Ertugrul Cubukcu

Plasmonic laser antenna

The authors have demonstrated a surface plasmon device composed of a resonant optical antenna integrated on the facet of a commercial diode laser, termed a plasmonic laser antenna. This device generates enhanced and spatially confined optical near fields. Spot sizes of a few tens of nanometers have been measured at a wavelength ∼0.8μm. This device can be implemented in a wide variety of semiconductor lasers emitting in spectral regions ranging from the visible to the far infrared, including quantum cascade lasers. It is potentially useful in many applications including near-field optical microscopes, optical data storage, and heat-assisted magnetic recording.

Split ring resonator sensors for infrared detection of single molecular monolayers

We report a surface enhanced molecular detection technique with zeptomole sensitivity that relies on resonant coupling of plasmonic modes of split ring resonators and infrared vibrational modes of a self-assembled monolayer of octadecanthiol molecules. Large near-field enhancements at the gap of split ring resonators allow for this resonant coupling when the molecular absorption peaks overlap spectrally with the plasmonic resonance. Electromagnetic simulations support experimental findings.

Transparent and flexible low noise graphene electrodes for simultaneous electrophysiology and neuroimaging

Calcium imaging is a versatile experimental approach capable of resolving single neurons with single-cell spatial resolution in the brain. Electrophysiological recordings provide high temporal, but limited spatial resolution, because of the geometrical inaccessibility of the brain. An approach that integrates the advantages of both techniques could provide new insights into functions of neural circuits. Here, we report a transparent, flexible neural electrode technology based on graphene, which enables simultaneous optical imaging and electrophysiological recording. We demonstrate that hippocampal slices can be imaged through transparent graphene electrodes by both confocal and two-photon microscopy without causing any light-induced artefacts in the electrical recordings. Graphene electrodes record high-frequency bursting activity and slow synaptic potentials that are hard to resolve by multicellular calcium imaging. This transparent electrode technology may pave the way for high spatio-temporal resolution electro-optic mapping of the dynamic neuronal activity.

Bowtie plasmonic quantum cascade laser antenna

We report a bowtie plasmonic quantum cascade laser antenna that can confine coherent mid-infrared radiation well below the diffraction limit. The antenna is fabricated on the facet of a mid-infrared quantum cascade laser and consists of a pair of gold fan-like segments, whose narrow ends are separated by a nanometric gap. Compared with a nano-rod antenna composed of a pair of nano-rods, the bowtie antenna efficiently suppresses the field enhancement at the outer ends of the structure, making it more suitable for spatially-resolved high-resolution chemical and biological imaging and spectroscopy. The antenna near field is characterized by an apertureless near-field scanning optical microscope; field confinement as small as 130 nm is demonstrated at a wavelength of 7.0 mum.

Plasmonic Laser Antennas and Related Devices

This paper reviews recent work on device applications of optical antennas. Localized surface plasmon resonances of gold nanorod antennas resting on a silica glass substrate were modeled by finite difference time-domain simulations. A single gold nanorod of length 150 or 550 nm resonantly generates enhanced near fields when illuminated with light of 830 nm wavelength. A pair of these nanorods gives higher field enhancements due to capacitive coupling between them. Bowtie antennas that consist of a pair of triangular gold particles offer the best near-field confinement and enhancement. Plasmonic laser antennas based on the coupled nanorod antenna design were fabricated by focused ion beam lithography on the facet of a semiconductor laser diode operating at a wavelength of 830 nm. An optical spot size of few tens of nanometers was measured by apertureless near-field optical microscope. We have extended our work on plasmonic antenna into mid-infrared (mid-IR) wavelengths by implementing resonant nanorod and bowtie antennas on the facets of various quantum cascade lasers. Experiments show that this mid-IR device can provide an optical intensity confinement 70 times higher than that would be achieved with diffraction limited optics. Near-field intensities ~ 1 GW/cm2 were estimated for both near-infrared and mid-IR plasmonic antennas. A fiber device that takes advantage of plasmonic resonances of gold nanorod arrays providing a high density of optical ldquohot spotsrdquo is proposed. Results of a systematic theoretical and experimental study of the reflection spectra of these arrays fabricated on a silica glass substrate are also presented. The family of these proof-of-concept plasmonic devices that we present here can be potentially useful in many applications including near-field optical microscopes, high-density optical data storage, surface enhanced Raman spectroscopy, heat-assisted magnetic recording, and spatially resolved absorption spectroscopy.

Optical properties of surface plasmon resonances of coupled metallic nanorods

We present a systematic study of optical antenna arrays, in which the effects of coupling between the antennas, as well as of the antenna length, on the reflection spectra are investigated and compared. Such arrays can be fabricated on the facet of a fiber, and we propose a photonic device, a plasmonic optical antenna fiber probe, that can potentially be used for in-situ chemical and biological detection and surface-enhanced Raman scattering.

Graphene-enabled silver nanoantenna sensors.

Silver is the ideal material for plasmonics because of its low loss at optical frequencies but is often replaced by a more lossy metal, gold. This is because of silvers tendency to tarnish and roughen, forming Ag(2)S on its surface, dramatically diminishing optical properties and rendering it unreliable for applications. By passivating the surface of silver nanostructures with monolayer graphene, atmospheric sulfur containing compounds are unable to penetrate the graphene to degrade the surface of the silver. Preventing this sulfidation eliminates the increased material damping and scattering losses originating from the unintentional Ag(2)S layer. Because it is atomically thin, graphene does not interfere with the ability of localized surface plasmons to interact with the environment in sensing applications. Furthermore, after 30 days graphene-passivated silver (Ag-Gr) nanoantennas exhibit a 2600% higher sensitivity over that of bare Ag nanoantennas and 2 orders of magnitude improvement in peak width endurance. By employing graphene in this manner, the excellent optical properties and large spectral range of silver can be functionally utilized in a variety of nanoscale plasmonic devices and applications.

Raman injection laser.

Stimulated Raman scattering is a nonlinear optical process that, in a broad variety of materials, enables the generation of optical gain at a frequency that is shifted from that of the incident radiation by an amount corresponding to the frequency of an internal oscillation of the material. This effect is the basis for a broad class of tunable sources known as Raman lasers. In general, these sources have only small gain (∼ 10-9 cm W-1) and therefore require external pumping with powerful lasers, which limits their applications. Here we report the realization of a semiconductor injection Raman laser designed to circumvent these limitations. The physics underlying our device differs in a fundamental way from existing Raman lasers: it is based on triply resonant stimulated Raman scattering between quantum-confined states within the active region of a quantum cascade laser that serves as an internal optical pump—the device is driven electrically and no external laser pump is required. This leads to an enhancement of orders of magnitude in the Raman gain, high conversion efficiency and low threshold. Our lasers combine the advantages of nonlinear optical devices and of semiconductor injection lasers, and could lead to a new class of compact and wavelength-agile mid-and far-infrared light sources.

Transmission and reflection properties of composite double negative metamaterials in free space

We report free space transmission and the first reflection measurements of a composite double negative (DNG) metamaterial, also known as a left-handed material (LHM). The metamaterial composes of the split-ring-resonators and discontinuous thin wires. Very high transmission values of the metamaterial are observed within a frequency range for which both effective permeability and permittivity are expected to be negative.

Strongly enhanced molecular fluorescence inside a nanoscale waveguide gap.

We experimentally demonstrate dramatically enhanced light-matter interaction for molecules placed inside the nanometer scale gap of a plasmonic waveguide. We observe spontaneous emission rate enhancements of up to about 60 times due to strong optical localization in two dimensions. This rate enhancement is a nonresonant nature of the plasmonic waveguide under study overcoming the fundamental bandwidth limitation of conventional devices. Moreover, we show that about 85% of molecular emission couples into the waveguide highlighting the dominance of the nanoscale optical mode in competing with quenching processes. Such optics at molecular length scales paves the way toward integrated on-chip photon source, rapid transfer of quantum information, and efficient light extraction for solid-state-lighting devices.