We review recent advances in laser cell surgery, and investigate the working mechanisms of femtosecond laser nanoprocessing in biomaterials with oscillator pulses of 80-MHz repetition rate and with amplified pulses of 1-kHz repetition rate. Plasma formation in water, the evolution of the temperature distribution, thermoelastic stress generation, and stress-induced bubble formation are numerically simulated for NA=1.3, and the outcome is compared to experimental results. Mechanisms and the spatial resolution of femtosecond laser surgery are then compared to the features of continuous-wave (cw) microbeams. We find that free electrons are produced in a fairly large irradiance range below the optical breakdown threshold, with a deterministic relationship between free-electron density and irradiance. This provides a large ‘tuning range’ for the creation of spatially extremely confined chemical, thermal, and mechanical effects via free-electron generation. Dissection at 80-MHz repetition rate is performed in the low-density plasma regime at pulse energies well below the optical breakdown threshold and only slightly higher than used for nonlinear imaging. It is mediated by free-electron-induced chemical decomposition (bond breaking) in conjunction with multiphoton-induced chemistry, and hardly related to heating or thermoelastic stresses. When the energy is raised, accumulative heating occurs and long-lasting bubbles are produced by tissue dissociation into volatile fragments, which is usually unwanted. By contrast, dissection at 1-kHz repetition rate is performed using more than 10-fold larger pulse energies and relies on thermoelastically induced formation of minute transient cavities with lifetimes <100 ns. Both modes of femtosecond laser nanoprocessing can achieve a 2–3 fold better precision than cell surgery using cw irradiation, and enable manipulation at arbitrary locations.

Mechanisms of femtosecond laser nanosurgery of cells and tissues

Evidence suggests that MOSFET degradation is due to interface-states generation by electrons having 3.7 eV and higher energies. This critical energy and the observed time dependence is explained with physical model involving the breaking of the ≡ Si s H bonds. The device lifetime τ is proportional to I_{sub}^{-2.9}I_{d}^{1.9}\Delta V_{t}^{1.5} . If I sub is large because of small L or large V d , etc., τ will be small. I sub (and possibly light emission) is thus a powerful predictor of τ. The proportionality constant has been found to vary by a factor of 100 for different technologies, offering hope for substantially better reliability through future improvements in dielectric /interface technologies. A simple physical model can relate the channel field E m to all the device parameters and bias voltages. Its use in interpreting and guiding hot-electron scaling are described. LDD structures can reduce E m and I sub and, when properly designed, reduce device degradation.

Hot-electron-induced MOSFET degradation&#8212;Model, monitor, and improvement

Results of laser‐induced breakdown experiments in fused silica (SiO2) employing 150 fs–7 ns, 780 nm laser pulses are reported. The avalanche ionization mechanism is found to dominate over the entire pulse‐width range. Fluence breakdown threshold does not follow the scaling of Fth∼ √τp, when pulses are shorter than 10 ps. The impact ionization coefficient of SiO2 is measured up to ∼3×108 V/cm. The relative role of photoionization in breakdown for ultrashort pulses is discussed.

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Laser‐induced breakdown by impact ionization in SiO2 with pulse widths from 7 ns to 150 fs

Laser-induced breakdown and damage to transparent materials has remained an active area of research for four decades. In this paper we review the basic mechanisms that lead to laser-induced breakdown and damage and present a summary of some open questions in the field. We present a method for measuring the threshold intensity required to produce breakdown and damage in the bulk, as opposed to on the surface, of the material. Using this technique, we measure the material band-gap and laser-wavelength dependence of the threshold intensity for bulk damage using femtosecond laser pulses. Based on these thresholds, we determine the relative role of different nonlinear ionization mechanisms for different laser and material parameters.

https://iopscience.iop.org/article/10.1088/0957-0233/12/11/305/pdf

Laser-induced breakdown and damage in bulk transparent materials induced by tightly focused femtosecond laser pulses

Evidence suggests that MOSFET degradation is due to interface-states generation by electrons having 3.7 eV and higher energies. This critical energy and the observed time dependence is explained with a physical model involving the breaking of the = Si/sub s/H bonds. The device lifetime /spl tau/ is proportional to...

Hot-Electron-Induced MOSFET Degradation - Model, Monitor, and Improvement

Utilizing changes in the carrier distribution function by magnitude and momentum scaling of scattering rates, the author a number of interesting results concerning ionization coefficients and transient drift and diffusion. Starting with a general definition of the ionization coefficient which includes nonlocal effects, the behavior of this coefficient under scaling is determined and used to find a simple analytical expression in terms of physical parameters valid for all field strengths. When fit to data for silicon, surprisingly large but consistent high‐field, effective ionization energies are found for electrons (3.6 eV) and holes (5.0 eV). This expression can also relate ionization near an interface to that in the bulk. Rate scaling is also used to predict changes in velocity overshoot and diffusion‐limited rise times between bulk and interface behavior. These comparisons are facilitated by a novel relationship between the time dependence of the spacial diffusion of a carrier pulse and it spacial displacement in an applied field. A classification of scattering rates suggested by momentum scaling as well as alternative scaling procedures applicable in special cases are briefly included. The former provides information on the behavior under momentum scaling of the positional‐displacement correlation factor of surface‐roughness scattering. The latter shows how scattering‐rate scaling and momentum scaling can be made to transform into each another.

K. K. Thornber

Papers

Applications of scaling to problems in high-field electronic transport