Christopher Gladden

Probing the electromagnetic field of a 15-nanometre hotspot by single molecule imaging

When light illuminates a rough metallic surface, hotspots can appear, where the light is concentrated on the nanometre scale, producing an intense electromagnetic field. This phenomenon, called the surface enhancement effect, has a broad range of potential applications, such as the detection of weak chemical signals. Hotspots are believed to be associated with localized electromagnetic modes, caused by the randomness of the surface texture. Probing the electromagnetic field of the hotspots would offer much insight towards uncovering the mechanism generating the enhancement; however, it requires a spatial resolution of 1–2 nm, which has been a long-standing challenge in optics. The resolution of an optical microscope is limited to about half the wavelength of the incident light, approximately 200–300 nm. Although current state-of-the-art techniques, including near-field scanning optical microscopy, electron energy-loss spectroscopy, cathode luminescence imaging and two-photon photoemission imaging have subwavelength resolution, they either introduce a non-negligible amount of perturbation, complicating interpretation of the data, or operate only in a vacuum. As a result, after more than 30 years since the discovery of the surface enhancement effect, how the local field is distributed remains unknown. Here we present a technique that uses Brownian motion of single molecules to probe the local field. It enables two-dimensional imaging of the fluorescence enhancement profile of single hotspots on the surfaces of aluminium thin films and silver nanoparticle clusters, with accuracy down to 1.2 nm. Strong fluorescence enhancements, up to 54 and 136 times respectively, are observed in those two systems. This strong enhancement indicates that the local field, which decays exponentially from the peak of a hotspot, dominates the fluorescence enhancement profile.

A carpet cloak for visible light.

We report an invisibility carpet cloak device, which is capable of making an object undetectable by visible light. The cloak is designed using quasi conformal mapping and is fabricated in a silicon nitride waveguide on a specially developed nanoporous silicon oxide substrate with a very low refractive index (n<1.25). The spatial index variation is realized by etching holes of various sizes in the nitride layer at deep subwavelength scale creating a local effective medium index. The fabricated device demonstrates wideband invisibility throughout the visible spectrum with low loss. This silicon nitride on low index substrate can also be a general scheme for implementation of transformation optical devices at visible frequencies.

Overcoming the bandgap limitation on solar cell materials

The thermodynamic efficiency of a single junction solar cell is bounded by the Shockley-Queisser detailed balance limit at ∼30% [W. Shockley and H. J. Queisser, J. Appl. Phys. 32, 510 (1961)]. This maximal efficiency is considered achievable using a semiconductor within a restricted bandgap range of 1.1-1.5 eV. This work upends this assumption by demonstrating that the optimal material bandgap can be shifted to lower energies by placing selective reflectors around the solar cell. This technique opens new possibilities for lower bandgap materials to achieve the thermodynamic limit and to be effective in high efficiency solar cells.

Macroscale Transformation Optics Enabled by Photoelectrochemical Etching

Photoelectrochemical etching of silicon can be used to form lateral refractive index gradients for transformation optical devices. This technique allows the fabrication of macroscale devices with large refractive index gradients. Patterned porous layers can also be lifted from the substrate and transferred to other materials, creating more possibilities for novel devices.

Carpet Cloak Device for Visible Light

We report a cloak device that makes objects undetectable by visible light. It is designed using conformal mapping and is fabricated in silicon nitride waveguide on nano-porous silicon oxide substrate with very low refractive index.

Three-dimensional nanoscale imaging by plasmonic Brownian microscopy

Abstract Three-dimensional (3D) imaging at the nanoscale is a key to understanding of nanomaterials and complex systems. While scanning probe microscopy (SPM) has been the workhorse of nanoscale metrology, its slow scanning speed by a single probe tip can limit the application of SPM to wide-field imaging of 3D complex nanostructures. Both electron microscopy and optical tomography allow 3D imaging, but are limited to the use in vacuum environment due to electron scattering and to optical resolution in micron scales, respectively. Here we demonstrate plasmonic Brownian microscopy (PBM) as a way to improve the imaging speed of SPM. Unlike photonic force microscopy where a single trapped particle is used for a serial scanning, PBM utilizes a massive number of plasmonic nanoparticles (NPs) under Brownian diffusion in solution to scan in parallel around the unlabeled sample object. The motion of NPs under an evanescent field is three-dimensionally localized to reconstruct the super-resolution topology of 3D dielectric objects. Our method allows high throughput imaging of complex 3D structures over a large field of view, even with internal structures such as cavities that cannot be accessed by conventional mechanical tips in SPM.

Macroscale transformation optics enabled by photoelectrochemical etching of silicon(Conference Presentation)

Transformation optics provides a powerful tool for controlling electromagnetic fields and designing novel optical devices. In practice, devices designed by this method often require material optical properties that cannot be achieved at visible or near IR light wavelengths. The conformal transformation technique can relax this requirement to isotropic dielectrics with gradient refractive indices. However, there are few effective methods for achieving large arbitrary refractive index gradients at large scales, so the limitation for building transformation optical devices is still in fabrication. Here we present a photoelectrochemical (PEC) silicon etching technique that provides a simple and effective way to fully control the macro scale profiles of refractive indices by structuring porous silicon on the nanoscale. This work is, to our knowledge, the first demonstration of using light to control porosity in p-type silicon. We demonstrate continuous index variation from n = 1.1 to 2, a range sufficient for many transformation optical devices. These patterned porous layers can then be lifted off of the bulk silicon substrate and transferred to other substrates, including patterned or curved substrates, which allows for the fabrication of three dimensional or other more complicated device designs. We use this technique to demonstrate a gradient index parabolic lens with dimensions on the order of millimeters, which derives its properties from the distribution of nanoscale pores in silicon.

A new analysis for solar cell efficiency: Rigorous electromagnetic approach

Current methods for the evaluation of solar cell efficiency cannot be applied to extremely thin cells where phenomena from the realm of near-field optics prevail. We use the fluctuation dissipation theorem to calculate the rate of power removal from a semiconductor/metal system. This establishes for the first time, to our knowledge, a rigorous electromagnetic basis for solar cell efficiency analysis. We demonstrate our approach by calculating the efficiency of a Si solar cell with an Au back reflector.

Semiconductor plasmon laser

Laser science has tackled physical limitations to achieve higher power, faster and smaller light sources. The quest for ultra-compact laser that can directly generate coherent optical fields at the nano-scale, far beyond the diffraction limit of light, remains a key fundamental challenge. Microscopic lasers based on photonic crystals3, metal clad cavities4 and nanowires can now reach the diffraction limit, which restricts both the optical mode size and physical device dimension to be larger than half a wavelength. While surface plasmons are capable of tightly localizing light, ohmic loss at optical frequencies has inhibited the realization of truly nano-scale lasers. Recent theory has proposed a way to significantly reduce plasmonic loss while maintaining ultra-small modes by using a hybrid plasmonic waveguide. Using this approach, we report an experimental demonstration of nano-scale plasmonic lasers producing optical modes 100 times smaller than the diffraction limit, utilizing a high gain Cadmium Sulphide semiconductor nanowire atop a Silver surface separated by a 5 nm thick insulating gap. Direct measurements of emission lifetime reveal a broad-band enhancement of the nanowires exciton spontaneous emission rate up to 6 times due to the strong mode confinement and the signature of apparently threshold-less lasing. Since plasmonic modes have no cut-off, we show downscaling of the lateral dimensions of both device and optical mode. As these optical coherent sources approach molecular and electronics length scales, plasmonic lasers offer the possibility to explore extreme interactions between light and matter, opening new avenues in active photonic circuits, bio-sensing and quantum information technology.