Stimulated scattering caused by the interactionof light with morphology-dependent
acoustic resonance
N. V. Tcherniega,1,* M. I. Samoylovich,2 A. D. Kudryavtseva,1 A. F. Belyanin,2
P. V. Pashchenko,3 and N. N. Dzbanovski3
1P. N. Lebedev Physical Institute, RAS, Leninskii Prospekt, 53, Moscow 119991, Russia2Central Scientific Technology Institute “Technomash,” I. Franko, 4, Moscow 121108, Russia
3Lomonosov Moscow State University Skobeltsyn Institute of Nuclear Physics (MSU SINP),Leninskie gory, GSP-1, Moscow 119991, Russia*Corresponding author: [email protected]
Received November 2, 2009; accepted December 1, 2009;posted December 23, 2009 (Doc. ID 119300); published January 22, 2010
Nanostructured materials exhibit a large variety ofpossible applications. The propagation of electromag-netic waves through such systems gives rise to anumber of interesting phenomena, particularly in thecase when the wavelength becomes comparable withthe nanostructure dimension. Electromagnetic fieldenhancement near the nanoscaled objects, photonicbandgap formation, decreasing of the wave group ve-locity, photon localization, and many others are phe-nomena of great fundamental interest that stimulateincreasing of the number of experimental and theo-retical investigations in this field. The study of theoptical properties of the nanostructured materialshas been the task of many theoretical and experi-mental works and still remains the task to be inves-tigated [1,2]. Like electromagnetic waves acousticwaves demonstrate a number of novel phenomenawhile propagating in a medium consisting of morpho-logical inhomogeneities with dimensions of the orderof the acoustic wavelength [3]. For a typical size ofthe morphologically defined inhomogeneities (rods,globules, and so on) of several hundred nanometersthe acoustic wavelength corresponds to hypersound.For nanostructured materials of different morpholo-gies the acoustic excitation of the morphological inho-mogeneities can lead to low-frequency Raman scat-tering for modes that obey the transition selectionrules. The low-frequency Raman scattering realizedin semiconductor, dielectric, and metallic nanoobjects[4–7] gives important information about their vibra-tional dynamics. The frequency shift of the scatteredlight is defined by the eigenfrequencies of the mor-phological inhomogeneities. The aim of this work wasto realize stimulated scattering of light caused by theinteraction of the laser light with confined acousticmodes of the morphological inhomogeneities. Poly-cluster diamond (PCD) films, aluminum nitride (AlN)films, and indium tin oxide (ITO) films were investi-gated. One can find in [8] the process of the deposi-
tions of the films used on the substrates and their
main structural and optical properties with a de-tailed description of the samples’ morphologies.
ITO films, because of their unique electrical andoptical properties (optical transmittance about 80%in the visible spectral region and high electrical con-ductivity), are being widely used for many practicalapplications in the field of optoelectronics. The ITOfilms used were deposited by magnetron sputteringon a glass substrate. The image of the ITO thin filmused is shown in Fig. 1.
The PCD films were prepared by the microwavedischarge method. Various numbers of extremephysical properties of PCD films define their applica-tions for creating field emission displays, surfaceacoustic waves filters, electrochemical sensors, andso on. The image of the PCD film used is shown inFig. 2.
Fig. 1. (Color online) Scanning electron microscopy (SEM)
AlN is a wide bandgap (6.2 eV) III–V compoundwhich is widely used in microelectronics. AlN filmsare transparent in near UV and visible regions of thespectrum and owing to their low extinction can beused in optoelectronics devices. AlN films are alsovery attractive owing to a number of interestingproperties such as high thermal conductivity, highelectrical insulation, and high chemical stability.These characteristics make it possible to design andfabricate high-frequency resonators and bandpass fil-ters for signal processing and communication de-vices. The structure of AlN films varies depending onthe substrate, but for our samples it had the typicaldimension of the morphological inhomogeneities,which is about 1 �m. The image of the AlN thin filmused is shown in Fig. 3.
All these materials can be characterized by the av-erage dimension of the structural inhomogeneities,which are defined by the conditions of growth. Thevalues of these inhomogeneities are about severalhundred nanometers (from 900 to 1000 nm), whichcorresponds to the acoustical frequencies lying in thegigahertz range.
Ruby laser giant pulses (�=694.3 nm, �=20 ns,Emax=0.3 J, and ��=0.015 cm−1) were used as thesource of excitation. Fabry–Perot interferometerswere used for the spectral structure investigations:
Fig. 2. (Color online) SEM image of PCD film on diamondmonocrystal.
Fig. 3. (Color online) AlN thin film grown by reactive mag-
netron sputtering on fused silica substrate.
the range of dispersion was changed from 0.42 to1.67 cm−1. All measurements were realized both forthe forward and backward directions.
The optical scheme of the experimental setup forthe spectral characteristics of the scattered lightmeasurement is shown in Fig. 4. Under the ruby la-ser giant pulse excitation we observed in the spec-trum of the light passing through the sample and re-flected from the sample additional spectralcomponents shifted from the laser line by the value ofseveral tenths of cm−1 if the threshold is reached.
The spectrum of the light scattered in the forwarddirection for the ITO film is shown in Fig. 5.
The threshold value for the PCD film was about0.1 GW/cm2, for the ITO film—0.14 GW/cm2, and forthe AlN film—0.16 GW/cm2. The maximum value ofthe energy conversion of the laser light into the scat-tered light (both for the forward and backward direc-tions) reached in the PCD film was about 20%, for theITO films—12%, and for the AlN films—10%. The di-vergency of the scattered light beam (in the back-ward direction) was of the order of 10−3 rad. The line-width of the scattered light was close to the laserlight linewidth. The results for the first Stokes com-ponent scattered in the forward direction are shownin Table 1.
In all the investigated samples the stimulated scat-tering of light caused by the interaction of the laserlight with the acoustic vibrations of the domains witha size of about several hundred nanometers was reg-istered. For all the samples measured the first Stokescomponent propagating in the backward directionwith the same frequency shift was also registered.The increasing of the average dimension of thesample structure led to the decreasing of the fre-quency shift of the wave scattered in the backwardand forward directions simultaneously. The PCD filmthickness was about 2.8 �m, the ITO film thicknesswas about 1.3 �m, and the AlN film thickness wasabout 3.2 �m. The high conversion efficiency thattakes place for all the samples investigated corre-sponds to the high efficiency of the hypersound exci-tation. All the samples used may be considered asconsisting of close packed units (typical size—severalhundred nanometers) and the gaps between the unitsmuch smaller than the wavelength of the visible
Fig. 4. Optical scheme for stimulated scattering studies:1—ruby laser; 2, 5, and 11—glass plates; 3—system for la-ser parameters control; 4 and 12—system for measuring ofthe scattered light energy in backward and forward direc-tions; 6 and 13—Fabry–Perot interferometers; 7 and 14—systems of spectra registration; 8—mirror; 9—lens;
light. From this point of view these materials can beconsidered as quasi-homogeneous films with a uni-form refractive index, but the acoustic properties ofsuch systems are totally defined by the acoustic prop-erties of the individual units. The morphology of thesamples defines the number and the values of acous-tic resonances, which can be excited and if thesemodes obey the selection rules they can be seen inlow-frequency Raman scattering and at some condi-tions of excitation the stimulated scattering of lightcan be excited.
Our experimental results show that at some condi-tions of excitation the interaction between the modesof the morphologically inhomogeneous films and la-ser radiation can lead to the excitation of the stimu-lated scattering of light. A large number of nano-structured films with different morphologies givesthe possibility for the experimental realization of thecoherent source of light with different frequencyshifts lying in the gigahertz range that can be usedfor the biharmonic pumping for the tasks of spectros-
