**Excitation of Periodical Shock Waves in Solid–State Optical Media (Yb:YAG, Glass) at SBS of Focused Low–Coherent Pump Radiation: Structure Changes, Features of Lasing**

N.E. Bykovsky and Yu.V. Senatsky *Lebedev Physical Institute, Russian Academy of Sciences, Moscow Russia* 

### **1. Introduction**

368 Acoustic Waves – From Microdevices to Helioseismology

Peng, W. P., Y. C. Yang, et al. (2006). Laser-induced acoustic desorption mass spectrometry

Perez, J., L. E. Ramirez-Arizmendi, et al. (2000). Laser-induced acoustic desorption/chemical

Phipps, C. R., T. P. Turner, et al. (1988). Impulse coupling to targets in vacuum by KrF, HF and CO2 single-pulse lasers. *Journal of Applied Physics* 64(3): 1083-1096.

Prokhorov, A. M., V. I. Konov, et al. (1990). Laser Heating of Metals. Bristol, Philadelphia,

Reid, G. E., S. E. Tichy, et al. (2001). N-terminal derivatization and fragmentation of neutral

Scruby, C. B. and H. N. G. Wadley (1978). Calibrated Capacitance Transducer for Detection of Acoustic-Emission. *Journal of Physics D-Applied Physics* 11(11): 1487-1494. Shea, R. C., C. J. Petzold, et al. (2007). Experimental investigations of the internal energy of

degradation? *Journal of the American Chemical Society* 123(6): 1184-1192. Royer, D. and E. Dieulesaint (2000). Elastic waves in solids. Berlin ; New York, Springer. Schiller, J., R. Suss, et al. (2007). Maldi-Tof Ms in Lipidomics. *Frontiers in Bioscience* 12: 2568-2579. Scruby, C. B. (1987). An introduction to acoustic emission. *Journal of Physics E (Scientific* 

ionization in Fourier-transform ion cyclotron resonance mass spectrometry. *Int. J.* 

peptides via ion-molecule reactions with acylium ions: Toward gas-phase Edman

molecules evaporated via laser-induced acoustic desorption into a Fourier transform ion cyclotron resonance mass spectrometer. *Analytical Chemistry* 79(5): 1825-1832. Smith, S. T. (2000). Flexures: elements of elastic mechanisms. Amsterdam, Gordon & Breach. Song, K. H. and X. Xu (1997). Mechanisms of absorption in pulsed excimer laser-induced plasma. *Applied Physics a-Materials Science & Processing* 65(4-5): 477-485. Spengler, B., U. Bahr, et al. (1988). Postionization of Laser-Desorbed Organic and Inorganic-

Compounds in a Time of Flight Mass-Spectrometer. *Anal. Instrum.* 17(1-2): 173-193.

*and Mechanisms for Producing Ions from Large Moleculres*. K. G. Standing and W. Ens.

Srinivasan, R. and B. Braren (1989). Ultraviolet-Laser Ablation of Organic Polymers.

Vertes, A. (1991). Laser Desorption of Large Molecules: Mechanisms and Models. *Methods* 

Vertes, A. and R. D. Levine (1990). Sublimation Versus Fragmentation in Matrix-Assisted

Veryovkin, I. V., W. F. Calaway, et al. (2004). A new time of flight instrument for quantative surface analysis. *Nucl. Instrum. Methods Phys. Res., Sect. B* 219-220: 473. White, R. M. (1963). Generation of elastic waves by transient surface heating *Journal of* 

Xu, B. Q., J. Feng, et al. (2008). Laser-generated thermoelastic acoustic sources and Lamb waves in anisotropic plates. *Applied Physics a-Materials Science & Processing* 91(1): 173-179. Young, C. E., J. E. Whitten, et al. (1989). Electron-Stimulated Desorption of Neutrals from

Zinovev, A. V., I. V. Veryovkin, et al. (2007). Laser-driven acoustic desorption of organic molecules from back-irradiated solid foils. *Analytical Chemistry* 79(21): 8232-8241.

6063 Aluminum - Velocity Distributions Detected by 193 Nm Non-Resonant Laser

of single bioparticles. *Angew. Chem., Int. Ed.* 45(9): 1423-1426.

*Mass Spectrom.* 198(3): 173-188.

New York, Adam Higler.

*Instruments)* 20(8): 946-953.

*Chemical Reviews* 89(6): 1303-1316.

Laser Desorption. *Chem. Phys. Lett.* 171(4): 284-290.

Ionization. *Surface and Interface Analysis* 14(10): 647-655.

New York, Plenum Press.

*Applied Physics* 34(12): 3559-&.

Pollard, H. F. (1977). Sound waves in solids. London, Pion.

During several last decades much attention was paid to the processes that occur in solidstate optical media under the interaction with high-power focused laser radiation. A great number of studies were devoted to the phenomena of optical breakdown, structure changes, stimulated scatterings, generation of hypersonic waves in transparent dielectrics under the action of nanosecond (ns) and picosecond (ps) laser pulses (Manenkov & Prokhorov, 1986; Nelson et al., 1982; Ready, 1971; Robinson et al., 1984; Stuart et al., 1995). Recent interest in these studies was stimulated by the appearance of lasers with femtosecond (fs) pulses (Gordienko et al., 2010; Merlin, 1997; Sakakura et al., 2007).

An experimental study of a small region with high pressure and temperature gradients formed in a medium at focusing high-power laser radiation had been performed, as a rule, outside the laser cavity. In our experiment (Basiev et al., 2004), a region with such properties happened to be formed directly in the 2-mirror laser cavity, when Yb:YAG samples were pumped by the focused wide-band (0,89-0,95 μm) radiation from a pulsed LiF: F2 + color center laser (ccl). Thus, in contrast to many studies on ytterbium lasers, conditions for generation in Yb-doped samples in this experiment had been distinguished by the very high intensity (over 1 GW/cm2) of the pump, which moreover had a low coherence. Experiments on pumping of Yb-doped and non-doped samples of different optical media (YAG, glass, LiF et al.) by powerful low-coherent radiation from LiF: F2+ ccl were continued in subsequent papers (Bykovsky, 2005, 2006; Bykovsky & Senatsky, 2008a,b, 2010). At intensities *I* ≥ 1 GW/cm2 interaction of ccl pump radiation with the medium in the focal region was essentially nonlinear. The interaction of ccl pulses with samples was accompanied by excitation of stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) of pump radiation. The scattering generated hypersonic waves of high amplitude, which were converted into a periodic sequence of shock waves with sharp pressure jumps on their fronts propagating along the direction of pump. Pressure jumps were so large that they caused a phase transition in an optical medium, which was observed near the sample surface in the form of small domains with spatial modulation of the refractive index caused by the interference of hypersonic waves.

Excitation of Periodical Shock Waves in Solid–State Optical Media (Yb:YAG, Glass)

range.

at SBS of Focused Low–Coherent Pump Radiation: Structure Changes, Features of Lasing 371

pumping of Yb-doped samples by the ccl had been, undoubtedly, inferior to laser diodes pumping in efficiency. However, at focusing ccl radiation in the Yb-doped medium the pump power densities typical of semiconductor diodes could be easily exceeded. This had been, of course, of interest for research. Therefore, just the ccl had been used later in experiments on study lasing in Yb-doped samples as well as nonlinear interaction of pump radiation with optical materials. The possible generation region of LiF: F2+ ccl extends from 0.83 to 1.1 μm (Basiev et al., 1982) and completely covers the absorption band of Yb:YAG. In particular experimental conditions the actual emission band of ccl depends on the selective properties of the resonator, and usually covers only part of the noted wavelength

Fig. 1. Energy level diagram of Yb3+ ions in a Yb:YAG crystal. Ovals combine the Stark

Fig. 2. Absorption and luminescence spectra of Yb+3 ions in the YAG crystal (1 mm plate,

Yb+3 ions concentration 20%)

components of levels with rapid (≈ 10-12 s) thermal relaxation (Krupke, 2000)

Under ccl pumping due to heat release and generation of intensive hypersonic waves a region with strong temperature, pressure and refractive index gradients and at the same time with a high-level of inversion was formed in the focus of the pump laser in Yb-doped materials. Despite the strong optical inhomogeneity of the medium, Yb lasing in 10-15 ns pulses was observed in Yb:YAG (with 20% concentration of Yb3+ ions) and Yb:glass (with 10% Yb3+ concentration) during the action of the 20-30 ns pump pulse and after it (due to inversion remaining in the medium). During the SBS of pump radiation the hypersonic wave spatial structure served as a resonator for Yb lasing. Shock waves (with phonon energies up to 1000 cm-1) affected the generation dynamics. The Yb lasing was distinguished by some specific characteristics such as a surprisingly wide spectrum (up to 50 nm) and a high directivity of the emission. In addition to the wide-band generation on the shock-waves grating there was also observed Yb lasing on resonator modes. After the end of the pump pulse another sequence of shock waves diverging outward the focal region affected the build-up of generation between 2 mirrors in the cavity. The line spectra of Yb generation in the resonator contained twisted spectral lines with structures of small-scale spots.

Description of these unusual phenomena observed under the interaction of short intense pump laser pulse of low coherence with optical media and their explanation are presented in this chapter. The optical scheme and the parameters of LiF:F2+ ccl are considered. The features of SBS and SRS and the appearance of the periodical shock waves in the optical medium at low-coherent pumping are discussed. The interpretation to observed specific optical damage is given. The mechanism of generation of broadband, high-directional short laser pulses in the spatial structure of thin layers with inversion produced in the region of propagation of intense hypersonic waves in the medium is discussed. Conditions for generation in a 2 mirror resonator containing active medium with a strong refractive index gradient are considered. The interpretation of the observed twisted lines with small-scale structures in generation spectra as well as temporal, spatial-angular characteristics of Yb lasing in the resonator is given.

#### **2. LiF: F2 + color center pump laser**

Optical pumping of Yb-doped materials can be performed only into the single Yb3+ ion absorption band at 2F7/2 -2F5/2 transition near 0.9 μm (Figs.1, 2). In this connection, the use of flash lamps as sources of broadband radiation for pumping an Yb-doped medium is ineffective. At the present moment, the most effective and widely used sources of Yb-doped materials pumping are semiconductor laser diodes operating within the spectral range near 0.94 μm. Along with semiconductor diodes solid-state laser pump sources have been used to investigate Yb-doped active media. Cr:LiSAF, Ti:Sa, Nd:YAG (0.94 μm transition) lasers have been used to pump Yb:S-FAP and Yb:YAG (Bykovsky et al., 2000; Kanabe et al., 2000; Marshall et al., 1997, as cited in Bykovsky & Senatsky, 2008b).

In our work a LiF: F2+ color center laser (ccl) was used to pump an Yb:YAG crystal (Basiev et al., 2004). The lithium fluorine color center (LiF: F2 +, LiF: F2 **-**) lasers are the sources of radiation in the near IR (0.8-1.3 μm) , and they effectively convert the neodymium and ruby laser radiation into this spectral range (Basiev et al., 1982). Possible room temperature operation, high conversion efficiency (up to 30%), and a large generation tuning range (more than 1000 cm-1) make such lasers very attractive for certain practical applications. The

Under ccl pumping due to heat release and generation of intensive hypersonic waves a region with strong temperature, pressure and refractive index gradients and at the same time with a high-level of inversion was formed in the focus of the pump laser in Yb-doped materials. Despite the strong optical inhomogeneity of the medium, Yb lasing in 10-15 ns pulses was observed in Yb:YAG (with 20% concentration of Yb3+ ions) and Yb:glass (with 10% Yb3+ concentration) during the action of the 20-30 ns pump pulse and after it (due to inversion remaining in the medium). During the SBS of pump radiation the hypersonic wave spatial structure served as a resonator for Yb lasing. Shock waves (with phonon energies up to 1000 cm-1) affected the generation dynamics. The Yb lasing was distinguished by some specific characteristics such as a surprisingly wide spectrum (up to 50 nm) and a high directivity of the emission. In addition to the wide-band generation on the shock-waves grating there was also observed Yb lasing on resonator modes. After the end of the pump pulse another sequence of shock waves diverging outward the focal region affected the build-up of generation between 2 mirrors in the cavity. The line spectra of Yb generation in the resonator contained twisted spectral lines with structures

Description of these unusual phenomena observed under the interaction of short intense pump laser pulse of low coherence with optical media and their explanation are presented in this chapter. The optical scheme and the parameters of LiF:F2+ ccl are considered. The features of SBS and SRS and the appearance of the periodical shock waves in the optical medium at low-coherent pumping are discussed. The interpretation to observed specific optical damage is given. The mechanism of generation of broadband, high-directional short laser pulses in the spatial structure of thin layers with inversion produced in the region of propagation of intense hypersonic waves in the medium is discussed. Conditions for generation in a 2 mirror resonator containing active medium with a strong refractive index gradient are considered. The interpretation of the observed twisted lines with small-scale structures in generation spectra as well as temporal, spatial-angular characteristics of Yb

Optical pumping of Yb-doped materials can be performed only into the single Yb3+ ion absorption band at 2F7/2 -2F5/2 transition near 0.9 μm (Figs.1, 2). In this connection, the use of flash lamps as sources of broadband radiation for pumping an Yb-doped medium is ineffective. At the present moment, the most effective and widely used sources of Yb-doped materials pumping are semiconductor laser diodes operating within the spectral range near 0.94 μm. Along with semiconductor diodes solid-state laser pump sources have been used to investigate Yb-doped active media. Cr:LiSAF, Ti:Sa, Nd:YAG (0.94 μm transition) lasers have been used to pump Yb:S-FAP and Yb:YAG (Bykovsky et al., 2000; Kanabe et al., 2000;

al., 2004). The lithium fluorine color center (LiF: F2+, LiF: F2**-**) lasers are the sources of radiation in the near IR (0.8-1.3 μm) , and they effectively convert the neodymium and ruby laser radiation into this spectral range (Basiev et al., 1982). Possible room temperature operation, high conversion efficiency (up to 30%), and a large generation tuning range (more than 1000 cm-1) make such lasers very attractive for certain practical applications. The

+ color center laser (ccl) was used to pump an Yb:YAG crystal (Basiev et

of small-scale spots.

lasing in the resonator is given.

 **color center pump laser** 

Marshall et al., 1997, as cited in Bykovsky & Senatsky, 2008b).

**2. LiF: F2**

**+**

In our work a LiF: F2

pumping of Yb-doped samples by the ccl had been, undoubtedly, inferior to laser diodes pumping in efficiency. However, at focusing ccl radiation in the Yb-doped medium the pump power densities typical of semiconductor diodes could be easily exceeded. This had been, of course, of interest for research. Therefore, just the ccl had been used later in experiments on study lasing in Yb-doped samples as well as nonlinear interaction of pump radiation with optical materials. The possible generation region of LiF: F2+ ccl extends from 0.83 to 1.1 μm (Basiev et al., 1982) and completely covers the absorption band of Yb:YAG. In particular experimental conditions the actual emission band of ccl depends on the selective properties of the resonator, and usually covers only part of the noted wavelength range.

Fig. 1. Energy level diagram of Yb3+ ions in a Yb:YAG crystal. Ovals combine the Stark components of levels with rapid (≈ 10-12 s) thermal relaxation (Krupke, 2000)

Fig. 2. Absorption and luminescence spectra of Yb+3 ions in the YAG crystal (1 mm plate, Yb+3 ions concentration 20%)

Excitation of Periodical Shock Waves in Solid–State Optical Media (Yb:YAG, Glass)

experiments used a fresh part of a sample.

at SBS of Focused Low–Coherent Pump Radiation: Structure Changes, Features of Lasing 373

variation allowed one to carry out both experiments on Yb lasing (see Section 4-6) and experiments on nonlinear interaction of ccl radiation with optical materials (see Section 3). In some of experiments, the ccl energy density was close to the damage threshold of Yb:YAG, glass and other studied materials. The material being damaged, the further

Fig. 4. Oscillograms of the ruby laser (1) and ccl (2) pulses – (a); densitograms of the LiF: F2+

The experiments on nonlinear interaction of ccl radiation with optical media have been mainly performed using non-doped samples out of the cavity, Fig. 5. The ccl radiation was focused at samples by L2, F=120 mm, and, so, within the sample thickness of 1-3 mm the pump power density changed insignificantly. The lens was tilted at 100 to the direction of ccl beam so that reflections from lens surfaces would not come back to the ccl resonator. As the samples there were used plates and slabs with polished surfaces made of the following materials: crystalline quartz (10х10х20mm), Yb:YAG crystal (1-2 mm thick plates), YAG crystal (4,5х30х30 mm), calcite (2 mm plates), LiF crystal (5 mm plate), 2 mm plexiglas plates, glass cube (20х20х30 mm). The ccl radiation was directed onto samples at a normal or at some angle (including the Brewster angle) to the sample's surface. The ccl pulse energy

In all materials a strong scattering of radiation was registered under the action of a ccl pulse. A diagnostic complex consisting of photodiodes, oscilloscopes, calorimeters and a spectrograph was arranged on the stand (Fig.5) in order to study the scattering of the low coherent wideband (0,89-0,95 μm) radiation of LiF: F2+ ccl. Due to very large spectrum width (tens of nm) and complicated space-angular structure of the scattered radiation components, one comes across difficulties in obtaining the spectral data on the scattered radiation, and it was not done. Strong scattering from the ccl focusing region in a wide angular range (tens of degrees) in forward and opposite directions (relative to the ccl beam) was observed for all

laser spectrum with the argon spectral lamp reference lines – (b)

**3. Interaction of ccl radiation with optical media** 

coming to samples varied within the range from 50 to 120 mJ.

Figure 3 illustrates the scheme of the ccl with elements of radiation transport and diagnostics. A LiF: F2+ crystal (40х20х6 mm) was placed in the resonator formed by a plane mirror M1 (≈ 100% reflection at 0.9 μm) and a glass plate M2. The length of the ccl resonator made 30 cm. The LiF: F2+ crystal was pumped through glass plate by a ruby laser operating in the single shot regime with pulse duration ≈ 30 ns and the energy up to 1 J. A multimode radiation at the wavelength of 0.694 μm was focused into the LiF: F2 + crystal by a lens L1 with the focal length F1=500 mm. The ccl multimode radiation was, in its turn, focused on the studied samples by a lens L2 with the focal length F2=120 mm. The Yb-doped samples were placed into a compact two-mirror resonator.

Fig. 3. Scheme of the experimental setup: (1) active element of a ccl; (2) Yb-dopped plate in the resonator with mirrors M8 and M9; (3) calorimeter; (M1, M2) ccl resonator mirrors; (M3 - M7) steering mirrors for ccl and Yb laser radiation; (L1 – L2) lenses; (PD1, PD2) photodiodes

The ruby laser and ccl pulses were registered with the help of photodiodes and a twochannel oscilloscope, and the laser energy was measured by calorimeters. The ccl radiation spectra were analyzed by the STE-1 spectrograph operating in the near IR region. The ccl emitted pulses of 20-30 ns duration and the energy of 100-150 mJ. For the most cases, the ccl pulse shape repeated the shape of the ruby laser pulse, Fig. 4a. Since the ccl resonator round-trip time made ≈ 2 ns, then during the ruby laser pumping the radiation made not more than 15 round trips in the ccl cavity. Due to high amplification in the LiF: F2+ medium the ccl pulse was formed in several round trips inside the resonator. Though the LiF: F2+ crystal was cut at a Brewster angle the ccl radiation was weakly polarized.

Figure 4b presents the densitograms of LiF: F2 + laser spectrum. The LiF: F2 + laser emission was observed within the range 0.89-0.95 μm. The lines of an argon spectral lamp were used as the wavelengths markers. Large spectrum width of ccl and short time of radiation development in the resonator speak about low coherency of the ccl emission. The ccl multimode radiation divergence was ≈ 2х10-3 rad. This allowed focusing the ccl pump at the sample into a ≈250μm spot. Moving samples along the axis of the focused pump beam one could change the size of the focal region within the limits of 250÷1000 μm, and the power density in the medium within the range of 0.5÷5 GW/cm2. Such a range of power density

Figure 3 illustrates the scheme of the ccl with elements of radiation transport and diagnostics. A LiF: F2+ crystal (40х20х6 mm) was placed in the resonator formed by a plane mirror M1 (≈ 100% reflection at 0.9 μm) and a glass plate M2. The length of the ccl resonator made 30 cm. The LiF: F2+ crystal was pumped through glass plate by a ruby laser operating in the single shot regime with pulse duration ≈ 30 ns and the energy up to 1 J. A multimode radiation at the wavelength of 0.694 μm was focused into the LiF: F2+ crystal by a lens L1 with the focal length F1=500 mm. The ccl multimode radiation was, in its turn, focused on the studied samples by a lens L2 with the focal length F2=120 mm. The Yb-doped samples

Fig. 3. Scheme of the experimental setup: (1) active element of a ccl; (2) Yb-dopped plate in the resonator with mirrors M8 and M9; (3) calorimeter; (M1, M2) ccl resonator mirrors; (M3 - M7) steering mirrors for ccl and Yb laser radiation; (L1 – L2) lenses; (PD1, PD2) photodiodes The ruby laser and ccl pulses were registered with the help of photodiodes and a twochannel oscilloscope, and the laser energy was measured by calorimeters. The ccl radiation spectra were analyzed by the STE-1 spectrograph operating in the near IR region. The ccl emitted pulses of 20-30 ns duration and the energy of 100-150 mJ. For the most cases, the ccl pulse shape repeated the shape of the ruby laser pulse, Fig. 4a. Since the ccl resonator round-trip time made ≈ 2 ns, then during the ruby laser pumping the radiation made not more than 15 round trips in the ccl cavity. Due to high amplification in the LiF: F2+ medium the ccl pulse was formed in several round trips inside the resonator. Though the LiF: F2+

Figure 4b presents the densitograms of LiF: F2+ laser spectrum. The LiF: F2+ laser emission was observed within the range 0.89-0.95 μm. The lines of an argon spectral lamp were used as the wavelengths markers. Large spectrum width of ccl and short time of radiation development in the resonator speak about low coherency of the ccl emission. The ccl multimode radiation divergence was ≈ 2х10-3 rad. This allowed focusing the ccl pump at the sample into a ≈250μm spot. Moving samples along the axis of the focused pump beam one could change the size of the focal region within the limits of 250÷1000 μm, and the power density in the medium within the range of 0.5÷5 GW/cm2. Such a range of power density

crystal was cut at a Brewster angle the ccl radiation was weakly polarized.

were placed into a compact two-mirror resonator.

variation allowed one to carry out both experiments on Yb lasing (see Section 4-6) and experiments on nonlinear interaction of ccl radiation with optical materials (see Section 3). In some of experiments, the ccl energy density was close to the damage threshold of Yb:YAG, glass and other studied materials. The material being damaged, the further experiments used a fresh part of a sample.

Fig. 4. Oscillograms of the ruby laser (1) and ccl (2) pulses – (a); densitograms of the LiF: F2+ laser spectrum with the argon spectral lamp reference lines – (b)
