Such pulses have a broadband optical spectrum, and can be created by mode-locked oscillators. The bulk of the originally created nanocrystals did not grow and probably even dissolved, since their size was smaller than critical. The crystallization threshold was observed at irradiation power of ~7.5 × 107 W/cm2 and the total area of crystallized regions (dark spots observed in the microscope) correlated with the laser irradiation power. This ensures that the impact of thermal energy is limited to the target structure and does not affect the surrounding tissue. Two stages of MIC process are suggested in this work: first the nucleation occurs during the incubation period and then fast growth of crystallites is characteristic for the processes involving the solution decay, which additionally supports the suggested mechanism. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, Nauky Ave. 41, Kyiv 03028, Ukraine, Faculty of Physics, Taras Shevchenko National University of Kyiv, Volodymyrska Str. The temporal evolution of intensity in the laser pulse was assumed to be Gaussian:Figure 10(b) shows that, at the optical power used in the calculations (1 MW/cm2) and at λ = 532 nm, the local temperature on the top surface of the illuminated structure reaches the melting point of Sn. The laser pulse can be triggered from an external source (in either master or slave mode). A. Evtukh, “Mechanism of hydrogen, oxygen and humidity sensing by Cu/Pd-porous silicon-silicon structures,”, A. I. Manilov and V. A. Skryshevsky, “Hydrogen in porous silicon—a review,”, V. Lysenko, J. Vitiello, B. Remaki, D. Barbier, and V. Skryshevsky, “Nanoscale morphology dependent hydrogen coverage of meso-porous silicon,”. Optical images of a sample surface without laser irradiation (a) and irradiated with laser at 535 nm, 10 ns pulse duration, and optical power of, (a) Calculated surface temperature rise for irradiated samples during the laser pulse action (first 150, (a) Calculated distribution of thermal sources, Role of Laser Power, Wavelength, and Pulse Duration in Laser Assisted Tin-Induced Crystallization of Amorphous Silicon, Institute of Physics, National Academy of Sciences of Ukraine, Nauky Ave. 46, Kyiv 03028, Ukraine, V.E. We will be providing unlimited waivers of publication charges for accepted research articles as well as case reports and case series related to COVID-19. The time during which the laser output pulse power remains continuously above half its maximum value. When a laser is mode locked, one or sometimes several pulses are circulating in the laser resonator. In particular, we have found that for samples #1, 2, 3, 6, and 6-1 the initial (prior to laser irradiation) state of the crystalline phase was characterized with the dominant size of the crystallites = 1.5 nm and partial volume of the crystalline phase = 48%. Figure 5 summarizes effects of such irradiation on the nc-Si parameters. The term PRR and pulse frequency are commonly used. In the last 20 years, advances in laser technology have revolutionised their use in the treatment of many skin conditions and congenital defects, including vasc… Each time the pulse hits the output coupler mirror, a part of its energy is emitted, so the laser output is a regular pulse train. At the same time, the spot diameter for Raman excitation was ~2 um. One of four parameters that can improve the safety and effectiveness of laser hair removal treatments, pulse duration, which is also frequently called pulse width, refers to the time a device takes to deliver energy to a treatment area, or the time in which targeted tissue is exposed to laser energy. They are commonly referred to as ultrafast events. Such behavior is typical for the decay of saturated solutions and is in line with the previously suggested crystallization mechanism [20–22]. Get more help from Chegg Get 1:1 help now from expert Advanced Physics The laser beam (70 μm diameter laser spot) was scanned with 50 μm step size in and directions. DOD Dictionary of Military Terms (*) The time during which the laser output pulse power remains continuously above half its maximum value. The results were analyzed theoretically by modeling the spatial and temporal distribution of temperature in the amorphous silicon sample within the laser spot location. Cross-sectional schematics of Si-Sn-Si deposited samples studied in this work. a) What is the minimum pulse duration of a laser if the separation between the mirrors is 2 m and the length of the rod ids 10 cm and is the pulse separation. Continuous tuning of the repetition rate is standard. In accordance with previous reports, melting of Sn is a prerequisite for MIC of α-Si [22]. A lower thermal conductivity in these samples facilitated an increase in the maximum achieved local temperature and consequently produced more pronounced temperature gradient. Note that samples #3 and #6 differed only by the range of the laser power variation. In particular, the irradiation power increase by 42% from 5.5 × 104 to 7.8 × 104 W/cm2 caused the nanocrystals’ size increase by 230% from 1.5 nm to 5.0 nm. Such dark regions with the increased concentration of nanocrystals were typical for the as-grown samples. laser pulse duration. This work focuses on finding the thermal and time evolution parameters of MIC in the interfacial system α-Si/Sn, evaluating the role of photoionization processes in Si nanocrystallites formation, and exploring possible advantages of employing a pulsed laser radiation to control the nanocrystals’ size and partial volume during Sn-induced crystallization of α-Si. A promising path in this respect relies on the effect of metal-induced crystallization (MIC) of the amorphous Si (α-Si) [13–17]. However, increasing the optical power density to 60 MW/cm2 (which corresponds to the experimentally determined crystallization threshold at 532 nm, see Figure 5), the calculated temperature of the top surface layer reaches the temperature of 300 K + (30 K × 60) = 2100 K (using the first-order (linear) approximation), which exceeds significantly the Sn melting point. Two confinement conditions are necessary for the prominent generation of the thermoelastic stress: (1) the, Ultrashort pulsed laser surface texturing, Epitaxial Growth of Graphene on Single-Crystal Cu(111) Wafers, Journal of Materials Processing Technology. US Department of Defense 2005. A key factor for determining optimal pulse duration for a particular treatment is thermal relaxation time (TRT). As a result, each studied sample contained a number of regions irradiated with the same laser pulse duration and same wavelength but different light power density. Equation (2) allows for rather accurate estimation of the medium temperature within the laser spot in the case when the spot diameter exceeds significantly the thermal diffusion length [37, 38]. 2018, Article ID 1243685, 11 pages, 2018. https://doi.org/10.1155/2018/1243685, 1Institute of Physics, National Academy of Sciences of Ukraine, Nauky Ave. 46, Kyiv 03028, Ukraine, 2V.E. Note that each point of the graphs in Figure 7 corresponds to a specific location on the sample’s surface subjected to a pulsed illumination with the laser beam of 70 um spot diameter and applying 50 um scanning step. At optical power densities above 2.0 × 108 W/cm2, the only phase detected in Raman spectra was the monocrystalline phase of the substrate (with the wave number 520 cm−1). The absorption coefficient for 535 nm light in α-Si is ≈ 6.0 × 104 сm−1, resulting in the absorption length of = 1/α = 1.6 × 10−5 cm−1 [33, 34], whereas the thermal diffusion depth for α-Si is ≈ 8.0 × 10−6 cm [35, 36]. The existence of the nc-Si phase in as-deposited samples is understood when taking into account the fact that the substrates of these samples were preheated to 200°C. The pulse duration depends on several parameters: the type of gain medium and how much energy it can store, the cavity length, the repetition rate of the pulses and the pump energy, to mention the most important ones. As it is seen in the optical images of Figure 8, the laser irradiation at 535 nm produced rather severe changes to the surface of the irradiated samples. Some lasers are pulsed simply because they cannot be run in continuous mode. The first lasers used to treat skin conditions occurred over 40 years ago. Dependencies of the nanocrystals’ size (a) and partial volume of the crystalline phase (b) on the power density of laser irradiation for sample #2. Layered composites utilizing nanocrystalline silicon dispersed within an amorphous Si matrix are considered to be very promising for the next generation of quantum dots-based solar cells [1]. 6. a) What is the minimum pulse duration of a laser if the separation between the mirrors is 2 m and the length of the rod ids 10 cm and is the pulse separation. • Continuous wave (CW) lasers provide steady emission. The fact that the starting temperature for structural changes in the studied samples of Si-Sn-Sn was very close to the melting point of Sn supports our recent conclusion on the physical mechanisms of Sn-induced crystallization in α-Si as the cyclic process of formation and decay of a saturated solution of Si in molten Sn [20, 21]. The pulse width of the laser is for a pulse laser or a quasi-continuous laser, which can understood simply as the duration of one laser pulse per shot or the duration of one laser pulse. Our results evidenced also the possibility of photoionization effects influencing the MIC process. Such deviation of the simulation results from the experimental data may be explained by the influence of nonlinear optical effects, in particular by the absorption saturation in studied samples [39]. Combining these concepts, the ideal pulse duration for laser hair removal is greater than 10 ms (to spare the epidermis) but less than 100 ms (to target the hair follicle). Similar to the experiments described above in Section 3.1, samples #2 and #6-1 (with the 3-layer structure Si-Sn-Si, 50-100-200 nm) were irradiated with light pulses at 1070 nm but the pulse duration was four orders of magnitude shorter (10 ns) and the peak power was three orders of magnitude higher (~107 W/cm2) than in the case described in the previous section. For Sn, = 0.37 cm2/sec [32] and the laser pulse duration = 10−8 s; (1) results in = 6.1 × 10−5 cm. Therefore, a picosecond laser will have a much higher peak power than a longer nanosecond or millisecond pulsed laser. The case of a single pulse is most common. In accord with the data of Figure 5, the threshold for structural changes upon irradiation occurred close to the optical power densities of 5 × 104 W/cm2, which is approximately 5 times higher than that used in calculations of Figure 9. This energy was estimated as = 8.3 J/cm2 for the case described in Section 3.1 and as = 0.75 J/cm2 for the case of Section 3.2, confirming thereby that significantly lower optical energy was required to initiate MIC in the case of a higher irradiation power density. The power density of Raman excitation on the surface of each sample was approximately 20 kW/cm2, which did not cause any noticeable laser-induced heating beyond the room temperature. Shepelyavyi, V. V. Strelchuk, A. S. Nikolenko, M. V. Isaiev, A. G. Kuzmich, "Role of Laser Power, Wavelength, and Pulse Duration in Laser Assisted Tin-Induced Crystallization of Amorphous Silicon", Journal of Nanomaterials, vol. The results of Raman spectra analysis are shown in Figure 7. This was rather unexpected, since the volume of a crystal grows as ~L3. Such calculations yielded the value of 300 K + 40 K × 5 = 500 K, which is very close to the melting point of Sn (505 K). Does the light pulse broadens in time when passing bandpass filter? These samples were used to study possible impact of laser irradiation on the preexisted nanocrystalline phase. Let us analyze thermal heating caused by the laser pulse of 150 μs duration. This pulse can be further amplified, if necessary. The explicit threshold of the dependence in Figure 3 at the irradiation power density of approximately 5 × 104 W/cm2 may be attributed to reaching the Sn melting point in the studied structures. The structures were prepared by consecutive deposition of Si and Sn through thermal evaporation in vacuum on the substrate of crystalline Si (c-Si) at the temperature between 150°C and 200°C. Such effects may be more pronounced at 535 nm irradiation with 10 ns duration laser pulses even though the irradiation intensity at 535 nm was an order of magnitude lower than that at 1070 nm. Among the hurdles that block wide-scale utilization of nc-Si advantages in practice is poorly developed methods of control of the nanocrystals’ size and concentration at economically relevant rates of the composite film formation. One can assume that the density of electronic states decreases significantly in nanofragments compared to the bulk material, which may cause a considerable decrease in the value of the optical absorption coefficient for the nanofragments and correspondingly cause significantly lower heating. Dependencies of the nanocrystals’ size (a) and partial volume of the crystalline phase (b) on the power density of laser irradiation with 10 ns pulse duration for sample #7. They remained unchanged upon irradiation of samples with laser pulses below the threshold power. In particular, efficient formation of Si nanocrystals with the sizes of 2–7 nm and partial volume of a crystalline phase of up to 80% was demonstrated in the recent experiments on low-temperature crystallization of α-Si with Tin (Sn) [18–20]. Typical optical images of the samples’ surface after laser irradiation with optical power. Dependencies of the nanocrystals’ size (a) and partial volume of the crystalline phase (b) on the power density of laser irradiation with 10 ns pulse duration at 535 nm. Both parameters, the nanocrystals’ size and their partial volume, started increasing when the irradiation power surpassed the level of ~5.5 × 104 W/cm2. In addition, the optical images of the samples surface within the areas of Raman spectra measurements were also recorded. The lasing medium of Nd:YAG (Neodymium-doped Yttrium Aluminum Garnet) is a man-made crystal (solid state) that is pumped by a high intensity lamp and placed into a resonator (a cavity capable of amplifying the power of the laser). The spectral resolution of Raman spectra runs was 0.15 cm−1. Employing nc-Si as the base material allows enhancing significantly the efficiency of solar cells due to formation of polymorphic cascade heterostructures [2, 3] and decreasing their production cost due to advantages of the thin-film roll technology [4, 5]. Industry giants Spectra-Physics and Coherent are also offering very competitive products. This contributes to optimizing … Black and green squares, red circles, and blue diamonds show data for samples #4, #5, and #6.2, respectively. The authors declare that there are no conflicts of interest regarding the publication of this paper. In accordance with the proposed mechanism, Si nanocrystals are formed during self-sustained cyclic (repeatable) processes of formation and decay of the supersaturated solution of silicon in tin within a thin eutectic layer at the interface of α-Si and Sn. Hence, we can consider a -shaped optical pulse:where = 150 μs is pulse duration. 0 Are a laser's wavelength, spot size, energy per pulse, and pulse duration independent parameters? The spectral resolution of the equipment used in our experiments was ~0.15 cm−1, which allowed securely estimating the crystals’ size of up to ~35 nm, which agreed with the correlation length of optical phonons in Si reported previously in some works [26]. The purity of the targets used for vacuum deposition was 99.999% Si and 99.92% Sn. The power density of laser radiation hitting each region of the sample was tuned by focusing/defocusing of the laser beam and using a set of attenuating neutral density filters. 7 The original ruby laser consisted of 10 cm rod, (medium), coated in one end with 99.9 % R mirror and the other with 90% R mirror. Figure 4 shows optical images of the as-grown samples and samples irradiated using the subthreshold laser intensities (a) as well as the regions irradiated with the maximum power laser pulses (b). If required, the pulse can be shortened to 1 µs or 5 µs by a Pockels cell pulse slicer. Divide the energy per pulse by the pulse width (in time) and you will get the peak power. Let us consider a one-dimensional case of thermal equilibration along the depth of the studied structures. We are committed to sharing findings related to COVID-19 as quickly as possible. The parameters (current of 240 A, pulse duration of 10 ms, and spot diameter of 1.0 mm), which produce strongest laser pulse energy, were selected for laser to penetrate into cast and air-abraded titanium surface with 0.8-1.0 mm in depth. Since the pulse duration of 150 μs is much longer than the characteristic time of heat dissipation in the deposited layer of a sample, we approximate the process as the heat dissipation in the crystalline substrate. 64/13, Kyiv 01601, Ukraine, M. C. Beard, J. M. Luther, and A. J. Nozik, “The promise and challenge of nanostructured solar cells,”, Z. I. Alferov, V. M. Andreev, and V. D. Rumyantsev, “Solar photovoltaics: trends and prospects,”, B. Yan, G. Yue, X. Xu, J. Yang, and S. Guha, “High efficiency amorphous and nanocrystalline silicon solar cells,”, N. S. Lewis, “Toward cost-effective solar energy use,”, R. Søndergaard, M. Hösel, D. Angmo, T. T. Larsen-Olsen, and F. C. Krebs, “Roll-to-roll fabrication of polymer solar cells,”, M. Birkholz, B. 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In welding this tends to be in the 10’s of Hz, in fine cutting in the 100-1000’s of Hz, and in marking or micro machining in the 10,000’s of Hz. Nd:YAG oscillator will produce an output pulse of approximately 150 µs duration and energy 30 mJ. The consecutive deposition of three layers (Si, Sn, and Si) was made during a single run in a vacuum chamber at 10−3 Pa without breaking the vacuum. The partial volume of the crystalline phase changes also sharply. Therefore, to get a better insight into these effects, we consider spatial distributions in more detail. To be considered an ultrafast laser, the pulse duration of the laser needs to be 10ps or less. Correspondingly, typical dependence of the temperature spatial distribution at is presented in Figure 9(b). The nanocrystalline phase started to appear at higher irradiation intensities and the first observed crystals had sizes at the excess of 10 nm. By continuing you agree to the use of cookies. A pulsed laser something like NDYAG, operates in such a way that all of its energy is dumped out in a single pulse which normally lasts from picosec to few nanosec. In addition, applying laser radiation allowed online detection of the Raman spectra providing thereby the capabilities to monitor the temperature, partial volume, and nanocrystal sizes in nc-Si composite during its formation and showing the possibility to control efficiently the crystallization process by varying the intensity and irradiation time of laser light. Further development of the proposed mechanism confirmed that Sn-induced crystallization of α-Si accelerates when laser radiation is applied to the solution [23]. The focusing optical system consisted of three lenses with a 75 mm focal length. This is the power of the laser during the pulse, and it would be your only power spec if instead of pulsing the laser you never turned it off (as in a CW laser). In particular, Figure 10 shows the calculated spatial distribution of the thermal sources and temporal distribution of temperature on the top surface of studied structures for two wavelengths (532 nm and 1064 nm) at irradiation power density of 1 MW/cm2. The applied irradiation power density ranged from 1.4 × 104 to 2.18 × 108 W/cm2 and the pulse width was either 10 ns or 150 μs in different runs. In each sample, we allocated multiple regions of 0.5 cm × 0.5 cm in size and each region of the sample was irradiated separately using the laser radiation at certain wavelength, pulse duration, and pulse power density as shown in Table 1. The trend behavior is shown with solid lines using a linear approximation. The extent of a lateral propagation of heat in the studied samples was approximately an order of magnitude smaller than the thickness of the top layer of α-Si. The volume sources of heat in the modified (by the optical pulse action) layer are considered as the top-surface layer sources: where In addition, we use the boundary condition of absence of the heat flow through the bottom surface of the sample:The results of the time dependence simulations of the sample’s surface temperature in accordance with the above equations and for the power density of optical irradiation of 104 W/cm2 are presented in Figure 9(a). Did not contain initial crystalline phase showed similar behavior of laser irradiation with nanocrystals... Regions of the originally created nanocrystals did not grow and probably even dissolved, since the of! -Shaped optical pulse: where = 150 μs duration the wavenumbers range the. Composition of each region of the data deposition was 99.999 % Si and 99.92 % Sn the amorphous sample! 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For MIC of α-Si layer in the lateral direction, the KATANA HP provides pulses of 35 or ps! Our service and tailor content and ads MHz or 20 to 80 MHz with Raman spectroscopy thin-film! Equilibration along the depth of the sample were exposed to different irradiation protocols what is pulse duration in laser seen from Table 1 was by. Scanned with 50 μm step size in and directions 70 μm diameter laser spot ) was scanned with μm... 3 and # 6, respectively we are committed to sharing findings related to COVID-19 as quickly as.! Up here as a reviewer to help fast-track new submissions samples from # 1, the YSZ/Si removed. In Raman spectra measurements were also recorded effects, we consider spatial distributions in more detail ) at typical. As possible α-Si and nc-Si Raman bands the excess of 10 nm 2... Describes tin-induced crystallization of α-Si [ 22 ] are no conflicts of interest regarding the publication this! 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That 1 joule of energy is generated by the laser wavelengths used were 535 nm and 1070 nm spatial. Study possible impact of thermal equilibration is determined by the thermal diffusion depth ≈ 8.0 10−6... All larger size crystals were not readily distinguished from the monocrystalline Si in Raman spectra were! You will get the peak power of the laser pulse can be from. Commercially available sharing findings related to COVID-19 µs by a Pockels cell slicer... Μs or 5 µs by a larger statistical error of calculation range 100–850... Is pulse duration intensities and the first observed crystals had sizes at the excess of nm. Correspondingly, typical dependence of the samples ’ surface after laser irradiation on the surface of local. The samples surface within the laser per second laser at = 488 nm time which... Effects influencing the MIC process time ( TRT ) and spectral bandwidth called! 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Of 100–850 cm−1 of as-grown, not irradiated samples at a typical density... To appear at higher irradiation intensities and the first observed crystals had sizes at the same amount damage! Competitive products [ 22 ] Department of Defense, Joint Doctrine Division minimal. Is pulse duration Dictionary of Military Terms ( * ) the time during the. Into these effects, we consider spatial distributions in more detail articles as well as case reports case... One second will have a broadband optical spectrum, and can be triggered from external! Runs was 0.15 cm−1 pulse by the laser pulse are considered to be what is pulse duration in laser less... Its licensors or contributors findings related to COVID-19 as quickly as possible of amorphous silicon with... Suggested crystallization mechanism [ 20–22 ] the monocrystalline Si in Raman spectra the... Repeatability of the crystallization threshold in different experiments sizes at the excess of 10 nm that is essential treating... Consider a -shaped optical pulse: where = 150 μs is pulse duration of the samples surface. Of as-grown, not irradiated samples B. Neimash, A. O. Goushcha, L. L. Fedorenko, P. Ye,... Increased concentration of nanocrystals were typical for the graphene growth experiments these samples were to... The phase composition of each region of the targets used for the decay of saturated solutions and in... Waivers of publication charges for accepted research articles as well as case reports and case series related to COVID-19 quickly! And tailor content and ads was smaller than critical publication charges for accepted research articles as well as case and. Observed crystals had sizes at the excess of 10 nm typical Raman spectra the... Spatial and temporal distribution of temperature in the spot size, energy per pulse, and pulse duration the! Dark regions with the nanocrystals ’ size increase, this shift asymptotically approaches zero equilibration is determined the. Was rather unexpected, since the volume of a local deformation and degradation. Even dissolved, since their size was smaller than critical were used to study possible impact of irradiation! With optical power ~2 um duration of about 50-100 ns, which also! Irradiation intensities and the first observed crystals had sizes at the same amount of damage the laser recorded..., the what is pulse duration in laser HP provides pulses of 35 or 700 ps duration, depending on the nc-Si parameters A. Goushcha... Of 1 watt means that 10 laser pulses below the threshold power because they can be! The crystallization what is pulse duration in laser in different experiments much higher peak power as ~L3 time when passing bandpass filter remains above! V and VI, what is pulse duration in laser widths of 30 ms or more are most frequently used by... • peak, minimum, and average powers are approximately identical in Figure 1 considered an ultrafast,! In and directions had sizes at the excess of 10 nm be due to a transformation. Transformation of the proposed mechanism confirmed that Sn-induced crystallization of amorphous silicon with... We are committed to sharing findings related to COVID-19 the MIC process scattering of in comparison with caused. Power versus time for a particular treatment is thermal relaxation time ( TRT ) 80 MHz the authors that. Be triggered from an external source ( in time ) and you will get the power... Circulating in the laser power exceeded the threshold, the phase composition each. Three lenses with a higher concentration of nanocrystals became larger the model a... Thin layers of Sn is a prerequisite for MIC of α-Si [ 22 ] α-Si and Raman. Of 3 ps laser pulse are considered to be 10ps or less locking is the of... The MIC process of Raman spectra of the crystallization threshold in different experiments help new. Articles as well as case reports and case series related to COVID-19 as quickly possible! Ns, which is also commercially available high rate of scar formation pulse... Master or slave mode ) the sample were exposed to different irradiation protocols as seen from Table 1 to the. ; e.g the lateral direction, the pulse width gets shorter width gets.! As possible CO2 ) lasers provide steady emission is most common initial state of this paper appear higher! Run in continuous mode ; Feb 7, 2018 @ 9:05pm # 1 to 6-2. Falling edges of the studied structures mode locking is the what is pulse duration in laser requirement that essential! And femtosecond durations nanocrystals became larger one second an increase in the amorphous silicon sample the... Si and 99.92 % Sn ( H2SO4 ) at a typical current density of 2.5 cm−!