LASER INDUCED REFRACTIVITY, LIMITING, SWITCHING OF LASER BEAM AND LASER STRENGTH IMPROVEMENT OF MATERIALS WITH NANOOBJECTS
Natalia V. Kamanina, Petr Ya. Vasilyev, VladislavI. Studenov
VavilovState Optical Institute, 12, Birzhevaya Line, St.-Petersburg, Russia
Abstract.Photophysics properties of organic conjugated systems doped with nanoobjects as well as the laser and mechanical parameters of the inorganic compounds covered with nanoobjects and treated with surface electromagnetic waves are now placed under detailed consideration. As effective nanosensitisers, the fullerene, nanotubes, nanoparticles, etc. can be considered. The promising nonlinear optical, photoconductive, and laser-induced dynamic properties can be activated, as well as the increase in the polarizability can be found under conditions of nanoobjects sensitization. Some improvement of the laser and mechanical characteristics can be revealed.
INTRODUCTION
It is well known that the systems with fullerenes and nanotubes have been used with good advantage in a different area of optoelectronics [1-5]. Fullerene- and nanotubes-doped organic structures have been applied in development of passive and active optical limiters, laser high-speed switchers, converters, spatial light modulators, new display elements, etc. High laser and mechanical strength can be revealed when inorganic matrixes covered with fullerene- or nanotubes have been treated.
In this paper the laser-induced change in the refractive index in fullerene- and nanotubes-doped conjugated systems due to high frequency Kerr effect has been considered, the optical limiting of the laser beam has been discussed. As an additional, the dynamic and laser strength properties improvement has been shown under nanoobjects (such as nanotubes) covering.
THEORETICAL BACKGROUND AND EXPERIMENTAL CONDITIONS
Let us to consider the Kerr effect in the fullerene- or nanotubes-doped organic systems, where the lowest nontrivial nonlinearity is the cubic one. The matter equation of that medium can be the same considered in [6]:
P = 1)E + (3)E3,(1)
where P is nonlinear polarization of the systems, E is field intensity of the light beam, 1)and (3) is linear and nonlinear optical susceptibilities, respectively. In that approximation, the refractive index n is defined by the following equation:
D=E+4P=E=n2E, (2)
that yields
.(3)
With Eq. (1) and neglecting the nonlinear term, one can obtain:
,(4)
where
,(5)
The light intensity is I=cE2/8. Therefore,
,(6)
where
.(7)
n0 is the linear refractive index and c is the light velocity.
It follows from Eq. (6) that the refractive index depends on the light intensity in the media with the cubic nonlinearity. This effect causes self-interaction of the light waves; resulting in self-focusing of a light beam, phase self-modulation of pulses, etc. n2 is an adequate characteristic of the cubic nonlinearity, for example, the fullerene- or nanotubes-doped structures. The mechanism of an anisotropic molecule turn can result in the Eq. (6) nonlinearity under the effect of the intense polarized light wave. The process is quite slow in comparison to the electron polarizability of the medium. Because this mechanism provides the birefringence induced by the dc field (the Kerr effect), the dependence of the refractive index on the light intensity is the high-frequency Kerr effect, and the Eq. (6) nonlinearity is the Kerr nonlinearity.
The results presented in Table 1 consider the laser-induced change in the refractive index in the nanoobjects-doped polyimide materials; the data shown in Table 1 provoke the estimation of the cubic optical susceptibility of fullerene- and nanotubes-doped systems with good advantage. These organic -conjugated systems have been considered as the materials with higher cubic optical susceptibility. Moreover, nanoobjects-doped 2-cyclooctylamine-5-nitropyridine (COANP), polymer-dispersed liquid crystals (PDLCs) based on them, etc. should be taken in to account too. It should be noticed that when these structures have been doped with fullerenes or nanotubes the efficient charge transfer complex (СTС) formation can be obtained. The spectral, mass-spectrometry, photoconductive and quantum chemical simulation evidences of CTC process have been shown in the papers [7-9]. It should be mentioned that the 2–4µm thick films of the polyimide or COANP solution in tetrachloroethane deposited on glass substrates were investigated. 10µm thick films of PDLCs were treated. Fullerene C60 and C70 as well as carbon nanotubes with concentration of 0.1-5wt.% was used in order to sensitized the photosensitive molecules. A holographic grating was recorded by the second harmonic (=532nm) of a pulsed Nd:YAG–laser with the pulsewidth of 10ns. Two beams applied to recording the sinusoidal diffraction grating formed the spot in 5mm diameter on the film surface. The write energy density was 0.01–3.5Jcm–2. The spatial frequency was 90-100mm–1. The films were investigated in self-diffraction mode under Raman-Nath diffraction conditions. The experimental set-up was the same shown in paper [10]. As an additional, the holographic experiment at wavelength of 1315 nm has been made to support the effect.
Table 1. Laser-induced change in the refractive index in the systems based on polyimide.
Structures / Nanoobjects contents,wt.% / Wavelength,
nm / Energy density, Jсm-2 / Laser pulse width, ns / Change in the refractive index, n
Pure polyimide / 0 / 532 / 0.6 / 10-20 / 10-4-10-5
Polyimide+ malachite green dye / 0.2 / 532 / 0.6 / 20 / 2.8710-4
Polyimide+C60 / 0.2 / 532 / 0.5-0.6 / 10-20 / 4.210-3
Polyimide+C60 / 0.5 / 532 / 0.5-0.6 / 10-20 / 4.4710-3
Polyimide+C70 / 0.2 / 532 / 0.6 / 10-20 / 4.6810-3
Polyimide+C70 / 0.5 / 532 / 0.6 / 10-20 / 4.8710-3
Polyimide+nanotubes / 0.1 / 532 / 0.5-0.8 / 10-20 / 5.710-3
Polyimide+C70 / 0.1-0.5 / 1315 / 0.2-0.8 / 50 / 10-3
The light-induced refractive index change ni in the thin fullerene- or nanotubes-doped films could be estimated from the experimental data of increase in diffraction efficiency using the Eq.from[11]:
,(8)
where is the diffraction efficiency, I1 is the intensity of the first diffraction order, I0 is the incident laser beam, d is the film thickness, and is the laser wavelength.
It should be noticed that the thermal part of delta n in the materials studied is close to value of 10-5. Thus, the increase in the diffraction efficiency in the current experiments and hence in the light-induced refractive index change could be explained by the photorefractive effect stimulated by CTC processes in these compounds. The electron affinity of fullerenes is ~2.65eV, it is twice as larger as that of an intramolecular acceptor fragment of polyimide, and it is forth times larger than that for COANP. Therefore, fullerenes are stronger sensitizers and they dominate the acceptor fragments of intramolecular complexes. As a result, complexes between fullerenes and donor fragments are formed enhancing phototransfer of charge in these systems. The path of the charge transfer changes from the intramolecular donor fragment of polyimide or COANP not to its acceptor fragment but to fullerene. In this case the field gradient is formed, that causes the photorefractive effect in these structures under the laser irradiation.
It should be noticed that the changes of the photorefractive properties correlated with a long-wave shift of the absorption spectrum and with the occurrence of an additional absorption band in the near IR range. Moreover, structural changes of the system were observed. They were associated, for example, for polyimide with a transition of the polyimide donor fragment from its neutral tetragonal form to the ionized planar one under the laser irradiation. As a result, electron shells of polyimide and fullerene overlapped, this effect was conducive to the complex formation between the donor fragment and fullerene.
In the fullerene-containing polyimide film with different fullerene content, ni changed from 4.210–3 to 4.8710–3 for nanosecond pulsewidth range. In this case, the incident laser energy density increased from 0.03 up to 0.5-0.6Jcm–2. It should be noticed that ni was not so drastically changed for non-sensitized polyimide. Moreover, the light-induced refractive index change was more significant on introducing fullerenes than dyes into the conjugated systems. The same situation has been observed for COANP matrix. For example, the introduction of 7,7,8,8–tetracyanoquinodimethane into COANP caused ni of 10–5–10–6[12] under the Kr+–laser irradiation with =676nm. These results were less than those for fullerene sensitization of COANP, namely, ni changed from 3.1610–4 to 6.8910–3 as the incident laser energy density increased from 0.03 to 0.9Jcm–2.
The spectral shift in the pyridine compounds was also more with the fullerene introduction[13]. It was reasonable that the light-induced refractive index change influenced the nonlinear absorption as a whole. The general tendency implied that the introduction of only 0.2wt.% of C60 into photorefractive polymers increased their diffraction efficiency 10 and more times.
The light-induced refractive index change established proposes larger nonlinear refractive index n2 and larger third order nonlinear optical susceptibility (3). Determine these values from Eqs.(9) and (10):
, (9)
.(10)
For example, for the polyimide film with 0.2wt.% of C70, and delta n of 4.6810-3 (see Table 1) the nonlinear refraction n2 is 0.7810–10cm2W–1 and (3) is 2.6410–9esu at the incident laser energy density of 0.6Jcm–2. For the COANP film with 5 wt% of C70, n2 and χ(3)are 0.77×10−10 cm2W−1 and 2.4×10−9 esu, respectively, at the incident energy density of 0.9 J cm−2 and ni = 6.89×10−3.Thus, the nonlinear refraction n2 and third order susceptibility (3) for conjugated organic strictures doped with nanoobjects (estimated from holographic recording data) could be respectively: ~10-7cm2kW-1 and ~10-9esu for thin films of the fullerene- or nanotubes-doped organic structures; ~10-6cm2kW-1 and ~10-8esu for the fullerene- or nanotubes-doped polymer-dispersed liquid crystals. The value of third order susceptibility (3) estimated above is in good coinciding with that patented for nanotubes in paper [14]; the authors of this patent testified that (3)=8.510-8 esu for nanotubes systems.
Moreover, it should be noticed that these nonlinear optical parameters are close to those for silicon (10−10 cm2 W−1 and 10−8 esu, respectively). Thus, for nonlinear optical aims the inorganic structures can be replaced with organic ones with good advantage. The results obtained suggest broad potentialities for application of fullerene- or nanotubes-doped materials not only to hologram recording, but to optical limiting too. The basic results of the optical limiting properties of the fullerene- and nanotubes-doped materials for the visible and IR spectral range have been shown in paper [15]. The energy loses due to diffraction on the reversible photorefractive grating have been considered as an additional mechanism.
Let us to consider the pronouns organic conjugated structures in display technique to increase the speed of liquid crystal (LC) electrooptical switchers and display elements. Really, due to drastic increase in (3), thus in local volume polarizability [16], the LC element with fullerene- or nanotubes СTС reveals better switching parameters than the one without nanoobjects. Some switching characteristics of LC mesophase with fullerene-doped СTС and main mechanism explained the accelerating effect have been shown in paper [16]. It should be mentioned that for typical nematic liquid crystals, such as 5CB or TN LC, the time-on of the electrooptic response fell in the range of 8-16 ms. After the self-arrangement under condition of СTC doping in the fullerene-doped structure, the time-on of the electrooptic response can be less than the 0.5 ms that is by one order of magnitude shorter. Moreover, namely nanoobjects-doped LC mesophase placed between conducting indium-tin oxide (ITO) layers treated with surface electromagnetic wave (SEW) reveals good high speed operation when direct alignment layers can be absent. In this case higher transparency of LC display element owing to absence of additional alignment layers can be found. The SEW source was a quasi-CW gap CO2 laser generating p-polarized radiation with a wavelength of 10.6 micrometers and a power of 30 W. The skin layer thickness for this radiation was ~0.05 micrometers. Due to larger number of CC bond in the nanoobjects we have observed the high laser strength of the ITO contacts with nanotubes and studied the laser strength of the ITO layers with and without nanoobjects placing. The results are shown in the Table 3.
Table 3. Improvement of the laser strength properties of ITO contacts.
Type of the layers / Laser energy density,Jcm-2 / Laser energy density destroyed layer, Jcm-2 / Number of pulse before destroy
Pure ITO / 0.35-0.5 / 0.65-0.67 / 10 at 0.66 Jcm-2
ITO with SEW / 1.025-1.05 / 1.25 / 7-10 at 1.25 Jcm-2
ITO covered with nanotubes / 0.4-0.7 / 0.75 / 10 at 0.75 Jcm-2
ITO covered with nanotubes than treated with SEW / 0.94-1.25 / 1.5-1.56 / 10 at 1.5 Jcm-2
Analyzing the Table 3 results, one can see that the laser strength of the ITO covered with nanotubes and than treated with SEW revealed the best laser strength. It should be mentioned, that this treatment has been made when glass or quartz substrates have been used. In this case at the level of energy density close to 1.5 Jcm-2 the output signal has been changed not more than on 10%.
Following the promising test of the ITO contacts, we have considered the wide groups of UV and IR range materials (for example, LiF, CaF2, MgF2, BaF2, ZnSe, etc.) as the perspective candidates in order to increase the mechanical and laser strength of the matrixes saving the transmittance spectra. The surface mechanical strength of these materials has been increased by the factor of 5-10 depended on the quality of the substrate. The mechanism of this improvement is now under discussion. Moreover, the improvement in transmittance spectra in the UV and IR spectra range has been found. Figures 1 and 2 demonstrate these results.
It should be noticed that in the IR spectra region these facts can be explained due to little value of the imaginary part of the dielectric constant (responsible for the absorption) of the nanotubes in the IR range.
Fig.1. The UV spectra of the BaF2 substrate treated with nanotubes.Fig.2. The IR spectra of the BaF2 substrate treated with nanotubes.
CONCLUSION
Laser-induced processes of the fullerene- and nanotubes-doped conjugated structures based on polyimide, 2-cyclooctylamino-5-nitropyridine, and polymer-dispersed liquid crystal systems have been studied to apply these materials as efficient nonlinear media. Estimated from holographic recording data, the nonlinear refraction n2 and third order susceptibility (3) for conjugated organic strictures doped with nanoobjects could be respectively: ~10-7cm2kW-1 and ~10-9esu for thin films of the fullerene- or nanotubes-doped organic structures; ~10-6cm2kW-1 and ~10-8esu for the fullerene- or nanotubes-doped polymer-dispersed liquid crystals. The high frecuency Kerr effect provoked by efficient charge transfer complex formation has been considered as the basic mechanism responsible for the laser-induced features. It has been discussed that the additional polarizability of the fullerene- or nanotubes-doped structures, for example LC, stimulates the easy control of these systems. It has been shown that switching time can be improved by at least one-two orders of magnitude. It permits to develop the nanoobjects-doped LC display element of new generation. Moreover, it has been obtained that the surface mechanical and laser strength of the materials covered with nanoobjects and treated with surface electromagnetic wave can be improved. It predicts to apply these structures in laser technique with good advantage.
ACKNOWLEDGEMENTS
The authors wish to thank Dr Yu.M. Voronin and Dr. A.P. Zhevlakov (Vavilov State Optical Institute, St. Petersburg, Russia) for their help in this study. This work was partially supported by ISTC Project IPP A-1484.
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