Efficient silicon metasurfaces for visible light (1609.06400v1)

Abstract: Dielectric metasurfaces require high refractive index contrast materials for optimum performance. This requirement imposes a severe restraint; devices have either been demonstrated at wavelengths of 700nm and above using high-index semiconductors such as silicon, or they use lower index dielectric materials such as TiO${2}$ or Si${3}$N$_{4}$ and operate in the visible wavelength regime. Here, we show that the high refractive index of silicon can be exploited at wavelengths as short as 532 nm by demonstrating a silicon metasurface with a transmission efficiency of 47% at this wavelength. The metasurface consists of a graded array of silicon posts arranged in a square lattice on a quartz substrate. We show full 2{\pi} phase control and we experimentally demonstrate polarization-independent beam deflection at 532nm wavelength. The crystalline silicon is placed on a quartz substrate by a bespoke layer transfer technique and we note that an efficiency >70% may be achieved for a further optimized structure in the same material. Our results open a new way for realizing efficient metasurfaces based on silicon in the visible wavelength regime.

• The paper demonstrates that crystalline silicon metasurfaces achieve 47% transmission at 532 nm with full 2π phase control.
• Methodology integrates RCWA and FDTD simulations with CMOS-compatible fabrication to validate a 21° beam deflection using a low aspect ratio design.
• Implications include scalable applications in flat lenses, holograms, and integrated optics by overcoming fabrication challenges in the visible range.

Overview of Efficient Silicon Metasurfaces for Visible Light

The research presented in this paper advances the field of dielectric metasurfaces by exploiting crystalline silicon (c-silicon) for wavefront control in the visible spectrum, specifically at a wavelength of 532 nm. Current dielectric metasurfaces typically operate above 700 nm, utilizing high-index semiconductors or lower index dielectric materials such as TiO2 and Si3N4, which present fabrication challenges due to their high aspect ratios. This paper demonstrates that c-silicon, known for its high refractive index and compatibility with CMOS processes, can overcome limitations in the visible light regime, achieving notable efficiency in optical transmission and phase control.

Key Results

The paper reports the successful deployment of a silicon metasurface comprising a graded array of c-silicon posts on a quartz substrate, achieving a transmission efficiency of 47% with room for improvement to 71%, according to simulations. The metasurface demonstrates full 2π phase control and polarization-independent beam deflection at 532 nm. Numerical simulations via rigorous coupled-wave analysis (RCWA) and finite-difference time-domain (FDTD) methods support the theoretical predictions, with simulations showing 59% efficiency, close to empirical results when manufacturing deviations are considered.

The fabricated metasurface exhibited a deflection angle of 21°, consistent with theoretical calculations, using a periodic array with post diameters selected for a full 0 to 2π phase coverage. Fabrication techniques involved transferring a 220 nm c-silicon layer from a SOI wafer to a quartz substrate using adhesive wafer bonding, followed by patterning via electron beam lithography. The aspect ratio of 2.47 is notably lower than required for other dielectric materials like TiO2, simplifying the manufacturing process.

Theoretical and Practical Implications

This work substantiates the use of c-silicon metasurfaces as an effective solution for visible light applications, overcoming absorption issues associated with amorphous and polycrystalline silicon. By achieving high transmission efficiency with a low aspect ratio, c-silicon metasurfaces hold promise for practical applications such as flat lenses, holograms, and nonlinear optical devices, potentially extending functionalities to flexible and integrated optics.

Future Directions

Further enhancements in the photonic performance of c-silicon metasurfaces could focus on reducing fabrication tolerances and optimizing design parameters to achieve transmission efficiencies closer to 71%. The insights gained from this paper suggest that c-silicon could drive innovations in metasurfaces, offering a scalable and efficient approach to wavefront shaping across various technological domains, including imaging systems, biomedical devices, and wearable electronics.

In summary, the research demonstrates that with appropriate material processing and structural design, silicon metasurfaces in the visible domain can reach proficiency levels comparable to, or exceeding, current high-performance metasurfaces in the infrared range, thereby broadening the potential application portfolio of dielectric metasurfaces.