We describe an experiment in which a laser beam is sent into a high-finesse optical cavity with a mirror coated on a mechanical resonator. We show that the reflected light is very sensitive to small mirror displacements. We have observed the Brownian motion of the resonator with a very high sensitivity corresponding to a minimum observable displacement of 2 × 10 −19 m/ √ Hz.PACS : 05.40.Jc, 04.80.Nn, 42.50.LcThermal noise plays an important role in many precision measurements [1]. For example, the sensitivity in interferometric gravitational-wave detectors [2-4] is limited by the Brownian motion of the suspended mirrors which can be decomposed into suspension and internal thermal noises. The latter is due to thermally induced deformations of the mirror surface. Experimental observation of this noise is of particular interest since its theoretical evaluation strongly depends on the mirror shape and on the spatial matching between light and internal acoustic modes [5][6][7][8][9]. It is also related to the mechanical dissipation mechanisms which are not well known in solids [10]. Mirror displacements induced by thermal noise are however very small and a highly sensitive displacement sensor is needed to perform such an observation.Monitoring extremely small displacements has thus become an important issue in precision measurements [11] and several sensors have been developed. A technique commonly used for the detection of gravitational waves by Weber bars is based on capacitive sensors [12]. Another promising technique consists in optical transducers [13,14]. Reflexion of light by a high-finesse Fabry-Perot cavity is very sensitive to changes in the cavity length. Such a device can thus be used to monitor displacements of one mirror of the cavity, as it has been proposed for gravitational wave bar detectors where the mirror is mechanically coupled to the bar [15,16], or for the detection of Brownian motion in gravitational wave interferometers [17,18]. In this letter we report a high-sensitivity observation of the Brownian motion of internal modes of a mirror. The sensitivity reached in our experiment is better than that of present sensors and comparable to

No abstract

We study the optical coupling between a gold nanowire and a silver ion-exchanged waveguide, with special emphasis on the nanowire antenna radiation pattern. We measure the radiation patterns of waveguide-coupled gold nanowires with a height of 70 nm and width of 50 or 150 nm in the 450-700 nm spectral range for TE and TM polarizations. We perform a systematic theoretical study on the wavelength, polarization, nanowire size, and material dependences on the properties of the radiation pattern. We also give some elements concerning absorption and near-field. Experiments and calculations show localized plasmon resonance for the polarization orthogonal to the wire (far-field resonance at 580 nm for the smallest wire and 670 nm for the widest). It is shown that a great variety of radiation patterns can be obtained, together with a high sensitivity to a change of one parameter, particularly near-resonance.

We report on 120-mW directly measured cw power at 532 nm from a tunable alpha -distributed-feedback laser diode near 1.064 microm frequency doubled in a KTP crystal operating room temperature inside a ring cavity. Our experimental setup allows us to scan frequencies up to 2 THz in the green-light domain and thus is extremely useful for iodine spectroscopy. We show good agreement between experimental results and theoretical predictions for the second-harmonic generation process.