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NONLINEAR OPTICAL PROPERTIES OF HYBRID NANOSTRUCTURES
Toyboev Fazliddinjon Fakhriddin ugli
Teacher of the Department of Physics and Astronomy, Navoi State University
e-mail:
Annotation:
Hybrid nanostructures, as nanomaterials created by integrating different materials
and phase states, are of great interest in the field of science and technology. Due to their
combinatorial nature, these materials have unique physical and optical properties, and are
especially notable for their non-exciting optical properties. Non-exciting optical properties
include complex processes that determine the spectral response of materials, their ability to emit
and absorb light, and their interaction with electromagnetic waves.
Keywords:
Hybrid nanostructures, non-polar optical properties, nanomaterials, optical
properties, electromagnetic waves, spectral response, emission and reception, photonics,
optoelectronics.
Abbreviations:
TY - Saturable absorption, TTY - Reverse saturable absorption, 2FY - Two-
photon absorption, 3FY - Three-photon absorption, NZ - Nanoparticle, KN - Quantum dot, TN -
Thionine, IR - Infrared.
We will consider the nonlinear optical properties of some organic dye solutions and
quantum dots. Organic dyes were among the first to be widely used in various fields of laser
physics and technology, including changing the frequency of laser radiation in dye vapors,
modulating its value, and synchronizing modes in various lasers. They have become the object of
studying the effect of increasing the transparency of the medium (bleaching) under the influence
of pulses of different durations and laser generation, etc. The high-frequency Kerr effect, TY,
and TTY are the main nonlinear optical processes arising from the interaction of high-frequency
laser pulses with dye molecules.
Nonlinear optical properties were first discovered in benzene vapor (C6H6), as well as in
acetylene vapor (C2H2), where the third harmonic is formed in the UV range [9]. In 1967, a
general overview of the spectroscopic properties of complex molecules was given, which
showed that a simple two-level scheme was not sufficient to describe the optical decolorization
of dye molecules [13]. Experimental data on the transfer of intense radiation from a ruby
laser to several types of dyes were presented. Analysis of the balance equations made it
possible to model photobleaching curves for the dyes under study. The optical decolorization
process involves the transition of molecules from the ground state to other states with smaller
absorption cross sections at the excitation frequency, and the decrease in absorption at this
frequency is characterized by a complex nonradiative transition mechanism. Hoffmann R.C. and
other dye examples demonstrated the ability of indanthrone and its derivatives to affect the
nonlinear optical properties of dyes by changing their chemical structure [14]. These dyes
exhibited TTY at wavelengths of 1064 and 532 nm in the nanosecond pulse range.
Due to the high similarity of their nonlinear sensitivities with the nonlinear sensitivities of
atoms, conjugated double-bonded molecules have attracted the greatest interest. The above
calculations showed that some organic dyes (tetracene, paraterphenyl, pentacene) have a third-
order sensitivity. In this case, the irreversible changes detected in the dye molecules in the laser
pulse range lead to a change in their nonlinear optical parameters, such as saturation intensity,
nonlinear absorption coefficients, etc. [10]. To analyze the complex behavior of dyes, it is
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necessary to know the nonlinear optical parameters such as 2FY, TY, TTY coefficients and, in
some cases, the nonlinear refractive index using different laser sources. The high-frequency Kerr
effect and induced nonlinear refraction rarely play a decisive role in the formation of optical
nonlinearities in dyes. However, TY is often performed in many dyes. The dynamics of the
singlet-singlet (S1 → S0) transition of dye solutions is determined by the properties of the dye
and the solvent. Accordingly, the high population of dye molecules in Sn (n>2) opens the path of
decomposition to the triplet state and photodissociation in the S1 and S0 states [11].
Photobleaching of dyes often occurs through intermediate states with the TY mechanism.
The initial process leading to the formation of a total nonlinear optical phenomenon in dye
molecules is absorption. The process of absorption of light quanta leads to a change in the
population of the ground and excited states and, as a result, to a change in the instantaneous
absorption spectrum. Thus, important information about the change in the absorption of dye
molecules can be obtained from the absorption spectra induced by the flash photolysis method.
Since the objects of study in this work are high-performance dyes, and thiazine and xanthene
dyes serve as model objects, let us consider the currently available experimental data on induced
absorption for them.
Thiotinine molecules are characterized by a clearing in the range of 510-630 nm (Fig. 1).
Experimental data on the study of nonlinear optical properties, as well as induced absorption
spectra for other thiazine dyes (azure A, azure B, azure C, etc.) were not found in the literature.
The paper presents the results of the study of induced absorption of Ery aqueous solution. With
zero time delay between the source pulse and the test pulse, a strong illumination of the
absorption band occurs, associated with the S0→S1 transitions. The observed decrease in optical
density is interpreted as a superposition of two processes: saturated absorption S0→S1 and
forced emission in the S1 state.
Figure
1-
shows the induced
absorption
spectra
(pulse
duration 6 ns) of a TN (2.5×10-5 M) ethanol solution recorded after excitation with a pulsed
laser at a wavelength of 532 nm [1]. As can be seen from Figure 2, a broad spectrum of induced
absorption appears for wavelengths greater than 550 nm. The maximum optical density of the
induced absorption is 0.05. This absorption does not decrease significantly until a delay time of
1.5 ns.
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It was concluded that the observed absorption is due to triplet-triplet transitions. The rapid
occurrence of triplet-triplet absorption indicates efficient interspin conversion. At shorter
wavelengths around 460 nm, the additional absorption band rapidly disappears simultaneously
with the stimulated emission at longer delay times, due to singlet-singlet transitions from the S1
excited state. Thus, the characteristic optical nonlinearity expected at the wavelength of the
second harmonic of the neodymium laser
(532 nm) for Er is TY.
Figure 2 -
Induced absorption spectra of an aqueous solution of Ery.
Previously, nonlinear optical effects of absorption in Ery were analyzed using pulsed laser
thermal lens spectrometry of this solution [12]. It should be noted that photothermal
spectroscopy is an indirect method for measuring nonlinear absorption. In this work, the laser
source was a He-Ne laser, which did not allow us to analyze short-term nonlinear optical
absorption processes.
The Z-scanning technique is widely used to determine the nonlinear optical parameters of
some dyes. Such studies are not available for Ery molecules. However, they have been
performed for other dyes. This method was used to find nonlinear absorption, refraction, and
optical power limitation for a neutral red organic dye in the field of 532 nm nanosecond laser
pulses [2].
Modeling the mechanisms of nonlinear optical response can be done by taking into account
certain literature data on the absorption cross sections of singlet and triplet transitions, as well as
the lifetimes of excited states. Similar studies have been conducted for dyes of the
phthalocyanine series. The authors have shown the possibility of controlling the degree of
induced absorption by modifying dyes with metal-coordinated complexes of phthalocyanines
using molecular engineering methods.
The next important objects of great interest in nonlinear optical properties are hybrid
organic-inorganic nanostructures. The components of such structures can be organic dyes,
semiconductor nanocrystals, plasmonic nanocrystals (nanoparticles), etc. The new properties of
such compounds are most clearly manifested in the vicinity of optical resonances of plasmonic
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nanocrystals, dye luminescence edges and semiconductor nanocrystals [5]. The laws that show
the manifestation of the interaction of the components of hybrid associations in the nonlinear
optical properties are of great interest due to various practical applications.
The authors of the literature observed a change in the nonlinear optical result in hybrid
associations of attenuated graphene oxide and Au nanocrystals grown on its surface (Fig. 3).
Thus, in these structures, upon pulsed laser irradiation (5 ns) at a wavelength of 532 nm, the
nonlinear optical response of cTY to TTY components changes upon coupling. According to the
authors, the nonlinear optical effect is due to charge transfer from the nanoparticles to the
reduced graphene oxide film.
Figure 3 -
The change of the nonlinear optical response of RGO and Au NZ from TY to
TTY during hybrid association in the field of nanosecond pulses at a wavelength of 532 nm [3].
The most discussed situation is related to the analysis of the nonlinear optical effect in the
associations of silver and gold NZ with dyes. The associations of metal NZ with molecules of
Phenothiazine, Methylene Blue, Uranium, Radomin 6J, betacyanin, etc. are considered.
For molecules with gold NZ, a decrease in the threshold for the onset of TTY and an
increase in nonlinear refractive indices were found for methylene blue molecules under the
influence of continuous radiation from a diode solid-state laser with a wavelength of 532 nm and
a power of 50 mW. In this work, a self-focusing of a thermal nature was observed using the Z-
scanning technique. The observed nonlinear absorption was associated with TTY in MV
molecules.
The asymmetric decrease in the Z-scan characteristic of Au NZ was associated with TTY,
which raises some doubts, especially since the contribution of the nonlinear optical response of
dynamic light scattering to the observed picture was not taken into account. In addition, in the
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discussion of the results of the work, the possible influence of the process of exchange of
electronic excitations between the components of the assemblies is indicated. Thus, despite the
significant changes in the nonlinear optical response accompanying the formation of hybrid
associations, the observed nonlinear mechanisms remain largely speculative.
An increase in the power threshold of nanosecond (7 ns) laser pulses with a wavelength of
532 nm was obtained by mixing phenothiazine with plasmonic gold nanoparticles (Fig. 4) [6]. A
photophysical method was used to synthesize hybrid nanostructures from Au NZ and
phenothiazine molecules with an average size of 28 nm. The resulting composite showed a
significant decrease in photoluminescence intensity, which the authors attributed to
electron/energy phototransfer between the components. The increase in depth in the Z-scan of
the composite compared to the original components is explained by the field effect and the
exchange of electronic excitations.
Figure 4 -
Nonlinear absorption enhancement during the association of phenothiazine and
gold nanoparticles in the region of nanosecond pulses at a wavelength of 532 nm.
The work [7] demonstrates the Rabi splitting and Fano effect in the absorption spectrum of
the IR-806 laser dye associated with gold nanocylinders. The authors attribute these effects to the
manifestation of plexitonic coupling. For such a hybrid association, an increase in nonlinear
absorption at a wavelength of 532 nm was found.
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Figure 5 -
Dependence of solution transmittance on radiation energy at λ= 532 nm. 1: KN
CdSe/ZnS; 2: 0.75C60+0.25CT; 3: C60; 4: 0.75C60+0.25KN+perylene. According to [4].
Non-stationary absorption spectroscopy was used to study the nonlinear optical process of
resonance-coupled systems of Ag NZ organic dyes (Uranine, Rhodamine 6Zh). It was found that
silver nanoparticles coated with dye molecules exhibit the Fano effect in the extinction spectra,
and at the same time, the nonlinear absorption coefficient and nonlinear changes increase
compared to the original Ag NZ [8]. The scope of studies for KN-based associations is very
small. Of interest is the work discussing the observed decrease in the optical power limiting
threshold of laser radiation at a wavelength of 532 nm in the hybrid association of CdSe/ZnS
QDs (average size 3.4 nm) with C60 fullerene and perylene molecules [4]. The association of
these materials leads to photoinduced electron transfer and charge separation in the QD, which in
turn contributes to the optical power limiting (Fig. 5).
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(1992) Excited-state nonlinearity in polythiophene thin films investigated by the Z-scan
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Quantum Dot: A Bright and Biocompatible Fluorescent Nanoprobe in the Second Near-
Infrared Window. ACS Nano, 6 (5), 3695–3702.
4. Ovchinnikov, O. V., Smirnov, M.S., Shapiro, B.I., Shatskikh, T.S., Perepelitsa, A.S., and
Korolev, N. V. (2015) Optical and structural properties of ensembles of colloidal Ag2S
quantum dots in gelatin. Semiconductors, 49 (3), 373–379.
5. Aleali, H., Mansour, N., and Mirzaie, M. (2015) Nonlinear Absorption and Scattering in
Wide Band Gap Silver Sulfide Nanoparticles Colloid and Their Effects on the Optical
Limiting. 8 (1), 1446–1449.
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6. Sendhil, K., Vijayan, C., and Kothiyal, M.P. (2006) Low-threshold optical power limiting of
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