Evaluation of Linear and Nonlinear Optical Properties of D−π–A Type 2‑Amino-5-Nitropyridinium Dihydrogen Phosphate (2A5NPDP) Single Crystal Grown by the Modified Sankaranarayanan–Ramasamy (SR) Method for Terahertz Generation

We report the growth process and linear and nonlinear optical characterization of high quality improvised semiorganic 2A5NPDP single crystal using the modified SR method for the first time. The major difficulties for the growth of 2A5NPDP single crystal have been resolved with the aid of a temperature lowering method. After optimization of the growth conditions, a transparent 2A5NPDP single crystal was harvested with a size of 85 mm length and 10 mm diameter over a period of 60 days. The growth rate of the modified SR method grown crystal was found to be 10 times higher than that of the conventional slow evaporation solution technique (SEST) grown crystal. The unit cell parameters of the grown crystal were ascertained by the single crystal X-ray diffraction (SXRD) and powder X-ray diffraction (PXRD) techniques. The high crystalline perfection of the crystal was ascertained by using a high-resolution X-ray diffraction (HRXRD) technique. The presence of multifarious functional groups of the 2A5NPDP crystal were confirmed by Fourier transform infrared (FT-IR) and Fourier transform Raman (FT-Raman) spectral analyses. The thermal property was assessed by the thermogravimetric (TG) and differential thermal analysis (DTA). It shows that the grown crystal withstands temperature up to 175 °C. UV–vis–NIR analysis was used to measure the linear optical property while third-order nonlinear optical properties were ascertained using Z-scan and optical limiting (OL) techniques. The optical limiting threshold of conventional method grown crystal is 5.7 mW/cm2, whereas the modified SR method grown crystal is 4.7 mW/cm2. Low value of limiting threshold indicates that the grown crystal has minimum defects. Finally, 800 nm wavelength of 140 fs at 80 MHz repetition rate obtained from a Ti:sapphire oscillator was used for the terahertz (THz) generation between 0.1 and 2.1 THz range. The efficiency of the generated THz signal was measured using another 800 nm wavelength of 40 fs, at 1 kHz repetition rate obtained from a Ti:sapphire amplifier. The generated THz signal is separated out from unconverted 800 nm using Teflon filters, and power is measured using a pyrometer. The maximum conversion efficiency of generated THz signal is on the order of 0.63 × 10–5%.