Variable ultrafast optical nonlinearity in bacteriorhodopsin achieved through simple chemical treatment

Abstract

Tunability of the third-order optical nonlinearity and optical limiting action of bacteriorhodopsin (BR) was achieved by altering the native structure through simple surfactant treatment like cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS) and Triton X-100. The variation in spectral position and intensity of the UV–visible absorption spectrum was due to aggregation (hyperchromic lipid absorption—280 nm), solubilization (redshift in chromophore absorption—600 nm, formation of retinal—396 nm) and monomerization (blueshift in chromophore absorption—553 nm) effects upon interaction with anionic, cationic and non-ionic surfactants. Further, changes in the Raman vibrational modes confirmed the alteration in C=C ethylenic chain and 13-cis retinal on surfactant treatment. Surfactant dose-dependent assay of nonlinear optical parameters was evaluated by using the Z-scan technique with ultrashort laser pulse excitation (800 nm, ~150 fs, 80 MHz). The closed aperture data demonstrated self-defocusing nature and notably higher monomerization (0.15–0.25 mM) effects endorsed sign reversal (self-defocusing to self-focusing) of the nonlinear refractive index coefficient. Bare BR exhibited saturable absorption, while reverse saturable absorption (RSA) was observed for all surfactant-treated samples (expect for 0.20 mM CTAB and 1.0 mM SDS). The observed RSA and optical limiting was ascribed due to two-photon absorption process. Interestingly, we observed that aggregation enhanced the nonlinear refraction, monomerization improved nonlinear absorption (NLA), and solubilization reduced the nonlinear optical (NLO) response of BR. The maximum nonlinear refractive index (n 2 = 8.78 × 10−19 m2/W) was observed for 0.20 mM CTAB, while the NLA coefficient (β = 4.40 × 10−12 m/W) and low optical limiting threshold (538 µJ/cm2) were observed to be highest for 0.25 mM TX-100 sample. The influence of aggregation, monomerization and solubilization effects of purple membrane in achieving the variation of the NLO coefficients is discussed.