Saturable absorption, optical limiting and ultrafast laser-induced two-photon fluorescence in Caesalpinia sappan (sappanwood) natural dye

Abstract

In this work, we extract the natural dye present in the bark of the medicinal tree Caesalpinia Sappan (sappanwood) and investigate its linear and nonlinear optical (NLO) properties. Nonlinear refraction and absorption are measured by Z-scan at 532 nm using continuous wave and pulsed (5 ns) Nd:YAG lasers, and two-photon fluorescence is measured using 800 nm ultrafast (100 fs) pulses obtained from a Ti:Sapphire laser. The dye exhibits a marked transition from saturable to reverse saturable absorption depending on its concentration and laser intensity. Nonlinear refraction and saturable absorption (SA) are observed under CW excitation at relatively lower laser intensities (≈10⁴ W/m²) whereas reverse saturable absorption (RSA) is observed at higher intensities (≈10⁶ W/m²). Nonlinear refractive indices (n₂) are in the order of 10⁻⁹ m²/W, saturation intensities (I_s) are in the order of 10⁵ W/m², and effective nonlinear absorption coefficients (β) are in the order of 10⁻⁵ to 10⁻³ m/W. For pulsed excitation, SA is observed for intensities in the range of 10¹⁰–10¹² W/m² while RSA is seen at 10¹³ W/m² β values obtained for pulsed excitation are in the range of 10⁻¹¹ to 10⁻¹⁰ m/W. Measured two-photon fluorescence spectra resemble the normal fluorescence spectrum of the dye. These NLO results are comparable to those reported for efficient NLO materials in literature, indicating the potential application of Caesalpinia Sappan dye in the generation of short laser pulses, optical switching, optical limiting for eye and sensor protection, and imaging of tissues and other biological matter.