Dangerous high-power laser radiation has been used in technology for many decades, capable of damaging CCD and CMOS matrices (used in video cameras and lidars), as well as causing irreparable damage to the organs of vision. Damage occurs at a high-energy fluence density above a certain threshold value. The main disadvantages of the passive protection devices used – light filters – are color distortion and a narrow wavelength range that requires an accurate value of the wavelength of laser radiation that falls on a photosensitive material or device. The operation of a nonlinear optical limiter of laser radiation power for passive protection is based on the use of a material with a strong nonlinear optical response. However, existing materials do not have the low energy dimming threshold required for practice and, moreover, have insufficiently effective optical attenuation. Further improvement of these characteristics is necessary, although the results already achieved at the moment may well be used in practice. A nonlinear absorption coefficient was used to evaluate the effect of optical limiting. As a result of the studies carried out in this work on the example of composites of single walled carbon nanotubes with a tetra(cyclotriphosphazene)-substituted phthalocyanine ligand, we found that depending on the hydrodynamic radius of fine particles, the efficiency of optical limiting can increase by an order of magnitude. At the same time, the efficiency of optical limiting of composites is more than 2 times higher compared to the original nanotubes.
To protect photosensitive detectors and micro-optoelectromechanical systems, protective equipment is needed against the ingress of powerful laser radiation. Conventional color filters pose a problem for such devices by cutting out the region of the spectrum in which the laser operates. To ensure the possible operation, passive limiters based on nonlinear optical effects can be used. They are capable of transmitting non-hazardous laser radiation with insignificant attenuation (transmission of the order of 70%), and when hit by powerful laser radiation they can quickly darken. The urgency of this problem only increases with the development and widespread use of laser systems. In particular, when using lidars, receiving photosensitive detectors are often damaged. The use of various conjugates with carbon nanotubes is promising for creating limiters. In this work, it is proposed to use conjugates of dimeric phthalocyanine complexes of Cu to create nonlinear optical materials. This material demonstrates the ability to attenuate laser radiation with a nanosecond duration of 16 ns, even in the case of single pulses, due to the synergistic effect of absorption and scattering of radiation. In addition, in the case of 140 fs femtosecond periodic radiation, with a pulse repetition rate of 80 MHz, this material demonstrates the attenuation of radiation when using a cut-off diaphragm due to the effect of self-phase modulation (SSPM). The characteristics of the SSPM pattern were studied, it was noted that the outer rings have the greatest width. In general, the width of the rings gradually increases with distance from the center of the beam. At the same time, the peak fluence (highest fluence value) of each ring is approximately the same, with the exception of the central part. In the central region, a strong effect is observed from heated flows tending upward and thus strongly violating symmetry.
In this study, J-type dimeric copper phthalocyanine in dimethylformamide (DMF) was investigated to show strong nonlinear absorption due to the reverse saturable absorption (RSA) under single nanosecond pulses (16 ns at a wavelength of 532 nm), which significantly exceeds the lifetimes of excited states. In the case of a femtosecond laser, the effect of spatial self-phase modulation was observed, with no contribution from nonlinear absorption being found. The only linear absorption was detected under pulses of 140 fs at a wavelength of 790 nm. The femtosecond pulse repetition rate was 80 MHz which corresponds to a delay between single pulses of about 12.5 ns. Both analytical wavelengths used were outside the intense absorption region (Q- and B- bands) of J-type dimeric copper phthalocyanine in the UV/Vis/NIR spectrum. In the case of femtosecond laser radiation, a sufficient population of excited states was not achieved due to the low peak fluence (~ 0.03 mJ/cm2). For the single nanosecond pulses, the threshold fluence density was ca. 30 mJ/cm2. In the switch circuit, such material should be placed horizontally to exclude asymmetric thermal convection resulting from the gravity effect.
Laser radiation limiters can be made on the basis of working substances, which have strong nonlinear effects after reaching a certain critical value (threshold limiting). Thus, it becomes possible to obtain a high transmission for a safe beam and a sharply reduced transmission for a hazardous beam. To determine the nonlinear and linear optical properties of these materials there were carried out comprehensive spectroscopic studies, experiments by Z-scan methods with an open aperture and a fixed location of the limiter. Working substances was developed which is suspension of conjugates J-type phthalocyanine dimers Zn or Mg with single-walled carbon nanotubes (SWCNTs) in water. Created conjugates can be used not only for protecting eyes and light-sensitivity elements, but for forming three-dimensional tissueengineered structures. Using conjugates J-type phthalocyanine dimers Zn and Mg with SWCNTs will increase the optical absorption in the wavelength range of laser processing by reducing the thermal effect on other substances in the composition of this structure. The Nd:YAG laser was used as the laser radiation source for generating pulses of 16 ns duration at a wavelength of 532 nm with the linearly polarized laser beam in the horizontal plane and a shape of Gaussian type. The threshold of limiting, linear and nonlinear absorption coefficients were determined by output characteristic, that was obtained by fixed location of the limiter. Created working substances have values of the following order: linear absorption coefficient ~ 3 cm-1 for layer of 0.2 cm thickness, low limiting threshold ~ 1 MW·cm-2 and high value of the nonlinear absorption coefficient ~ 550 cm GW-1 . Knowing the nonlinear optical parameters, Z-scan data with an open aperture can be calculated for comparison with experimental data.
Creation of effective means of protection from laser radiation of high power requires the development of optical materials (working substance), with their transparence being decreased sharply above a certain critical value of the laser intensity due to the appearance of non-linear optical properties (limiting threshold). Based on the threshold model, the working substance of the optical limiter was characterized. Experimental data of z-scan with open aperture are used to determine the nonlinear optical parameters of solutions of dimeric phthalocyanine complexes of Mg and Zn of J-type in tetrahydrofuran (THF) and thin films of their conjugates with single-walled carbon nanotubes (SWCNTs). The output characteristic (output (peak) fluence vs input (peak) fluence), that describes the basic properties of optical limiters, was obtained with the fixed location of the optical limiter. Dimeric phthalocyanine complexes were found to have low limiting threshold ~ 2 MW·cm-2 and high value of the nonlinear absorption coefficient ~ 330 and 370 cm GW-1, respectively. Conjugates of these dimeric phthalocyanines with SWCNTs have been produced for the improving of the limiting parameters and increasing of the optical nonlinearity. Size of J-type dimeric phthalocyanine complexes of Mg and Zn were determined by the scanning electron microscopy (SEM). The atomic force microscopy (AFM) allowed to determine the dimensions of nanotubes. The structure parameters, such as diameter and defects as well as the strength of aggregates were estimated with the Raman spectroscopy. For our experiments, the lens with a focal length of 20 cm was used. As the laser radiation source, the Nd:YAG laser was used to generate pulses of 16 ns duration at a wavelength of 532 nm with the linearly polarized laser beam in the horizontal plane and a shape closed to Gaussian type.
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