Effect of probe beam intensity on all-optical switching based on excited-state absorption

We theoretically analyze the effect of probe beam intensity on all-optical switching based on nonlinear absorption, using the pump-probe configuration. To draw general inferences that are applicable to a wide range of polyatomic molecules, we consider as a typical example, switching in pharaonis phoborhodopsin (ppR) protein and its mutants that exhibit a complex photocycle similar to bacteriorhodopsin (bR), having a number of intermediates with respective absorption spectra spanning the entire visible region. The switching of the transmission of a cw probe beam by a pulsed pump beam has been studied in detail at different wavelength combinations. Interesting consequences emerge from the present analysis. It is shown that by controlling the probe intensity, the switching characteristics can be inverted, switching time can be reduced and the profile of the switched probe beam and the switching contrast can be controlled. For some cases, the switching contrast can also be maximized by optimizing the probe intensity. Increase in probe intensity also leads to increase in switching contrast under certain conditions. At particular spectral and kinetic conditions, the nonlinear optical material appears linear for a given probe intensity and pump-probe wavelengths, respectively. Variation in probe intensity thus provides an effective means to modify the switching characteristics instead of using mutants with different rate constants for a variety of nonlinear absorption based all-optical devices

Design of Ultrafast All-Optical Pseudo Binary Random Sequence Generator, 4-bit Multiplier and Divider using 2 x 2 Silicon Micro-ring Resonators

All-optical devices are essential for next generation ultrafast, ultralow-power and ultrahigh bandwidth information processing systems. Silicon microring resonators (SiMRR) provide a versatile platform for all-optical switching and CMOS-compatible computing, with added advantages of high Q-factor, tunability, compactness, cascadability and scalability. A detailed theoretical analysis of ultrafast all-optical switching 2 x 2 SiMRRs has been carried out incorporating the effects of two photon absorption induced free-carrier injection and thermo optic effect. The results have been used to design simple and compact all-optical 3-bit and 4-bit pseudo-random binary sequence generators and the first reported designs of all-optical 4 x 4-bit multiplier and divider. The designs have been optimized for low-power, ultrafast operation with high modulation depth, enabling logic operations at 45 Gbps.Comment: 13 pages, 4 figures. Submitted at Journal (Optik) for publicatio

Recent Advances in Optogenetic Retinal Prostheses

Optogenetics has emerged as a revolutionary technology that enables circuit-specific restoration of neuronal function with millisecond temporal resolution. Restoring vision is one of the most promising and forefront applications of optogenetics. This chapter discusses essential components, mechanisms, present challenges, and future prospects of optogenetic retinal prostheses. The theoretical framework and analysis of optogenetic excitation of retinal ganglion neurons are also presented, which are useful in developing a better understanding and guidance for future experiments. It shows that the newly discovered ChRmine opsin provides control at light powers that are two orders of magnitude smaller than that required with experimentally studied opsins that include ChR2, ReaChR, and ChrimsonR, while maintaining single-spike temporal resolution, in retinal ganglion neurons

All-optical switching in Pharaonis phoborhodopsin protein molecules.

Low-power all-optical switching with pharaonis phoborhodopsin (ppR) protein is demonstrated based on nonlinear excited-state absorption at different wavelengths. A modulating pulsed 532-nm laser beam is shown to switch the transmission of a continuous-wave signal light beam at: 1) 390 nm; 2) 500 nm; 3) 560 nm; and 4) 600 nm, respectively. Simulations based on the rate equation approach considering all seven states in the ppR photocycle are in good agreement with experimental results. It is shown that the switching characteristics at 560 and 600 nm, respectively, can exhibit negative to positive switching. The switching characteristics at 500 nm can be inverted by increasing the signal beam intensity. The profile of switched signal beam is also sensitive to the modulating pulse frequency and signal beam intensity and wavelength. The switching characteristics are also shown to be sensitive to the lifetimes of$mmb pbf pR_bf M$and$mmb pbf pR_bf O$intermediates. The results show the applicability of ppR as a low-power wavelength tunable all-optical switch

Modelling of light modulation processes in D85N bacteriorhodopsin

A simplified model for the complex photocycle of the D85N genetic variant of the bacteriorhodopsin (bR) protein molecule is presented. Steady state population densities of the various intermediate states of the molecule induced by photo-absorption of modulation light beam are obtained using the rate equations approach. All-optical modulation of various probe signals at wavelengths corresponding to absorption peaks of each of the intermediate states by a pump signal at 570 nm is presented in the form of optical densities. The analysis presented here is useful for designing the molecular spatial light modulators using D85N variant of bR molecules

Modelling of light modulation processes in D85N bacteriorhodopsin

A simplified model for the complex photocycle of the D85N genetic variant of the bacteriorhodopsin (bR) protein molecule is presented. Steady state population densities of the various intermediate states of the molecule induced by photo-absorption of modulation light beam are obtained using the rate equations approach. All-optical modulation of various probe signals at wavelengths corresponding to absorption peaks of each of the intermediate states by a pump signal at 570 nm is presented in the form of optical densities. The analysis presented here is useful for designing the molecular spatial light modulators using D85N variant of bR molecules

Analysis of all-optical switching in bacteriorhodopsin

All-optical switching has been demonstrated in bacteriorhodopsin (bR) based on nonlinear, intensity-induced excited state absorption. The transmission of a cw probe laser beam at 640 nm corresponding to the peak absorption of O-state through a bR film, is switched by a pulsed pump laser beam at 570 nm that corresponds to the maximum initial B-state absorption. The switching characteristics have been analysed using the rate equation approach and the six intermediate states in the bR photocycle. The effect of various parameters such as pump pulse width and intensity, rate constants of M and O states and absorption cross-section of the B-state at probe wavelength on switching, have been analysed in detail. It has been shown that the probe laser beam can be completely switched off by the pump laser beam at relatively low pump powers, if the B-state does not absorb the probe beam

Generalized model for all-optical light modulation in bacteriorhodopsin

We present a generalized model for the photochemical cycle of bacteriorhodopsin bR protein molecule. Rate equations have been solved for the detailed light-induced processes in bR for its nine states: $(B\rightarrow K \leftrightarrow L \leftrightarrow M^I \rightarrow (M^{II}) \leftrightarrow N \leftrightarrow$ $O \leftrightarrow P \rightarrow Q \rightarrow B)$. The complete steady-state intensity-induced population densities in various states of the molecule have been computed to obtain a general, exact, and analytical expression for the nonlinear absorption coefficient for multiple modulation pump laser beams. All-optical light modulation of different probe laser beam transmissions by intensity induced population changes due to one and two modulation laser beams has been analyzed. The proposed model has been shown to accurately model experimental results