Ewold Verhagen

Light trapping in ultrathin plasmonic solar cells

We report on the design, fabrication, and measurement of ultrathin film a-Si:H solar cells with nanostructured plasmonic back contacts, which demonstrate enhanced short circuit current densities compared to cells having flat or randomly textured back contacts. The primary photocurrent enhancement occurs in the spectral range from 550 nm to 800 nm. We use angle-resolved photocurrent spectroscopy to confirm that the enhanced absorption is due to coupling to guided modes supported by the cell. Full-field electromagnetic simulation of the absorption in the active a-Si:H layer agrees well with the experimental results. Furthermore, the nanopatterns were fabricated via an inexpensive, scalable, and precise nanopatterning method. These results should guide design of optimized, non-random nanostructured back reflectors for thin film solar cells.

Quantum-coherent coupling of a mechanical oscillator to an optical cavity mode

Optical laser fields have been widely used to achieve quantum control over the motional and internal degrees of freedom of atoms and ions, molecules and atomic gases. A route to controlling the quantum states of macroscopic mechanical oscillators in a similar fashion is to exploit the parametric coupling between optical and mechanical degrees of freedom through radiation pressure in suitably engineered optical cavities. If the optomechanical coupling is ‘quantum coherent’—that is, if the coherent coupling rate exceeds both the optical and the mechanical decoherence rate—quantum states are transferred from the optical field to the mechanical oscillator and vice versa. This transfer allows control of the mechanical oscillator state using the wide range of available quantum optical techniques. So far, however, quantum-coherent coupling of micromechanical oscillators has only been achieved using microwave fields at millikelvin temperatures. Optical experiments have not attained this regime owing to the large mechanical decoherence rates and the difficulty of overcoming optical dissipation. Here we achieve quantum-coherent coupling between optical photons and a micromechanical oscillator. Simultaneously, coupling to the cold photon bath cools the mechanical oscillator to an average occupancy of 1.7 ± 0.1 motional quanta. Excitation with weak classical light pulses reveals the exchange of energy between the optical light field and the micromechanical oscillator in the time domain at the level of less than one quantum on average. This optomechanical system establishes an efficient quantum interface between mechanical oscillators and optical photons, which can provide decoherence-free transport of quantum states through optical fibres. Our results offer a route towards the use of mechanical oscillators as quantum transducers or in microwave-to-optical quantum links.

Nanofocusing in laterally tapered plasmonic waveguides

We investigate the focusing of surface plasmon polaritons (SPPs) excited with 1.5 microm light in a tapered Au waveguide on a planar dielectric substrate by experiments and simulations. We find that nanofocusing can be obtained when the asymmetric bound mode at the substrate side of the metal film is excited. The propagation and concentration of this mode to the tip is demonstrated. No sign of a cutoff waveguide width is observed as the SPPs propagate along the tapered waveguide. Simulations show that such concentrating behavior is not possible for excitation of the mode at the low-index side of the film. The mode that enables the focusing exhibits a strong resemblance to the asymmetric mode responsible for focusing in conical waveguides. This work demonstrates a practical implementation of plasmonic nanofocusing on a planar substrate.

Electric and Magnetic Dipole Coupling in Near-Infrared Split-Ring Metamaterial Arrays

We present experimental observations of strong electric and magnetic interactions between split ring resonators (SRRs) in metamaterials. We fabricated near-infrared planar metamaterials with different inter-SRR spacings along different directions. Our transmission measurements show blueshifts and redshifts of the magnetic resonance, depending on SRR orientation relative to the lattice. The shifts agree well with simultaneous magnetic and electric near-field dipole coupling. We also find large broadening of the resonance, accompanied by a decrease in effective cross section per SRR with increasing density due to superradiant scattering. Our data shed new light on Lorentz-Lorenz approaches to metamaterials.

Direct imaging of propagation and damping of near-resonance surface plasmon polaritons using cathodoluminescence spectroscopy

Cathodoluminescence imaging spectroscopy is used to determine the propagation distance of surface plasmon polaritons near the surface plasmon resonance on both silver and gold films. Surface plasmon polaritons are generated by a focused (diameter of 5nm) electron beam spot in the metal and coupled out through a grating. By gradually varying the distance between the excitation spot and the grating the damping is probed. Propagation lengths as small as several hundred nanometers are probed, and an increase in propagation length is observed if the wavelength is increased above resonance. The measured data are compared with the calculated propagation lengths taking into account both absorption in the film and leakage radiation, and it is found that other loss mechanisms appear to be significant as well.

Field enhancement in metallic subwavelength aperture arrays probed by erbium upconversion luminescence

Upconversion luminescence from erbium ions placed in the near field of subwavelength aperture arrays is used to investigate field enhancement of incident near-infrared light in such nanostructures. We study field enhancement due to the excitation of both propagating and localized surface plasmon resonances in arrays of square and annular apertures in a Au film. The conversion of 1480 nm excitation light to 980 nm emission is shown to be enhanced up to a factor 450 through a subwavelength hole array. The effects of array periodicity and aperture size are investigated. It is shown that a Fano model can describe both far-field transmission and near-field intensity. The upconversion enhancement reveals the wavelength and linewidth of the surface plasmon modes that are responsible for extraordinary transmission in such arrays. Angle-dependent measurements on annular aperture arrays prove that the field enhancement due to localized resonances is independent of the incident angle. These experiments provide insight in the mechanisms responsible for extraordinary transmission and are important for applications that aim to exploit near-field enhancement in nanostructured metal films.

Are negative index materials achievable with surface plasmon waveguides? A case study of three plasmonic geometries

We present a theoretical analysis of planar plasmonic waveguides that support propagation of positive and negative index modes. Particular attention is given to the modes sustained by metal-insulator-metal (MIM), insulator-metal-insulator (IMI), and insulator-insulator-metal (IIM) geometries at visible and near-infrared frequencies. We find that all three plasmonic structures are characterized by negative indices over a finite range of visible frequencies, with figures of merit approaching 20. Moreover, using finite-difference time-domain simulations, we demonstrate that visible-wavelength light propagating from free space into these waveguides can exhibit negative refraction. Refractive index and figure-of-merit calculations are presented for Ag/GaP and Ag/Si(3)N(4) - based structures with waveguide core dimensions ranging from 5 to 50 nm and excitation wavelengths ranging from 350 nm to 850 nm. Our results provide the design criteria for realization of broadband, visible-frequency negative index materials and transformation-based optical elements for two-dimensional guided waves. These geometries can serve as basic elements of three-dimensional negative-index metamaterials.

Near-Field Visualization of Strongly Confined Surface Plasmon Polaritons in Metal−Insulator−Metal Waveguides

A nanoscale gap between two metal surfaces can confine propagating surface plasmon polaritons (SPPs) to very small dimensions, but this geometry makes it inherently difficult to image SPP propagation at high resolution. We demonstrate the near-field probing of these SPPs, propagating within a 50 nm thick Si 3N 4 waveguide with Ag cladding layers for frequencies ranging from the blue to the near-infrared. Using near-field SPP interferometry, we determine the wave vector, showing that the wavelength is shortened to values as small as 156 nm for a free-space wavelength of 532 nm.

Controlling Fano lineshapes in plasmon-mediated light coupling into a substrate

Metal nanoparticles are efficient resonant plasmonic scatterers for light, and, if placed on top of a high-index substrate, can efficiently couple light into the substrate. This coupling, however, strongly depends on particle shape and surrounding environment. We study the effect of particle shape and substrate refractive index on the plasmonic resonances of silver nanoparticles and we systematically relate this to the efficiency of light scattering into a substrate. The light coupling spectra are dominated by Fano resonances for the corresponding dipolar and quadrupolar scattering modes. Varying the particle shape from spherical to cylindrical leads to large shifts in the Fano resonance for the dipolar mode, reducing the light incoupling integrated over the AM1.5 spectral range. Using a dielectric spacer layer, good light coupling is achieved for cylinders in the near-infrared. An asymmetric environment around the particles turns quadrupolar resonances into efficient radiators as well.

Nonreciprocity and magnetic-free isolation based on optomechanical interactions

Nonreciprocal components, such as isolators and circulators, provide highly desirable functionalities for optical circuitry. This motivates the active investigation of mechanisms that break reciprocity, and pose alternatives to magneto-optic effects in on-chip systems. In this work, we use optomechanical interactions to strongly break reciprocity in a compact system. We derive minimal requirements to create nonreciprocity in a wide class of systems that couple two optical modes to a mechanical mode, highlighting the importance of optically biasing the modes at a controlled phase difference. We realize these principles in a silica microtoroid optomechanical resonator and use quantitative heterodyne spectroscopy to demonstrate up to 10 dB optical isolation at telecom wavelengths. We show that nonreciprocal transmission is preserved for nondegenerate modes, and demonstrate nonreciprocal parametric amplification. These results open a route to exploiting various nonreciprocal effects in optomechanical systems in different electromagnetic and mechanical frequency regimes, including optomechanical metamaterials with topologically non-trivial properties.