All-optical modulation by continuous wave light using two-color excited-state absorption at visible wavelengths

Abstract

We demonstrated all-optical modulation with a nonlinear medium, i.e., indigo carmine, an aromatic conjugated structure with delocalized π-electrons, using non-high power continuous wave light for pump and probe of different visible wavelengths. Pump-induced probe transmission increase occurred through absorption saturation of probe light by pump-induced linear and nonlinear absorption including two-color excited-state absorption (ESA). The two-color ESA occurred only when both pump light and probe light co-propagated through a medium, leading to nearly pump power-independent increase in probe transmission for appropriately chosen wavelengths of pump and probe light, given the optical transition structure of electronic energy levels in the medium.

Anomalous spectral shift of localized surface plasmon resonance

“…Our group previously reported n 2 ∼ −1.0 × 10 −7 cm 2 W −1 of IC solutions (∼1 mM) at 405 nm, which was higher than those of silica fibers or inorganic semiconductors, and its large magnitude was due to delocalized π electrons in the aromatic conjugation structure of IC molecules. It was also noted that n 2 was negative at near 405 nm, 23–25 which was close to the AgNP plasmon wavelengths (400–420 nm) of the current studies.…”

“…Our group previously reported n 2 ∼ −1.0 × 10 −7 cm 2 W −1 of IC solutions (∼1 mM) at 405 nm, which was higher than those of silica bers or inorganic semiconductors, and its large magnitude was due to delocalized p electrons in the aromatic conjugation structure of IC molecules. It was also noted that n 2 was negative at near 405 nm, [23][24][25] which was close to the AgNP plasmon wavelengths (400-420 nm) of the current studies. When increasing the AgNP concentration in colloids, the plasmon-enhanced intensity of local elds near AgNPs enhanced n NL (<0) due to its intensity dependence, while n L remained nearly unchanged as shown in Fig.…”

“…We adopted IC as a nonlinear medium due to its large third-order optical nonlinearity at visible wavelengths. [23][24][25] Absorbance spectroscopy of colloidal AgNPs demonstrated the spectral blue shi of plasmonic peaks with increasing AgNP concentration only when IC solution was used as solvent. For a given AgNP concentration, the blue shi was also observed when increasing IC concentration, which was in contrast to the spectral peak red shi of LSPR typically observed when increasing the concentration of an ambient medium.…”