Scientific report
on the implementation of the project IDEI 77/2011 in the period of
January – December 2015
Project title : Nonlinear optical processes manifesting as Anderson localization of light
in mesoscopic materials.
Mesoscopic physics of structures is a sub-discipline of intermediate size of condensed matter
physics, which is based on the structures with dimensions between the size of molecular and macroscopic
type bulk material. Understanding and explaining the physical processes in such materials is exciting and
in this context, optical processes taking place in mesoscopic disordered systems effects result from light
scattering and interference processes. In this context there are revealed two important processes, which
are dependent optical coherence properties, Anderson localization of light or coherent backscattering
processes. Although these optical processes are spectacular, in the quantum mechanics, the description of
these phenomena remains intuitively in the optical physics area. In this context, the optical phenomena
abovementioned represented ideas of the fundamental research project. A review of scientific results
obtained during previous stages in the period 2011-2014 is mentioned by:
i)
Abnormal anti-Stokes Raman scattering  represents an Anderson localization signature
light process in strong diffusing systems;
ii)
The particularities of a SERS process (Surface Enhanced Raman Scattering) generated under
resonant or nonresonant optical excitation were established ;
iii)
In this context, SERS studies performed on highly separated metallic and semiconducting
carbon nanotubes were established for the first time as an abnormal anti-Stokes Raman
scattering effect (like a Coherent Anti-Stokes Raman Scattering process) and can be
associated only for the semiconducting carbon nanotubes.
iv)
It was released a kinematic scheme for the generation of an abnormal Anti-Stokes Raman
effect by a Coherent Anti-Stokes Raman Scattering process.
In the context of these results specified for a mesoscopic material, expanding scientific activities
in 2015-2016 has necessitated the introduction of a subject with the title: “Stimulated Raman scattering
generated by exciton-phonon interaction in mesoscopic structures of CdS and the development of a
kinematic process scheme”. The purpose of this additional activity reveals the possibility of another
nonlinear optical process, such as a Stimulated Raman Scattering (SRS), which is a result of excitonphonon interaction. In this context, it demonstrates the character of nonlinear optical effects of CARS and
SRS, both localized in mesoscopic systems.
The physical processes generated by optical excitation at the edge of the fundamental absorption
band remain of interest in the study of semiconductor materials. In this context, exciton-phonon
interaction, termed Frohlich interaction, is one of the topics much discussed mainly with regard to the
optical properties of nanoscaled materials. This process has been the target of several elaborate theoretical
models [1–6]. The most convincing experimental data indicates that the exciton-phonon interaction on the
one hand is obtained by changing the excitonic photoluminescence (PL) spectra based on the appearance
of a modulated PL spectra by phonon replicas [7–10], which transfer an energy from phonon to excitonic
luminescence and another part by a revers process i.e., a transfer from the excitonic luminescence to
phonon spectrum from which results an enhancement of Raman lines, sometimes also accompanied by
multi-phonon Raman emission and a considerable Raman line-narrowing [11–18].
CdS, in its wurtzite form, is a stable wide-gap (2.42 eV) semiconductor of great interest for basic
research and multiple applications in optics and optoelectronic devices. Raman spectroscopic studies
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under resonant and non-resonant optical excitation have been intensively used in the characterization of
CdS in different morphological forms. Under resonant excitation light of energy higher than 2.42 eV, it
exhibits a PL that increases at low temperature and consists of two main emission bands, a broad one
peaking in the range of 600-650 nm and a narrower band situated around 490 nm. The broader band,
characterized by decay times of order 10−7 −10−5 s, is assigned to the emission generated by the radiative
recombination of carriers trapped in states located in the forbidden region of the band gap, while the
narrow band of decay time < 10−10 s originates in excitonic recombination. Excitonic emission acts as a
coherent monochromatic light source following a Poisson distribution, while the wide-band emission
behaves as a pseudo-thermal source following the Bose-Einstein distribution [19]. This detail is very
important for understanding and explaining the origin of the exciton-phonon interaction in CdS revealed
experimentally by resonant Raman scattering studies, as shown by the enhancement and narrowing of
Raman lines.
Careful observation highlights several details that remain incompletely elucidated:
i) what is the real meaning of resonant optical excitation, or, in other words, what are the optical
excitation conditions to optimize this effect?
ii) why does the enhancement of the Raman emission occur unequally on different lines and in the Stokes
and anti-Stokes branches?
iii) why, for the same material, is the strength of this effect dependent on the sample morphology?
Below, it will be tried to answer these questions using correlated photoluminescence (PL) and
Raman spectroscopy studies on different samples of cadmium sulfide (CdS) as bulk single crystals,
compressedmicrometric powder, and thin film. Considering the different enhancements in the Stokes and
anti-Stokes Raman branches that occur at low temperatures and is conditioned by the overlapping of
the excitation laser light over PL excitonic emission band, it will demonstrate that the exciton-phonon
interaction manifests as a stimulated Raman effect. This non-linear optical phenomenon results from the
mixing of two coherent optical fields, namely the pump laser light and the excitonic PL, which in turn is
dependent on the optical diffusing power of the sample.
The most representative figure to highlight the exciton-phonon interaction is given in Figure 1 for
three different morphological samples: a polished single crystalline sample, approximately 10x10x1 mm,
of orientation (0001) purchased from SurfaceNet Gmbh, hereinafter called sample S1; (ii) a platelet
obtained by non-hydrostatic compression at 0.58 GPa of a micrometric crystalline powder, sample S2;
and (iii) a film approximately 100 nm in thickness deposited by thermal evaporation in vacuum of 10−5
mbar on a quartz substrate, sample S3.
120000
a
CdS (S1)
Figure 1. Influence of the sample
morphology on the exciton-phonon
interaction that manifests in an
enhancement of Raman intensity for the
CdS samples labeled S1 (a), S2 (b) and S3
(c). Black curves show the emission
spectra at 88 K that summarizes the
excitonic PL band, over which the
Raman contribution is superposed. Red
curves are the Raman spectra obtained
by subtraction of the PL band. Blue
curves indicate two components in the
PL band of sample S1. Insets show the
optical microscopic backscattering
images obtained using a 50X objective.
T = 88 K
PL + Raman
Raman
610
40000
exc = 476.5 nm
80000
0
500
1000
1500
2000
CdS (S2)
T = 88 K
PL + Raman
Raman
0
0
500
1000
1220
915
610
40000
305
80000
exc = 476.5 nm
b
1500
2000
2000
CdS (S3)
c
915
500
T = 88 K
PL + Raman
Raman
610
1000
305
1500
exc = 476.5 nm
(PL+Raman) intensity (counts)
0
120000
0
0
500
1000
1500
2000
Wavenumber (cm-1)
2
The exciton-phonon interaction appears as an optical phenomenon resulting from the mixing of
the two optical fields, namely the excitonic photoluminescence and the exciting laser light. A priori, the
achievement of such a process must depend on the effectiveness of overlapping of the two optical fields,
which can be performed energetically and geometrically, that is, under a coincidence of the laser
excitation light with the PL excitonic band and a superposition of the two lights, which is more efficient
in highly diffusive media. Based on this reasoning, we performed our studies on three types of samples:
S1, S2 and S3, the first being characterized by a very low light scattering power and the last by a very
small quantity of material submitted to optical excitation.
i) Due to the lack of defects the single crystal S1 sample shows an intense and narrow excitonic
emission band that results from the dominant contribution to the radiative recombination of free excitons
and is less scattered in the volume of the sample. In Fig. 1(b) and 1(c), the contribution of bound excitons
and the size of the optically excited CdS particles found in samples S2 and S3 is illustrated by a widening
towards the low-energy side of the excitonic PL band, which in turn determines a different enhancement
along the Stokes Raman spectrum, clearly highlighted in the case of the S2 and S3 samples.
ii) In a strongly diffusive medium, such as sample S2, each particle is subjected simultaneously to
the laser exciting light and PL excitonic light originating from a neighboring particle, which can mix
together. In this context, the scattering of light in all direction extends the optical path length inside the
material so that the probability of the light interacting with the matter increases. Therefore, a nonlinear
optical effect such as stimulated Raman scattering (SRS) revealed by an enhancement of Raman intensity
as signature of exciton-phonon interaction becomes a very tempting mechanism to invoke. Likewise, the
mixing of two optical fields in a thin film (S3) is less probable, which leads to a rapid attenuation of the
light, as revealed by an important limitation on the enhancement of Raman intensity of approximately 60
times less than the other two samples (Fig. 1(c)).
iii) Normally, under non-resonant optical excitation, the ratio of intensities in the anti-Stokes and
Stokes Raman branches is defined by the Boltzmann law [19]. At resonance, ()aS  (), so that the
deviations of the IaS/IS ratio from the Boltzmann law may reveal the occurrence of a nonlinear optical
process. More specifically, the terms of ()aS ~ VN1 and ()S ~ VN0 are related to the populations N1
and N0 of the excited and ground vibration levels, respectively, and to volume V of optically excited
material.
In conclusions, the strength of the exciton-phonon interaction that results from the superposition
of the laser excitation light over the excitonic PL at different low temperatures was studied in different
samples of CdS. The presence of two optical fields, the incident laser light and the PL excitonic band,
suggests that the different enhancement of Raman lines in the Stokes or anti-Stokes branches can be
considered as result of a stimulated Raman scattering process. This nonlinear optical effect is dependent
on the diffusing power of the sample, which ensures a wave mixing process in larger volumes.
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Dissemination of obtained results:
1) Works published in 2015 :  Anti-Stokes Raman Spectroscopy as a method to identify metallic and
mixed metallic/semiconducting configurations of multi-walled carbon nanotubes ; by Mihaela Baibarac,
Adelina Matea, Mirela Ilie, Ioan Baltog*, Arnaud Magrez;Analytical Methods, 7, 6225–6230 (2015);
2) Works submitted for publication in 2015 :  Exciton-phonon interaction as stimulated Raman effect
in BiI3 of different morphological forms; by A. Nila, A. Matea, M. Baibarac, L. Mihut, I. Baltog*; Journal
of Applied Physics;  Stimulated Raman scattering generated by exciton-phonon interaction in CdS of
different morphological forms; by M. Baibarac, A. Nila, I. Baltog*; Physical Review B; Exciton-phonon
intercation in the Cs3Bi2I9 crystal structure revealed by Raman spectroscopic studies ;by A. Nilă,
M.Baibarac, A.Matea, R. Mitran2, I. Baltog;Physica Status Solidi (b)
3) General seminars: Exciton-phonon interaction in layered crystals and CdS evidenced by
photoluminescence and Raman spectroscopy; by M.Baibarac, L.Mihut, A.Matea, A.Nila, I.Baltog*
INCDFM, 17-03-2015
5) Leadership masters theses and doctoral activities: i) Andreea Nila; masters; Faculty of Physics,
Univ.Bucuresti, 2015; ii) Adelina Matea research in the Doctoral School; Faculty of Physics;
iii) Andreea Nila; admitted to doctoral and research in the Doctoral School; University of
Physics, Univ.Bucuresti, 2015
Project director,
Dr. Ioan Baltog
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