Time-Resolved Studies of Nd:YAG Loser-Induced Breakdown Plasma Formation, Acoustic Wave Generation, and Cavitation Jomes G. Fujimoro,* Wei Z. Liaf Erich P. Ippen,* Carmen A. PuliQfiro4 and Roger F. Sreinerr^: The use of high intensity ultrashort pulsed laser radiation to produce optical breakdown is an important approach for the surgical treatment of intraocular structures. We have investigated the transient properties of Nd:YAG laser induced breakdown in a saline model using time-resolved spectroscopic techniques. Spatially resolved pump and probe techniques are applied to study the dynamic behavior of the plasma formation, acoustic wave generation, and cavitation processes which accompany the optical breakdown. Measurements of plasma shielding and luminescence indicate that the laser induced plasma forms on a subnanosecond time scale and has a lifetime of several nanoseconds. An acoustic transient is generated at the breakdown site and propagates spherically outward with an initial hypersonic velocity, then loses energy and propagates at sound velocity. Transient heating following the plasma formation produces a liquid-gas phase change and gives rise to cavitation or gas bubble formation. This gas bubble expands rapidly for several microseconds, then slows to reach its maximum size and finally collapses. Invest Ophthalmol Vis Sci 26:1771-1777, 1985 Short pulsed Q-switched and modelocked Nd:YAG important role in protecting the retina from the inci- lasers have recently achieved widespread use in dent laser energy,10"12'25"28 while the clinically useful ophthalmic surgery. The ability of high intensity short effects are primarily produced by the acoustic wave laser pulses to induce optical breakdown and produce and cavitation.10111923'28 lesions from acoustic transient and cavitation effects In this paper we describe the experimental investi- has been especially important for the surgical treat- gation of the transient phenomena associated with ment of intraocular structures in the anterior eye which laser-induced optical breakdown through the applica- are nominally transparent to the laser wavelength.1"12 tion of time-resolved spectroscopic techniques. In par- The transient physical phenomena which accompany ticular we emphasize pump-probe measurements as a the optical breakdown process include laser-induced method for characterizing the transient temporal and plasma formation, acoustic wave generation and prop- spatial evolution of the breakdown process. The pump- agation, as well as cavitation or gas bubble forma- probe technique relies on the use of two laser beams: tion.13"28 These processes are relevant to the clinical one beam, the pump, is used to produce a change in effectiveness of optical breakdown and have direct im- the sample, while the other beam, the probe, measures plications on its physiological and pathological effects the effects produced by the pump. In this investigation in the eye. The increased absorption of the laser-in- a Q-switched Nd:YAG is used as the pump laser to duced plasma has been shown to play a potentially induce optical breakdown in a saline sample while a continuous wave HeNe laser is used to measure the transient changes in optical properties. The measure- From the Department of Electrical Engineering and Computer ment of both spatial and temporal behavior allows the Science and Research Laboratory of Electronics,* Massachusetts In- characterization of plasma formation, acoustic wave stitute of Technology, Cambridge, Massachusetts; the Howe Labo- generation, and cavitation which accompany the ratory of Ophthalmology, Massachusetts Eye and Ear Infirmary,! breakdown as well as a determination of the physical Boston, Massachusetts; Visiting from Zhongshan University,! dimensions and propagation velocities associated with Guangzhou, People's Republic of China. Supported in part by the Joint Services Electronics Program under these phenomena. While previous researchers have ex- Grant DAAG 29-83-K-003. amined laser-induced breakdown using a variety of Submitted for publication: January 22, 1985. techniques including high speed photography and ho- Reprint requests: James G. Fujimoto, PhD, Department of Elec- lography,1516'20'21'24 transient luminescence and pulse trical Engineering and Computer Science and Research Laboratory transmission detection,17'26'27 and acoustic transducer of Electronics, Massachusetts Institute of Technology, 77 Massachu- measurements,131419'23 the spatially resolved pump- setts Avenue, Cambridge, MA 02139. 1771 Downloaded from iovs.arvojournals.org on 09/26/2021 1772 INVESTIGATIVE OPHTHALMOLOGY 6 VISUAL SCIENCE / December 1985 Vol. 26 SCHEMATIC OF PUMP - PROBE MEASUREMENT stability. The pulse energy delivered to the sample was varied using beamsplitters and calibrated neutral den- sity filters. Pulse energy measurements were performed with a Scientech Model 361 (Boulder, CO) pyroelectric He-Ne PROBE energy meter. The laser output beam was 10 mm in diameter and focused into the sample cell with a 25- mm focal length lens. Since the Nd:YAG laser em- ployed a diffraction coupled resonator, the spatial mode profile of the output beam was a "doughnut" rather Nd< YAG than a Gaussian mode, and diffraction limit focusing PUMP was not expected. The transient properties of the laser-induced break- down were probed by a low intensity, 1 mW, contin- SAMPLE CELL uous wave helium-neon (HeNe) laser (X = 632.8 nm). In order to obtain a spatial resolution of better than FILTER 10 nm, the probe beam was first expanded to a diameter of 6 mm before being focussed into the sample ceil with a 32-mm focal length lens. The focal point of the FAST PHOTODIODE HeNe probe laser was carefully adjusted to coincide with the center of the breakdown region with the probe beam orthogonal to the pump beam. Since the break- Fig. 1. Schematic diagram of experimental configuration for pump probe time resolved measurements of laser-induced breakdown. Nd: down region extends along the axis of the pump beam YAG laser pump pulses are focused into a sample cell containing with increasing pump pulse energy, the probe align- saline. The transient optical properties are probed by a HeNe laser ment was adjusted after changes in the pump energy. and a high speed photodiode. Spatially resolved measurements are Spatially resolved pump-probe measurements of the performed by displacing the pump and probe laser beams in a di- rection perpendicular to the plane of the figure. breakdown process were obtained by varying the rel- ative separation of the probe laser beam focus from the center of the breakdown region. This was accom- plished by translating the pump beam steering and fo- probe technique is one of the few approaches which cusing optics in a direction orthogonal to the plane of permits the investigation of a wide variety of physical Figure 1 using a l-/um resolution stepping motor me- processes in a simple and consistent experimental chanical stage (Klinger Scientific; Richmond Hill, NY). framework. The center of the breakdown region was determined by noting that the probe beam behavior should be Materials and Methods symmetric for positive and negative displacements. The use of this spatially resolved technique facilitates the The experimental configuration for pump-probe measurement of physical dimensions as well as the dy- measurements of laser-induced optical breakdown is namic behavior of processes involving transient prop- shown schematically (Fig. 1). For our investigations, agation away from the center of the breakdown region. optical breakdown was studied in a simple model sys- The transmitted HeNe probe beam was focussed tem consisting of physiological (0.9%) saline solution. onto a small area, high speed, avalanche photodiode The saline was contained in a 10 X 20-mm quartz flu- detector (Telefunken BPW28; Sommerville, NJ). A orimeter cell. In order to prevent the accumulation of narrowband dielectric interference filter centered at gas bubbles or other impurities produced by the optical 632.8 nm was used to block luminescence and scattered breakdown process, the solution was circulated and light from the optical breakdown. The small area of filtered with a 3-nm filter. the detector combined with this detection geometry A frequency doubled Q-switched Nd:YAG laser makes the probe laser sensitive not only to changes in (Quanta-Ray DCR1; Mountain View, CA) was used transmission arising from induced absorption or scat- as the pump laser to produce optical breakdown in the tering, but also to changes in the index of refraction sample. The Nd:YAG laser generates ~10 ns long which will influence the imaging of the probe beam pulses at 532 nm with a repetition rate of 10 Hz. The onto the detector. The temporal behavior of the laser- second harmonic of the Nd.YAG laser was used in induced plasma luminescence was also measured by order to facilitate accurate measurements of beam spa- turning off the probe laser and using a low pass, red tial profile and focusing characteristics. The laser was filter in front of the detector to block the scattered pump adjusted to optimize temporal pulse shape and energy laser. Downloaded from iovs.arvojournals.org on 09/26/2021 No. 12 ND:YAG LASER-INDUCED BREAKDOWN / Fujimoro er ol. 1773 The output of the photodiode was measured with a high speed oscilloscope (Tektronix 7104; Beaverton, (a) OR) with a 1 GHz bandwidth. Measurements which Nd=YAG PULSE did not require as high temporal resolution were per- formed using a high speed storage oscilloscope (Tek- 5 ns/div tronix 7834). The temporal resolution was ~4 ns as limited by the duration of the Nd:YAG laser pulses. Experimentally, the effective temporal measurement resolution was limited by fluctuations in the laser-in- duced breakdown processes which occurred on a shot (b) to shot basis rather than bandwidth restrictions in the electronic instrumentation. LUMINESCENCE 5 ns/div Results A sharp threshold for the onset of optical breakdown is observed with increasing Nd:YAG pump laser in- tensity. For our experimental conditions, using purified (c) saline solution, threshold was ~-6 mJ per pulse.