Aberration-free ultra-thin flat lenses and axicons at telecom wavelengths based on plasmonic metasurfaces (1207.2194v1)

Abstract: The concept of optical phase discontinuities is applied to the design and demonstration of aberration-free planar lenses and axicons, comprising a phased array of ultrathin subwavelength spaced optical antennas. The lenses and axicons consist of radial distributions of V-shaped nanoantennas that generate respectively spherical wavefronts and non-diffracting Bessel beams at telecom wavelengths. Simulations are also presented to show that our aberration-free designs are applicable to high numerical aperture lenses such as flat microscope objectives.

• The paper presents a design for ultra-thin, aberration-free flat lenses and axicons leveraging plasmonic metasurfaces at telecom wavelengths.
• The paper employs V-shaped nanoantenna arrays and FDTD simulations to precisely control phase shifts over a full 0 to 2π range, enabling multiple focal lengths and non-diffracting Bessel beams.
• The paper validates its approach with experimental results closely matching simulations, despite challenges from substrate reflections and optical losses.

Aberration-Free Ultra-Thin Flat Lenses and Axicons at Telecom Wavelengths Based on Plasmonic Metasurfaces

The paper presents the development and experimental verification of aberration-free planar lenses and axicons leveraging the novel capabilities of plasmonic metasurfaces. This research primarily addresses the fundamental challenges associated with designing thin, lightweight optical elements that operate efficiently at telecom wavelengths (1.55 µm), particularly in combating monochromatic aberrations.

Overview of Optical Phase Discontinuities and Metasurfaces

Central to the paper's advancement is the concept of optical phase discontinuities. These involve controlling the optical wavefronts via metasurfaces consisting of ultrathin, subwavelength-spaced resonators. In this methodology, the phase shifts in the wavefronts are achieved through radiation scattering from the metasurface, as opposed to traditional optical devices where phase alterations transpire over longer light propagation distances. The proposed lenses and axicons in this paper are crafted from V-shaped nanoantennas strategically arranged in radial distributions.

Design and Experimental Results

The team successfully fabricated and tested two metasurface-based lenses with focal lengths of 3 cm and 6 cm and a metasurface axicon with a base angle of 0.5°. Notable achievements include the generation of non-diffracting Bessel beams, a capability not easily attainable with conventional optics at this wavelength. The lenses and axicons display excellent agreement with numerical simulations, reinforcing the robustness of the design approach. The effective implantation of these metastructures enables tailoring the phase over the entire 0 to 2π range, overcoming a key limitation in other planar focusing devices.

FDTD Simulations and Fabrication Techniques

The finite-difference time-domain (FDTD) simulations complement the experimental work, offering insight into the phase shifts and scattering amplitudes of the designed V-shaped antennas. Enumeration of eight distinct antenna elements with controlled phase increments allows for a diverse range of focal distances. Fabrication was realized on a double-side-polished silicon substrate using electron beam lithography, a technique ensuring precision placement of gold nanoscale antennas.

Optical Performance and Limitations

One remarkable feature of these flat optics is their aberration-free nature even at high numerical apertures (NAs). The integration of a hyperboloidal phase profile into the metasurface ensures that the emergent wavefront remains spherical beyond paraxial conditions, a critical feature for high-accuracy focusing and imaging tasks. However, current limitations in focusing efficiency, predominantly caused by internal substrate reflections and optical losses, are noted. Suggestions to enhance performance include utilizing low-loss metals and impedance matching techniques.

Implications and Future Directions

The implications of this research extend across various domains in optical engineering. These metasurface lenses and axicons pave the way for ultra-thin, lightweight, and high-performance optical components suitable for telecommunications and potentially other spectral regions, such as terahertz and mid-infrared. Future research could focus on overcoming the diffraction limit in far-field applications, potentially using structures with subwavelength resolution capabilities for phase and amplitude control.

In conclusion, the demonstrated plasmonic metasurface optics offer a promising platform for developing advanced optical devices with significant implications in both theoretical optics and practical applications. Leveraging further innovations in material science and nanofabrication, the issues of efficiency and expandability to other spectral ranges will be crucial areas for future exploration.