Variable Fluence Research Articles

Thermal lensing in pulsed laser amplifiers: an analytical model

An analytical expression for the thermal-lensing effects in pulsed laser amplifiers is given. The model permits a detailed investigation of the range of parameters where thermal lensing can have severe effects on the operation of crystal-based amplifiers. In particular, the behavior of Ti:sapphire-crystal amplifiers operated at variable repetition rate and pump fluence is discussed.

AFM studies of swift heavy ion-irradiated surface modification in Si and GaAs

Atomic force microscopy (AFM) studies of Swift Heavy Ions (SHI ∼100 MeV Si 7+ and Au 7+) irradiated Si and GaAs surfaces have been performed for a variable fluence in the range of 10 10– 10 13 ions cm −2 . The craters with piled up material, which is called hill, are clearly seen in the micrographs. A significant direct observation of amorphization (or melting due to SHI irradiation damage), plastic flow and subsequent recrystallization in the form of platelets has been made. The quantitative estimation of the features revealed that the volume of the craters for silicon ion irradiation is smaller than the gold ion irradiation. However, surface roughness has been found to be enhanced after the irradiation. Moreover, the GaAs surfaces were found to be less rough than the Si surface. The features are related to the difference in electronic energy loss of incident ions, thermal diffusivity, thermal conductivity and density of the target materials.

A variable fluence step clustering and segmentation algorithm forstep and shoot IMRT

A step and shoot sequencer was developed that can be integrated into an IMRT optimization algorithm.The method uses non-uniform fluence steps and is adopted to the constraints of an MLC.It consists of a clustering, a smoothing and a segmentation routine. The performance of thealgorithm is demonstrated for eight mathematical profiles of differing complexity and two optimizedprofiles of a clinical prostate case. The results in terms of stability, flexibility, speed andconformity fulfil the criteria for the integration into the optimization concept.The performance of the clustering routine is compared with another previously published one(Bortfeld et al 1994 Int. J. Radiat. Oncol. Biol. Phys. 28 723-30) and yields slightlybetter results in terms of mean and maximum deviation between the optimized and theclustered profile. We discuss the specific attributes of the algorithmconcerning its integration into the optimization concept.

Enhancement of tissue lesion depth by dual wavelength irradiation with the Nd-YAG/KTP laser: Perspectives for laser prostatectomy.

The Nd-YAG/KTP laser coagulates and vaporizes prostate tissue. The objective of this study was to investigate the combined effects of both wavelengths and to determine the irradiation parameters allowing the largest lesion volume. Chicken breast tissue was irradiated ex vivo. Consecutive 1064 and 532 nm Nd-YAG/KTP laser irradiations were performed for different combinations (30 W/10 W, 20 W/20 W, 10 W/30 W) with variable total fluence (1200 J, 2400 J, 3600 J) and compared to isofluent single wavelengths at 40 W irradiation. The depths, diameters and volumes of the total lesion as well as the vaporization effects of the 532 nm wavelength on normal and on priorly coagulated tissue were analysed. Maximum total lesion depths (p< 0.001) were found under combined Nd-YAG/KTP (20 W/20 W) irradiation conditions. Ablation efficacy of the 532 nm wavelength was reduced after prior 1064 nm irradiation, but crater depths were increased. Dual wavelength irradiation with the Nd-YAG/KTP laser induces a specific denaturation process. This may represent a new approach to increase the depth of coagulation necrosis, and thus the treated volume, thereby improving long-term results.

Light-controlled adaptation kinetics in Phycomyces: evidence for a novel yellow-light absorbing pigment.

When sporangiophores of the fungus Phycomyces blakesleeanus adapt from high to low fluence rate, dark adaptation (sensitivity recovery) can be accelerated by dim subliminal light [Galland et al. (1989) Photochem. Photobiol. 49, 485-491]. We measured fluence rate-response curves for this acceleration under the following conditions. After sporangiophores were initially adapted symmetrically to a fluence rate of 1 W m-2 (447 nm), they were exposed to unilateral subliminal light (subthreshold for phototropism) of variable wavelength and fluence rate, and then to unilateral test light (447 nm) of fluence rate either 10(-3) or 10(-5) W m-2. The duration of the subliminal light was chosen so that phototropism would not occur during this period. Phototropic latencies could be shortened by subliminal light that was less intense than the test light by several orders of magnitude. In experiments with the final unilateral light of fluence rate 10(-3) W m-2, the 447 nm subliminal light had a threshold (for the acceleration effect) of about 10(-11) W m-2. Yellow light of wavelength 575 nm, which itself is extremely ineffective for phototropism was extremely effective in shortening phototropic latencies in response in response to the test light. At 575 nm, the threshold was about 2 x 10(-12) W m-2. Conversely, near-UV light of wavelength 347 nm, which is highly effective for phototropism, was relatively ineffective (threshold approximately 7 x 10(-8) W m-2) in shortening the phototropic latency. Our results suggest the presence of a novel yellow-light absorbing pigment in Phycomyces that specifically regulates dark adaptation.(ABSTRACT TRUNCATED AT 250 WORDS)

ARE THERE SEVERAL PHOTORECEPTORS INVOLVED IN PHOTOTROPISM OF Phycomyces blakesleeunus? KINETIC STUDIES OF DICHROMATIC IRRADIATION*

Abstract— Photogeotropic equilibrium angles were measured for Phycomyces blakesleeanus wild type firstly by means of dichromatic fluence rate response curves using simultaneous irradiation with near threshold 450 nm reference light (constant at 1.2 × 10−8 W m−2) and variable fluence rates of test light (498–630 nm) from the same side. These curves showed minima for test light fluence rates that were close to the photogeotropic threshold for these wavelengths. Secondly, the time course of this inhibitory effect was studied with both the inductive reference 450 nm light (2 × 10−‐7 W m−2) and the test light (606 or 450 nm) given as light pulses of 2 s duration (2 s light/48 s dark periods for 6 h). The dark period between the onset of the inductive reference light and test light pulses was varied between 0 and 48 s. No inhibitory effects were observed for simultaneous pulses; however, inhibitory effects were demonstrated for delay times of 2 s and 20 s for 606 nm as well as 450 nm test light. If the test light pulses were given immediately before the inductive reference light, only 606 nm test light was effective in producing a significant inhibitory effect.The results are discussed with regard to a multichromophoric photoreceptor system and to the wavelength dependence of the effects observed. The data and conclusions favor a photoreceptor system with at least two separate chromophoric absorptions of the blue light receptor type, one acting positively, the other acting inhibitorily, and at least one other photoreceptor of presumably minor influence.