RU2710389C1 - Information recording method in nanoporous quartzic glass - Google Patents

Abstract

FIELD: optics.

SUBSTANCE: invention relates to optical material science, particularly to a method of recording information on a nanoporous quartzoid glass carrier under laser radiation. This is achieved by a method of recording information by directing polarization-dependent birefringence by modifying nanoporous quartzoid glass with a focused low-range-range laser beam with a reduced number of pulses from 100 to 3, high repetition frequency of pulses of up to 10 MHz at pulse duration of 150–220 fs using a lens with a numerical aperture in range of 0.65–0.9.

EFFECT: invention increases recording speed of information carried out by directing polarization-dependent birefringence in nanoporous quartzide glass.

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Description

The invention relates to the field of optical material science, in particular to a method for recording information on a carrier made of nanoporous quartzoid glass under the influence of laser radiation. The obtained result can be used to create devices of durable multidimensional optical memory on glass with an ultra-dense information storage capacity.

The process of recording information with a laser beam on an optical medium consists in the formation of a region with properties changed under the influence of laser radiation and having a contrast with respect to the initial medium. The created contrast, determined by the conditions of laser modification, in which the data is encoded, subsequently represents the signal source when reading information.

A known method of multilayer information recording in transparent plastic - polymethyl methacrylate [Kallepalli, Deepak LN, et al. "Ultra-high density optical data storage in common transparent plastics." Scientific reports 6 (2016)]. Using a focused femtosecond beam of a titanium-sapphire laser, recording was recorded by two-photon absorption of twenty layers consisting of points that had luminescence due to the formation of fluorophores. At the luminescence level of each point, 5 bits of information were encoded. The main disadvantage of the recording method is the thermal stability of the optical carrier based on polymethyl methacrylate, not exceeding 200 ° C, which is significantly lower in comparison with oxide glasses.

A known method of recording information in a photosensitive glass doped with silver ions [Method for recording optical information in glass (RU 2543670)] using a femtosecond laser beam with a wavelength in the near infrared range of 0.8-1.1 μm. The method consists in the fact that under local exposure to femtosecond laser pulses with a duration of 200 fs, an energy of 1.67-10 μJ and a repetition rate of 300 kHz on the glass, Ag ⁺ ions are reduced due to multiphoton absorption and photoionization, which increases the luminescence intensity of the irradiated region . The density of data storage in the above method is limited by the ability to record 1 bit of information at one point. The disadvantage of this method is also the low thermal stability (below 400 ° C) of the optical carrier in comparison with nanoporous quartzoid glass.

A known method of three-dimensional recording of information by a laser beam in quartz glass due to the contrast of the refractive index [Imai, Ryo, et al. "100-Layer recording in fused silica for semi permanent data storage." Japanese Journal of Applied Physics 54.9S (2015): 09MS02]. For recording, a titanium-sapphire laser was used, which generates 300 nJ energy pulses at a wavelength of 800 nm and a duration of 120 fs with a repetition rate of 1 kHz. Focusing into the glass volume was carried out using a lens with a numerical aperture of 0.55, and 50 layers were recorded on each side of the optical disk with a distance of 60 μm between them. To record several points simultaneously, a spatial light modulator was used. The layers included points that differ in brightness when observed under a microscope; their minimum depth was 4 μm when correcting aberrations. In this case, the thermal stability of the optical medium with the recorded information at 1000 ° C for 2 hours is shown [Shiozawa, Manabu, et al. Simultaneous multi-bit recording in fused silica for permanent storage // Japanese Journal of Applied Physics - 52.9S2 (2013). 09LA01]]. The main disadvantages of the method is the limitation of the recording density of information by one bit in one spatial recording point (pita), a laser source with a low pulse repetition rate and an expensive spatial light modulator, which also has a slow speed.

Closest to the essence of the invention are works that describe a method for multidimensional recording of information by a femtosecond laser beam in the volume of quartz glass due to birefringence of a shape, the magnitude of which depends on the irradiation conditions [Zhang, Jingyu, et al. "Seemingly unlimited lifetime data storage in nanostructured glass." Physical review letters 112.3 (2014): 033901], taken as a prototype. The information was encoded into a phase shift and a slow birefringence axis of the nanogrid, an anisotropic nanoperiodic structure formed when linearly polarized light is exposed to glass. Earlier in previous papers [Shimotsuma, Yasuhiko, et al. "Self-organized nanogratings in glass irradiated by ultrashort light pulses." Physical review letters 91.24 (2003): 247405, Beresna, Martynas, et al. "Exciton mediated self-organization in glass driven by ultrashort light pulses." Applied Physics Letters 101.5 (2012): 053120.] the authors of the prototype found that the orientation of the slow axis of the birefringence of the nanogrid is perpendicular to the plane of polarization of the laser beam. The data were read out by analyzing the birefringence parameters of the recorded nanogratings. The phase shift of the nanogrid increases with increasing number or energy of laser pulses. Thus, coding of information is possible not only in three spatial dimensions, but also in several directions of the slow axis and phase shift levels, which allows you to encode more than one bit of information in one spatial point (i.e., the principle of multilevel memory is implemented). This allows you to increase the density of recording information on an optical medium in proportion to the number of bits recorded at one point. To write nanogrids, a femtosecond laser system based on a potassium gadolinium tungstate doped with ytterbium was used. Laser pulses with a wavelength of 1030 nm and a duration of 280 fs with a repetition rate of 200 kHz were focused using a water-immersion lens with a numerical aperture of 1.2 into quartz glass.

For quartz glass, a record of three layers of information with a recording density of 26.75 KB / mm ^{2 was} shown and read from them was demonstrated, and the nanogratings, in each of which 3 bits of information were encoded, were recorded at a depth of 130-170 μm every 3.7 microns in layers with a distance between the layers of 20 microns. To accelerate the recording in the prototype, a laser beam with an initial pulse energy of 6.3 μJ was split using a spatial light modulator into a maximum of 100 laser beams, i.e. the minimum pulse energy for pit formation was 63 nJ. The write speed under these conditions was 6.3 KB / s. The mechanism of the formation of an anisotropic structure under the action of a femtosecond laser beam is still in question [Beresna, Martynas, et al. "Exciton mediated self-organization in glass driven by ultrashort light pulses." Applied Physics Letters 101.5 (2012): 053120.]. Therefore, to accelerate the process of recording information using birefringent nanogratings, experimental optimization of not only the parameters of laser radiation, but, as it turned out, the structure of the optical carrier is required. For nanoporous quartzoid glass with a pore size of less than 5 nm, it was found that for the formation of birefringence, only 2-3 laser pulses are needed, which is 4 times less than the number of pulses required for the formation of a nanogrid in quartz glass [Cerkauskaite, Ausra, et al. "Ultrafast laser-induced birefringence in various porosity silica glasses: from fused silica to aerogel." Optics Express 25.7 (2017): 8011-8021], and allows you to proportionally increase the speed of recording information. It is worth noting that the prototype uses the step-by-step formation of nanogrids, which in turn significantly limits the recording speed of information, and it is obvious that by using continuous broadcasting of an optical medium, for example, its rotation, the recording speed can be increased. However, to further increase the speed of recording information on an optical carrier made of nanoporous silica glass, experimental optimization of such parameters of laser irradiation is required that determine the physical basis for the interaction of laser radiation with the nanoporous structure of quartzoid glass, namely, the duration of laser pulses and the maximum pulse repetition rate, which allow locally forming birefringence of the form has not yet been determined. The value of the duration determines the peak intensity of the laser radiation, and the pulse repetition rate determines the effect of heat accumulation, which in turn significantly affects the size of the region modified by the laser beam [Eaton, M. Shane, et al. "Heat accumulation effects in femtosecond laser-written waveguides with variable repetition rate." Optics Express 13.12 (2005): 4708-4716.]. At the same time, to increase the recording density, it is necessary to achieve a minimum size of the birefringent region in order to minimize the distance between the nanogratings in nanoporous quartzoid glass, which provides the subsequent possibility of reading them with a minimum number of errors.

The problem to which this invention is directed is to repeatedly increase the speed of recording information and the capacity of an optical carrier made of nanoporous silica glass.

The problem is solved in this way of recording information in which microregions with birefringence are formed by modifying glass with a laser beam emitting femtosecond pulses with a repetition rate of up to 10 MHz and a duration of 150 to 220 fs, and the laser beam is focused using a lens with a numerical aperture from 0.65 to 0.9 to record nanogratings with a distance of 1.4 μm from each other in nanoporous quartzoid glass, which has a composition of 97-99 mol. % SiO ₂ , 0.2-0.6 mol. % Na ₂ O, 1.4-1.8 mol. % B ₂ About ₃ and an average pore size of 1-10 nm.

It was found that a decrease in the pulse duration in the range of 600–150 fs at a frequency of 1 MHz leads to an increase in the phase shift of microregions. The higher the level of the phase shift, the lower the number of errors when reading data. The threshold was experimentally set at 10 MHz for the maximum pulse repetition rate of 180 fs, at which the effect of the formation of birefringence of the shape in the regions modified by the laser beam is observed.

Thus, the main difference from the prototype is to optimize the repetition rate of laser pulses and their duration, which allow the formation of birefringence in nanoporous quartzoid glass and ensure stable reading of information. In turn, the use of nanoporous quartzoid glass composition 97-99 mol. % SiO ₂ , 0.2-0.6 mol. % Na ₂ O, 1.4-1.8 mol. % B ₂ O ₃ and an average pore size of 1-10 nm, the structure of which minimizes the stresses arising from laser irradiation, and lenses with a numerical aperture of 0.65 to 0.9 provide a birefringent microregion size that allows them to be placed at a distance of 1, 4 μm from each other and determines the value of a confidently detected phase shift - more than 20 nm, with multilayer information recording - at least 7 recorded layers on each side of the sample.

In order to form birefringent microregions in the volume of an optical carrier made of nanoporous quartzoid glass polished on both sides, a setup was used in which the near-infrared radiation of a wavelength of 1030 nm from a femtosecond regenerative amplifier is attenuated to the desired pulse energy using an optical attenuator consisting of a rotating half-wave Glan plates and prisms passes through a system of mirrors and another half-wave plate, the rotation angle of which determines the orientation tatsiyu linear polarization of the laser beam into the lens with a numerical aperture in the range of 0,45-0,9 and focused in the volume window. The magnitude of the laser pulse energy was measured after the optical attenuator. The optical carrier was located on a motorized rotating table with an accuracy of radial and angular coordinates of 0.1 μm and 0.0002 degrees, respectively. The minimum focusing depth of the laser beam was 20 μm to avoid the possibility of cracking. When 3 laser pulses are exposed to nanoporous quartzoid glass, local regions are formed - nanolattices less than 1.2 microns in size, having birefringence. In the orientation of the slow birefringence axis of each region, 3 bits of information were encoded. To analyze the birefringence, namely the orientation of the slow axis of the irradiated regions, the Abrio Microbirefringence system [Retardance measurement system and method US 7372567 B2] was used based on an Olympus BX51 optical polarizing microscope.

The achievement of the claimed technical result is confirmed by the following examples.

Example 1

Nanoporous quartzoid glass composition 97-99 mol. % SiO ₂ , 0.2-0.6 mol. % Na ₂ O, 1.4-1.8 mol. % В ₂ О ₃ with an average pore size of 10 nm is irradiated with a focused lens with a numerical aperture of 0.9 at a depth of 100 μm into a spot with a diameter of 1.0 μm by femtosecond laser pulses with a wavelength of 1.03 μm, a pulse duration of 150 fs, and a pulse repetition rate 10 MHz and an average power of 0.5 W (pulse energy 50 nJ). The number of pulses varies from 3 to 6 pulses per pit. As a result of irradiation, pit arrays were obtained with a “slow” axis orientation in the interval 0-157.5 ° in increments of 22.2 ° relative to the initial direction of laser radiation polarization, with a phase shift in the range of 15 25 nm. The recorded information is not affected by heat treatment at 600 ° C for 2 hours.

Example 2

In the volume of nanoporous quartzoid glass composition 97-99 mol. % SiO ₂ , 0.2-0.6 mol. % Na ₂ O, 1.4-1.8 mol. % В ₂ О ₃ with an average pore size of 10 nm, an array of pits is recorded with different orientations of the “slow” axis, into which 3 bits of information are encoded (Fig. 1), using a femtosecond laser emitting pulses of a duration of 220 at a wavelength of 1.03 μm fs, an energy of 60 nJ and a repetition rate in the range of 0.01-1000 kHz (Fig. 2). The laser radiation was focused with a 0.65 numerical aperture lens into a spot with a diameter of about 12 μm. 7 layers were obtained, including recorded arrays of pits with a phase shift of 20–30 nm, on each side of the nanoporous glass sample. The distance between the recorded pits was 1.4 μm with the number of pulses per pit equal to 3. The recording of pit arrays was carried out in layers, starting from the lower layer at a depth of 200 μm, and the distance between the layers was 20 μm. After recording 7 layers, the glass sample was turned over to the other side. The reduction in the number of pulses and the use of a repetition rate of 1000 kHz in the above example provides an increase in the write speed up to 1050 KB / s.

Claims (1)

A method of recording information in nanoporous quartzoid glass by inducing polarization-dependent birefringence by modifying the glass with a focused laser beam that emits femtosecond pulses at a near infrared wavelength, characterized in that they reduce the number of pulses from 100 to 3 and increase the pulse repetition rate to 10 MHz with a pulse duration of 150-220 fs using a lens with a numerical aperture in the range of 0.65-0.9.

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