Imaging femtosecond laser-induced electronic excitation in glass

Abstract

While substantial progress has been achieved in understanding laser ablation on the nanosecond and picosecond time scales, it remains a considerable challenge to elucidate the underlying mechanisms during femtosecond laser material interactions. We present experimental observations of electronic excitation inside wide band-gap silica glass during single femtosecond laser pulse (100 fs, 800 nm) irradiation. Using a femtosecond time-resolved imaging technique, we measured the evolution of a laserinduced electronic plasma inside the silica glass and calculated the electron number density to be on the order of 10 19 cm -3 . 2Pulsed laser ablation has been demonstrated to be a viable technology for an increasing number of applications. These include pulsed laser deposition of high criticaltemperature superconductor films 1,2 , femtosecond laser micro-machining and structural modification of dielectrics 3,4 , ultrafast laser-assisted material and chemical analysis 5,6 , and ultrashort X-ray pulse generation 7,8 . However, high power, short pulse laser ablation is still largely unexplored at the fundamental level.Mechanisms underlying laser ablation processes are quite complex, most previous investigations have focused on laser ablation at relatively long (e.g., nanosecond and picosecond) time scales 9,10 . Since the femtosecond time scale is much shorter than the electron-lattice energy relaxation time, thermal equilibrium cannot be established during the time the laser interacts with a solid material. The absence of a quantitative model at least in part contributes to the lack of understanding of femtosecond laser ablation.Systematic time-resolved measurements can help establish a predictive theory for laser ablation on the femtosecond time scale. In this study, we aim to elucidate some of the characteristics resulting from femtosecond laser interactions with a transparent solid. A detailed examination is presented of the development of laser-induced electronic exciation inside silica glass on the femtosecond time scale. An electronic plasma generated by a 100 fs, 800 nm laser incident in the silica glass was measured.Laser ablation experiments were performed using a femtosecond time-resolved pump-probe setup to image the plasma. Details of the experimental setup will be published elsewhere. Briefly, a Spectra-Physics TSA high power femtosecond laser at its 3 fundamental wavelength (800 nm) was used as the pump beam, which has a pulse duration of approximately 100 femtosecond (FWHM) and energy output up to 25 mJ per pulse. The 800 nm laser beam was focused to a spot size of 50 µm in diameter onto a silica glass sample using an f = 15 cm focal-length lens. After a beam splitter, one arm of the 800 nm output passes an optical delay stage and a KDP crystal, forming a probe beam at 400 nm that is perpendicular to the ablation laser beam. By moving the delay stage on an optical rail, the optical path of the probe beam can be varied, so the time difference between the ablation beam and the probe beam is changed. In...

Sequentially Timed All-Optical Mapping Photography for Real- Time Monitoring of Laser Ablation: Breakdown and Filamentation in Picosecond and Femtosecond Regimes

“…In frames 2-6, the process of carrier-phonon scattering is indicated by the Laser Ablation -From Fundamentals to Applicationsdark region (white arrow in Figure 10) that corresponds to the high density of electrons in the plasma state caused by the material's absorption of light. This observation agrees well with previous reports [17].…”

Strong nonlinear growth of energy coupling during laser irradiation of transparent dielectrics and its significance for laser induced damage

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Laser assisted plasma spectrochemistry: laser ablation

“…Depending on the lasers and configuration, femtosecond time resolution imaging of the 'plume' in (transparent sample only) or above a sample surface is possible. 79 One laser beam is used to ablate the sample (target) and a second beam is used as a time-resolved probe to image events during/after the ablation process. As configured, the system also forms an interferometer at the sample, providing the capability for determining the number density of species in the plume.…”