A new, safer lasing technique for laser-facilitated coronary angioplasty

In vitro studies during cold pulsed-wave laser angioplasty have demonstrated production of gas bubbles within the target tissue, creation of shock wave and formation of multi-layer dissections accompanied by an increase in the plaque and vessel wall temperature. These processes account for certain complications of coronary lasing, including acute vessel closure, dissections, spasm, and even perforation. The traditional lasing technique in which a large number of pulses is continually emitted across the lesion, may in fact contribute to the development and acceleration of the above mentioned processes. To overcome the shortcomings we have developed a new, safe lasing technique that consists of multiple trains of a small number of pulses each. Between laser sessions the laser catheter is retracted into the guiding catheter and nitroglycerin is injected intracoronary, thus providing time for dispersion of produced gas bubbles, cooling of the target artery, and adequate coronary vasodilatation. This new technique results in a significant reduction of laser associated complications.     

Gain-swept superradiance applied to the stand-off detection of trace impurities in the atmosphere

We show that gain-swept superradiance can be used to detect low (parts per million) concentrations of various gases at distances on the order of kilometers, which is done by using pulse timing to create small regions of gain at positions that sweep toward a detector. The technique is far more sensitive than previous methods such as light detection and ranging or differential absorption light detection and ranging.     

Monitoring molecular dynamics using coherent electrons from high harmonic generation

We report a previously undescribed spectroscopic probe that makes use of electrons rescattered during the process of high-order harmonic generation. We excite coherent vibrations in SF(6) using impulsive stimulated Raman scattering with a short laser pulse. A second, more intense laser pulse generates high-order harmonics of the fundamental laser, at wavelengths of approximately 20-50 nm. The high-order harmonic yield is observed to oscillate, at frequencies corresponding to all of the Raman-active modes of SF(6), with an asymmetric mode most visible. The data also show evidence of relaxation dynamics after impulsive excitation of the molecule. Theoretical modeling indicates that the high harmonic yield should be modulated by both Raman and infrared-active vibrational modes. Our results indicate that high harmonic generation is a very sensitive probe of vibrational dynamics and may yield more information simultaneously than conventional ultrafast spectroscopic techniques. Because the de Broglie wavelength of the recolliding electron is on the order of interatomic distances, i.e., approximately 1.5 A, small changes in the shape of the molecule lead to large changes in the high harmonic yield. This work therefore demonstrates a previously undescribed spectroscopic technique for probing ultrafast internal dynamics in molecules and, in particular, on the chemically important ground-state potential surface.     

Single-electron pulses for ultrafast diffraction

Visualization of atomic-scale structural motion by ultrafast electron diffraction and microscopy requires electron packets of shortest duration and highest coherence. We report on the generation and application of single-electron pulses for this purpose. Photoelectric emission from metal surfaces is studied with tunable ultraviolet pulses in the femtosecond regime. The bandwidth, efficiency, coherence, and electron pulse duration are investigated in dependence on excitation wavelength, intensity, and laser bandwidth. At photon energies close to the cathode's work function, the electron pulse duration shortens significantly and approaches a threshold that is determined by interplay of the optical pulse width and the acceleration field. An optimized choice of laser wavelength and bandwidth results in sub-100-fs electron pulses. We demonstrate single-electron diffraction from polycrystalline diamond films and reveal the favorable influences of matched photon energies on the coherence volume of single-electron wave packets. We discuss the consequences of our findings for the physics of the photoelectric effect and for applications of single-electron pulses in ultrafast 4D imaging of structural dynamics.     

Generation of ultra-intense single-cycle laser pulses by using photon deceleration

A scheme to generate single-cycle laser pulses is presented based on photon deceleration in underdense plasmas. This robust and tunable process is ideally suited for lasers above critical power because it takes advantage of the relativistic self-focusing of these lasers and the nonlinear features of the plasma wake. The mechanism is demonstrated by particle-in-cell simulations in three and 2(1/2) dimensions, resulting in pulse shortening up to a factor of 4, thus making it feasible to generate few-femtosecond single-cycle pulses in the optical to IR domain with intensities I > 10(20) W/cm(2) by using present-day laser technology.     

Coherence brightened laser source for atmospheric remote sensing

We have studied coherent emission from ambient air and demonstrated efficient generation of laser-like beams directed both forward and backward with respect to a nanosecond ultraviolet pumping laser beam. The generated optical gain is a result of two-photon photolysis of atmospheric O₂, followed by two-photon excitation of atomic oxygen. We have analyzed the temporal shapes of the emitted pulses and have observed very short duration intensity spikes as well as a large Rabi frequency that corresponds to the emitted field. Our results suggest that the emission process exhibits nonadiabatic atomic coherence, which is similar in nature to Dicke superradiance where atomic coherence is large and can be contrasted with ordinary lasing where atomic coherence is negligible. This atomic coherence in oxygen adds insight to the optical emission physics and holds promise for remote sensing techniques employing nonlinear spectroscopy.     

Ultrafast Third-Order Nonlinear Optical Response Excited by fs Laser Pulses at 1550 nm in GaN Crystals

The ultrafast third-order optical nonlinearity of c-plane GaN crystal, excited by ultrashort (fs) high-repetition-rate laser pulses at 1550 nm, wavelength important for optical communications, is investigated for the first time by optical third-harmonic generation in non-phase-matching conditions. As the thermo-optic effect that can arise in the sample by cumulative thermal effects induced by high-repetition-rate laser pulses cannot be responsible for the third-harmonic generation, the ultrafast nonlinear optical effect of solely electronic origin is the only one involved in this process. The third-order nonlinear optical susceptibility of GaN crystal responsible for the third-harmonic generation process, an important indicative parameter for the potential use of this material in ultrafast photonic functionalities, is determined.     

FAST CARS: engineering a laser spectroscopic technique for rapid identification of bacterial spores

Airborne contaminants, e.g., bacterial spores, are usually analyzed by time-consuming microscopic, chemical, and biological assays. Current research into real-time laser spectroscopic detectors of such contaminants is based on e.g., resonance fluorescence. The present approach derives from recent experiments in which atoms and molecules are prepared by one (or more) coherent laser(s) and probed by another set of lasers. However, generating and using maximally coherent oscillation in macromolecules having an enormous number of degrees of freedom is challenging. In particular, the short dephasing times and rapid internal conversion rates are major obstacles. However, adiabatic fast passage techniques and the ability to generate combs of phase-coherent femtosecond pulses provide tools for the generation and utilization of maximal quantum coherence in large molecules and biopolymers. We call this technique FAST CARS (femtosecond adaptive spectroscopic techniques for coherent anti-Stokes Raman spectroscopy), and the present article proposes and analyses ways in which it could be used to rapidly identify preselected molecules in real time.     

Cross-peak-specific two-dimensional electronic spectroscopy

Intermolecular electronic coupling dictates the optical properties of molecular aggregate systems. Of particular interest are photosynthetic pigment-protein complexes that absorb sunlight then efficiently direct energy toward the photosynthetic reaction center. Two-dimensional (2D) ultrafast spectroscopy has been used widely in the infrared (IR) and increasingly in the visible to probe excitonic couplings and observe dynamics, but the off-diagonal spectral signatures of coupling are often obscured by broad diagonal peaks, especially in the visible regime. Rotating the polarizations of the laser pulses exciting the sample can highlight certain spectral features, and the use of polarized pulse sequences to elucidate cross-peaks in 2D spectra has been demonstrated in the IR for vibrational transitions. Here we develop 2D electronic spectroscopy using cross-peak-specific pulse polarization conditions in an investigation of the Fenna-Matthews-Olson light harvesting complex from green photosynthetic bacteria. Our measurements successfully highlight off-diagonal features of the 2D spectra and, in combination with an analysis based on the signs of features arising from particular energy level pathways and theoretical simulation, we characterize the dominant response pathways responsible for the spectral features. Cross-peak-specific 2D electronic spectroscopy provides insight into the interchromophore couplings, as well as into the energetic pathways giving rise to the signal. With femtosecond resolution, we also observe dynamical processes that depend on these couplings and interactions with the protein environment.     

Photoacoustic fibrinolysis: Pulsed-wave, mid-infrared laser-clot interaction

Objectives The purpose of this study was to determine whether a mid-infrared laser can induce selective fibrinolysis and to analyze the effect of altered fibrin structure (thin vs. thick fibers) on laser-clot interaction.Background Mechanical disruption of thrombus can be achieved with balloon angioplasty, sonication, and thermal energy. Thrombi avidly absorb light in the mid-infrared optical spectrum due to their high water content. This phenomenon provides a potential for mid-infrared lasers as a source for selective thrombolysis. As fibrin is the essential component of clot, a study of mid-infrared laser-fibrin interaction is warranted.Methods Clots of varying fibrin structure were lased in cuvettes with a solid-state, pulsed-wave, mid-infrared laser (2.1 micron, 500 mJ/pulse, 250 msec pulse length). Total pulse energies of 5 Joules (J), 10 J, 37.5 J, 75 J, and 112.5 J were tested. Protein content of the extruded fluid was measured by optical density absorbance at 280 nm. The amount of released material was studied as a function of lasing energy and clot structure. SDS-polyacrylamide gel electrophoresis was applied for analysis of protein bands in order to identify unique protein bands released by the selective effect of laser fibrinolysis.Results A threshold for mid-infrared laser induced fibrinolysis was found; application of up to 20 J of energy did not result in dissolution. As lasing energy was increased above 37.5 J, the structure of these gels was mechanically destroyed and 12.4 ± 6.7% (mean ± SEM) of the original content of protein was released. Electrophoresis revealed that lased gels did not release any unique protein band. Lased, thin fibers released significantly less protein than thick fibers, indicating that they are more resistant to the effect of this wavelength of energy.Conclusions Mid-infrared laser can induce in-vitro photoacoustic dissolution of fibrin clots. However, this wave-length laser achieves fibrinolysis by mechanical destruction of the target clot rather than by a selective effect, as induced by the pulsed-dye laser. A threshold exists for energy levels required. Thin fibrin fibers, with their high elastic modulus (i.e., gel rigidity) appear more resistant than thick fibers to the effect of lasing at this wavelength.This work was supported in part by a research grant from Boston Scientific, Boston, MA.     

Modified Fiber Tips for Laser Angioplasty: Mechanisms of Action

Laser angioplasty with modified fiber tips has become a common procedure for the recanalization of totally occluded peripheral arteries. We evaluated the contribution of optical, thermal, and mechanical effects to the mechanism of recanalization of various probes, theoretically and experimentally. Temperature behavior and tissue penetration were measured in relation to axial force exerted by metal laser probes, and optical contact probes coupled to continuous-wave and pulsed Nd:YAG lasers. Modified fiber tips only penetrated tissue when the contact surface of the probe exceeded a temperature threshold of about 225°C in the fatty tissue model used. Metal laser probes had to be insulated from a liquid environment to attain this temperature. Optical probes needed to have an absorbing layer of carbonized tissue particles to attain this temperature. Tissue penetration by modified fiber tips was force dependent, especially with optical probes. Since the diameter of the probe was larger than the ablative laser beam, the atraumatic probe had to distend soft tissues mechanically. Because the metal laser probes delivered their energy in all directions, undesired heating in the radial direction has to be reduced by motion. The recanalization mechanism of modified fiber tips will depend on the properties of the obstruction. Sometimes the probes will recanalize the obstruction without the use of laser energy (Dotter effect). Moderate heating of the tissue by either direct absorption of light, or by heat conduction from the heated surface of the probe, may help to remodel the obstruction. Tissue temperatures around 100°C will vaporize the water compound, and temperatures higher than 225°C will ablate the solid compounds of the tissue creating a channel. The present unsteerable probes will not penetrate heavily calcified obstructions. Calcific deposits may deflect the probe into the wall with the risk of perforation.     

Mechanism of four-level laser action in solution excimer and excited-state proton-transfer cases

Four-level laser energy level schemes are compared from the mechanistic spectroscopic viewpoint: (i) noble-gas excimer, (ii) solution molecular excimer, (iii) conventional dye laser, and (iv) intramolecular proton transfer. The lasing action of the chlorophyll special pair is discussed as an example of a solution excimer laser, and the lasing action of 3-hydroxyflavone and other molecules is discussed as an example of an intramolecular proton-transfer laser.     

Dynamics of molecules in extreme rotational states

We have constructed an optical centrifuge with a pulse energy that is more than 2 orders of magnitude larger than previously reported instruments. This high pulse energy enables us to create large enough number densities of molecules in extreme rotational states to perform high-resolution state-resolved transient IR absorption measurements. Here we report the first studies of energy transfer dynamics involving molecules in extreme rotational states. In these studies, the optical centrifuge drives CO₂ molecules into states with J ∼ 220 and we use transient IR probing to monitor the subsequent rotational, translational, and vibrational energy flow dynamics. The results reported here provide the first molecular insights into the relaxation of molecules with rotational energy that is comparable to that of a chemical bond.     

Visible and UV coherent Raman spectroscopy of dipicolinic acid

We use time-resolved coherent Raman spectroscopy to obtain molecule-specific signals from dipicolinic acid (DPA), which is a marker molecule for bacterial spores. We use femtosecond laser pulses in both visible and UV spectral regions and compare experimental results with theoretical predictions. By exciting vibrational coherence on more than one mode simultaneously, we observe a quantum beat signal that can be used to extract the parameters of molecular motion in DPA. The signal is enhanced when an UV probe pulse is used, because its frequency is near-resonant to the first excited electronic state of the molecule. The capability for unambiguous identification of DPA molecules will lead to a technique for real-time detection of spores.     

Holmium-YAG laser for gall stone fragmentation: an endoscopic tool.

A systematic review of the 2.1 mu holmium-YAG laser for gall stone lithotripsy was undertaken. This infrared laser, which can be used endoscopically and percutaneously, has safety advantages over other lasers and has potential as a general purpose vascular and surgical tool. Twenty nine gall stones (mean mass 1.3 g) were fragmented in vitro using pulse energies of 114 to 159 mJ/pulse at 5 Hz with a 0.6 mm fibre, while being held in an endoscopy basket. All stones were successfully fragmented, requiring an average of 566 pulses with a 5 Hz pulse repetition frequency. The number of pulses required increased with gall stone size and mass (p < 0.01), and decreased with both pulse energy (p < 0.01) and operator experience (p < 0.05). The biochemical content of the stone did not significantly affect the number of pulses needed. The potential hazard of the laser to the biliary endothelium was investigated. At the pulse energies used, five pulses at close contact penetrated into the serosa of fresh gall bladder wall. No damage was seen when two pulses were fired. This laser shows considerable promise in gall stone lithotripsy. Until further safety data are available, however, its use with endoscopic vision is advised.     

A novel approach for under-expanded stent: excimer laser in contrast medium

The novel approach of using excimer laser in a contrast medium for treating an under-expanded stent in a calcified coronary artery is described. In this first reported case, the laser in a contrast medium was used to treat an under-expanded stent deployed 18 months prior in a calcified non-dilatable coronary artery lesion. The normal laser approach in saline medium flush during lasing to clear any contrast was not effective. The use of concurrent contrast during lasing was effective and successful to help expand the stent with a successful outcome.     

Influence of Pulse Duration on X-ray Emission during Industrial Ultrafast Laser Processing

Soft X-ray emissions during the processing of industrial materials with ultrafast lasers are of major interest, especially against the background of legal regulations. Potentially hazardous soft X-rays, with photon energies of >5 keV, originate from the fraction of hot electrons in plasma, the temperature of which depends on laser irradiance. The interaction of a laser with the plasma intensifies with growing plasma expansion during the laser pulse, and the fraction of hot electrons is therefore enhanced with increasing pulse duration. Hence, pulse duration is one of the dominant laser parameters that determines the soft X-ray emission. An existing analytical model, in which the fraction of hot electrons was treated as a constant, was therefore extended to include the influence of the duration of laser pulses on the fraction of hot electrons in the generated plasma. This extended model was validated with measurements of H (0.07) dose rates as a function of the pulse duration for a constant irradiance of about 3.5 × 10¹⁴ W/cm², a laser wavelength of 800 nm, and a pulse repetition rate of 1 kHz, as well as for varying irradiance at the laser wavelength of 1030 nm and pulse repetition rates of 50 kHz and 200 kHz. The experimental data clearly verified the predictions of the model and confirmed that significantly decreased dose rates are generated with a decreasing pulse duration when the irradiance is kept constant.     

"Shim pulses" for NMR spectroscopy and imaging

A way to use adiabatic radiofrequency pulses and modulated magnetic-field gradient pulses, together constituting a "shim pulse," for NMR spectroscopy and imaging is demonstrated. These pulses capitalize on phase shifts derived from probe gradient coils to compensate for nonlinear intrinsic main magnetic field homogeneity for spectroscopy, as well as for deviations from linear gradients for imaging. This approach opens up the possibility of exploiting cheaper, less-than-perfect magnets and gradient coils for NMR applications.