The aim of our experiment is to observe the optical response of gold
aggregates deposited on a silica surface near the percolation threshold.
Their size (about 20 nm) is very small compared to the laser beam wavelength
(647 nm) used to reveal them and therefore smaller than the resolution
limit of a classic optical microscope . Indeed for a given illumination
wavelength, only low spatial frequencies can propagate beyond the object
whereas high spatial frequencies stay confined in an evanescent wave whose
amplitude decreases with the distance from the object. To achieve the requested
sub-wavelength resolution, Scanning Near-field Optical Microscopy (SNOM)
probes the electromagnetic field near the sample surface.
The SNOM setup developed at the Laboratoire d’Optique Physique of the
ESPCI [1] uses a tungsten tip to locally perturb and scatter the evanescent
field. To achieve a good optical resolution the size of the tip end must
be the same or smaller than the studied structures (about 10 nm).
|
The experimental setup is depicted in the figure on the left. The sample is illuminated by total internal reflection which generates an evanescent wave at the sample surface. This wave is locally perturbed by the aggregates presence. The tip oscillates above the sample with an amplitude of about 100 nm and a frequency near the 5 kHz. At its lowest position (about 1 nm from the surface) it scatters the evanescent wave. The signal collected in the far field at the oscillation frequency carries local optical information at the nanometer scale. A feedback control loop maintains a constant oscillation amplitude and simultaneously collects topographical information during the sample scanning motion under the tip. A computer system drives the motion and records the requested signals. |
Using a similar setup working in reflection or transmission mode, we have brought to the fore [2] local electromagnetic field exaltations generated by aggregates distributed at the percolation threshold, as predicted by theoretical works [3].
1 Threshold at which a continuous
electric path is set between the sample edges.
2 Given by the Rayleigh rule
: 1.22l/2n0sinq
[1] : R. Bachelot, P. Gleyses and A. C. Boccarra, Optics Letters, 20,
1924 (1995).
[2] : L. Aigouy, A. C. Boccara, H. Cory, S. Ducourtieux, S. Gresillon
and J. C. Rivoal, International Congress of Optics XVIII, 1999, San Francisco.
[3] : V. M. Shalaev and A. K. Sarychev, Physical Review B 57, 13265
(1998)
A strong effort is now given to study optical properties of an ensemble
of nano-particles and nano-structures, which possess strong optical nonlinearities
and high potential for various applications in optoelectronics. Apertureless
SNOM remains one of the most promising techniques to achieve an optical
resolution better than 10 nanometers. Our near-field optical microscope
working in transmission has been presented[1]. In our experiment, we use
an apertureless tungsten tip to probe the near field. We have demonstrated
a resolution better than 10-nanometer on gold aggregates with this transmission
mode set-up.
| Polarization images of a metallic step confirmed the good resolution by comparison with an analytical model. They demonstrate also the capability of the microscope to obtain images with polarization effects. The good resolution was used for the observation of small gold aggregates which confirmed that this microscope is able to make spectroscopic measurements of the optical effect induced by a nanometric scale particle. . |
The ability of this set-up to measure the optical signal with a good
resolution was used to study the semicontinuous film of gold granules.
The samples are formed by irregularly shaped clusters of gold that form
fractal structures near the percolation threshold. When the sizes of the
particles in composite films become much smaller than the wavelength of
illumination, the distribution of the electromagnetic field may be localized.
It has been recently predicted that interaction of electromagnetic field
with a semicontinuous film consisting of metal granules randomly distributed
on an insulating substrate gives rises to strongly localized sharp peaks
in the field distribution. Positions and magnitudes of the intensities
of these peaks ("hot" spots) are wavelength dependent. The intensity of
the electric field in the peaks can exceed the applied field by several
orders of magnitude. The sizes of the areas where these "hot" spots occur
are much smaller than the wavelength of illumination. Near-field microscopy
is ideally suitable for the characterization of this effect.
SNOM studies were made for various visible and infrared wavelengths
with tunable Titanium/Sapphire (near IR, transmission mode), Kripton and
CO2 (Visible and mid IR, reflection mode) lasers. The enhancements of the
electric field we found are in good agreement with the ones predicted by
the theory.
This work is a collaboration with:
-Patrice Gadenne and Xavier Quélin, LMOV,
Université de Versailles Saint Quentin
-Vladimir A. Shubin, A. K. Sarichev and Vladimir
M. Shalaev, Department of Physics, New Mexico States University, Las
Cruces, NM, USA.
[1]-Transmission mode aperturless near-field microscopy: optical and magneto-optical studies, S. Grésillon, H. Cory, JC Rivoal, AC Boccara, Journal of Optics A : Pure and Applied Optics, 2 (1999).