Hanhong Gao

Design of thin–film photonic metamaterial Lüneburg lens using analytical approach

We design an all-dielectric Lüneburg lens as an adiabatic space-variant lattice explicitly accounting for finite film thickness. We describe an all-analytical approach to compensate for the finite height of subwavelength dielectric structures in the pass-band regime. This method calculates the effective refractive index of the infinite-height lattice from effective medium theory, then embeds a medium of the same effective index into a slab waveguide of finite height and uses the waveguide dispersion diagram to calculate a new effective index. The results are compared with the conventional numerical treatment - a direct band diagram calculation, using a modified three-dimensional lattice with the superstrate and substrate included in the cell geometry. We show that the analytical results are in good agreement with the numerical ones, and the performance of the thin-film Lüneburg lens is quite different than the estimates obtained assuming infinite height.

Broadband surface-wave transformation cloak

Significance Guiding surface electromagnetic waves around disorder without disturbing the wave amplitude or phase is in great demand for modern photonic and plasmonic devices. In this work, we introduce a class of cloaks capable of remarkable broadband surface electromagnetic waves guidance around ultrasharp corners and bumps with no perceptible changes in amplitude and phase. This work provides strong support for the application of transformation optics to plasmonic circuits and could pave the way for high-performance, large-scale integrated photonic circuits. Guiding surface electromagnetic waves around disorder without disturbing the wave amplitude or phase is in great demand for modern photonic and plasmonic devices, but is fundamentally difficult to realize because light momentum must be conserved in a scattering event. A partial realization has been achieved by exploiting topological electromagnetic surface states, but this approach is limited to narrow-band light transmission and subject to phase disturbances in the presence of disorder. Recent advances in transformation optics apply principles of general relativity to curve the space for light, allowing one to match the momentum and phase of light around any disorder as if that disorder were not there. This feature has been exploited in the development of invisibility cloaks. An ideal invisibility cloak, however, would require the phase velocity of light being guided around the cloaked object to exceed the vacuum speed of light—a feat potentially achievable only over an extremely narrow band. In this work, we theoretically and experimentally show that the bottlenecks encountered in previous studies can be overcome. We introduce a class of cloaks capable of remarkable broadband surface electromagnetic waves guidance around ultrasharp corners and bumps with no perceptible changes in amplitude and phase. These cloaks consist of specifically designed nonmagnetic metamaterials and achieve nearly ideal transmission efficiency over a broadband frequency range from 0+ to 6 GHz. This work provides strong support for the application of transformation optics to plasmonic circuits and could pave the way toward high-performance, large-scale integrated photonic circuits.

Iterative nonlinear beam propagation using Hamiltonian ray tracing and Wigner distribution function

We present an iterative method for simulating beam propagation in nonlinear media using Hamiltonian ray tracing. The Wigner distribution function of the input beam is computed at the entrance plane and is used as the initial condition for solving the Hamiltonian equations. Examples are given for the study of periodic self-focusing, spatial solitons, and Gaussian-Schell model in Kerr-effect media. Simulation results show good agreement with the split-step beam propagation method. The main advantage of ray tracing, even in the nonlinear case, is that ray diagrams are intuitive and easy to interpret in terms of traditional optical engineering terms, such as aberrations, ray-intercept plots, etc.

Aperiodic subwavelength Lüneburg lens with nonlinear Kerr effect compensation.

We introduce a Lüneburg lens design where Kerr nonlinearity is used to compensate for the focal point shift caused by diffraction of a Gaussian source. A computationally efficient iterative method introduced in [Opt. Lett. 35, 4148 (2010)] is used to provide ray diagrams in the nonlinear case and verify the focal shift compensation. We study the joint dependence of focal shift on waist size and intensity of Gaussian source, and show how to compensate spherical aberration caused by the nonlinearity by a small perturbation of the Lüneburg profile. Our results are specific to Lüneburg lens but our approach is applicable to more general cases of nonlinear nonperiodic metamaterials.

Design of volume hologram filters for suppression of daytime sky brightness in artificial satellite detection

We present a design methodology for volume hologram filters (VHFs) with telephoto objectives to improve contrast of solar-illuminated artificial satellites observed with a ground-based optical telescope and camera system operating in daytime. VHFs provide the ability to selectively suppress incoming light based on the range to the source, and are used to suppress the daylight background noise since signal (satellite) and noise (daylight scatterers) are located at different altitudes. We derive the overall signal-to-noise ratio (SNR) enhancement as the system metric, and balance main design parameters over two key performance considerations--daylight attenuation and spectral bandwidth--to optimize the functioning of VHFs. Overall SNR enhancement of 7.5 has been achieved. Usage of multi-pixel cameras can potentially further refine this system.

Mass replication of multifunctional surface by nanoimprint of high aspect ratio tapered nanostructures

Tapered nanostructures with high aspect ratio were fabricated using high-throughput nanoimprint process. The replicated nanostructures have axially increasing effective refractive index, which enhances the optical transmission over a wide range of wavelengths and incident angles.

Guidance condition correction into the design of two dimensional nanophotonic devices

We quantify the importance of including finite thickness effects into the design of two dimensional nanophotonic metamaterial devices. Direct band diagram method and analytical guidance condition method are introduced and verified with nanostructured Liineburg lens.

Nonlinear Kerr effect aperiodic Lüneburg lens

A subwavelength-modulated dielectric nonlinear Lu¨neburg lens is proposed and numerically verified. In the linear limit, the structure focuses a plane wave to an ideal geometrical point image. In the presence of Kerr nonlinearity, focal shift to a Gaussian beam input is compensated at a tunable intensity value. Nonlinear Lu¨neburg lens is also capable of finite conjugate imaging. A modified aperiodic Lu¨neburg is also proposed to greatly reduce the aberration caused by the Kerr effect.

Background suppression in long-distance imaging using volume hologram filters.

We performed experiments using a volume hologram filter (VHF) coupled with a telephoto objective lens to detect weak distant signals masked by strong background noise. The VHF was able to selectively pass light originating from a certain distance while attenuating background noise contributions from other distances, resulting in a higher signal-to-noise ratio (SNR). The proposed method is useful in remote sensing applications such as daytime artificial satellite and space debris detection.

Hamiltonian and phase-space representation of spatial solitons

The authors thank Baile Zhang for useful discussions and the anonymous reviewers for constructive criticism and suggestions. Financial support was provided by Singapores National Research Foundation through the Center for Environmental Sensing and Modeling (CENSAM) and BioSystems and bioMechanics (BioSyM) independent research groups of the Singapore-MIT Alliance for Research and Technology (SMART) Centre, and by the Chevron-MIT University Partnership Program. (Singapores National Research Foundation through the Center for Environmental Sensing and Modeling (CENSAM); BioSystems and bioMechanics (BioSyM) independent research groups of the Singapore-MIT Alliance for Research and Technology (SMART) Centre; Chevron-MIT University Partnership Program)