These papers report on the first successful observation of coherent emission from organic gain media using direct electric excitation. This mode of excitation is also known as electrical excitation, or, electrical pumping.
The interferogram to the left corresponds to the emission of a high-power optically-pumped coumarin 545 tetramethyl dye laser. Under identical geometrical conditions the interferogram to the right corresponds to the emission from the electrically-pumped sub-micrometer-cavity organic semiconductor using coumarin 545 tetramethyl dye as gain medium. The emission wavelength is ~ 540 nm. These interferograms were recorded on silver-halide film (from Duarte et al., Opt. Lett. 30, 3072-3074 (2005)).
Note: the organic semiconductors used in these experiments are tandem organic light-emitting diodes also known as tandem oleds.
Basics on Lasers and Coherence
"Therefore the occurrence of fringes must be caused by the spatial coherence present during maser action." (D. F. Nelson and R. J. Collins, Spatial coherence in the output of an optical maser, J. Appl. Phys. 32, 739-740 (1961)).
"A conventional source is spatially almost completely incoherent... such a source gives rise to an intensity distribution in the far zone that changes slowly with direction, i.e. the field exhibits little dependence on direction. On the other hand a source with a high degree of spatial coherence, such as a laser, generates light that is highly directional." (E. Wolf and W. H. Carter, Opt. Commun. 13, 205-209 (1975)).
This paper reports on highly directional emission in a near-Gaussian beam, with a divergence of ~ 1.1 times the diffraction limit, and on high visibility interferograms originating from the same emission. The interferometric emitter generating this beam comprises an electrically-excited coumarin 545 tetramethyl-doped semiconductor confined within a sub-micrometer resonator. This beam profile was originally recorded on silver-halide film.
F. J. Duarte, Coherent electrically-excited organic semiconductors: visibility of interferograms and emission linewidth,Opt. Lett. 32, 412-414 (2007).
The upper interferogram corresponds to the emission from the electrically-excited organic semiconductor and has a visibility of V ~ 0.9. Using the same interferometric configuration an interferogram generated with radiation from a He-Ne laser (at 543.3 nm) is included for comparison. Using an interferometric approach the linewidth from the electrically-excited organic semiconductor was determined to be ~ 11 nm. Threshold behavior is observed at a current density of ~ 0.9 A/square cm.
In this paper the spatial and spectral characteristics of coherent, or laser, emission are discussed. It is indicated that the radiation from the electrically-excited doubly interferometrically confined organic semiconductor emitter corresponds to single-transverse-mode and single-longitidinal-mode emission. Further the visibility of the interferograms are found to be well within range of the visibility of laser radiation. In summary the emission is highly directional, low diverence, with the spectral properties of dye laser emission.
Note: coherent emission from an electrically pumped organic semiconductor device, also doped with a tetramethyl dye, has been subsequently reported by X. Liu et al., Opt. Lett. 34, 503-505 (2009). Using a similar experimental arrangement these authors report on visibilities of V = 0.89, at ~ 622 nm. A critical commentary to the paper of Liu et al. has been published by I. D. W. Samuel et al., Nat. Photon. 3, 546-549 (2009), and more recently by Duarte (2010).
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