R. Sauerbrey

Pulsed ultraviolet laser ablation

The radiation transport involved in pulsed ultraviolet laser ablation of organic materials has been studied both theoretically and experimentally. A mathematical description of the absorption process that includes nonlinear effects such as chromophore saturation and multiphoton absorption is presented. This theoretical model accurately describes the observed polymer etch depth versus incident laser fluence relationship for various target materials, laser wavelengths, and pulse durations varying from nanoseconds to subpicoseconds. The theoretical analysis can also be used to explain observed nonlinearities in the transmission characteristics of polyimide subjected to intense KrF excimer laser irradiation.

Theory for the etching of organic materials by ultraviolet laser pulses

A theoretical description of the ultraviolet laser etching process is developed. The threshold for laser ablation is reached when the density of absorbed photons is approximately equal to the density of chromophores in the material. Saturation of the absorption coefficient, absorption by the plume of ablated products, and multiphoton effects are considered. Agreement with all available experimental etch data, including femtosecond ultraviolet laser ablation, is found. The description is based on an analysis of the radiation transport at high intensities and is independent of the question as to whether ultraviolet laser ablation is photochemical or thermal.

Sub‐100 nm lines produced by direct laser ablation in polyimide

Periodic line structures with a period of 167 nm and linewidths varying from 35 to 100 nm have been produced on polyimide by direct ablation with a KrF laser using an interferometric technique. Since ablation is a nonlinear process, the resolution can exceed that expected from the wavelength and numerical aperture of the system and the linewidth can be controlled by varying the laser fluence. This externally generated period of 167 nm prevents the spontaneous growth of periodic surface structures due to radiation remnants.

Autofluorescence maps of atherosclerotic human arteries--A new technique in medical imaging

A new medical imaging technique for arterial walls based on laser-induced autoflnorescence spectroscopy is reported. The internal surface of isolated arteries with or without atherosclerosis is irradiated with an argon ion laser (458 nm) and the peak intensity of the excited autofluorescence spectrum is related to the composition of the arterial wall. The higher autofluorescence intensity in the range between approximately 480 and 630 nm for grossly calcified tissue compared to normal or noncalcified atherosclerotic tissue is used to produce maps of the arterial wall. These images delineate the calcified areas of the sample with good spatial resolution. If this technique can be adapted to the endoscopic visualization of arteries in vivo (angios, Copy), it could become an important tool for the diagnosis of atherosclerosis and for the monitoring of atheroma ablation during laser angioplasty.

Ultraviolet-Laser-Induced Permanent Electrical Conductivity in Polyimide

When polyimide (Kapton) is irradiated by a krypton fluoride (KrF) laser, an increase of the electrical conductivity of up to 16 orders of magnitude is observed. In the high conduction regime, the resistivity is about 0.1 Ω cm, the current voltage characteristic is ohmic and the contacts of gold and silver with the irradiated conducting polymer are also ohmic. The conduction mechanism is phonon-assisted variable range hopping, evident from the observed temperature and electric field dependence of the resistivity at low conductivities. The laser-induced conductivity depends on the ambient atmosphere during irradiation. Transmission spectroscopy in the visible region and infrared Fourier transform spectroscopy have been used to characterize the material. A thermal mechanism is proposed for the formation of conducting polyimide, by excimer-laser irradiation.

Direct laser ablation of sub-100 nm line structures into polyimide

Periodic line structures with a period of 167 nm and linewidths varying from 30 to 100 nm have been produced in polyimide by direct ablation with a KrF laser using an interferometric technique. The characteristics of this interferometer as it applies to the ablation of these line structures, including linewidth and alignment sensitivity, are analyzed. The ability to control the linewidth by varying the average incident fluence is described theoretically and demonstrated experimentally. This externally generated period of 167 nm also prevents the spontaneous growth of laser induced periodic surface structures (LIPSS).

Spectral blue shifting of a femtosecond laser pulse propagating through a high-pressure gas

Plasma-induced spectral blue shifting in rare gases has been investigated with a subpicosecond KrF excimer laser focused to a peak intensity in the region of 1014 W/cm2 (adiabaticity parameter in the range 8 < γ < 10). The quiver energy of a free electron under these conditions is sufficiently small to ensure that ionization occurs solely by optical-field-induced processes. Blue shifts as large as 2 nm have been observed, and the blue-shifted spectrum shows an interferencelike oscillatory structure. Experimental results are compared with numerical simulations to show that the blue-shifted spectra are the result of plasma-induced self-phase modulation and can be modeled qualitatively by assuming tunneling ionization and plane-wave pulse propagation. The structure in the spectrum is closely related to that observed in earlier experiments on self-phase modulation in quite different systems.

Femtosecond single-shot phase-sensitive autocorrelator for the ultraviolet

A single-shot phase-sensitive autocorrelator for the investigation of high-intensity ultraviolet laser pulses is developed and tested. The fluorescence of the two-photon-excited self-trapped excitons in BaF(2) is used as a nonlinear detector. Simultaneous measurements of subpicosecond KrF laser are reported.

Excimer laser photoablation of silicon

The ultraviolet and visible emission spectra from excimer laser‐produced silicon plasmas were studied and the ablation rate measured as a function of laser energy density and wavelength. A spectroscopic investigation of the laser‐produced plasma showed Si i, Si ii, and Si iii spectral lines with higher laser intensity causing a higher degree of ionization in the plasma. Both time‐integrated and time‐resolved spectroscopic studies showed electronic transitions superimposed on a weak continuum over the entire range from 250 to 640 nm. The photoablation rate of Si was independent of laser wavelength (193 or 248 nm), and had an energy density threshold of ≊1.3 J/cm2. The threshold was almost independent of the buffer gas pressure between vacuum and 1000 Torr. These results are described in the framework recently developed for excimer laser ablation of metals.

Fluence‐dependent transmission of polyimide at 248 nm under laser ablation conditions

Transmission experiments on thin polyimide films under laser ablation conditions using 248 nm KrF laser radiation have been performed. The transmitted temporal pulse shapes and the transmittted intensity show fluence‐dependent absorption as predicted by a recent theoretical description of the pulsed ultraviolet laser ablation process.