Kresimir Franjic

Laser selective cutting of biological tissues by impulsive heat deposition through ultrafast vibrational excitations

Mechanical and thermodynamic responses of biomaterials after impulsive heat deposition through vibrational excitations (IHDVE) are investigated and discussed. Specifically, we demonstrate highly efficient ablation of healthy tooth enamel using 55 ps infrared laser pulses tuned to the vibrational transition of interstitial water and hydroxyapatite around 2.95 microm. The peak intensity at 13 GW/cm(2) was well below the plasma generation threshold and the applied fluence 0.75 J/cm(2) was significantly smaller than the typical ablation thresholds observed with nanosecond and microsecond pulses from Er:YAG lasers operating at the same wavelength. The ablation was performed without adding any superficial water layer at the enamel surface. The total energy deposited per ablated volume was several times smaller than previously reported for non-resonant ultrafast plasma driven ablation with similar pulse durations. No micro-cracking of the ablated surface was observed with a scanning electron microscope. The highly efficient ablation is attributed to an enhanced photomechanical effect due to ultrafast vibrational relaxation into heat and the scattering of powerful ultrafast acoustic transients with random phases off the mesoscopic heterogeneous tissue structures.

High-power femtosecond infrared laser source based on noncollinear optical parametric chirped pulse amplification

The optical parametric chirped pulse amplication (OPCPA) concept has been extended to the infrared range, providing a new approach, to our knowledge, to high-power femtosecond pulses in the 1.5 μm range. This amplifier is based on 1.05 μm pumping of bulk KTiOAsO4 with broadband phase matching in a noncollinear geometry. The output pulse energy of 1 mJ was generated with 50 nm of bandwidth by 3.8 mJ of pump energy from a 100 ps, 1 kHz Nd:YLF amplified laser. Total gain of 108 was achieved in three stages with 32% conversion of the pump energy in the final amplifier stage. Amplified pulses were compressed to 130 fs with peak intensities of 3.85 GW.

Ultrafast noncollinear optical parametric chirped pulse amplification in KTiOAsO 4

Amplification of femtosecond pulses at 1.56 microm based on noncollinear parametric chirped pulse amplification in a potassium titanyl arsenate (KTA) crystal with pumping at 1.05 microm is reported. The 100 fs pulses of an erbium fiber laser are parametrically amplified while synchronously pumped by an amplified mode-locked Nd:YLF laser. This amplifier has a saturated gain of 65 dB with 30% conversion efficiency and has produced 160 fs pulses with peak powers of up to 0.75 GW. The system produced 380 mW before compression and can be readily scaled to the multiwatt range with bandwidths to support sub-100 fs pulses.

Ambient Mass Spectrometry Imaging with Picosecond Infrared Laser Ablation Electrospray Ionization (PIR-LAESI)

A picosecond infrared laser (PIRL) is capable of cutting through biological tissues in the absence of significant thermal damage. As such, PIRL is a standalone surgical scalpel with the added bonus of minimal postoperative scar tissue formation. In this work, a tandem of PIRL ablation with electrospray ionization (PIR-LAESI) mass spectrometry is demonstrated and characterized for tissue molecular imaging, with a limit of detection in the range of 100 nM for reserpine or better than 5 nM for verapamil in aqueous solution. We characterized PIRL crater size using agar films containing Rhodamine. PIR-LAESI offers a 20-30 μm vertical resolution (∼3 μm removal per pulse) and a lateral resolution of ∼100 μm. We were able to detect 25 fmol of Rhodamine in agar ablation experiments. PIR-LAESI was used to map the distribution of endogenous methoxykaempferol glucoronide in zebra plant (Aphelandra squarrosa) leaves producing a localization map that is corroborated by the literature. PIR-LAESI was further used to image the distribution inside mouse kidneys of gadoteridol, an exogenous magnetic resonance contrast agent intravenously injected. Parallel mass spectrometry imaging (MSI) using desorption electrospray ionization (DESI) and matrix assisted laser desorption ionization (MALDI) were performed to corroborate PIR-LAESI images of the exogenous agent. We further show that PIR-LAESI is capable of desorption ionization of proteins as well as phospholipids. This comparative study illustrates that PIR-LAESI is an ion source for ambient mass spectrometry applications. As such, a future PIRL scalpel combined with secondary ionization such as ESI and mass spectrometry has the potential to provide molecular feedback to guide PIRL surgery.

Bone surgery with femtosecond laser compared to mechanical instruments: healing studies

Femtosecond (fs) ablation is mediated via electron avalanche and multiphoton ionization and is characterized by very precise cutting and undetectable thermal damage in biological tissues. We have used a 775nm, 150 fs, 1kHz laser system compared to two conventional bone cutting techniques using carbide and diamond tip burs in a mice calvarial wound healing model. Wound healing was evaluated using micro computerized tomographs and histological techniques. Good healing outcomes were found for fs laser surgery in comparison to the conventional surgical methods. However, the degree of healing was highly variable in all treatment groups. The realization of healing comparable to that observed for conventional surgical tools demonstrates the possible use of fs lasers for clinical surgery involving small bones where a much higher degree of precision is required than that possible with current methods.

Pressure tuning of thermal lensing: application to high brightness source development

The development of high power diode-pumped solid-state lasers (DPSSLs) with diffraction limited beam quality has been significantly hampered by pump induced inhomogeneous temperature profiles in the laser gain medium which results in thermal lensing and birefringence.