Rémi Carminati

Measuring the transmission matrix in optics: an approach to the study and control of light propagation in disordered media.

We introduce a method to experimentally measure the monochromatic transmission matrix of a complex medium in optics. This method is based on a spatial phase modulator together with a full-field interferometric measurement on a camera. We determine the transmission matrix of a thick random scattering sample. We show that this matrix exhibits statistical properties in good agreement with random matrix theory and allows light focusing and imaging through the random medium. This method might give important insight into the mesoscopic properties of a complex medium.

Thermal radiation scanning tunnelling microscopy

In standard near-field scanning optical microscopy (NSOM), a subwavelength probe acts as an optical ‘stethoscope’ to map the near field produced at the sample surface by external illumination. This technique has been applied using visible, infrared, terahertz and gigahertz radiation to illuminate the sample, providing a resolution well beyond the diffraction limit. NSOM is well suited to study surface waves such as surface plasmons or surface-phonon polaritons. Using an aperture NSOM with visible laser illumination, a near-field interference pattern around a corral structure has been observed, whose features were similar to the scanning tunnelling microscope image of the electronic waves in a quantum corral. Here we describe an infrared NSOM that operates without any external illumination: it is a near-field analogue of a night-vision camera, making use of the thermal infrared evanescent fields emitted by the surface, and behaves as an optical scanning tunnelling microscope. We therefore term this instrument a ‘thermal radiation scanning tunnelling microscope’ (TRSTM). We show the first TRSTM images of thermally excited surface plasmons, and demonstrate spatial coherence effects in near-field thermal emission.

Image formation in near-field optics

Abstract An overview is presented of the image formation theory in near-field optical microscopy. The emphasis is placed on the basic concepts and the understanding of the images. We briefly recall the general principles used in near-field optics to break the resolution limit. Since some of the concepts widely used in optics become meaningless in near field, a brief critical review of basic concepts is given. A theory of scattering of electromagnetic waves by inhomogeneous surfaces is then presented. For objects much smaller than the wavelength, a closed-form expression of the scattered field is derived, which provides a link between the near field and the structure of the sample. The different set-ups and their imaging capabilities are analysed. A general relationship between the signal and the induced currents in the sample is derived by means of the reciprocity theorem. The set-ups are compared and an equivalence between illumination and collection mode is proven. It is shown that, when multiple scattering between the sample and the rest of the system can be neglected, an impulse response can be defined for the three different types of set-ups: illumination mode, collection mode and apertureless. The importance of coherence in the near field is studied. Finally, the influence of the different control modes (constant height, constant intensity, constant tip-sample distance) is analysed and the existence of artifacts is discussed.

Near-field thermophotovoltaic energy conversion

We report a quantitative model of a near-field thermophotovoltaic (TPV) device consisting in a thermal source located in the near field of a TPV cell. The enhanced radiative transfer at short distance leads to an increase of the photogeneration current. We analyze quantitatively other potential near-field effects, in particular, on the dark current. We also study the influence of the modification of the spectrum of the sources in the near field, comparing the case of a tungsten source with the case of a quasimonochromatic source. Our model leads to a quantitative evaluation of the near-field TPV device output electric power and efficiency.

ENHANCED RADIATIVE HEAT TRANSFER AT NANOMETRIC DISTANCES

We study in this article the radiative heat transfer between two semi-infinite bodies at subwavelength scale. We show that this transfer can be enhanced by several orders of magnitude when the surfaces support resonant surface waves. In these conditions, we show that the transfer is almost monochromatic.

Single-molecule spontaneous emission close to absorbing nanostructures

The spontaneous emission of a single molecule is substantially modified close to a metallic nanostructure. We study the spectral behavior of the radiative and nonradiative decay rates and of the local-field factor in the vicinity of a plasmon resonance. We show that the highest fluorescence enhancement is obtained for an emission wavelength redshifted from the plasmon resonance, and that quenching always dominates at plasmon resonance. These results may have experimental implications in spectroscopy and monitoring of elementary light sources.

Nanoscale radiative heat transfer between a small particle and a plane surface

We study the radiative heat transfer between a small dielectric particle, considered as a point-like dipole, and a surface. In the framework of electrodynamics and using the fluctuation-dissipation theorem, we can evaluate the energy exchange in the near field, which is dominated by the contribution of tunneling waves. The transfer is enhanced by several orders of magnitude if the surface or the particle can support resonant surface waves. An application to local heating is discussed.

Theory of infrared nanospectroscopy by photothermal induced resonance

We present a theoretical investigation of the physics involved in a recently developed spectromicroscopy technique, called photothermal induced resonance (PTIR). With this technique, one measures the local infrared absorption spectrum of a sample shined with a tunable infrared laser pulse, and detects the induced photothermal expansion with the tip of an atomic force microscope (AFM). Simple physical assumptions allow us to describe analytically the heating and expansion of the sample, the excitation of the vibration modes of the AFM cantilever, and the detected signal. We show that the signal depends on the thermal expansion velocity rather than on the absolute displacement of the tip, and we investigate the influence of the laser pulse length. Eventually, we express the PTIR signal in terms of relevant parameters, and prove its proportionality to the sample absorbance. This analytical approach complement our experimental results and validates the PTIR method as a technique of choice for infrared spectro...

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.

Two-dimensional numerical simulation of the photon scanning tunneling microscope. Concept of transfer function

Abstract A numerical simulation is used to calculate the signal detected by a photon scanning tunneling microscope when scanning a surface. We show that the signal provides an image very different from the actual surface profile. However, it is possible to define a transfer function relating the signal and the near-field intensity which exists without the presence of the detecting probe tip. Such a transfer function cannot be defined between the signal and the surface.