Ultrafast Laser Energy Density and Retinal Absorption Cross-Section Determination by Saturable Absorption Measurements

Abstract

Laser pulse nonlinear transmission measurements through saturable absorbers of known absorption parameters allow the measurement of their energy density. On the other hand , nonlinear transmission measurements of laser pulses of known energy density through absorbing media allow their absorption parameter determination. The peak energy density w 0P of second harmonic pulses of a mode-locked titanium sapphire laser at wavelength λ P = 400 nm is determined by nonlinear energy transmission measurement T E through the dye ADS084BE (1,4-bis(9-ethyl-3-car - bazovinylene)-2-methoxy-5-(2’-ethyl-hexyloxy)-benzene) in tetrahydrofuran. T E ( w 0 P ) calibration curves are calculated for laser pulse peak energy density reading w 0P from measured pulse energy transmissions T E . The ground-state absorption cross-section σ P and the excited-state absorption cross-section σ ex at λ P , and the number density N 0 of the retinal Schiff base isoform RetA in pH 7.4 buffer of the blue-light adapted recombinant rhodopsin fragment of the histidine kinase rhodopsin HKR1 from Chlamydomonas reinhardtii were determined by picosecond titanium sapphire second harmonic laser pulse energy transmission measurement T E through RetA as a function of laser input peak energy density w 0P . The complete absorption cross-section spectrum σ