Abstract
Light emitted from single-mode semiconductor lasers generally has large divergence angles, and high numerical aperture lenses are required for beam collimation. Visible and near infrared lasers are collimated using aspheric glass or plastic lenses, yet collimation of mid-infrared quantum cascade lasers typically requires more costly aspheric lenses made of germanium, chalcogenide compounds, or other infrared-transparent materials. Here we report mid-infrared dielectric metasurface flat lenses that efficiently collimate the output beam of single-mode quantum cascade lasers. The metasurface lenses are composed of amorphous silicon posts on a flat sapphire substrate and can be fabricated at low cost using a single step conventional UV binary lithography. Mid-infrared radiation from a 4.8 μm distributed-feedback quantum cascade laser is collimated using a polarization insensitive metasurface lens with 0.86 numerical aperture and 79% transmission efficiency. The collimated beam has a half divergence angle of 0.36° and beam quality factor of M2=1.02.
Highlights
Quantum cascade lasers (QCLs) are compact and efficient sources of coherent mid-infrared radiation
Integration of directional antennas with the laser [6,7,8], and modification of the laser emission aperture [9,10,11,12] are shown to increase the effective area of the emission aperture and reduce beam divergence angle
We propose and experimentally demonstrate high nu-merical aperture (NA) diffractive mid-IR lenses based on dielectric metasurfaces
Summary
Quantum cascade lasers (QCLs) are compact and efficient sources of coherent mid-infrared (mid-IR) radiation. Due to the wavelength-scale emission aperture of single-mode QCLs, their output beams have large divergence angles. We propose and experimentally demonstrate high NA diffractive mid-IR lenses based on dielectric metasurfaces. Diffractive elements based on metallic and dielectric metasurfaces have recently attracted significant attention [15, 16] These metasurfaces are composed of two-dimensional arrays of subwavelength scatterers that modify the wavefront, polarization, or amplitude of light. They accurately realize lithographically defined phase, polarization, or amplitude profiles, and are fabricated using standard micro- and nano-fabrication techniques with potential for low cost wafer scale production. For efficient operation in mid-IR, the metasurface lenses should be fabricated using scatterers and substrates with low absorption. The metasurface lens has a focal length of 300 μm and its plane is parallel to the laser facet
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