Amplified spontaneous emission and optical gain characteristics of sexithiophene single crystals

This paper reports optical properties of a single crystal thiophene/phenylene co-polymer 5,5""'-diphenyl-2,2':5',2":5",2"':5"',2"":5"", 2""-sexithiophene (P6T). The P6T crystals demonstrate high charge carriers mobility and large photoluminescence quantum efficiency (PLQE). Herein, the stimulated emission of P6T is studied within wide range of pump energies - refers to energy per pulse - (from 19 +/- 0.5 mu J/cm(2) to 64.2 +/- 0.5 mu J/cm(2)) through amplified spontaneous emission (ASE) measurements by using Nd-YAG femtoseconds tunable pulsed laser at 445 nm excitation wavelength (lambda(exc)). ASE measurements exhibit lower threshold energy density (E-th) i.e., similar to 26 +/- 0.5 mu J/cm(2) which confirms optical amplification in P6T at 632 nm emission wavelength. The ASE characteristics of P6T are also studied as a function of different lambda(exc) ranging from 425 to 475 nm to find the wavelength for optimum amplification. It is observed that the maximum optical amplification for P6T occurs at a lambda(exc) of 445 nm. These crystals exhibit large optical gains which are measured to be 69 cm-1, 53 cm-1 and 29 cm-1 at excitation energies of 91 +/- 0.5 mu J/cm(2), 66 +/- 0.5 mu J/cm2 and 43 +/- 0.5 mu J/cm(2), respectively. PL measurements of P6T crystals demonstrate large value of the PLQE similar to 57.36%. This study suggests the potential of P6T single crystal as one of the good candidates among high optical gain materials of its kind for its possible applications in solid-state lasers, photodetectors and optical amplifiers.