L. De Dominicis

Second- and third- harmonic generation in single-walled carbon nanotubes at nanosecond time scale

Second- and third-harmonic generation in single-walled carbon nanotube films is experimentally investigated with the fundamental 1064nm radiation from a Q-switched Nd:YAG laser. Measurements were performed both on commercially available carbon nanotubes and on samples of carbon nanotubes grown with a catalyst-free method. Third-harmonic generation was observed in both samples while only the sample grown with a catalyst-free method generated a second-harmonic signal. The quantum yield of second- and third-harmonic signal was proven to scale, respectively, with the second and third power of the pump pulse energy up to intensities of 109W∕cm2.

Analysis of simultaneous chlorophyll measurements by lidar fluorosensor, MODIS and SeaWiFS

The ENEA (Italian Agency for New Technologies, Energy and the Environment) lidar fluorosensor (ELF), aboard the research vessel Italica, measured continuously surface chlorophyll-a concentrations during the Italy–New Zealand and New Zealand–Italy transects (13 November–18 December 2001 and 28 February–1 April 2002, respectively). The ELF measurements were compared with the data collected by the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and the Moderate Resolution Imaging Spectroradiometer (MODIS). This study pointed out advantages, disadvantages and possible synergies of lidar fluorosensor and spaceborne radiometers. In particular, the SeaWiFS and MODIS bio-optical algorithms have been calibrated with the ELF measurements. The differences between the performances of the two spaceborne radiometers are also briefly discussed.

Pressure dependence of degenerate four-wave mixing in NO and NO2: effects of population and thermal gratings

Abstract A simple phenomenological modification to the two-level density matrix theory of DFWM is presented, which models the DFWM signal intensity produced by both population and thermal gratings as a function of buffer gas pressure. A comparison is made between this model and data from DFWM for NO and NO 2 , showing effects due to both population and thermal gratings. The model is able to describe the experimental observations when either population or thermal gratings are the dominant mechanism for signal generation.

Techniques for effective optical noise rejection in amplitude-modulated laser optical radars for underwater three-dimensional imaging

Amplitude-modulated (AM) laser imaging is a promising technology for the production of accurate three-dimensional (3D) images of submerged scenes. The main challenge is that radiation scattered off water gives rise to a disturbing signal (optical noise) that degrades more and more the quality of 3D images for increasing turbidity. In this paper, we summarize a series of theoretical findings, that provide valuable hints for the development of experimental methods enabling a partial rejection of optical noise in underwater imaging systems. In order to assess the effectiveness of these methods, which range from modulation/demodulation to polarimetry, we carried out a series of experiments by using the laboratory prototype of an AM 3D imager ( = 405 nm) for marine archaeology surveys, in course of realization at the ENEA Artificial Vision Laboratory (Frascati, Rome). The obtained results confirm the validity of the proposed methods for optical noise rejection.

Experimental evidence of signal-optical noise interferencelike effect in underwater amplitude-modulated laser optical radar systems.

We report experimental evidence that in an amplitude-modulated laser optical radar system for underwater 3D imaging the observed contrast oscillations as a function of the modulation frequency originate from an interference-like effect between target signal VT and water backscattered radiation VW. The demonstration relies on the ability to perform a direct measurement of VW in a 25 m long test tank. The proposed data processing method enables one to remove the contribution of water backscattering from the detected signal and drastically reduce signal fluctuations due to the medium. Experiments also confirm the possibility to improve the signal to optical noise ratio and contrast by increasing the modulation frequency.

Collisional relaxation and internal energy redistribution in NO2 investigated by means of laser-induced thermal grating technique

Abstract Optical scattering from laser-induced thermal gratings (LITGs) was employed in investigations of molecular relaxation processes in gas mixtures containing a small amount of NO 2 diluted with different buffer gases (N 2 ,Ar,CO 2 ). The thermal gratings were generated by the thermalization of the molecular internal energy stored in the NO 2 molecules after laser resonant excitation of the 2 B 1 ← 2 A 1 Huber–Douglas band. The temporal evolution of LITGs was detected by scattering a cw read-out laser beam. A theoretical simulation of experimental results was performed in which the linearized hydrodynamic equations relevant to collisions responsible for the relaxation process were solved by following a two-step rate equations approach.

Probe of the Si nanoclusters to Er3+ energy transfer dynamics by double-pulse excitation

In this letter the dynamics of the energy transfer (ET) process from amorphous silicon nanoclusters (Si-nc) to Er3+ ions is investigated in co-doped silica glasses. Using a double pulse setup the recovery time of the sensitizing effect is monitored and direct evidence is found for the occurrence of a fast (few hundreds ns) ET process in a sample containing large (2nm) aggregates, while a slower transfer time of few microseconds is revealed in a sample containing very small (<1nm) aggregates.

Polarimetry as tool to improve phase measurement in an amplitude modulated laser for submarine archaeological sites inspection

The propagation of polarized laser beams in turbid water is a subject of relevant interest in the field of underwater quantitative visualization with active sensors like amplitude modulated laser systems. In such devices, target range determination is based on the measurement of the phase difference ΔΦ between the fraction of the amplitude modulated laser beam reflected by the target and a reference signal. As water turbidity increases, the laser radiation backscattered from the water column shined by the sounding laser beam gives rise to an optical background with detrimental effects on the accuracy of range measurement. In this paper we analyze the possibility to increase the apparatus accuracy with a polarimetric technique based on the adoption of polarized laser radiation and polarization selective detection scheme for improving the underwater imaging of real scenes (e.g. archaeological sites). The method fully takes advantages of the different polarization properties of the laser radiation backscattered by turbid water and of the Lambertian component diffusively reflected by the target as described by the associated Mueller matrices. Measurements have been performed by adopting both a co-polarized and cross-polarized detection scheme with linearly and circularly polarized laser radiation. Various degrees of turbidity were realized by adding, as diffusive element, skim milk to water in order to obtain different scattering conditions. The effect of the transition from Rayleigh to Mie scattering regime on phase accuracy determination has been investigated together with the role played by high order scatterings as the medium approaches the optical thickness condition.

Improvement in underwater phase measurement of an amplitude-modulated laser beam by polarimetric techniques

The phase of the amplitude-modulated radiation reflected by a Lambertian target immersed in water was measured by using a linearly and circularly polarized sounding laser beam. Different values of the water extinction coefficient in the range of 0.06 - 2 m(-1) were realized by adding skim milk as a scattering element. It is shown that very efficient rejection of optical noise, resulting in reliable phase measurements, is accomplished with a cross-polarized and copolarized detection scheme for linear and circular polarization, respectively. The experiment demonstrates that phase measurements are very sensitive to optical noise suppression and that, as far as single scattering is the main involved mechanism, significant improvements can be achieved by adopting a polarization control on both the transmitter and the receiver stages of the apparatus.

Polarimetry as a valid means to reduce optical noise in underwater 3D imaging by means of amplitude-modulated laser optical radar systems

We report the results of a series of underwater imaging experiments in the visible, carried out at ENEA (Frascati, Rome) by using a bistatic, amplitude-modulated laser optical radar system. In these experiments, polarimetry is used for minimizing the water backscattering signal and improving the accuracy of phase measurements directly related to distance. The presented technique enables one to obtain 3D images of underwater real scenes characterized by high quality, space resolution, and contrast. The results are of remarkable importance for applications in the 3D imaging of submerged objects, such as submarine archaeological sites.