Optical techniques form a powerful characterization tool of these nanomaterials, but as all parameters fluctuate from object to object, only averaged properties are observed in conventional experiments performed on large ensembles. In many studies of elementary mechanisms, the inclusion of statistical averaging complicates interpretation of the effects. In this context, development of optical techniques for detection and characterizion of single nano-objects are of central interest in fundamental and applied nanosciences.
Luminescent single objects, such as molecules or semiconductor quantum dots, have been extensively investigated, yielding much information on their individual properties. Observation of single non-luminescent objects such as metallic clusters is more difficult since it requires detection of very weak light absorption or scattering. The high spatial resolution of near-field optical methods has been exploited to detect single gold nanoparticles. However, the strong coupling between the measuring tip and the studied metallic particle complicates the interpretation of the observed signals. Development of far-field techniques free from spurious influence of the observation apparatus is thus of central interest for the investigation of metallic nano-objects.
Figure 1 Principle of experiment: the position of one single nanoparticle is periodically modulated, at the frequency f and with amplitude dy under a focused laser beam. The modulated transmitted power of the laser is detected, either at f or 2f, with a photodiode and a lock-in amplifier. The sample is mounted on a XY scanner, and the signal is recovered for each (x,y) sample position. A photonic crystal fiber is used to generate a supercontinuum using a Ti:sapphire laser source. A spectral bandwith of ~5 nm is selected using a double grating pair.
We have developed a novel far-field technique for directly detecting and measuring the optical absorption of a single metal cluster. Its simple approach is based on spatial modulation of the particle position in the focal spot of a laser beam ( Figure 1) . Lock-in detection of the transmitted intensity allows for a quantitative determination of the absorption cross section of the individual nano-object. The limited spatial resolution of the far-field setup can be overcome by using a dilute system so that only one nanoparticle is present in the probed zone.
An image of a surface covered with gold nanoparticles of 10 nm diameter is shown in Figure 2. Synchronous detection at either the fundamental or the harmonic of the modulation frequency results in a signal proportional to the first or second order spatial derivative of the laser profile in the direction of modulation, respectively.
Figure 2 Spatial xy dependence of the transmission change DT/T induced by a single gold nanoparticle with diameter D ~ 10nm on a glass substrate. The laser, of wavelength l = 532 nm and 10 µW optical power, is focused on a 0.34 µm FWHM focal spot centered at (x0, y0) = (1 µm, 1µm). The sample position is scanned over a 2x2 µm2 area and modulated along the y axis at f = 1.5 kHz, with amplitude dy = 0.27µm. a) The signal is detected at f. b) Same for 2f detection. c) and d) Cut along the y direction for x = x0 of the a) and b) data, respectively (points). The dashed lines are the computed profiles using FWHM = 0.34 µm, dy = 0.27µm and the parameter sext set to s= 53 nm2.
The absolute value of the DT/T signal is directly proportional to the extinction cross section of the gold nanoparticle at the experimental wavelength. Results can be well reproduced using the value s = 53 ± 2 nm2 [dashed lines in Fig. 2(c,d)]. This is in excellent agreement with calculated estimates for D ~ 10nm. The high sensitivity of the setup permits detection of gold nanoparticles with diameter down to D = 5 nm with a signal to noise ratio of about 3.
Using a white light supercontinuum from a photonic crystal fiber we have obtained the quantitative absorption spectra of single nanoparticles, as shown in Figure 3. This permits full optical characterization of the nanoparticle properties: size, shape and orientation on the substrate. Furthermore, the absorption spectra can be used as probe for the local dielectric environment of the host medium.
Figure 3 Absorption spectra of a single 20-nm gold nanoparticle measured with (black, squares) unpolarized light, and (red, circles), (green, triangles) light linearly polarized along the particle main axes in the focal plane. Inset shows the absorption cross-section at a wavelength of 550 nm against polarization direction.
These first results open up many possibilities for optical detection and absorption spectroscopy of single metal nanoparticles as a function of their size, shape, structure and environment, in particular when the latter is inhomogeneous. They offer the possibility of both precisely testing theoretical models using controlled samples (deposited mass selected clusters for instance) or to extract information on the object by comparing the experimental and theoretical responses.
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