Fluorescence is widely used in optical devices, microscopy imaging, biology, medical research and diagnosis. Improving fluorescence sensitivity, all the way to the limit of single-molecular detection needed in many applications, remains a great challenge. The technique of surface enhanced fluorescence (SEF) is based upon the design of surfaces in the vicinity of the emitter. SEF yields an overall improvement in the fluorescence detection efficiency through modification and control of the local electromagnetic environment of the emitter. Near-field coupling between the emitter and surface modes plays a crucial role in SEF. In particular, plasmonic surfaces with localized and propagating surface plasmons are efficient SEF substrates. Recent progress in tailoring surfaces at the nanometre scale extends greatly the realm of SEF applications. This review focuses on the recent advances in the different mechanisms involved in SEF, in each case highlighting the most relevant applications.

Pulsed x-ray scattering is used to examine the lattice dynamics in gold nanoparticles in water following excitation with intense femtosecond laser pulses. At lower excitation power the initial lattice heating is followed by cooling on the nanosecond time scale. The decay can be described by solving the heat transfer equations including both the bulk conductivity in water and a finite thermal boundary resistance at the particlewater interface. The lattice expansion rises linearly with excitation power, up to an excitation power corresponding to a lattice temperature increase of 529 K. At higher temperatures the lattice shows a loss of long-range order due to pre-melting of the particles. At the bulk melting temperature, complete melting occurs within the first 100 ps after laser excitation.

We investigate the vibration dynamics of ellipsoidal silver nanoparticles, using time-resolved optical pump-probe spectroscopy. When excited with femtosecond laser pulses, the particles execute anisotropic shape oscillations. We show that these vibrations are triggered by the thermal expansion of the optically heated particles. The time dependence of the vibrations indicates that this expansion is caused by two mechanisms: The lattice anharmonicity and the extremely large pressure of the hot conduction electrons.

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We report broadband characterization of the propagation of light through a multiply scattering medium by means of its Multi-Spectral Transmission Matrix. Using a single spatial light modulator, our approach enables the full control of both spatial and spectral properties of an ultrashort pulse transmitted through the medium. We demonstrate spatiotemporal focusing of the pulse at any arbitrary position and time with any desired spectral shape. Our approach opens new perspectives for fundamental studies of light-matter interaction in disordered media, and has potential applications in sensing, coherent control and imaging.Propagation of coherent light through a scattering medium produces a speckle pattern at the output [1], due to light scrambling by multiple scattering events [2]. The phase and amplitude information of the light are spatially mixed, thus limiting resolution, depth and contrast of most optical imaging techniques. Ultrashort pulses, generated by broadband modelocked lasers, are very useful for multiphotonic imaging and non-linear physics [3][4][5]. In the temporal domain, an ultrashort pulse is temporally broadened during propagation in a scattering medium, due to the long dwell time within it [6,7], which therefore limits its range of applications.However, this scattering process is linear and deterministic. Therefore, one can control the input wavefront to design the output field. In this respect, spatial light modulators (SLMs) offer more than a million degrees of freedom to control the propagation of coherent light. These systems have played an important role in the development of wavefront shaping techniques to manipulate light in complex media. Iterative optimization algorithm [8][9][10] and phase conjugation methods [11,12] have been proposed to focus light at a given output position, an essential ingredient for imaging. An alternative method for light control is the optical transmission matrix (TM). The TM is a linear operator that links the input field (SLM) to the output field (CCD camera) [1,13]. The measurement of the TM allows imaging through [15] or inside a scattering medium [16], and potentially access mesoscopic properties of the system [17].The possibility of shaping the pulse in time is also essential for coherent control [18]. Temporally, photons exit a scattering medium at different times, giving rise to a broadened pulse at its output [7,19]. Temporal spreading of the original pulse is characterized by a confinement time τ m [20] related to the Thouless time [21]. Equivalently, from a spectral point of view, the scattering medium responds differently for distinct spectral components of an ultrashort pulse, with a spectral correlation bandwidth ∆ω m ∝ 1/τ m , giving rise to a very complex spatio-temporal speckle pattern [22][23][24][25]. With a single SLM, one can manipulate spatial degrees of freedom to adjust the delay between different optical paths. Therefore spatial and temporal distortions can be both compensated using wavefront shaping techniques. This approach allows th...

We present a method to measure the spectrally-resolved transmission matrix of a multiply scattering medium, thus allowing for the deterministic spatiospectral control of a broadband light source by means of wavefront shaping. As a demonstration, we show how the medium can be used to selectively focus one or many spectral components of a femtosecond pulse, and how it can be turned into a controllable dispersive optical element to spatially separate different spectral components to arbitrary positions.

Local field distributions in random metal-dielectric films near a percolation threshold are experimentally studied using scanning near-field optical microscopy ͑SNOM͒. The surface-plasmon oscillations in such percolation films are localized in small nanometer-scale areas, ''hot spots,'' where the local fields are much larger than the field of an incident electromagnetic wave. The spatial positions of the hot spots vary with the wavelength and polarization of the incident beam. Local near-field spectroscopy of the hot spots is performed using our SNOM. It is shown that the resonance quality-factor of hot spots increases from the visible to the infrared. Giant local optical activity associated with chiral plasmon modes has been obtained. The hot spot's large local fields may result in local, frequency and spatially selective photomodification of percolation films.