of refractive and conventional diffractive optics1-3. Dielectric metasurfaces demonstrated thus far4-10 are limited to transparency windows at infrared wavelengths because of significant optical absorption and loss at visible wavelengths. It is critical that new materials and fabrication techniques be developed for dielectric metasurfaces at visible wavelengths to enable applications such as three-dimensional displays, wearable optics and planar optical systems11. Here, we demonstrate high performance titanium dioxide dielectric metasurfaces in the form of holograms for red, green and blue wavelengths with record absolute efficiency (>78%). We use atomic layer deposition of amorphous titanium dioxide that exhibits low surface roughness of 0.738 nm and ideal optical properties. To fabricate the metasurfaces we use a lift-off-like process that allows us to produce highly anisotropic nanofins with shape birefringence. This process is applicable to any optical element and can be implemented using a broad class of materials.

• The paper demonstrates titanium dioxide metasurfaces achieving record absolute efficiencies (>78%) at red, green, and blue wavelengths using ALD.
• It employs a modified lift-off process to fabricate TiO₂ nanofins with subwavelength dimensions and precise PB phase control for robust optical performance.
• Experimental and simulation results validate the high efficiency and versatility of these metasurfaces, highlighting their potential in advanced display and wearable optical systems.

High Efficiency Dielectric Metasurfaces at Visible Wavelengths

The paper presents the successful development of high-efficiency dielectric metasurfaces operating at visible wavelengths, a noteworthy accomplishment given the historical limitations due to significant optical absorption and loss in the visible spectrum. This advancement addresses the necessity for innovative materials and fabrication methods necessary for the proliferation of metasurfaces in applications such as 3D displays and wearable optics.

The research demonstrates the use of titanium dioxide (TiO₂) metasurfaces fashioned as holograms that function at red, green, and blue wavelengths, achieving record absolute efficiencies greater than 78%. The basis of this success lies in the employment of atomic layer deposition (ALD) to deposit amorphous TiO₂, characterized by a low surface roughness of 0.738 nm and optimal optical properties. The selected material ensures minimal absorption losses and a high refractive index — criteria crucial for effective phase manipulation of the exiting wavefront at a nanoscale.

Fabrication Process and Material Advantages

The fabrication procedure diverges from conventional liftoff techniques, involving a modified lift-off-like process conducive to the construction of TiO₂ nanofins with shape anisotropy and birefringence. The self-limiting nature of ALD provides precise monolayer control and conformal coverage necessary for high aspect-ratio nanostructures, thus facilitating the creation of metasurfaces with dimensions as small as 40 nm.

The choice of titanium dioxide is strategic due to its transparency window for visible wavelengths longer than ~350 nm and appropriate interband transition just outside the visible spectrum. The refractive index of the deposited TiO₂ remains sufficiently high across the visible range, supporting robust light-matter interactions and allowing extensive phase control. ALD further ensures a defect-minimized film compared to traditional deposition methods.

Optical Performance and Implications

Empirical data from TiO₂ metasurfaces show a refractive index ranging from 2.63 to 2.34 within the visible spectrum, aligning with the theoretical models. Optical performances are characterized using meta-holograms designed for peak efficiencies at 480 nm, 532 nm, and 660 nm, exhibiting maximum efficiencies of 82%, 81%, and 78% respectively. Such commendable efficiency levels are confirmed through simulations and experimental validation, exceeding previously reported metrics for metasurfaces and reflective analogs.

Notably, the amplitude and phase of the generated holographic images are manipulated using Pancharatnam-Berry (PB) phase principles, ensuring wavelength-independent phase impartation with varying efficiency levels. The high-resolution images produced validate the superior performance of TiO₂ metasurfaces, with subwavelength spacing ensuring the precise rendering of complex features.

Conclusion and Future Prospects

The research validates the capability of dielectric metasurfaces to function efficiently across the visible spectrum, introducing new possibilities for the integration of metasurfaces in compact optical systems. By leveraging the advantages of ALD and the tailored optical properties of TiO₂, the fabrication of metasurfaces becomes not only feasible but modular across different materials and designs.

The demonstrated versatility of these metasurfaces posits them as viable candidates for diverse practical applications such as planar lenses, axicons, and polarizers. Future research could extend this fabrication approach to other materials and configurations, broadening the theoretical and practical horizons of metasurface research and emerging optical technologies.