Mahmood Bagheri

Dielectric metasurfaces for complete control of phase and polarization with subwavelength spatial resolution and high transmission

Metasurfaces are planar structures that locally modify the polarization, phase and amplitude of light in reflection or transmission, thus enabling lithographically patterned flat optical components with functionalities controlled by design. Transmissive metasurfaces are especially important, as most optical systems used in practice operate in transmission. Several types of transmissive metasurface have been realized, but with either low transmission efficiencies or limited control over polarization and phase. Here, we show a metasurface platform based on high-contrast dielectric elliptical nanoposts that provides complete control of polarization and phase with subwavelength spatial resolution and an experimentally measured efficiency ranging from 72% to 97%, depending on the exact design. Such complete control enables the realization of most free-space transmissive optical elements such as lenses, phase plates, wave plates, polarizers, beamsplitters, as well as polarization-switchable phase holograms and arbitrary vector beam generators using the same metamaterial platform.

Subwavelength-thick lenses with high numerical apertures and large efficiency based on high-contrast transmitarrays

Flat optical devices thinner than a wavelength promise to replace conventional free-space components for wavefront and polarization control. Transmissive flat lenses are particularly interesting for applications in imaging and on-chip optoelectronic integration. Several designs based on plasmonic metasurfaces, high-contrast transmitarrays and gratings have been recently implemented but have not provided a performance comparable to conventional curved lenses. Here we report polarization-insensitive, micron-thick, high-contrast transmitarray micro-lenses with focal spots as small as 0.57 λ. The measured focusing efficiency is up to 82%. A rigorous method for ultrathin lens design, and the trade-off between high efficiency and small spot size (or large numerical aperture) are discussed. The micro-lenses, composed of silicon nano-posts on glass, are fabricated in one lithographic step that could be performed with high-throughput photo or nanoimprint lithography, thus enabling widespread adoption.

Photonic cavity synchronization of nanomechanical oscillators.

Synchronization in oscillatory systems is a frequent natural phenomenon and is becoming an important concept in modern physics. Nanomechanical resonators are ideal systems for studying synchronization due to their controllable oscillation properties and engineerable nonlinearities. Here we demonstrate synchronization of two nanomechanical oscillators via a photonic resonator, enabling optomechanical synchronization between mechanically isolated nanomechanical resonators. Optical backaction gives rise to both reactive and dissipative coupling of the mechanical resonators, leading to coherent oscillation and mutual locking of resonators with dynamics beyond the widely accepted phase oscillator (Kuramoto) model. In addition to the phase difference between the oscillators, also their amplitudes are coupled, resulting in the emergence of sidebands around the synchronized carrier signal.

Sapphire-bonded photonic Crystal microcavity lasers and their far-field radiation patterns

Room-temperature continuous-wave lasing was demonstrated in photonic crystal microcavities with diameters of approximately 3.2 /spl mu/m. Far-field radiation patterns of these lasers were experimentally measured and compared with numerical simulation predictions.

High-quality-factor photonic crystal heterostructure laser

A high-quality-factor (Q) photonic crystal heterostructure laser was designed and characterized. Good agreement was obtained between the experimental lasing data and three-dimensional finite-difference time-domain numerical predictions.

Efficient dielectric metasurface collimating lenses for mid-infrared quantum cascade lasers.

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.

Reliable mid-infrared laterally-coupled distributed-feedback interband cascade lasers

We report on the performance and reliability of laterally-coupled distributed-feedback (DFB) interband cascade lasers designed to operate at 3.6 μm wavelength. A two-step ridge etch process ensures single-transverse-mode operation with minimal lateral current spreading, and a second-order Bragg grating etched alongside the ridge waveguide imposes single-mode DFB operation. Life tests performed on four randomly selected lasers, continuously operating at 40 °C with output power >10 mW, showed no measurable degradation after each laser was operated continuously for more than 1500 h.

Experimental characterization of the optical loss of sapphire-bonded photonic crystal laser cavities

Sapphire-bonded photonic crystal laser cavities with varying number of photonic crystal periods were studied in order to determine the optical loss in these cavities. The lasing threshold increases as the number of lattice periods decreases, and the quality factors of these cavities were calculated from the lasing threshold data. Continuous-wave operation was achieved for cavities with eight or more cladding periods

Linewidth and modulation response of two-dimensional microcavity photonic crystal lattice defect lasers

Linewidth and small-signal modulation response measurements are reported for room-temperature-operating microcavity two-dimensional photonic crystal lasers. The measured optical linewidth versus the output power was found to saturate at values on the order of 1 GHz. The modulation bandwidth for these low-power lasers was demonstrated to be on the order of 10 GHz

Photonic crystal lasers in InGaAsP on a SiO 2 /Si substrate and its thermal impedance

Two-dimensional photonic crystal defect lasers in InGaAsP membranes directly bonded to a SiO(2)/Si substrate have been demonstrated. Lasing at wavelengths near 1550 nm was obtained with incident threshold pump powers as low as 1.5 mW. Good agreement between experimental data and three-dimensional finite-difference time-domain (FDTD) simulation was achieved. The thermal impedance of this laser is also characterized.