Engineering optical properties using artificial nanostructured media known as metamaterials has led to breakthrough devices with capabilities from super-resolution imaging to invisibility. In this paper, we review metamaterials for quantum nanophotonic applications, a recent development in the field. This seeks to address many challenges in the field of quantum optics using advances in nanophotonics and nanofabrication. We focus on the class of nanostructured media with hyperbolic dispersion that have emerged as one of the most promising metamaterials with a multitude of practical applications from subwavelength imaging, nanoscale waveguiding, biosensing to nonlinear switching. We present the various design and characterization principles of hyperbolic metamaterials and explain the most important property of such media: a broadband enhancement in the electromagnetic density of states. We review several recent experiments that have explored this phenomenon using spontaneous emission from dye molecules and quantum dots. We finally point to future applications of hyperbolic metamaterials, using the broadband enhancement in the spontaneous emission to construct single-photon sources.

Quantum nanophotonics using hyperbolic metamaterials

Engineering the optical properties using artificial nanostructured media known as metamaterials has led to breakthrough devices with capabilities from super-resolution imaging to invisibility. In this article, we review metamaterials for quantum nanophotonic applications, a recent development in the field. This seeks to address many challenges in the field of quantum optics using recent advances in nanophotonics and nanofabrication. We focus on the class of nanostructured media with hyperbolic dispersion that have emerged as one of the most promising metamaterials with a multitude of practical applications from subwavelength imaging, nanoscale waveguiding, biosensing to nonlinear switching. We present the various design and characterization principles of hyperbolic metamaterials and explain the most important property of such media: a broadband enhancement in the electromagnetic density of states. We review several recent experiments that have explored this phenomenon using spontaneous emission from dye molecules and quantum dots. We finally point to future applications of hyperbolic metamaterials of using the broadband enhancement in the spontaneous emission to construct single photon sources.

/pdf/quantum-nanophotonics-using-hyperbolic-metamaterials-puksxnvf7b.pdf

Metasurfaces are artificially structured thin films with unusual properties on demand. Different from metamaterials, the metasurfaces change the electromagnetic waves mainly by exploiting the boundary conditions, rather than the constitutive pa-rameters in three dimensional (3D) spaces. Despite the intrinsic similarities in the operational principles, there is not a universal theory available for the understanding and design of metasurface-based devices. In this article, we propose the concept of metasurface waves (M-waves) and provide a general theory to describe the principles of them. Most importantly, it is shown that the M-waves share some fundamental properties such as extremely short wavelength, abrupt phase change and strong chromatic dispersion, which make them different from traditional bulk waves. It is shown that these properties can enable many important applications such as subwavelength imaging and lithography, planar optical devices, broadband anti-reflection, absorption and polarization conversion. Our results demonstrated unambiguously that traditional laws of diffraction, refraction, reflection and absorption should be revised by using the novel properties of M-waves. The theory provided here may pave the way for the design of new electromagnetic devices and further improvement of metasurfaces. The exotic properties of metasurfaces may also form the foundations for two new sub-disciplines called “subwavelength surface electromagnetics” and “subwavelength electromagnetics”.

Principles of electromagnetic waves in metasurfaces

We demonstrate a method to fabricate ultra-thin, ultra-smooth and low-loss silver (Ag) films using a very thin germanium (Ge) layer as a wetting material and a rapid post-annealing treatment. The addition of a Ge wetting layer greatly reduces the surface roughness of Ag films deposited on a glass substrate by electron-beam evaporation. The percolation threshold of Ag films and the minimal thickness of a uniformly continuous Ag film were significantly reduced using a Ge wetting layer in the fabrication. A rapid post-annealing treatment is demonstrated to reduce the loss of the ultra-thin Ag film to the ideal values allowed by the quantum size effect in smaller grains. Using the same wetting method, we have also extended our studies to ultra-smooth silver-silica lamellar composite films with ultra-thin Ag sublayers.

Ultra-thin ultra-smooth and low-loss silver films on a germanium wetting layer

This paper reports an effective method to enhance the surface plasmon resonance (SPR) on Ag films by using a thin Ni seed layer assisted deposition. Ag films with a thickness of about 50 nm were deposited by electron beam evaporation above an ultrathin Ni seed layer of approximately 2 nm on both silicon and quartz substrates. The root-mean-square (rms) surface roughness and the correlation length have been reduced from >4 nm and 28 nm for a pure Ag film to approximately 1.3 and 19 nm for Ag/Ni films, respectively. Both experimental and simulation results show that the Ag/Ni films exhibit an enhanced SPR over the pure Ag film with a narrower full width at half-maximum. Ag films with a Ge seed layer have also been prepared under the same conditions. The surface roughness can be reduced to less than 0.7 nm, but narrowing of the SPR curve is not observed due to increased absorptive damping in the Ge seed layer. Our results show that Ni acts as a roughness-diminishing growth layer for the Ag film while at the same time maintaining and enhancing the plasmonic properties of the combined structures. This points toward its use for low-loss plasmonic devices and optical metamaterials applications.

Enhanced surface plasmon resonance on a smooth silver film with a seed growth layer.

We demonstrate a smooth and low loss silver (Ag) optical superlens capable of resolving features at 1/12th of the illumination wavelength with high fidelity. This is made possible by utilizing state-of-the-art nanoimprint technology and intermediate wetting layer of germanium (Ge) for the growth of flat silver films with surface roughness at subnanometer scales. Our measurement of the resolved lines of 30 nm half-pitch shows a full-width at half-maximum better than 37 nm, in excellent agreement with theoretical predictions. The development of this unique optical superlens leads promise to parallel imaging and nanofabrication in a single snapshot.

A smooth optical superlens

Herein, we report the results in the search for optimized parameters to fabrication of substrate free microwalled photovoltaic devices for maximum light harvesting. By using the fracture transfer printing method, silicon (Si)-based microwalls (MWs) and micropilars (MPs) with an aspect ratio of 24 were successfully fabricated and transferred into PMMA on secondary substrates. Also, MW solar cell with the filling ratio ∼40% was fabricated by further processing a commercial solar cell with surface texture using deep reactive ion etching (DRIE). The results show that reflection is minimized for light incidence angle greater than 5° with respect to the surface normal to the solar cell plane. The power conversion efficiency (PCE) of the MW PV device starts to increase from 2% as the incident light angle increases from 0° and reaches its maximum at 3.5% for 70°. On the other hand, a textured commercial solar cell, with PCE of 9.5% exhibits decreasing PCE when the incident light angle is varied from orthogonal to parallel orientation with respect to the solar cell plane.

Incident light angle dependence of microwalled silicon solar cell efficiency for fracture transfer printing applications

We demonstrate a smooth and low loss silver (Ag) optical superlens capable of resolving features at 1/12th of the illumination wavelength with high fidelity. This is made possible by utilizing state-of-the-art nanoimprint technology and intermediate wetting layer of germanium (Ge) for the growth of flat silver films with surface roughness at sub-nanometer scales. Our measurement of the resolved lines of 30nm half-pitch shows a full-width at half-maximum better than 37nm, in excellent agreement with theoretical predictions. The development of this unique optical superlens lead promise to parallel imaging and nanofabrication in a single snapshot, a feat that are not yet available with other nanoscale imaging techniques such as atomic force microscope or scanning electron microscope.

Molecular Scale Imaging with a Smooth Superlens

Recent theory[1] suggested a novel approach of optical imaging with resolution far beyond the diffraction limit. Resolution as high as 60 nanometers or 1/6 of wavelength has been achieved experimentally[2]. This unique optical superlens will enable parallel imaging and nanofabrication in a single snapshot, a feat that are not yet available with other nanoscale imaging techniques such as atomic force microscope or scanning electron microscope.

VJ Logeeswaran

Papers

A smooth optical superlens

Incident light angle dependence of microwalled silicon solar cell efficiency for fracture transfer printing applications

Molecular Scale Imaging with a Smooth Superlens

Molecular Scale Imaging with A Smooth Superlens