Activity 3. 4 - Interface phenomena and interaction gas-surface in
nanosctructured materials for sensors
P. Maddalena, S. Lettieri*
Second harmonic generation in single and doubly-resonant microcavities
In the framework of a scientific collaboration with University of Pavia (Prof. L. C.
Andreani) and Politecnino of Turin (Prof. F. Giorgis), we studied the second
harmonic generation in resonant Fabry-Perot microcavities obtained by amaorphous
silicon nitride multilayers. The experimental work consists in the measurements of
the second harmonic spectra in all the different possible polarization configurations.
Single and doubly-resonant microcavities have been realized by Poluitecnico group
and have been experimentally analysed in Napoli. In particular, we investigated some
doubly resosant microcavities based on periodic Bragg reflectors. The use of periodic
reflectors improves on previously reported experiments based on non-periodic Bragg
mirrors, as the periodic mirror can be much easiely fabricated and it also minimizes
the consequences systematic errors in the estimation of the deposition rate. The
second harmonic spectra have been studiend experimentally, evidencing resonant
enhancement of the harmonic conversion efficiency corresponding to the critical
points of the photonic band. Moreover, the crucial effect of double resonance
condition has been clerly evidenced. The experimental findings have been
interpretated theoretically by the group coordinated by Prof. L.C. Andreani
(University of Pavia), showing that the results can be well explained by the
hypothesis of an interface-related second harmonic generation within the
multilayered structure.
Sufrace SHG optical spectroscopy
Optical spectroscopic analysis based on the detection of the second harmonic
generation (SHG) signal permits to obtain, in centrosymmetric media, important
informations on the electronic and structural properties of surfaces and interfaces,
such as, for example, surface electronic states resonances, stress, roughness,
symmetries, etc. Spectroscopic SHG analysis have been performed on nonstechiometric hydrogenated amorphous silicon nitride (a-Si1-xNx:H ) films, in order
to investigate on their surface states. The experimental analysis which have been
developed so far permitted us to determine, in arbitrary units, the values of nonvanishing elements of the second-order susceptibility tensor in a-Si1-xNx:H samples
having differnt stechiometries. Moreover, we developed a new experimental setup
which will allow us to perform SHG spectroscopic analysis of thin films and
surfaces as a function of the temperature and also allowing to study the surface/gas
interaction by using different gas mixtures.
Gas - nanostructure interaction in light emitting metal oxide nanostructures
Recent experiments reported in literature evidenced that quasi-unidimensional
nanostructures based on metal oxides show interesting gas-sensing features, as their
electric transport and photoluminescence properties are significantly influenced by
the interaction with gas. In particular, our attention has been focused on the light
emission properties and decay time measurements in quasi-unidimensional structures
(“nanobelts”) of ZnO, SnO2 ed In2O3. Such nanostructures are interesting in the light
of possible gas sensing applications, and the research on those materials is developed
in the framework of the PON-SVISENARIA project, in collaboration with the gas
sensing group coordinated by Prof. G. Sberveglieri (University of Brescia). During
this year, we studied the effect of nitrogen dioxide (NO2) of the photoluminescence
efficiency of ZnO and SnO2 nanobelts. The study of the gas interaction effect on the
non-radiative recombionation probabilities in such materials has also been performed
by means of time-resolved photoluminescence technique, which allows the direct
measurement of the electronic decay times in absence and/or in presence of the gas
quencher. The study of the effect of NO2 on In2O3 nanowires is also in progress.
Program for 2005
Second Harmonic Generation in single and doubly resonant microcavities
During the 2005, we will continue the investigations on non-linear optical effects in
doubly resonant microcavities by studying third harmonic generation. In particular,
we will investigate on the possibility cavità-enhanced cascading effects, in which the
third harmonic beam is optained by sum frequency generation of dundamental and
second harmonic beam within the microcavity.
Surface spectroscopy by SHG analysis
We will continue the study of surface electronic spectra of silicon nitrides. At the
same time, in the framework of the Coherentia activities concerning the study and
the characterization of metal oxides (Prof. L. Marrucci and U. Scotti di Uccio) we
will perform spectral SHG analysis on Strontium Titanate oxides in order to
investigate on the possibility of evidencing the different possible surface termination
of this material.
Gas-nanostructure interaction in nanostructured light-emitting metal oxides
In the framework of a collaboration with the University of Brescia, we will pursue a
complete characterization of the response of different nanostructured metal oxides
(ZnO, SnO2 and In2O3) to NO2 in order to investigate on the possible applications of
these materials in gas sensing.
In particolar, the basics mechanisms of gas-nanostructure interactions will be
investigated by means of electronic lifetimes measurements using the time-resolved
photoluminescence techinques.