Stéphane Collin

λ3/1000 Plasmonic Nanocavities for Biosensing Fabricated by Soft UV Nanoimprint Lithography

Arrays of plasmonic nanocavities with very low volumes, down to λ(3)/1000, have been fabricated by soft UV nanoimprint lithography. Nearly perfect omnidirectional absorption (3-70°) is demonstrated for the fundamental mode of the cavity (λ ≃ 1.15 μm). The second-order mode exhibits a sharper resonance with strong angular dependence and total optical absorption when the critical coupling condition is fulfilled (45-50°, λ ≃ 750 nm). It leads to high refractive index sensitivity (405 nm/RIU) and figure of merit (∼21) and offers new perspectives for efficient biosensing experiments in ultralow volumes.

Waveguiding in nanoscale metallic apertures

We study the optical properties of subwavelength metallic waveguides made of nanoscale apertures in a metal. We develop analytical expressions for the fundamental optical modes in apertures. The results are in excellent agreement with finite element calculations. This model provides a physical understanding of the role of non-perfect metallic walls, and of the shape and size of the apertures. They reveal the effect of the skin depth and of the surface plasmon polariton coupling on the waveguide modes. The nanoscopic origin of the increase of the cut-off wavelength due to the electromagnetic penetration depth in the metal is described. Simple expressions and universal curves for the effective index and the cut-off wavelength of the fundamental guided mode of any rectangular metallic waveguide are presented. The results provide an efficient tool for the design of nanoscale waveguides with real metal.

Probing carrier dynamics in nanostructures by picosecond cathodoluminescence

Picosecond and femtosecond spectroscopy allow the detailed study of carrier dynamics in nanostructured materials. In such experiments, a laser pulse normally excites several nanostructures at once. However, spectroscopic information may also be acquired using pulses from an electron beam in a modern electron microscope, exploiting a phenomenon called cathodoluminescence. This approach offers several advantages. The multimode imaging capabilities of the electron microscope enable the correlation of optical properties (via cathodoluminescence) with surface morphology (secondary electron mode) at the nanometre scale. The broad energy range of the electrons can excite wide-bandgap materials, such as diamond- or gallium-nitride-based structures that are not easily excited by conventional optical means. But perhaps most intriguingly, the small beam can probe a single selected nanostructure. Here we apply an original time-resolved cathodoluminescence set-up to describe carrier dynamics within single gallium-arsenide-based pyramidal nanostructures with a time resolution of 10 picoseconds and a spatial resolution of 50 nanometres. The behaviour of such charge carriers could be useful for evaluating elementary components in quantum computers, optical quantum gates or single photon sources for quantum cryptography.

Highly directional radiation generated by a tungsten thermal source

We report the design of a tungsten thermal source with extraordinarily high directivity in the near infrared, comparable to the directivity of a CO2 laser. This high directivity is the signature of the long-range correlation of the electromagnetic field in the source plane. This phenomenon is due to the resonant thermal excitation of surface-plasmon polaritons.

Efficient light absorption in metal-semiconductor-metal nanostructures

A nanoscale metal–semiconductor grating is proposed for efficient and ultrafast photodetection. Theoretical and experimental results of efficient absorption in nanoscopic semiconductor wires are presented. The strong confinement of light in subwavelength metal–semiconductor gratings is achieved by Fabry–Perot resonances involving vertical transverse magnetic surface-plasmon waves and transverse electric guided waves. Photodetectors have been fabricated with 40×100nm cross sections of Ag and GaAs wires. The reflectivity and photocurrent mesurements are in good agreement with theoretical estimates.

Strong discontinuities in the complex photonic band structure of transmission metallic gratings

Complex photonic band structures (CPBS) of transmission metallic gratings with rectangular slits are shown to exhibit strong discontinuities that are not evidenced in the usual energetic band structures. These discontinuities are located on Wood-Rayleighs anomalies and reveal unambiguously two different types of resonances, which are identified as horizontal and vertical surface-plasmon resonances. Spectral position and width of peaks in the transmission spectrum can be directly extracted from CPBS for both kinds of resonances.

Resonant transmission through a metallic film due to coupled modes

Enhanced transmission and absorption by a silver film with a periodic array of slits has been studied numerically. We find that transmission and absorption peaks coincide and can be attributed to resonances of the structure. We show that these modes can be viewed as a coupling between cavity modes and surface plasmon polaritons. A quantitative analysis shows that the coupled mode can have a cavity mode character or a surface plasmon character depending on the distance to the crossing point of their dispersion relation. Finally, we provide a simple model for the peak transmission value by introducing the concept of radiative yield.

Nanostructure arrays in free-space: optical properties and applications

Dielectric and metallic gratings have been studied for more than a century. Nevertheless, novel optical phenomena and fabrication techniques have emerged recently and have opened new perspectives for applications in the visible and infrared domains. Here, we review the design rules and the resonant mechanisms that can lead to very efficient light-matter interactions in sub-wavelength nanostructure arrays. We emphasize the role of symmetries and free-space coupling of resonant structures. We present the different scenarios for perfect optical absorption, transmission or reflection of plane waves in resonant nanostructures. We discuss the fabrication issues, experimental achievements and emerging applications of resonant nanostructure arrays.

Free-standing subwavelength metallic gratings for snapshot multispectral imaging

A mosaic of ten spectral filters has been fabricated in a single 20 mm2 membrane drilled by nanoslits and coated by a gold layer. The nanostructured core-shell gratings exhibit 70% average maximum transmission efficiency in 15% aperture area, which represents a fivefold enhancement compared to the geometrical transmission. This mosaic of bandpass filters regularly spaced in the 3–5 μm wavelength range is used to demonstrate real-time spectral imaging in a multichannel camera.

Resonant-cavity-enhanced subwavelength metal–semiconductor–metal photodetector

We propose a resonant-cavity-enhanced subwavelength metal–semiconductor–metal structure for efficient and ultrafast photodetection. A Fabry–Perot cavity is composed by a bottom multilayer Bragg reflector and a top subwavelength metallic grating. The structure was fabricated on a GaAs substrate with 75 nm finger width and finger spacing, and theoretical results were validated experimentally by reflection spectra and photocurrent measurements. 75% efficiency is predicted theoretically in a 40-nm-thick GaAs layer, leading to potential cutoff frequencies between 300 and 500 GHz in TE polarization.