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.     

The excimer laser: gross, light microscopic and ultrastructural analysis of potential advantages for use in laser therapy of cardiovascular disease

Excimer lasers are pulsed gas lasers that use a mixture of a rare gas and halogen as the active medium to generate pulses of short wavelength, high energy ultraviolet light. A krypton-fluoride gas mixture was used to achieve an excimer emission at a wavelength of 248 nm. A total of 30 atherosclerotic coronary artery segments were irradiated over a range of pulse energies (250 to 750 mJ), repetition rates (2 to 25 Hz), average powers (1.9 to 18.8 watts) and cumulative exposures (3 to 12 seconds). In no case was there gross, light microscopic or ultrastructural evidence of the pathologic injury typically associated with continuous wave laser irradiation of coronary artery segments. Similar results were achieved after excimer laser irradiation of 30 samples of myocardium. Excimer irradiation of calcified aortic valve leaflets accomplished focal debridement without pathologic tissue injury; when total debridement was attempted, however, gross charring was observed. The paucity of pathologic alterations observed after excimer irradiation of cardiovascular tissue may prove beneficial in precisely controlling laser ablation of pathologic tissue without injury to the surrounding normal tissue. Clinical application of excimer laser irradiation requires resolution of several issues, including the development of suitable fiber optics and laser coupling, evaluation of potential ultraviolet toxicity, and demonstration that ultraviolet light can be transmitted through a blood-filled system.     

X-ray Emission Hazards from Ultrashort Pulsed Laser Material Processing in an Industrial Setting

Interactions between ultrashort laser pulses with intensities larger than 10¹³ W/cm² and solids during material processing can lead to the emission of X-rays with photon energies above 5 keV, causing radiation hazards to operators. A framework for inspecting X-ray emission hazards during laser material processing has yet to be developed. One requirement for conducting radiation protection inspections is using a reference scenario, i.e., laser settings and process parameters that will lead to an almost constant and high level of X-ray emissions. To study the feasibility of setting up a reference scenario in practice, ambient dose rates and photon energies were measured using traceable measurement equipment in an industrial setting at SCHOTT AG. Ultrashort pulsed (USP) lasers with a maximum average power of 220 W provided the opportunity to measure X-ray emissions at laser peak intensities of up to 3.3 × 10¹⁵ W/cm² at pulse durations of ~1 ps. The results indicate that increasing the laser peak intensity is insufficient to generate high dose rates. The investigations were affected by various constraints which prevented measuring high ambient dose rates. In this work, a list of issues which may be encountered when performing measurements at USP-laser machines in industrial settings is identified.     

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.     

Do thermal lasers increase the risk of distal embolism in angioplasties? An experimental study with cw Nd: YAG and excimer lasers.

A thermal (contact cw Nd:YAG) and non-thermal (Xe-Cl excimer) laser were used to irradiate fresh human blood in an experimental setting to investigate the possible thrombogenic properties of lasers and to compare the two laser modalities. Blood was obtained into 10 ml citrate test tubes from healthy volunteers. Laser irradiations were performed with cw Nd: YAG laser (10W, 3 and 6 secs) and with excimer laser (20 mJ, 60 ns pulses, 15 secs). Altogether, 80 samples were collected. A thromboelastographic (TEG) analysis was performed for each sample as well as the controls, which were taken simultaneously. Serum potassium levels indicating haemolysis were also determined. Thermal laser energy seems to have more thrombogenic effect than excimer laser. There appear to be no previous reports available in the literature using TEG to determine thrombogenicity of different lasers.     

Fragmentation of biliary calculi with tunable dye lasers

The feasibility of using lasers to fragment biliary calculi was examined in vitro. Flashlamp-pumped tunable dye lasers were coupled to small-diameter flexible quartz fibers that were placed in direct contact with biliary calculi. The minimum laser energy necessary to damage a calculus was measured for wavelengths between 450 and 700 nm and for pulse durations between 0.8 and 360 microseconds. This threshold energy increased with increasing wavelength but was not significantly affected by pulse duration. Cholesterol stones had uniformly higher thresholds than pigmented ones. When a repetitively pulsed laser was used, complete fragmentation required fewer than 500 pulses and fragments were predominantly less than 2 mm. The pulsed dye laser can effectively fragment biliary calculi when transmitted through a small-diameter quartz fiber and may be useful as a tool for fragmenting retained common duct stones.     

Origin of arterial wall dissections induced by pulsed excimer and mid-infrared laser ablation in the pig.

To study adjacent tissue damage after delivery of holmium, thulium and excimer laser pulses, porcine thoracic aortas were irradiated in vivo. After 3 days, microscopic analysis of 67 craters produced by all three lasers demonstrated large dissections extending from the craters. The mean diameter of the dissections was smaller for excimer-induced craters (1.38 +/- 0.42 mm; n = 22) than for holmium-induced (2.7 +/- 0.87 mm; n = 22) and thulium-induced (2.37 +/- 0.42 mm; n = 14) craters (p less than 0.01 vs. mid-infrared dissections). In addition, microscopic analysis demonstrated necrosis adjacent to the crater. The lateral necrotic zones of the thulium-induced craters were smaller than the holmium- and excimer-induced necrotic zones (p less than 0.01). To identify the origin of the excessive tissue tearing, laser-saline and laser-tissue interaction were compared in vitro by time-resolved flash photography. In saline solution, the mid-infrared lasers showed bubble formation on a microsecond time scale. The excimer laser produced similar bubbles in the vicinity of tissue. For all three lasers, elevation of the tissue surface was shown during in vitro ablation. Dimension (diameter up to 4 mm) and time course (rise time of 100 to 300 microseconds) of bubble formation and tissue elevation were strikingly similar. Thus, tissue dissections are caused by the expansion of a vapor bubble within the target tissue. Coronary dissections after excimer and mid-infrared laser angioplasty might be related to the forceful bubble expansion.     

Comparison of tissue disruption caused by excimer and midinfrared lasers in clinical simulation

Laser coronary angioplasty is a useful therapy for selected complex coronary lesions. Laser-induced acoustic trauma is postulated to be a cause of dissection and acute vessel occlusion. Controversy exists regarding the relative degree of photoacoustic effects of midinfrared and excimer lasers in clinical practice. To date, these systems have not been compared at clinical energy doses and with clinical pulsing strategies. Therefore, we studied the photoacoustic effects of both midinfrared and excimer lasing at clinically accepted doses. Human atherosclerotic iliofemoral artery segments were obtained at autopsy (n = 36) and placed lumen side up in a saline bath. Clinical laser catheters were advanced over an 0.018′ guide wire, perpendicular to the tissue. A 10-g down force was applied to the catheter for full-thickness lasing. Pulsing strategies were, for midinfrared laser: 5 pulses, 1-sec pause, 5 pulses, 1-sec pause, 5 pulses, withdraw; for excimer: 5 sec of pulses, wait 10 sec, 5 sec of pulses. Several clinically acceptable energy levels were used; for excimer: 25 mJ/mm², 40 mJ/mm², 60 mJ/mm²; for midinfrared: 3 W (400 mJ/mm²), 3.5 W (467 mJ/mm²). Photoacoustic effect was assessed histologically by determining the number of lateral cleavage planes (dissections) arising from the lased crater border and extending into the surrounding tissue. In normal tissue, midinfrared lasing produced less acoustic damage than excimer lasing (2.79 ± 0.78 vs. 5.27 ± 0.75 cleavage planes, mean ± SD, P < 0.05, data for lowest energy for each system). The same was true in noncalcified atheroma (2.48 ± 0.71 vs. 6.43 ± 1.09, P < 0.05) and calcified atheroma (2.47 ± 1.21 vs. 6.27 ± 1.13, P < 0.05). This effect was similar at all energy levels, with a trend for more damage at higher energies in both systems. This study demonstrates that midinfrared lasing causes less acoustic damage than excimer lasing when using clinical catheters, energy levels, and pulsing strategies. This effect is independent of tissue-type but tends to be dose-related. These findings may explain, in part, the differences in dissection rates seen clinically. © 1996 Wiley-Liss, Inc.     

Physics of surgery with lasers

The characteristics of the principal lasers used in surgery are summarized in Table 1. Their diverse effects on biologic tissues permit the following generalizations: The CO2 laser is best suited for precise, visually controllable tissue removal by vaporization with minimal marginal damage. Hemostasis is excellent for bleeding from capillary vessels, but difficult for larger ones. The Nd:YAG laser is best suited for the coagulation of larger tissue volumes of the order of 10 mm3 or more. Tissue heating inherently extends for several millimeters, leading to excellent hemostasis. Radiation from this laser is well transmitted through flexible optical fibers and clear fluids. The argon ion laser emits radiation in the visible range and is ideally suited for treating the retina and other tissues in the eye without damage to its transparent structures. Radiation of this laser is strongly absorbed by pigmented tissues, scattered and reflected by others, and transmitted by fluids. Its radiation can be focused to very small spot sizes, leading to high precision and high-power densities. It has hemostatic properties intermediate between those of the CO2 and of the Nd:YAG laser radiations. It is well transmitted through optical fibers and clear fluids. It is used extensively in ophthalmology and dermatology. Selected applications to neurosurgery and otology are being investigated. These lasers have become indispensable adjuncts to the surgical armamentarium of several specialties. The very success of these lasers is leading to a critical examination of their shortcomings and to a search for improved systems. Examples are (1) the ongoing search for optical fibers to transmit the radiation of the CO2 laser; (2) the development of systems for the sequential delivery to tissues of several wavelengths from a single unit (Fig. 14); and (3) investigations of tissue effects of laser beams in the ultraviolet and in the infrared at wavelengths intermediate between those of the Nd:YAG and CO2 lasers. The use of lasers has already contributed to improved medical care in many surgical disciplines. Additional areas of application can be confidently anticipated.     

A Novel Process for Manufacturing High-Friction Rings with a Closely Defined Coefficient of Static Friction (Relative Standard Deviation 3.5%) for Application in Ship Engine Components

In recent years, there has been an increased uptake for surface functionalization through the means of laser surface processing. The constant evolution of low-cost, easily automatable, and highly repeatable nanosecond fibre lasers has significantly aided this. In this paper, we present a laser surface-texturing technique to manufacture a surface with a tailored high static friction coefficient for application within driveshafts of large marine engines. The requirement in this application is not only a high friction coefficient, but a friction coefficient kept within a narrow range. This is obtained by using nanosecond-pulsed fibre lasers to generate a hexagonal pattern of craters on the surface. To provide a suitable friction coefficient, after laser processing the surface was hardened using a chromium-based hardening process, so that the textured surface would embed into its counterpart when the normal force was applied in the engine application. Using the combination of the laser texturing and surface hardening, it is possible to tailor the surface properties to achieve a static friction coefficient of ≥0.7 with ~3–4% relative standard deviation. The laser-textured and hardened parts were installed in driveshafts for ship testing. After successfully performing in 1500 h of operation, it is planned to adopt the solution into production.     

Generation of bright isolated attosecond soft X-ray pulses driven by multicycle midinfrared lasers

High harmonic generation driven by femtosecond lasers makes it possible to capture the fastest dynamics in molecules and materials. However, to date the shortest subfemtosecond (attosecond, 10⁻¹⁸ s) pulses have been produced only in the extreme UV region of the spectrum below 100 eV, which limits the range of materials and molecular systems that can be explored. Here we experimentally demonstrate a remarkable convergence of physics: when midinfrared lasers are used to drive high harmonic generation, the conditions for optimal bright, soft X-ray generation naturally coincide with the generation of isolated attosecond pulses. The temporal window over which phase matching occurs shrinks rapidly with increasing driving laser wavelength, to the extent that bright isolated attosecond pulses are the norm for 2-µm driving lasers. Harnessing this realization, we experimentally demonstrate the generation of isolated soft X-ray attosecond pulses at photon energies up to 180 eV for the first time, to our knowledge, with a transform limit of 35 attoseconds (as), and a predicted linear chirp of 300 as. Most surprisingly, advanced theory shows that in contrast with as pulse generation in the extreme UV, long-duration, 10-cycle, driving laser pulses are required to generate isolated soft X-ray bursts efficiently, to mitigate group velocity walk-off between the laser and the X-ray fields that otherwise limit the conversion efficiency. Our work demonstrates a clear and straightforward approach for robustly generating bright isolated attosecond pulses of electromagnetic radiation throughout the soft X-ray region of the spectrum.High-order harmonic generation (HHG) is the most extreme nonlinear optical process in nature, making it possible to coherently upconvert intense femtosecond laser light to much shorter wavelengths (1, 2). High harmonics are radiated as a result of a coherent electron recollision process that occurs each half-cycle of the driving laser field while an atom is undergoing strong-field ionization. The short pulse duration of HHG (which must be shorter than the driving laser pulse) has made it possible to directly access the fastest timescales relevant to electron dynamics in atoms, molecules, and materials. The unique properties of attosecond HHG in the extreme UV (EUV) have uncovered new understanding of fundamental processes in atoms, molecules, plasmas, and materials, including the timescales on which electrons are emitted from atoms (3), the timescale for spin–spin and electron–electron interactions (4, 5), the timescale that determines molecular dissociation and electron localization (6–9), the timescale and mechanisms for spin and energy transport in nanosystems (10–12), as well as new capabilities to implement EUV microscopes with wavelength-limited spatial resolution (13).The temporal structure of HHG is related to the number of times a high-energy electron undergoes a coherent recollision process, as well as the time window over which bright harmonics emerge. Using multicycle 0.8-µm driving lasers, HHG generally emerges as a train of attosecond (as) pulses (14, 15) corresponding to a series of harmonic peaks in frequency space. This emission can narrow to a single isolated as burst when the driving laser field is a few optical cycles (∼5 fs) in duration (16, 17), with an associated broad continuous spectrum. Other techniques can isolate a single burst using a combination of multicolor fields and polarization control (18–26) or spatial lighthouse gating of the driving laser pulses (27, 28). Phase matching can also result in bright isolated as pulse generation for short driving laser pulses (29, 30). To obtain bright, phase-matched, high harmonic beams, the laser and HHG fields must both propagate at the speed of light c so that emission from many atoms interferes constructively. Above a critical ionization level, the phase velocity of the laser exceeds c, which terminates the HHG temporal emission. The chirp present on attosecond bursts can be compensated by using thin materials, gases, or chirped mirrors (31–33). To date, however, most schemes for creating isolated attosecond pulses require either very short-duration few-cycle 0.8-µm driving laser pulses that are difficult to reliably generate, or complex polarization modulation schemes. In addition, the carrier envelope phase (CEP) of the driving laser pulse must be stabilized.A more general understanding of how to efficiently sculpt the temporal, spatial, and spectral characteristics of HHG emission over an extremely broad photon energy range (from the EUV to the keV and higher) has emerged in recent years (34–39). This understanding is critical both for a fundamental understanding of strong-field quantum physics, as well as for applications which have fundamentally different needs in terms of the HHG pulse duration, spectral bandwidth, and flux. By considering both the microscopic single-atom response as well as the macroscopic coherent buildup of HHG, efficient phase-matched HHG can now be implemented from the EUV to >keV photon energies, simply by driving HHG with midinfrared (mid-IR) femtosecond driving lasers. This advance represents, to our knowledge, the first general-purpose, tabletop, coherent soft X-ray light source (39). Furthermore, theory suggested that bright isolated attosecond X-ray bursts would be achievable using multicycle mid-IR driving lasers in a phase-matched geometry (35). However, the low repetition rate of the driving lasers precluded experimental testing of these predictions. Moreover, formidable computation requirements meant that advanced simulations could not be fully extended into the mid-IR region at 2–4 µm.In this paper, we experimentally demonstrate a beautiful convergence of physics for mid-IR (2-µm) driving lasers by showing that the conditions for optimal bright, soft X-ray generation naturally coincide with the generation of bright isolated attosecond soft X-ray bursts. We combine advanced theory with a novel experimental method equivalent to high-resolution Fourier transform spectroscopy to measure bright, attosecond soft X-ray pulses for the first time, to our knowledge. Specifically, we measure a field autocorrelation pulse width of 70 as, corresponding to a transform-limited 35-as pulse, that is supported by a coherent supercontinuum spectrum extending to photon energies around 180 eV. We also validate experimentally, for the first time, to our knowledge, the most intuitive dynamic picture of phase matching of HHG in the time domain by clearly demonstrating that the temporal window during which phase matching occurs shrinks rapidly with increasing driving laser wavelength. Finally, we show through advanced theory that the isolated attosecond pulse is chirped to 300 as. Most surprisingly, we find that bright attosecond pulse generation in the soft X-ray region requires the use of longer-duration, multicycle, mid-IR driving lasers to mitigate group velocity walk-off issues that would otherwise reduce the conversion efficiency. By harnessing the beautiful physics of phase matching, this work represents the simplest and most robust scheme for attosecond soft X-ray pulse generation, and will make attosecond science and technology accessible to a broader community.     

The Use of Laser Energy for Etching Enamel Surfaces in Dentistry—A Scoping Review

Background: In dental practice, different situations require etching the enamel layer. Acid etching, the present golden standard, may be replaced by other methods, such as laser etching. The main focus of our scoping review is to assess the existent literature regarding the effectiveness of different types of lasers, to identify the main aspects studied so far, and to understand where new search strategies are needed. Methods: The search was conducted in several databases focusing on the laser etching of human definitive enamel. We included English language articles published between January 2000 and December 2021. Results: The 34 articles reviewed showed that hard lasers, Er:YAG, Er,Cr:YAG, may represent an alternative etching method on enamel surfaces. They create a fractured, irregular surface and open dentin tubules, highly suitable for adhesion but with a lower risk of cavity formation. Nd:YAG, CO₂, and Diode lasers do not help in creating sufficient shear bond strength. There is, however, evidence suggesting that microcracks in the enamel layer may appear after thermomechanical ablation using laser energy. Conclusions: While the use of acid etching is still successfully used for enamel conditioning, some researchers have emphasized the role played by saliva in the enamel-remineralization process a few days after the procedure. In this context, laser energy can be used, especially for bonding ceramic brackets in the case of orthodontic treatments. However, as thermomechanical ablation can generate microcracks, further research is required in order to establish clear findings concerning the use of laser energy on enamel etching.     

Single-dose vaccine against tick-borne encephalitis

Tick-borne encephalitis (TBE) virus is the most important human pathogen transmitted by ticks in Eurasia. Inactivated vaccines are available but require multiple doses and frequent boosters to induce and maintain immunity. Thus far, the goal of developing a safe, live attenuated vaccine effective after a single dose has remained elusive. Here we used a replication-defective (single-cycle) flavivirus platform, RepliVax, to generate a safe, single-dose TBE vaccine. Several RepliVax-TBE candidates attenuated by a deletion in the capsid gene were constructed using different flavivirus backbones containing the envelope genes of TBE virus. RepliVax-TBE based on a West Nile virus backbone (RV-WN/TBE) grew more efficiently in helper cells than candidates based on Langat E5, TBE, and yellow fever 17D backbones, and was found to be highly immunogenic and efficacious in mice. Live chimeric yellow fever 17D/TBE, Dengue 2/TBE, and Langat E5/TBE candidates were also constructed but were found to be underattenuated. RV-WN/TBE was demonstrated to be highly immunogenic in Rhesus macaques after a single dose, inducing a significantly more durable humoral immune response compared with three doses of a licensed, adjuvanted human inactivated vaccine. Its immunogenicity was not significantly affected by preexisting immunity against WN. Immunized monkeys were protected from a stringent surrogate challenge. These results support the identification of a single-cycle TBE vaccine with a superior product profile to existing inactivated vaccines, which could lead to improved vaccine coverage and control of the disease.     

In vitro opening of human atheromatous coronary arteries using a pulsed laser

This study was undertaken to assess the respective values of pulsed and continuous laser emission for in vitro recanalisation of very stenosed atheromatous human coronary arteries. The Nd-YAG laser used emitted a 10 Hz 10 ns burst in the infrared band (1 064 microns). Previous spectroscopic studies had shown no specific band of absorption in the spectral field of emission of the usual lasers. The laser beam was focused in the axis of the segment of coronary artery irradiated. The crater or neo lumen obtained usually had irregular walls. No perforation of the arterial wall or macroscopic debris were observed. Histological studies showed minimal burn lesions with sparse coagulation necrosis limited to a few tens of micron thickness. The percentage recanalisation obtained with pulses of 200 mJ attained 50% for a total energy of 450 J delivered in 2 mn. This study confirmed the feasibility of disobliteration of atheromatous coronary arteries by pulsed laser. Our results suggest that ultra short pulsed laser acts more by a mechanical than by a thermal mechanism which may lead to less side effects than observed in vivo with continuous laser emission.     

NdYAG laser photocoagulation in the dog stomach.

Considerable discussion still centres around the relative merits of the Argon and Neodymium Yttrium Aluminum Garnet (NdYAG) lasers for the endoscopic treatment of gastrointestinal haemorrhage, although both are undoubtedly effect. We have carried out experiments to elucidate which factors determine the safety and efficacy of NdYAG laser photocoagulation. Histological studies on normal gastric mucosa showed that the depth of tissue damage depended mainly on the total incident laser energy, whereas the effectiveness of photocoagulation of induced gastric ulcers in heparinised animals depended on the laser power and the exposure time used. Optimum haemostasis with minimum tissue damage was obtained using pulses of 300 to 500 ms duration with energies of 25 to 40 J. We consider these parameters safe and effective for use in pilot clinical studies.     

Enhanced X-ray Emissions Arising from High Pulse Repetition Frequency Ultrashort Pulse Laser Materials Processing

The ongoing trend in the development of powerful ultrashort pulse lasers has attracted increasing attention for this technology to be applied in large-scale surface engineering and modern microfabrication. However, the emission of undesired X-ray photon radiation was recently reported even for industrially relevant laser irradiation regimes, causing serious health risks for laser operators. In the meantime, more than twenty influencing factors have been identified with substantial effects on X-ray photon emission released by ultrashort pulse laser processes. The presented study on enhanced X-ray emission arising from high pulse repetition frequency ultrashort pulse laser processing provides new insights into the interrelation of the highest-contributing parameters. It is verified by the example of AISI 304 substrates that X-ray photon emission can considerably exceed the legal dose rate limit when ultrashort laser pulses with peak intensities below 1 × 10¹³ W/cm² irradiate at a 0.5 MHz pulse repetition frequency. The peak intensity threshold value for X-ray emissions decreases with larger laser spot sizes and longer pulse durations. Another key finding of this study is that the suction flow conditions in the laser processing area can affect the released X-ray emission dose rate. The presented results support the development of effective X-ray protection strategies for safe and risk-free ultrashort pulse laser operation in industrial and academic research applications.     

Coronary endarterectomy using an excimer laser. Preliminary peroperative results

From experimental and clinical experience, safe coronary angioplasty cannot be performed with CW lasers. The excimer laser does present a number of advantages in vitro: non-thermal ablation of plaques and a linear relationship between the number of pulses and the depth of the crater, so that tissue ablation is quantitatively predictable. A 308 nm, 20 ns pulse duration, 1 to 5 repetition rate laser was specifically designed for clinical application. During cardiopulmonary bypass prior to bypass grafting in 10 symptomatic patients, a 1 mm diameter core UV-tipped fiberoptic was introduced via the coronary arteriotomy and directed in contact with the coronary stenosis. Laser power was progressively increased until the stenosis or occlusion was recanalized. The quality of this angioplasty was controlled by calibration of he neo-lumen, cardioplegia solution flow through the lased segment, and 8th day coronary angiography. The laser treated coronary segments of the first 4 patients showed clearly parallel-lined patent neo-lumen despite competitive bypass graft flow. The main limitation of the method is that laser coronary recanalization is confined to the fiber core diameter. The authors conclude that: 1) excimer laser angioplasty is a safe and efficient intra-operative procedure; 2) the most critical problem for percutaneous laser angioplasty remains flexibility of the apparatus as the fiber diameter must be large enough to provide an adequate arterial neo-lumen.     

Safety and efficacy of Neodymium-Yag laser photocoagulation: an experimental study in dogs.

Acute and chronic experiments were carried out in 26 beagle dogs to study the safety and efficacy of Neodymium-Yag laser photocoagulation in the treatment of bleeding gastric lesions. Continuous high power (50-60 W) Neodymium-Yag laser photocoagulation applied to the exposed stomach of the dog produced evaporation lesions that reached the muscle layer after six to 10 seconds and caused free perforation after 10 to 12 seconds. The tissue damage caused by these long lasting exposures was closely related to the working distance. Moreover, long pulses of high power photocoagulation were not always effective in stopping experimentally induced gastric bleedings. Short pulses (1/2-1 s) of very high power (60-70 W) caused less tissue evaporation, which reached the muscle layers only after 14 to 18 pulses and caused free perforation after 22 to 24 pulses. The tissue damage was not related to the working distance when short pulses were used. Repeated shots of high power Yag laser radiation always resulted in stopping the experimental bleedings without deep injury. It is concluded that high power Neodymium-Yag laser photocoagulation is safe and may be used with success in the treatment of bleeding gastric lesions if the radiation is performed in shots of short duration (1 s or less). Clinical studies in man are warranted and indicated.     

Standoff spectroscopy via remote generation of a backward-propagating laser beam

In an earlier publication we demonstrated that by using pairs of pulses of different colors (e.g., red and blue) it is possible to excite a dilute ensemble of molecules such that lasing and/or gain-swept superradiance is realized in a direction toward the observer. This approach is a conceptual step toward spectroscopic probing at a distance, also known as standoff spectroscopy. In the present paper, we propose a related but simpler approach on the basis of the backward-directed lasing in optically excited dominant constituents of plain air, N(2) and O(2). This technique relies on the remote generation of a weakly ionized plasma channel through filamentation of an ultraintense femtosecond laser pulse. Subsequent application of an energetic nanosecond pulse or series of pulses boosts the plasma density in the seed channel via avalanche ionization. Depending on the spectral and temporal content of the driving pulses, a transient population inversion is established in either nitrogen- or oxygen-ionized molecules, thus enabling a transient gain for an optical field propagating toward the observer. This technique results in the generation of a strong, coherent, counterpropagating optical probe pulse. Such a probe, combined with a wavelength-tunable laser signal(s) propagating in the forward direction, provides a tool for various remote-sensing applications. The proposed technique can be enhanced by combining it with the gain-swept excitation approach as well as with beam shaping and adaptive optics techniques.     

Bioinspired optofluidic FRET lasers via DNA scaffolds

Optofluidic dye lasers hold great promise for adaptive photonic devices, compact and wavelength-tunable light sources, and micro total analysis systems. To date, however, nearly all those lasers are directly excited by tuning the pump laser into the gain medium absorption band. Here we demonstrate bioinspired optofluidic dye lasers excited by FRET, in which the donor-acceptor distance, ratio, and spatial configuration can be precisely controlled by DNA scaffolds. The characteristics of the FRET lasers such as spectrum, threshold, and energy conversion efficiency are reported. Through DNA scaffolds, nearly 100% energy transfer can be maintained regardless of the donor and acceptor concentration. As a result, efficient FRET lasing is achieved at an unusually low acceptor concentration of micromolar, over 1,000 times lower than that in conventional optofluidic dye lasers. The lasing threshold is on the order of μJ/mm². Various DNA scaffold FRET lasers are demonstrated to illustrate vast possibilities in optofluidic laser designs. Our work opens a door to many researches and applications such as intracavity bio/chemical sensing, biocontrolled photonic devices, and biophysics.