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We report on organic semiconductor lasers based on spiro-terphenyl as active gain medium in a second-order distributed feedback (DFB) structure. In contrast to neat p-terphenyl, the spiro-linked compound forms vitreous amorphous films—an essential prerequisite for low loss laser structures. Spiro-terphenyl shows amplified spontaneous emission in the ultraviolet part of the spectrum at 381 nm. Thin-film lasers with DFB grating periods between 200 and 230 nm exhibit emission wavelengths between 361.9 and 393.8 nm. The minimum threshold energy density under optical pumping is as low as 8.9μJ∕cm2 (383 nm). Therefore, the novel spiro-linked compound paves the way for low-threshold, widely tunable organic semiconductor lasers advancing into the ultraviolet spectral region.

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Organic distributed-feedback lasers based on spiro-linked quarterphenyl and terphenyl with a tunable emission between 361.9 nm and 395 nm are presented. As yet, spiro-terphenyl allows for both the shortest emission wavelength and low lasing thresholds. ©2005 Optical Society of America OCIS codes: (160.4890) Organic materials; (160.3380) Laser materials Amplified spontaneous emission and lasing have been observed in many organic small molecules and polymers throughout the visible spectrum [1][2][3][4][5][6]. However, only recently the first organic semiconductor laser in the ultraviolet (UV) has been demonstrated [7]. These quaterphenyl based lasers had a tuning range of 18 nm with a minimum lasing wavelength of 377.7 nm, accompanied by rather high minimum threshold densities of 62 µJ/cm 2 under optical excitation. To proceed further into the UV we employ racemic spiro-terphenyl as active material. The relevant chromophore in our compound is p-terphenyl, which itself is a well known laser dye in liquid based dye lasers in the UV. In thin films neat p-terphenyl tends to form crystallites, which effectively renders the laser devices inoperable, as the grain boundaries between adjacent micro crystallites act as scattering centers introducing significant waveguide losses. Our approach is to suppress crystallization by increasing the steric demand of the molecule via spiro-linkage of two (or more) chromophore units with a central sp 3 -carbon atom [8]. The linkage results in star shaped molecules which allow the fabrication of morphologically stable amorphous films by either spin coating or molecular beam deposition. In a 100 nm thick film the threshold for the onset of amplified spontaneous emission under pulsed optical pumping (N 2 laser, 500 ps pulses, 20 Hz repetition rate) was determined to be 0.5 µJ/cm 2 (collimated excitation beam). With this material we succeeded to realize thin-film second-order DFB lasers operating in the ultraviolet between 361.9 nm and 393.8 nm. The modulation period of the DFB grating on the substrates ranged from 200 nm to 230 nm. To the best of our knowledge, the emission at 361.9 nm is the shortest lasing wavelength for organic thin film lasers reported to date. The lasers exhibit a large tuning range of almost 32 nm, unsurpassed by inorganic semiconductor lasers in this spectral region. The minimum threshold is as low as 8.9 µJ/cm 2 at λ=383 nm, almost an order of magnitude lower compared to the quaterphenyl based thin film lasers. Low threshold operation is a crucial issue, both to increase the operating lifetime and for future electrical operation of the devices. The pulse powers of the organic lasers reach approx. 7 W, which we show is sufficient to excite fluorescent dye solutions for spectroscopic applications. A potentially cost effective mass production of the organic lasers through the use of micro-molded plastic DFB substrates is discussed.