RU2671150C1 - Method for forming defects in volume of dielectric sample with laser radiation - Google Patents

Abstract

FIELD: technological processes.SUBSTANCE: invention relates to the field of laser processing of materials and concerns the method of forming defects in the volume and on the surface of a dielectric. Method includes the generation of femtosecond laser pulses in the short-wave and long-wave ranges, determination of threshold energies for each radiation, reduction of two focused pulses, determination of the time delay of the long-wave pulse, the effect on the sample of laser pulses with the parameters found, control of the process of creating defects. Reduction of the two focused pulses in the first stage is performed by visualizing the photoluminescence regions of the plasma from both pulses, and the time delay between the pulses is determined from the minimum value of the nonlinear transmission of the long-wave pulse. At the second stage, the reduction is carried out by recording the maximum signal of the third harmonic from the long-wave pulse. Parameters of the defects are monitored by recording the signal of an unsynchronized third harmonic from the long-wave pulse generated on the defects formed.EFFECT: technical result consists in increasing the accuracy of the radiation guidance, reducing the energy of the pulses and providing the possibility of monitoring the processes in real time.12 cl, 8 dwg, 1 tbl

Description

Technical field

The present invention relates to a technology for the formation of micro- and nanoscale defects in the volume and / or on the surface of transparent materials - dielectrics by means of laser femtosecond volumetric processing of dielectrics (micromachining), and can be used including when creating optical schemes, Bragg gratings inside fibers, biosensors, laboratory-on-a-chip, in tissue engineering when creating scaffold structures, three-dimensional information recording, etc.

State of the art

Technical solutions are known from the prior art that provide micro-nano-processing (guidance of point defects or multiple point defects) of a surface and / or volume of a material with a single-pulse action, a sequence of the same femtosecond pulses, a non-Gaussian pulse, or combined pulses of different wavelengths. In this case, crystals (≈10 * 10 * 5 mm) or films (≈25 * 25 * 2 mm) of wide-band dielectrics, such as fused silica, sapphire, as a rule, act as an object of processing. The result of processing is a point ablation of the material on the surface or a plurality of such defects on the surface or a point change in the internal structure of the material, most often recorded as a change in the refractive index, in the bulk of the sample or a plurality of such defects.

In particular, a prior art method for modifying the surface of a material is known (Xiaoming Yu and others, 'Damage Formation on Fused Silica Illuminated with Ultraviolet-Infrared Femtosecond Pulse Pairs', 9511 (2015), 95110C <http://dx.doi.org/ 10.1117 / 12.2182633>.) To obtain round craters with a diameter of 500 nm and a depth of 70 nm, resulting from the processing of the material by two femtosecond pulses with different wavelengths (first pulse: 70 fs 266 nm; second pulse: 60 fs 800 nm) with a delay between them 60 fs. The proposed method for two-color exposure to a substance combines the advantages associated with operating in the short-wavelength range of wavelengths, namely, the small size of micromodification (the dimensions of induced defects are close to the diffraction limit for a given wavelength) and the small degree of photonity of the multiphoton ionization process, which allows to reduce energy of the pulse acting on the material, in the long-wavelength range - effective avalanche ionization. The first (seed) short-wave pulse, by multiphoton ionization, creates seed electrons in the medium, which are used by the second (heating) long-wave pulse to trigger avalanche ionization. The method of two-color exposure is characterized by a two-stage effect on the material, which allows controlling the processes of plasma formation, and, consequently, the size of induced modifications. This article explores the possibility of reducing energy in the first short-wavelength pulse without changing the spatial resolution (that is, maintaining the possibility of inducing defects with a diameter close to the diffraction limit), which will make it possible to use cheaper equipment in the future for processing materials (commercial fiber lasers instead of high-power lasers) .

The method for producing defects on the surface of fused silica, presented in this article, involves focusing on the sample surface in one region of two radiations with different lenses, with the possibility of independent change in the pulse energy using ND filters, reducing two pulses in time using the differential frequency generation effect in an additional BBO nonlinear crystal. The necessary energies in the beams, and the threshold energies of optical breakdown in the single-pulse mode and in the double-pulse mode, were found by visualizing the induced defect using a CCD camera. The optimal delay between pulses, at which the minimum threshold energy of the short-wavelength pulse is reached, was found by visualizing the induced defect using a CCD camera for different values of the delay and for different energy of the long-wavelength pulse. Also, the modifications obtained in the experiment in the above work were investigated using an optical microscope and a scanning electron microscope. It was shown that the use of a long-wavelength pulse with circular polarization makes it possible to reduce the threshold energy of the first short-wavelength pulse in comparison with the use of a linearly polarized second pulse, however, there was no difference in the sizes of induced defects.

However, this solution does not provide the possibility of creating defects in the volume of the material, including due to the lack of visualization tools for the resulting defects in volume and the information of two pulses in space and in the volume of the sample. In addition, filter sets are used to change the energies in both channels, which does not allow smoothly (continuously) changing pulse energies. An additional nonlinear crystal is used to determine the time of femtosecond pulses in time, which complicates the design of the setup and increases its cost. Determining the optimal delay between pulses is associated with the need to register the appearance of modifications, which requires appropriate equipment and is not an optimal solution. In addition, there is no possibility of monitoring the process of creating modifications in real time. The size of the induced defect is measured using a scanning electron microscope, which is located separately from the microprocessing unit for materials, and therefore, for carrying out control studies of defects, it is necessary to extract a sample from the unit.

Closest to the claimed solution is a femtosecond laser system with process control and feedback (US 2011139760), including a femtosecond laser, optical frequency conversion, beam control optics, target (target) motion control, processing camera, diagnostic systems and system control modules providing control of the processing of laser materials. The system allows real-time changes to processing parameters, in particular, you can configure the system for a specific application and make sure you achieve the desired result. The system includes a device for generating optical pulses, in which each pulse can have individual characteristics. The device comprises laser means for generating pulses, control means that controls the laser means and beam manipulation means for controlling the pulse width, wavelength, repetition rate, polarization characteristics and / or time delay of pulses containing burst pulses. The device generates feedback data based on the measured pulse width, wavelength, repetition rate, polarization characteristics and / or time delay for the control means. In one embodiment of the invention, the laser means may comprise a fiber amplifier that uses tensile grids and compressor grids. The beam manipulation means may comprise a plurality of devices, for example, an optical gating device that measures the duration of the laser pulses, a power meter that measures the power of the laser pulses output from the laser means, or a photodiode that measures the repetition frequency of the laser pulses.

In another embodiment, the beam control means optically converts the fundamental frequency of the generated laser pulses into one or more other optical frequencies and includes at least one optical element that converts a portion of the fundamental laser pulses into at least one harmonic of a higher order. The optical device may comprise a non-linear crystalline device that controls the orientation of the crystal. Preferably, the means for converting the optical frequency include a spectrometer that measures the predetermined parameters of the pulses output from the non-linear crystalline device and generates feedback for the control means. Another embodiment of the beam control means comprises telescopic optical devices for controlling the size, shape, divergence or polarization of the input laser pulses and steering optics for controlling the location of the laser pulse impact on the target substrate. The device may further comprise a device that monitors the characteristics of the laser pulses and generates feedback for the controls.

The above device can be used to change the refractive index of the target substrate; surface marking, subsurface marking and texturing of the sample surface; making holes, channels or holes in the target substrate; deposition or removal of thin layers of material. The device can operate both in single-pulse mode and in double-pulse mode (with one or different wavelengths) or in multi-pulse mode with real-time monitoring of the induced modifications.

However, the system is complex and large-sized, includes a large number of devices, which reduces the reliability of the installation and complicates the process of caring for it. The control system in the inventive device consists only of a sensor for determining the third harmonic of long-wave radiation. Whereas for providing feedback, the known solution for controlling the beam parameters contains large-sized devices - a spectrometer, a pulse amplifier (stretcher, compressor). Another disadvantage of the known system is the use of pulses with an energy of more than 1 μJ, which requires the use of a powerful laser. The inventive technology (method and device) is simpler and more economical, requires less energy to form structured samples (microdefects) (the energy of both pulses is less than 1 μJ) while ensuring high accuracy of positioning of micro-nano defects.

Disclosure of invention

The present invention is the creation of size-controlled point modifications in the volume of wide-gap dielectrics by controlling the energy input (volume density of the energy absorbed by the material) when a sample is exposed to a pair of femtosecond low-energy (E <1 μJ) pulses with different wavelengths with the registration and control of processes occurring when the sample is modified in real time, as well as the possibility of subsequent investigation of induced defects without removing the sample from installation.

The technical result of the invention is the provision of high-precision guidance of size-controlled micromodifications in the volume of a wide-gap dielectric by two low-energy (E <1 μJ) femtosecond sharply focused (NA = 0.4) pulses of different wavelengths with the possibility of real-time monitoring of ongoing processes and subsequent visualization of the induced defect while simplifying the technology (method and device).

The problem is solved in that the method of forming defects in the volume and / or on the surface of the dielectric sample includes:

- generation of laser radiation at two wavelengths of femtosecond duration (Gaussian pulses) in the short-wave (UV-visible) range with linear polarization and the long-wave (near-medium IR) range with circular polarization, respectively, with their subsequent focusing at a given point in the volume and / or on the surface of the sample,

- determination of threshold energies for each radiation,

- the reduction of two focused pulses in the volume of the sample in space and time with the determination of the time delay of the second long-wave pulse relative to the first short-wave,

- subsequent exposure to the sample with laser pulses with the found parameters, and control of both the process of creating defects and the parameters of the created defects,

at the same time, two focused pulses are reduced in space and in time in two stages, in the first of which “rough” reduction of focused pulses without a sample is performed by visualizing the plasma photoluminescence regions from both pulses and combining the obtained images, while the time delay between pulses is determined the minimum value of the nonlinear transmission of a long-wave pulse from the value of the time delay between pulses, at the second stage, focuses are reduced ovannyh pulses in the volume and / or on the surface of the sample by pointing modification short-pulse with energy greater E _{threshold q} and subsequent alignment of focus longwave pulse with an energy smaller than E _{threshold dd} before the registration of the maximum of the third harmonic signal from the long wavelength pulse, then the maximum value of the third harmonic determined in the mode of movement of the sample to obtain the intersection of the constrictions of two focused pulses with the highest possible micron accuracy, while repeatedly predelyayut time delay between the pulses in a sample similar to the first step;

and the control of the process of creating defects is carried out by recording the signal of the nonsynchronous third harmonic from the long-wave pulse generated by the inhomogeneities formed by the induced plasma in the waist region, while registering the transmitted long-wave radiation, and the parameters of the created defects are controlled by recording the signal of the non-synchronous third harmonic from the long-wave pulse, generated by formed defects.

To create more than one defect, the sample is moved synchronously with the pulse repetition rate by a given distance. In the best embodiment of the invention, long-wave radiation is used in the range 1.2-5.0 μm, short-wave - 0.2-0.7 μm, the energy range of long-wave radiation is 0.9-0.95 E _{threshold dv} , short-wave - 0, 3-0.9 E _{threshold sq} . The threshold radiation energies are determined by recording non-linear transmission of the corresponding radiation, or registering a third harmonic signal from long-wave radiation generated by laser-induced plasma generated by short-wave radiation. Visualization of plasma photoluminescence regions from two focused pulses is carried out using CCD cameras using micro lenses in two projections.

The problem is also solved by the fact that the system for creating defects in the volume and / or on the surface of the dielectric sample by laser radiation includes:

- a laser radiation source configured to generate laser radiation at two wavelengths of femtosecond duration (Gaussian shape) in the short-wave (UV-visible) range with linear polarization and the long-wave (near-average IR) range with circular polarization, respectively,

- two radiation channels, a short-wavelength and a long-wavelength, connected to a laser radiation source, with blocks for changing and controlling the radiation energy located in them, while in the long-wave channel there is a lens to compensate for the difference in focal lengths for different wavelengths,

- a control unit for the time delay of the second long-wave pulse relative to the first short-wave pulse by changing the optical path length of the radiation, located in one of the channels,

- block focusing short-wave and long-wave radiation,

- a unit for visualizing regions of plasma photoluminescence from two focused pulses, including two CCD cameras with micro lenses aimed at the sample and connected to a PC located in mutually perpendicular directions, arranged to move along three axes (x y z);

- means of moving the sample in three directions,

- a control unit for the process of creating defects and parameters of the created defects in or on the surface of the sample,

at the same time, the unit for changing and controlling the radiation energies, the time delay control unit, the visualization unit, the sample moving means, the control unit for the process of creating defects and the parameters of the created defects are made with the possibility of connecting to a PC.

The unit for changing and controlling the radiation energy in one embodiment of the invention includes a quarter-wave plate, a Glan prism and a semiconductor radiation intensity sensor of the desired wavelength, while the plate and prism are designed to change the radiation energy.

The time delay control unit in one embodiment of the invention includes two dielectric mirrors configured for axial movement.

The focusing unit in one embodiment of the invention includes a focusing lens with a parameter of NA in the range of 0.3-0.5, made with the possibility of movement along three axes.

The lens to compensate for the difference in focal lengths has a focal length of 30-70 cm, depending on the radiation wavelengths.

The control unit for the process of creating defects and parameters of created defects in one embodiment of the invention includes a collimating lens that collects radiation emanating from the sample, and three intensity sensors for three different wavelengths — long-wave radiation, short-wave radiation, and third harmonic of long-wave radiation.

Brief Description of the Drawings

The invention is illustrated by drawings, where in FIG. 1 schematically shows the arrangement of the functional blocks described in the claims, FIG. 2 is a schematic representation of an experimental setup with all optical elements; FIG. 3 shows in detail (with all optical elements) a block for visualizing the regions of plasma photoluminescence, FIG. Figure 4 shows the image obtained on the PC screen of the plasma luminescence regions, where the arrows show the directions of alignment of the plasma photoluminescence regions. In FIG. 5 shows in detail the control unit for the process of creating defects and the parameters of the created defects. FIG. 6-8 show the experimental results. In particular, in FIG. Figure 6 shows the results of determining the threshold plasma-forming energy of long-wavelength radiation (1240 nm, 200 fs) in fused silica by measuring nonlinear transmission of long-wavelength radiation 25 and measuring the signal of the non-synchronous third harmonic 26 generated on the plasma (the focusing lens A240TM was used in the experiment). From FIG. Figure 6 shows that when using the third-harmonic generation effect, the plasma formation threshold is determined more precisely. The plasma formation threshold corresponds to the growth point of the third harmonic signal. The evolution of the signals of the third harmonic 28 and non-linear transmission of long-wave radiation 27 depending on the delay between pulses is shown in FIG. 7. Zero on the time axis corresponds to the moment when the maxima of the envelopes of the pulses coincide in time (with negative delays, the long-wave pulse arrives earlier than the short-wave on Wednesday). The optimal delay corresponds to the minimum of the transmission signal of the long-wave pulse and the local minimum in the third harmonic signal. In FIG. Figure 8 presents the results of experiments on the modification of fused silica. In FIG. Figure 8a shows a defect in fused silica obtained in a 100X optical microscope induced using radiation with parameters of 620 nm, 150 fs, 0.3 E _{threshold q} and a long-wave pulse of 1240 nm, 200 fs, 0.9 as a short-wavelength pulse. E _{threshold dv} . In FIG. 8b, c show the transverse and longitudinal profiles of the defect, respectively, obtained by means of recording the signal of the third harmonic 29, and using an optical microscope 30. In FIG. 8d is a schematic representation of the obtained modifications. In FIG. 8e shows images of modifications on an optical microscope obtained at different energies of a short-wavelength pulse. The dark region (reflection coefficient is lower than that of pure material) is the region of discharged density, the light region (reflection coefficient is higher than that of pure material) is a densified shell.

The positions in the drawings indicate:

1 - laser radiation source

2 - unit for changing and controlling energy in channels (it is possible to use a quarter-wave plate and a Glan prism to determine the energy of a semiconductor intensity sensor for the measured wavelength)

3 - lens to compensate for the difference in focal lengths for different wavelengths

4 - block focusing short-wave and long-wave radiation

5 - time delay control unit

6 - block visualization of the areas of plasma photoluminescence

7 - control unit for the process of creating defects and parameters of created defects

8 - beam divider into two channels

9 - half-wave plate

10 - Glan prism

11 - semiconductor sensor for controlling incident energy

12 - nonlinear crystal, used to convert radiation to a higher harmonic in one of the channels

13 - linear stepper motor that changes the optical path length of one of the channels

14 - dielectric mirror mounted on a stepper motor 13

15 - dielectric mirror mounted on a stepper motor 13

16 - three-axis motorized stepper motor, on which the sample is mounted

17 - CCD camera in the XZ plane

18 - a micro lens to the CCD camera in the XZ plane

19 - CCD camera in the XY plane

20 - a micro lens to the CCD camera in the XY plane

21 - sample

22 - region of plasma photoluminescence in a fused quartz crystal from long-wave radiation

23 - region of plasma luminescence in a fused silica crystal from short-wave radiation

24 - collimating lens, collecting radiation, diverging from the sample

25 - semiconductor sensor for the appropriate wavelength, measuring the intensity of the transmitted radiation (either long-wave radiation or short-wave radiation)

26 is a semiconductor sensor for an appropriate wavelength, measuring the intensity of the transmitted radiation (either long-wave radiation or short-wave radiation)

27 - photomultiplier tube that records the third harmonic signal

28 is a graph of nonlinear transmission of long wavelength radiation

29 is a graph of the dependence of the third harmonic signal on the incident energy of long-wave radiation

30 is a graph of the nonlinear transmission of long-wave radiation from the magnitude of the time delay between pulses

31 is a graph of the dependence of the third harmonic signal on the value of the time delay between pulses

32 is a modification profile (transverse FIG. 8b, longitudinal FIG. 8c) obtained by means of recording the third harmonic signal

33 is a modification profile (transverse FIG. 8b, longitudinal FIG. 8c) obtained by means of image analysis obtained in an optical microscope (FIG. 8a).

The implementation of the invention

The claimed group of inventions can be implemented using the system (experimental setup) shown in FIG. 1-2. In one embodiment, an apparatus for influencing the volume and / or surface of a dielectric by a pair of sharply focused femtosecond laser pulses includes the following elements: a laser system 1, then a beam splitter into two channels 8 (one channel of a long-wave pulse and a second short-wave pulse). The radiation energy in each channel is varied independently using a half-wave plate 9 and a Glan prism 10, and energy is monitored using a semiconductor sensor at a measured wavelength 11 connected to a PC (these three elements together make up the unit for changing and controlling radiation energy 2). In the channel of the short-wave pulse there is a non-linear crystal 12, which serves to convert the radiation coming out of the laser system (main radiation) into a higher harmonic, as well as a filter for cutting off the main radiation. In the same channel, there is a time delay control unit 5, which includes a linear stepper motor 13, controlled by a PC, moving two dielectric mirrors 14-15. In the channel of long-wave radiation, after the unit for changing and controlling the radiation energy 2, there is a lens to compensate for the difference in focal lengths 3 arising from the use of radiation of different wavelengths, with the possibility of alignment along three axes. The short-wave and long-wave radiation “enter” the focusing unit along one track, the block itself consists of a focusing aspherical lens 4 with NA values in the range 0.3-0.5 with the possibility of alignment along three axes. The focus of the lens is a motorized three-axis stepper motor 16, on which stands a sample holder. On two sides (top and side) in two perpendicular directions there are two CCD cameras with 17-20 micro lenses that transmit images of the plasma photoluminescence region (Fig. 4) to a PC - this makes up imaging unit 6. Behind imaging unit 6 is a unit for controlling the process of creating defects and parameters of the created defects in the volume and / or on the surface of the sample 7. The block consists of a collimating lens 24 and two dichroic mirrors and a set of filters used to separate radiation of the same wavelength, and two semiconductor sensors radiation intensity of the corresponding wavelength 25, 26. The third harmonic signal is recorded by a photomultiplier tube 27, before which a set of filters and a polarizer are installed to cut off the short-wave radiation signal (the polarization of the short-wave radiation and the third harmonic are perpendicular).

The method of micromodification of the volume and / or surface of the sample is carried out by sequentially exposing the sample to a pair of femtosecond laser pulses of different wavelengths.

The method begins with the generation of femtosecond pulses of a Gaussian shape with different wavelengths in the UV-visible ranges (short-wavelength radiation) with linear polarization and near-medium IR range (long-wavelength radiation) with circular polarization. The next step is to determine the threshold energy of plasma formation (E _threshold ) for each of the pulses separately. Field ionization processes are nonthreshold; therefore, the incident radiation energy at which the electron density of the induced plasma affected the passage of the pulse through the medium is called the plasma formation threshold. Two methods can be used to determine the threshold: the first is based on the detection of nonlinear transmission of the sample upon absorption of the laser pulse energy in the plasma 28, the second is based on the efficient generation of the third harmonic on the laser-induced plasma 29. Radiation of the required wavelength is focused into the volume of the sample automatically transferred using a motorized three-axis stepper motor 16 (movement is necessary in order to avoid the interaction of the laser pulse with already damaged material - single-pulse the mode of interaction with the target), after which it is collimated by the lens 24 and sent to a semiconductor detector 25, 26. Nonlinear transmission of the fourth harmonic and the third harmonic signal from long-wave radiation are recorded using a photoelectron multiplier 27. The threshold plasma formation energy is determined by the inflection point of the nonlinear transmittance (Fig. 6), for example, see FV Potemkin, G Bravy, and et al., 'Overcritical Plasma Ignition and Diagnostics from Oncoming Interaction of Two Color Low Energy Tightly Focused Femtosecond Laser Pulses inside Fused Silica', Laser Physics Letters , 13 (2016), 45402 <http: // dx .doi.org / 10.1088 / 1612-2011 / 13/4/045402>.

A smooth change in the energy incident on the sample occurs in the unit for changing and controlling the radiation energy 2 in each channel separately. This is done by rotating the half-wave plate 9 at a fixed position of the Glan prism 10. The energy of the incident radiation is transmitted to the PC from the semiconductor sensor, where 4% of the radiation from the channel is received due to Fresnel reflection from the transparent plate.

The next step in the inventive method is to reduce two sharply focused pulses falling into the sample volume on the one hand, in space with microns and time with femtosecond accuracy. The unique technique of spatial mixing of beams with an accuracy of 2 microns and temporal with an accuracy of 8.3 fs consists of 2 stages. At the first stage, the image of the plasma luminescence regions obtained from CCD cameras 17-20 (resolution of CCD cameras with micro lenses of the order of 1 μm per pixel) is combined by means of alignment of dielectric mirrors in the radiation channels. The image obtained on the PC is shown in FIG. 4, where the arrows indicate the direction of adjustment. With such a “rough” setting in the air, the constriction of radiation is achieved with an accuracy of up to 15 μm in space and with an accuracy of the duration of the pulse in time. At this stage of the information, the time delay between pulses is determined by the minimum value of the nonlinear transmission of the long-wavelength pulse 27. For this, the energy of the long-wavelength pulse is reduced to the threshold energy in air, and its nonlinear absorption is recorded 28. When combining two pulses up to the duration of the pulses, the long-wavelength pulse begins to be absorbed by the plasma created by the short-wave pulse, which is recorded as a sharp decrease in the energy of the transmitted signal 28. For further reference, When adjusting the circuit, the time delay is set to the position of the maximum absorption. The next stage is a more accurate reduction of constrictions of focused pulses with micron accuracy in space and with an accuracy of 8.3 fs in time. To do this, the location of the short-wave pulse constriction is fixed by creating a modification in the sample volume in a single-pulse mode (by a short-wave pulse with an energy greater than E _{threshold sq} ). A long-wavelength pulse with an energy below the threshold (0.2 E _{threshold dv} ) scans the sample volume in the direction of propagation of the pulses by moving the lens to compensate for the difference in focal lengths along the radiation axis. The combination of the constriction of the long-wave pulse with the induced defect is recorded by the effect of generating a non-synchronous third harmonic. The third harmonic in this case is generated at the edges of the defect, that is, in the place where the jump in the refractive index occurred. With this reduction, the accuracy of the reduction in space is determined by the length of the beam waist at an energy of 0.2 E _{threshold dv} . Further, the reduction is carried out in a dynamic mode when the crystal moves by maximizing the effect of third harmonic generation at the energy of both pulses below the plasma formation threshold. In this mode, the microplasma is localized in the waist region of the short-wave radiation, and the third harmonic signal will be maximum when two foci are combined. The accuracy of reducing the beams in space is determined by the sensitivity of the third harmonic generation effect from long-wave radiation on the inhomogeneities of the refractive index, which is equal to 1 micron. To more accurately reduce the beams in time, experiments are carried out to study the evolution of the electron density of the tandem microplasma 31. After the beams are reduced in space, the dependence of the nonlinear transmission of the long-wave pulse on the delay between pulses with energies in the channels is less than the threshold 30 is taken. Reduction of the beams with an accuracy of 8.3 fs in time (accuracy is determined by the step of a linear stepper motor, which is used in the time delay control unit 5) is determined in two ways. The minimum of the transmission signal and the local minimum in the third harmonic signal between the two peaks are observed at the moment when the electron concentration is maximum, that is, when two pulses follow each other with an optimal time delay (the short-wave pulse arrives on Wednesday first) Fig. 7. For each experiment, the value of the optimal delay will vary depending on the duration of the pulses used and the material, namely, the relaxation time of the electrons in it. To select the optimal delay, that is, the pulse information over time, the dependences of the third harmonic signal 31 and non-linear transmission of the long-wave pulse 30 on the delay between pulses are taken.

After converging the radiation, the pulse energy necessary for the required size of the modification is set and the delay between pulses is set (to obtain a defect with minimum dimensions, the optimal delay is set). The energy in each channel can be varied independently using a half-wave plate and a Glan prism. For minimal modification of the dielectric, the energy in the long-wave impulse is set equal to 0.95 E _{threshold dv} , and in the short-wave impulse 0.4 E _{threshold sq} .

Next, the sample is set to the desired depth by moving it along the axis of the radiation, and then when the sample moves, so that the required number of pulses gets to one point (to modify the minimum size it is necessary to influence the material with one pair of pulses), create modifications about the volume or on the surface of the sample .

In this case, to change the size of the induced modification, it is necessary to change the energy in the short-wave radiation, change the polarization of the long-wave pulse, and change the delay between pulses. Changing the wavelengths of the pulses and changing the duration of the pulses also allows you to control the size of the induced modification.

The control unit for the process of creating defects begins with lens 24, which collimates the radiation transmitted through the sample at three wavelengths (shortwave, longwave, and third harmonic). Control over the process of formation of modifications is carried out by recording the signal of the nonsynchronous third harmonic from the long-wave pulse generated by inhomogeneities (of the produced plasma or the defect itself) in the waist region, and the signal of the transmitted long-wave radiation. To study induced defects, it is not necessary to extract the sample from the setup; only the signal of the non-synchronous third harmonic from the long-wave pulse generated by the inhomogeneities, which in this case are the defect boundary, is recorded. For this, at a fixed position of the focusing and collimating lenses, the sample is moved, while scanning along one of the coordinates with a weak long-wave impulse (Е = 0.2 Е _{threshold dv} ) of the sample volume with induced micromodifications. The modification profile is obtained from the dependence of the third harmonic signal on coordinate 32.

An experimental design for exposing a dielectric to a pair of sharply focused femtosecond laser pulses is shown in FIG. 2. The installation used radiation from a chromium-forsterite laser system (wavelength 1240 nm, pulse duration 200 fs, energy per pulse up to 1.5 mJ, pulse repetition rate 10 Hz). At the entrance to the circuit, the main radiation (1240 nm) was divided into a 30/70 beam splitter 8 into two channels: a long-wave pulse channel (1240 nm) and a short-wave radiation channel with a wavelength of 620 nm (second harmonic from the main radiation). The radiation in the second channel using a BBO 12 crystal (10 × 10 × 1 mm, type I, θ = 21 °, ϕ = 0 °, conversion efficiency 30%) was converted to the second harmonic - the polarization of the short-wave radiation was obtained perpendicular to the polarization of the long-wave radiation. The main radiation was blocked using the SZS-27 filter, after which the short-wave radiation fell on a linear stepper motor 16, which was included in the time delay control unit (1 step = 6.67 fs), controlled by a PC. The energy in each channel was varied independently using a half-wave plate 9 and a Glan prism 10. A quarter-wave plate was installed in the long-wave radiation channel to rotate the radiation polarization to circular. The long-wavelength and short-wavelength radiation were focused into the sample using the sharp focusing (f = 8 mm; NA-0.5 / f = 3.3 mm; NA = 0.4) aspherical lens 4 A240TM / CAY033, however, in order to compensate for the difference in focal lengths arising from the use of radiation of different wavelengths, an additional lens 3 was put into the channel of long-wave radiation to compensate for the difference in focal lengths (f = 50 cm).

The input energy in each channel was monitored using photodetectors 11 PDA-100A in the channel of short-wave radiation and PDA-50B in the channel of long-wave radiation. After passing through the collimating lens 21 of the control unit for the process of creating defects 7, the long-wave radiation together with the third-harmonic signal (generated in the sample volume) passed through a dichroic mirror (the mirror does not significantly change the amplitude of these emissions), where, using another dichroic mirror (at 1240 nm ) two signals were separated: the long-wavelength was focused into the receiving aperture 26 of the PD PDA-100A, and the third harmonic was collected by a photomultiplier 27 (Hammamatsu H5784-04). Additionally, a set of filters and a polarizer were installed in front of the photomultiplier to cut off the short-wave radiation signal (the polarizations of the short-wave radiation and the third harmonic are perpendicular). In the course of the studies, the dependences of the third harmonic signal and the long wavelength signal transmitted through the sample on time were recorded, as well as the dependence of the third harmonic signal on the input energy of the long wave and short wave radiation at their fixed energy, respectively. To exclude work on the sample volume modified by the previous pulse, it was mounted on a motorized three-axis stepper motor 16, moving with a pulse repetition rate.

In the course of experiments (28, 29), the following threshold energies were obtained for short-wave and long-wave radiation

Further, according to the dependences of the nonlinear transmission signal of the long-wavelength pulse 30 and the third harmonic signal 31 on the time delay between pulses, the optimal delay value was obtained at which a minimum size modification of 140 fs was induced (FV Potemkin, E I Mareev, and et al., ' Enhancing Nonlinear Energy Deposition into Transparent Solids with an Elliptically Polarized and Mid-IR Heating Laser Pulse under Two-Color Femtosecond Impact ', Laser Physics Letters, 2017).

To obtain defects, the CAY033 aspherical lens (NA = 0.4; f = 3.3 mm) was used as the focusing lens 4. In the experiment, the energy of the long-wavelength pulse with linear polarization was 0.8 E _{threshold dv} , and the energy of the short-wavelength pulse varied from 0.3 E _{threshold q} to 1 E _{threshold q} . The laser focus was set at 50 μm below the surface of the sample. This value was chosen for the convenience of further study of the modifications using an optical microscope with a magnification of 100X.

In FIG. 8b, the micromodification profiles are obtained by registering the generation of an asynchronous third harmonic. As these data and the image obtained with an optical microscope show, see FIG. 8a, the defect has the shape of an ellipsoid, the long axis of which is located along the axis of the beam (schematic representation of FIG. 8d). The method for controlling the process of creating defects using the third-harmonic generation effect allows one to determine the presence of both the central micromodification region and the outer ring created by the shock wave without extracting the sample from the experimental setup.

The experiment showed that an increase in energy in a short-wave impulse from 0.3 E _{threshold sq} to 1 E _{threshold sq} leads to an increase in the size (diameter of the outer circumference of the densified shell and discharged core) of the induced defect from 3 μm to 8 μm of FIG. 8d Thus, the ability to control the size of the created defects was shown. Moreover, the use of the claimed invention with the parameters of the Gaussian beam profile and the corresponding polarization with control over the size of the modification during its formation will allow the formation of defects with reduced sizes in comparison with those presented in the experiment, in which the size of the induced defects was 1.3 times smaller than the created , in a single-pulse mode with a radiation wavelength of 620 nm.

Claims (26)

1. A method of forming defects in the volume and / or on the surface of a dielectric sample, including: - generation of laser radiation at two wavelengths of femtosecond duration in the short wavelength range with linear polarization and the long wavelength range with circular polarization, respectively, with their subsequent focusing at a given point in the volume and / or on the surface of the sample, - determination of threshold energies for each radiation, - the reduction of two focused pulses in the volume of the sample in space and time with the determination of the time delay of the second long-wave pulse relative to the first short-wave, - and the subsequent exposure of the sample to laser pulses with the parameters found, and control of both the process of creating defects and the parameters of the created defects, at the same time, two focused pulses are reduced in space and time in two stages, the first of which carries out “rough” reduction of focused pulses without a sample by visualizing the plasma photoluminescence regions from both pulses and combining the obtained images, while the time delay between pulses is determined by the minimum value of nonlinear transmission of a long-wave pulse from the value of the time delay between pulses; in the second stage, the focusing of pulses in the volume and / or on the surface of the sample by inducing modification by a short-wavelength pulse with an energy greater than the E _{threshold kv} and then adjusting the focus of the long-wavelength pulse with an energy lower than E _{threshold dv} until the maximum signal of the third harmonic from the long-wave pulse is registered, after which the maximum value of the third harmonic determined in the mode of movement of the sample to obtain the intersection of the constriction of two focused pulses, while re-determine the time delay between the pulse and a sample similar to the first step; and the control of the process of creating defects is carried out by recording the signal of the nonsynchronous third harmonic from the long-wave pulse generated by the inhomogeneities formed by the induced plasma in the waist region, while registering the transmitted long-wave radiation, and the parameters of the created defects are controlled by recording the signal of the non-synchronous third harmonic from the long-wave pulse, generated by formed defects. 2. The method according to p. 1, characterized in that to create more than one defect, the sample is moved synchronously with the pulse repetition rate by a predetermined distance. 3. The method according to p. 1, characterized in that using long-wave radiation in the range of 1.2-5.0 microns, short-wave 0.2-0.7 microns. 4. The method according to p. 1, characterized in that using long-wave radiation in the energy range of 0.9-0.95 E _{threshold dv} , short-wavelength 0.3-0.9 E _{threshold square} . 5. The method according to p. 1, characterized in that the threshold radiation energies are determined by recording non-linear transmission of the corresponding radiation or registering a third harmonic signal from the long-wave radiation generated by the laser-induced plasma generated by the short-wave radiation. 6. The method according to p. 1, characterized in that the visualization of the areas of plasma photoluminescence from two focused pulses is carried out using CCD cameras using micro lenses in two projections. 7. A system for creating defects in the volume and / or on the surface of a dielectric sample, including: - a laser radiation source configured to generate laser radiation at two femtosecond wavelengths in the short wavelength range with linear polarization and the long wavelength range with circular polarization, respectively, - two radiation channels, a short-wavelength and a long-wavelength, connected to a laser radiation source, with blocks for changing and controlling the radiation energy located in them, while in the long-wave channel there is a lens to compensate for the difference in focal lengths for different wavelengths, - a control unit for the time delay of the second long-wave pulse relative to the first short-wave pulse by changing the optical path length of the radiation, located in one of the channels, - block focusing short-wave and long-wave radiation, - a unit for visualizing regions of plasma photoluminescence from two focused pulses, including two CCD cameras with micro lenses, aimed at the sample located in mutually perpendicular directions, made with the possibility of movement along three axes (x y z), - means of moving the sample in three directions, - a control unit for the process of creating defects and parameters of the created defects in or on the surface of the sample, at the same time, the unit for changing and controlling the radiation energies, the time delay control unit, the visualization unit, the sample moving means, the control unit for the process of creating defects and parameters of the created defects are connected to the PC. 8. The device according to p. 7, characterized in that the unit for changing and controlling the radiation energy includes a quarter-wave plate, a Glan prism and a semiconductor radiation intensity sensor of the desired wavelength, while the plate and prism are designed to change the radiation energy. 9. The device according to claim 7, characterized in that the time delay control unit includes two dielectric mirrors made with the possibility of axial movement. 10. The device according to p. 7, characterized in that the focusing unit includes a focusing lens with a parameter of NA in the range of 0.3-0.5, configured to move along three axes. 11. The device according to claim 7, characterized in that the lens for compensating for the difference in focal lengths has a focal length of 30-70 cm, depending on the wavelengths of the radiation. 12. The device according to claim 7, characterized in that the control unit for the process of creating defects and parameters of the created defects includes a collimating lens configured to collect radiation coming out of the sample, and three intensity sensors for three different wavelengths - long-wave radiation, short-wave radiation, third harmonic of long-wave radiation.

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