Broadband Optical Limiting and CCD Sensor Protection From Nanosecond-Pulsed Laser Threat With Reverse Saturable Absorber
- PDF / 1,266,180 Bytes
- 7 Pages / 414.72 x 648 pts Page_size
- 104 Downloads / 219 Views
ABSTRACT We study in detail the linear and nonlinear optical properties of a heavy metal substituted naphthalocyanine reverse saturable absorber (RSA). The excited states involved in the RSA process under nanosecond pulsed laser excitation in the visible are fully characterized. We then demonstrate that this material can be used as an efficient optical limiter for protecting silicium CCD arrays from destruction due to nanosecond-pulsed laser irradiation in the visible. INTRODUCTION Metal-substituted phthalocyanines and naphthalocyanines are known to exhibit reverse saturable absorption (RSA) properties with (sub)nanosecond response time in the visible and have been studied in the past few years as potential candidates for optical limiting applications [1-4]. It is known that heavy metal substitution of phthalocyanines increases the inter-system crossing rate [4], and that naphthalocyanine derivatives present Q-band absorption maximum red-shifted as compared to the corresponding phthalocyanine derivative. These interesting features in view of nanosecond optical limiting in the visible are unfortunately often couterbalanced by the relatively low solubility and stability of such compounds. We synthesized a soluble tin naphthalocyanine (SnNc) with good photostability which presents a Q absorption band peaked at 790 nm and a region of weak absorption from 450 to 650 nm. In the following, we investigate in detail the linear and nonlinear absorption properties of this molecule. We then demonstrate that a solution of SnNc with 60% photopic transmission, inserted in the intermediate focal plane of a laboratory built magnifying sight, increases the incident energy leading to surface damage of a CCD array by a factor > 60. EXPERIMENTAL The SnNc molecul6 is dissolved in distilled tetrahydrofuran and placed in quartz cells with different path lengths and concentrations ranging from 5 10-6 up to 5 10- 3 M/l. Solutions are not degassed before measurements. Ground state spectroscopy, The transmission spectra of the solutions are recorded on a dual beam spectrometer (PerkinElmer Lambda-9). Values of the ground-state absorption cross-sections GG are extracted from measurements performed on 1 cm path-length cells on dilute solutions (5 10-6 to 5 10-5 M/1). Absorption spectra recorded for concentrations up to 5 10-3 M/1 indicate that no aggregation occurs at high concentrations. Moreover, from the linear absorption spectrum of a 2 mm thick cell of an undegassed solution with a concentration = 5 10- 4 M/I , a degradation of only = 14% can be estimated after a period of 5 months when the cell is kept in the dark. Photo-induced nanosecond transient spectroscopy, Laser flash photolysis is performed in a crossed beam configuration to infer the triplet-triplet absorption spectrum. Triplet states are generated by pumping the solution with a tripled Q-switch 261 Mat. Res. Soc. Symp. Proc. Vol. 479 01997 Materials Research Society
Nd:YAG laser (k = 355 nm, pulse duration - 10 ns, repetition rate 0.5 Hz) and the transient absorption is probed
Data Loading...
