Switching light with light in chlorophyll-A molecules based on excited-state absorption.

We analyze all-optical switching in chlorophyll-A (Chl-A) molecules for different combinations of pump-probe wavelengths, based on nonlinear intensity-induced excited-state absorption. It is shown that for a pulsed pump beam at 672 nm with peak pump intensity of 5 kW/cm(2) and Chl-A concentration of 1.5 mM, the transmission of a continuous-wave probe beam at 476 nm can be completely switched off (100% modulation) with switch on-off time of 0.58 and 0.18 micros, respectively. It is also shown that the switching characteristics can be inverted by changing the probe beam wavelength. The effect of various parameters, such as concentration, pump beam intensity, pump pulse width, absorption cross section of the ground state, and lifetimes of different states, on the switching characteristics has been analyzed in detail. It is shown that there exists an optimum value of concentration of Chl-A for maximum switching contrast, for the case when the ground state also absorbs the probe beam. The switching characteristics of Chl-A have also been compared with Chl-B and Bchl. Experimental results for all-optical switching in Chl-A with a train of pulses are in good agreement with theoretical results. It is shown that higher contrast and faster switching can be achieved as opposed to what was reported recently in other biomolecules such as archael rhodopsin and phototropin proteins. The results have also been used to design switches and logic gates.

All-optical switching in plant blue light photoreceptor phototropin.

We theoretically analyze all-optical switching in the recently characterized LOV2 domain from Avena sativa (oat) phot1 phototropin, a blue-light plant photoreceptor, based on nonlinear intensity-induced excited-state absorption. The transmission of a cw probe laser beam at 660 nm corresponding to the peak absorption of the first excited L-state, through the LOV2 sample, is switched by a pulsed pump laser beam at 442 nm that corresponds to the maximum initial D state absorption. The switching characteristics have been analyzed using the rate equation approach, considering all the three intermediate states and transitions in the LOV2 photocycle. It is shown that for a given pump pulse intensity, there is an optimum pump pulsewidth for which the switching contrast is maximum. It is shown that the probe laser beam can be completely switched off (100% modulation) by the pump laser beam at 50 kW/cm2 for a concentration of 1 mM with sample thickness of 5.5 mm. The switching characteristics are sensitive to various parameters such as concentration, rate constant of L-state, peak pump intensity and pump pulse width. At typical values, the switch-off and switch-on time is 1.6 and 22.3 micros, respectively. The switching characteristics have also been used to design all-optical NOT and the universal NOR and NAND logic gates.