Laser Oscillation of Organic Distributed-Feedback Lasers at the Edge of a Mini Stopband

Organic semiconductor lasers have continued to attract attention because of their broad spectrum and diversity of materials, covering all wavelengths from UV to near IR.[1] In recent years, laser oscillation by current pumping, which does not require an external pumping source, has been actively studied. As a resonator structure for this purpose, DFB (Distributed Feedback) resonators with a single longitudinal mode and low threshold are often used.[2, 3] Although most of the laser oscillation wavelengths can be explained by the Bragg condition, several experimental results have been reported for polymer DFB lasers doped with organic semiconductors that do not match the Bragg condition in any mode.[4]<br/>Here, we report the conditions that determine the oscillation wavelength of organic DFB lasers using 5,5″-bis(biphenyl-4-yl)-2,2′:5′,2″-terthiophene (BP3T) single crystals, a promising material for injection lasers,[5-9] with various thicknesses as an active layer. Analysis using experimentally obtained refractive indexes revealed that the oscillation wavelengths of some BP3T DFB lasers are at the edge of a stopband, while the others are at an edge of the mini stopband,[10] which satisfies the phase-matching condition between different transverse modes and has not been considered in DFB lasers so far. Finite-element optical-waveguide simulations of the BP3T DFB laser verified the coupling of different transverse modes.<br/><br/>[1] H. Shang et al., "Comparative Study of Single and Dual Gain-Narrowed Emission in Thiophene/Furan/Phenylene Co-Oligomer Single Crystals", J. Phys. Chem. C 121, 2364 (2017).<br/>[2] T. Kanagasekaran et al., "Towards electrically driven organic semiconductor laser with field-effective transistor structure", arXiv:1903.08869 [physics.optics] (2019).<br/>[3] A. S. D. Sandanayaka et al.,"Indication of current-injection lasing from an organic semiconductor" Appl. Phys. Express 12, 061010 (2019).<br/>[4] V. Navarro-Fuster et al., "Film thickness and grating depth variation in organic second-order distributed feedback lasers", J. Appl. Phys. 112, 043104 (2012).<br/>[5] S. Z. Bisri et al., "High Mobility and Luminescent Efficiency in Organic Single-Crystal Light-Emitting Transistors", Adv. Funct. Mater. 19, 1728 (2009).<br/>[6] K. Oniwa et al., "Single crystal biphenyl end-capped furan-incorporated oligomers: influence of unusual packing structure on carrier mobility and luminescence", J. Mater. Chem. C 1, 4163 (2013).<br/>[7] H. Tamura et al.,"Theoretical Analysis on the Optoelectronic Properties of Single Crystals of Thiophene-furan-phenylene Co-Oligomers: Efficient Photoluminescence due to Molecular Bending", J. Phys. Chem. C 117, 8072 (2013).<br/>[8] T. Kanagasekaran et al., "Equivalent ambipolar carrier injection of electrons and holes with Au electrodes in air-stable field effect transistors", Appl. Phys. Lett. 107, 043304 (2015).<br/>[9] T. Kanagasekaran et al., "A new electrode design for ambipolar injection in organic semiconductors", Nature Commun. 8, 999 (2017).<br/>[10] T. Miura et al., "Laser oscillation of an organic distributed-feedback laser at the edge of a mini stopband", Appl. Phys. Express 14, 052007 (2021)