Z. Burshtein

Spectroscopy and laser action of the “red perylimide dye” in various solvents

Abstract Optical properties of the red perylimide laser dye in various solvents are studied. The absorption spectrum exhibits two main bands, in the ranges 480–600 and 400–460 nm, due to the S0–S1 and S0–S2 transition. The fluorescence spectrum is a mirror image of the S0–S1 absorption (shift of ∼ 30–50 nm). The Stokes shift increases with solvent polarity. Such dye-solvent interactions are compared to theoretical predictions. The fluorescence quantum yields approaches unity in all the solvents studied. Laser tunability around 30 nm was obtained each time, covering the spectral range 580–640 nm. This interval is important for medical applications in photodynamic therapy and fluorescence diagnostics. The laser threshold energy varied from 0.35 mJ/pulse in cyclohexane to 1.87 mJ/pulse in methanol, and the slope efficiency from about 6.6% in methanol to 14% in xylenes. The laser output was stable for several hours of operation under an average pump energy of about 20 mJ/pulse at 1 Hz repetition rate, without flow.

Cr/sup 4+/:YAG as passive Q-switch and Brewster plate in a pulsed Nd:YAG laser

We demonstrate the performance of a Nd:YAG laser, passively Q-switched with a Cr/sup 4+/:YAG plate, which plays the double role of a passive Q-switch and a Brewster plate. The Brewster plate configuration contributes an intracavity loss of approximately 3.2-10/sup -3/ cm/sup -1/ along the cavity length. Losses contributed by the active Cr/sup 4+/ ions in the plate relate to their excited state absorption. A freshly measured transmission saturation curve of Cr/sup 4+/:YAG suggests a ground state absorption cross section /spl sigma//sub gs/=(8.7/spl plusmn/0.8)-10/sup -19/ cm/sup 2/, and an excited state absorption cross section /spl sigma//sub es/=(2.2/spl plusmn/0.2)-10/sup -19/ cm/sup 2/ of the Cr/sup 4+/ ions at /spl lambda/=1064 nm. >

Excited-state absorption studies of Cr/sup 4+/ ions in several garnet host crystals

An experimental study of saturable absorption and excited-state absorption (ESA) in several inorganic saturable absorbers, Cr/sup 4+/:YAG, Cr/sup 4+/:GGG, and Cr/sup 4+/:YSGG, is presented. We provide the theoretical background of absorption characteristics in saturable absorbers that exhibit ESA, with some new results: approximate analytical solutions are proposed for the optical transmission in the case of a slow absorber, and for various light intensity conditions of spatially or temporally Gaussian beams in fast and slow absorbers. Experimentally, partial bleaching of the first excited state itself could be observed in Cr/sup 4+/:YAG at /spl lambda/=1064 nm, yielding the higher excited-state lifetime as /spl tau/*=(0.55/spl plusmn/0.1) ns. The regular transmission bleaching curve was measured in Cr/sup 4+/:GGG, for the first time in this material, yielding /spl sigma//sub ga/=(58/spl plusmn/5)/spl times/10/sup -1/ cm/sup 2/, and /spl sigma/e/sub s/=(13/spl plusmn/2)/spl times/10/sup -19/ cm/sup 2/ at /spl lambda/=1064 nm, ESA spectra were measured for the three materials between /spl sim/700 and 900 nm. All three exhibit crossing between saturable absorption at longer wavelengths and inverse saturable absorption at shorter wavelengths.

Repetitive Q-switching of a CW Nd:YAG laser using Cr/sup 4+/:YAG saturable absorbers

Repetitive Q-switching of a CW pumped Nd:YAG laser using a Cr/sup 4+/:YAG saturable absorber was achieved for the first time, providing pulses 80-300 ns wide (FWHM) with repetition rates ranging between 2 and 29 kHz. Different ranges of repetition rates and pulse widths are obtained by using absorbers of different optical densities. Satisfactory quantitative description of the experimental results is obtained by a full numerical solution of the system rate equations according to the theory of Powell and Wolga. These equations involve the dynamics of the laser population inversion, the absorber state population, and the photon density in the laser cavity.

Passive Q-switching in Nd:YAG/Cr4+:YAG monolithic microchip laser

Abstract Composite laser devices of passively Q-switched Nd:YAG were prepared by optical contacting between Nd:YAG and Cr,Ca:YAG crystal wafers followed by prolonged heating at elevated temperatures. Heating of the composite devices under reducing and/or oxidizing environments allowed to control the Cr4+ ion concentration in the Cr,Ca:YAG, thus affecting its absorption saturation behavior. Optical absorption saturation measurements on partially reduced Cr,Ca:YAG crystal were performed. Residual absorption of the saturable absorber at 1064 run results from the Cr4+ ion excited-state absorption. Laser damage threshold at the gain/absorber interface of the composite device, 14.7 J/cm2, is higher than at the entrance face. The device thus obtained was end-pumped by a fiber-optic-coupled diode laser, and exhibited short (∼5 ns), high repetition-rate pulsing.

Some optical properties of Cr4+-doped crystals

We summarize briefly some of our past studies, and report of preliminary recent new results concerning the optical properties of Cr 4+ -doped crystals that are important for utilization as passive Q-switching devices in Nd:YAG laser systems. The host crystals involved are YAG, YSGG, GGG, LuAG and forsterite. Excited-state lifetimes of 4.0, 1.0 and 1.7 μs were measured by the Cr 4+ fluorescence decay (1.3-1.7 μm) following pulsed excitation at 1064 nm in YAG, YSGG and GGG, respectively. The ground- and excited-state absorption (ESA) cross-sections at λ = 1064 nm were estimated from transmission saturation measurements. For [Cr 4+ ,Mg 2+ ]:YAG the respective results were σ gs = (3.25 ± 0.15) × 10 -18 cm 2 and σ es = (6.25 ± 0.5) × 10 -19 cm 2 . In the orthorhombic forsterite, the cross-sections were polarization dependent. We got σ gs < (3.3 ± 1) × 10 -19 cm 2 for E∥a∥, σ gs = (23 ± 2) × 10 -19 cm 2 and σ es = (9.0 ± 0.7) × 10 -19 cm 2 for E∥b, and σ gs = (16 ± 1) × 10 -19 cm 2 and σ es = (5.7 ± 0.4) × 10 -19 cm 2 for E∥c. Polarised ESA spectra were measured between 680 and 960 nm using the pulsed pump/probe technique. At 750 nm we got σ gs = (110 ± 10) × 10 - 19 cm 2 and σ es = (25 ± 3) × 10 -19 cm 2 for E∥b. Passive Q-switching performance of a flashlamp-pumped Nd:YAG laser using an intracavity [Cr 4+ ,Ca 2+ ]:GGG sample is also demonstrated.

Spectroscopy and laser characteristics of copper-vapor-laser pumped Pyrromethene-556 and Pyrromethene-567 dye solutions

We measured the basic optical properties of Pyrromethene-567 (P567) and Pyrromethene-556 (P556) dye solutions that are relevant to their application as dye lasers. The fluorescence spectra of methanol solutions show mirror images in relation to the absorption spectra, with Stokes shifts of 29.5 and 37.5 nm, respectively, for the two dyes. The central fluorescence peaks were at 546 and 535 nm, with widths of ~40 and ~50 nm (FWHM). The quantum yields were 97% ? 5% and 78% ? 5% for P567 and P556, respectively. Fluorescence lifetimes of 6.0 ? 0.2 ns were obtained for both dyes in methanol. Laser action, obtained by pumping with the green emission line (510.6 nm) from a copper-vapor laser, was measured in a Hänsch-type cavity. Tunability ranged from 531 to 590 nm for P567 and from 522 to 590 nm for P556. Lasing thresholds were ~0.27 and ~0.16 mJ/pulse, with 25% and 27% slope efficiencies for P567 and P556, respectively. Spectroscopy and lasing were studied in other solvents as well.

Photostability and loss mechanism of solid-state red perylimide dye lasers

The lasing photostability of the red perylimide dye (RPD) in various solid matrices was measured under frequency-doubled Nd:YAG laser excitation. The RPD: composite glass laser intensity decayed to 50% of its initial value after approximately 20,000 pump pulses of 13 mJ/pulse. The output of RPD:ormosil glass and RPD:PMMA glass lasers decayed to 50% of their initial value after 1,200 and 1,000 pump pulses of the same energy, respectively. For rhodamine-6G:silica-gel and rhodamine-6G:ormosil glass lasers, the 50% decay occurred already after 1,000 and 300 pulses, respectively. The decay was non-exponential, suggesting that the dye bleaching was not a single-photon process. The average laser output decay rates increased linearly with the pump energy.Singlet-singlet excited state absorption of the RPD dye in the solid matrices was also measured between 550 and 730 nm. At ∼600 nm the cross section was ∼2×10−16 cm2/molecule. The excited-state absorption competes with the lasing, and is a main factor that limits the laser efficiency.

Nanocrystals of PbSe core, PbSe/PbS, and PbSe/PbSe x S 1-x core/shell as saturable absorbers in passively Q-switched near-infrared lasers

The saturable optical absorption properties of PbSe core nanocrystals (NCs), and their corresponding PbSe/PbScore/shell and PbSe/PbSexS(1-x) core/alloyed-shell NCs, were examined at lambda = 1.54 microm. Saturation intensities of approximately 100 MW/cm2 were obtained. The NCs act as passive Q switches in near-infrared pulsed lasers. Q-switched output pulse energies up to 3 mJ, with a pulse duration of 40-55 ns were demonstrated. Analysis of the optical transmission versus pulse light intensity was carried out according to a model that includes ground-state as well as excited-state absorption. For pulses approximately 10 ns long, the NCs act as fast saturable absorbers. The theoretical fits yield a ground-state absorption cross section of 10-16-10-15 cm2, an excited-state absorption cross section of sigma(es) is congruent to 10(-16) cm2, and an effective lifetime of tau(eff) is congruent to 5 x 10(-12) s.

The perylene derivative BASF-241 solution as a new tunable dye laser in the visible

Basic optical properties of the perylene derivative BASF-241 solution in chloroform relevant to its application as a dye laser were measured. The fluorescence spectrum, excited by frequency-doubled (532-nm) Nd:YAG laser pulses, shows three pronounced peaks centered at 540, 580, and 630 nm, with a Stokes shift and mirror image in relation to the absorption spectrum. The quantum yield was about a unity, with a lifetime of 5.0+or-0.5 ns. Addition of trichloroacetic acid (CCl/sub 3/COOH) causes a red shift of absorption and fluorescence spectra by about 750 cm/sup -1/-L/mol. In a cavity consisting of a approximately 100% reflecting back mirror and an 8% reflecting output coupler, lasing was obtained at 577 nm, with a threshold of approximately 10 mu J/pulse. Using a grating, a tuning range of almost 20 nm was obtained, centered at the 0-1 fluorescence emission peaks. >