Randy Polson

Integrated metamaterials for efficient and compact free-space-to-waveguide coupling.

We applied nonlinear optimization to design nanophotonics-based metamaterials for efficient free-space-to-waveguide coupling. Three devices were designed, fabricated and characterized. The first device couples incident light into a multi-mode waveguide, the second device couples incident light into a single-mode waveguide directly, and the third device couples and separates two orthogonal polarizations into two multi-mode waveguides. All devices offer comparable or higher coupling efficiencies, are easier to fabricate, and demonstrate higher bandwidth when compared to conventional devices. Furthermore, each device is at least an order of magnitude smaller in area than previously reported devices. The highly efficient single-mode waveguide-coupler is a unique device that has not been experimentally demonstrated before. We further performed careful simulations to underscore the tolerance of these devices to fabrication errors. Their robustness is primarily a result of the large number of coupled guided-mode resonances that are responsible for each device performance.

Integrated digital metamaterials enables ultra-compact optical diodes

We applied nonlinear optimization to design integrated digital metamaterials in silicon for unidirectional energy flow. Two devices, one for each polarization state, were designed, fabricated, and characterized. Both devices offer comparable or higher transmission efficiencies and extinction ratios, are easier to fabricate, exhibit larger bandwidths and are more tolerant to fabrication errors, when compared to alternatives. Furthermore, each device footprint is only 3μm × 3μm, which is the smallest optical diode ever reported. To illustrate the versatility of digital metamaterials, we also designed a polarization-independent optical diode.

Broadband asymmetric light transmission via all-dielectric digital metasurfaces

We demonstrate broadband asymmetric transmission or optical-diode behavior via a digital metasurface, that is, a surface that is digitally patterned at subwavelength dimensions. Enhanced light-matter interactions at the interfaces of the metasurface break the symmetry in the propagation direction, and enables high light-transmission in one direction, while strongly reflecting the light in the opposite direction. We measured a peak extinction ratio of 11.18 dB and peak forward transmission efficiency of 74.3% at the design wavelength of 1.55μm. The operational bandwidth of the device was 201nm. We further designed, fabricated and experimentally characterized a digital metasurface that enables polarization-independent optical-diode behavior, which we believe is the first device of its kind. Our digital metasurfaces enable the optical-diode behavior in a single layer of sub-wavelength thickness for several input modes and therefore, can perform as a passive, albeit imperfect optical isolator.

Increasing the density of passive photonic-integrated circuits via nanophotonic cloaking

Photonic-integrated devices need to be adequately spaced apart to prevent signal cross-talk. This fundamentally limits their packing density. Here we report the use of nanophotonic cloaking to render neighbouring devices invisible to one another, which allows them to be placed closer together than is otherwise feasible. Specifically, we experimentally demonstrated waveguides that are spaced by a distance of ∼λ0/2 and designed waveguides with centre-to-centre spacing as small as 600 nm (−2 dB and an extinction ratio >15 dB over a bandwidth larger than 60 nm. This performance can be improved with better design algorithms and industry-standard lithography. The nanophotonic cloak relies on multiple guided-mode resonances, which render such devices very robust to fabrication errors. Our devices are broadly complimentary-metal-oxide-semiconductor compatible, have a minimum pitch of 200 nm and can be fabricated with a single lithography step. The nanophotonic cloaks can be generally applied to all passive integrated photonics.

Metamaterial-waveguide bends with effective bend radius < λ_0/2

We designed, fabricated, and characterized broadband, efficient, all-dielectric metamaterial-waveguide bends (MWBs) that redirect light by 180 deg. The footprint of each MWB is 3  μm×3  μm and redirection is achieved for single-mode waveguides spaced by 1.3 μm, which corresponds to an effective bend radius of 0.65 μm (<λ₀/2 for λ₀=1.55  μm). The designed and measured transmission efficiencies are >80% and ∼70%, respectively. Furthermore, the MWBs have an operating bandwidth >66 nm (design) and >56  nm (experiments). Our design methodology that incorporates fabrication constraints enables highly robust devices. The methodology can be extended to the general routing of light in tight spaces for large-scale photonic integration.

Spatially mapping random lasing cavities

A mapping technique is developed to spatially resolve random laser-emission spectra from disordered solid media with an optical gain above the threshold excitation intensity for lasing; the technique is applied to pi-conjugated polymer 1 ms. By mapping the spatial extent of emission peaks in the random laser spectrum, bright areas that correspond to naturally formed lasing microcavities are unraveled. The size of the obtained microcavities matches the size extracted from the Fourier transform analysis of the laser-emission spectrum. Mapping at increased excitation intensities shows multiple resonant microcavities that lase at increasing threshold intensities.

Ultra-compact polarization rotation in integrated silicon photonics using digital metamaterials

Polarization controlling devices such as polarization splitters and rotators are critical elements in integrated-photonic circuits that function via polarization-diversity schemes. Here, we present the design of an ultra-compact nanophotonic-polarization rotator (NPR) that rotates the polarization state from TE to TM with a simulated extinction ratio of 23dB over a coupling length of 5µm and an operating bandwidth of 40nm. This all-silicon device can be fabricated in a single lithography step and we have fabricated and characterized a preliminary device exhibiting 9dB extinction ratio. To emphasize the generality of our methodology, we also designed a NPR that can rotate the polarization state from TM to TE as well. A small device footprint is enabled by the evanescent coupling of guided modes enabled by computationally designed digital metamaterials.

Passive and active light control using computational metamaterials

We report on our latest developments in computational metamaterials based nanophotonics devices. Non-linear optimization is used to design passive metamaterials for ultracompact on-chip polarization rotator and waveguide cloaks. We also report a novel method of implementing active control to such devices and present the design of an all-optical modulator. Our devices exhibit efficient performance at a much smaller footprint compared to similar conventional devices.

Ultra-compact nanophotonic devices designed by computational metamaterials

Computationally designed metamaterials are used to realize a number of novel nanophotonic devices. We report on the development of ultra-compact on-chip polarization rotators, waveguide cloaks and all optical modulators made possible by the use of a non-linear optimization algorithm.