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Proceedings Paper

Narrow linewidth emissions from organic crystals with diffraction gratings engraved directly on their surface (Conference Presentation)
Author(s): Hiroyuki Yamamoto; Takeshi Yamao; Shu Hotta

Paper Abstract

In combination with optical cavities, organic semiconductor crystals are powerful candidates for current-injected organic laser devices. For such cavities diffraction gratings are applicable. In common with external diffraction gratings made of dielectrics and oxides, built-in ones are effective for producing narrow linewidth emissions. However, direct fabrication of the diffraction gratings on the surfaces of the organic crystals is still challenging. In the present studies, we directly engraved one-dimensional (1D) diffraction gratings on the flat surfaces of organic semiconductor crystals by using focused ion beam (FIB) lithography, and shaped a distributed feedback resonator (DFB) structure. We chose as the organic semiconductor material 5,5'''''-diphenyl-2,2':5',2'':5'',2''':5''',2'''':5'''',2'''''-sexithiophene (P6T) from among thiophene/phenylene co-oligomers. We grew plate-like crystals of P6T in a vapor phase. The P6T crystals showed emissions with a maximum intensity around 630 nm. We laminated them on Si wafer substrates covered with 300-nm-thick silicon dioxide and 160-nm-thick Al-doped ZnO (AZO) layers. The AZO layer was used to prevent the crystals from being charged during the FIB lithography. We precisely controlled FIB process conditions and obtained the gratings having the equally-spaced 200 (400) grooves with the periods of 240 (200) nm. When we observed the emissions perpendicular to the grating grooves as well as parallel to the crystal surface under ultraviolet light from a mercury lamp, these crystals with the 1D diffraction gratings indicated narrowed emission peaks at 745 (670) nm. From the grating period and the emission peak position, we estimated an order of diffraction and an effective refractive index. We related the effective refractive index with a phase refractive index of the crystal and decided the optimum grating period to be 190 nm to produce the narrowed peak at 630 nm. We fabricated the diffraction grating having the optimum grating period on the P6T crystal by engraving 400 grooves, and succeeded in observing strong single peak at 629 nm. Using the present method, we can design the diffraction grating periods for the optical devices of organic oligomer crystals that show the narrow linewidth emissions.

Paper Details

Date Published: 5 December 2016
PDF: 1 pages
Proc. SPIE 9994, Optical Materials and Biomaterials in Security and Defence Systems Technology XIII, 99940F (5 December 2016); doi: 10.1117/12.2243450
Show Author Affiliations
Hiroyuki Yamamoto, Kyoto Institute of Technology (Japan)
Takeshi Yamao, Kyoto Institute of Technology (Japan)
Shu Hotta, Kyoto Institute of Technology (Japan)

Published in SPIE Proceedings Vol. 9994:
Optical Materials and Biomaterials in Security and Defence Systems Technology XIII
Roberto Zamboni; François Kajzar; Attila A. Szep; Katarzyna Matczyszyn, Editor(s)

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