Influence of the excitation area on the thresholds of organic second-order distributed feedback lasers

Eva M. Calzado, José M. Villalvilla, Pedro G. Boj, José A. Quintana, Victor Navarro-Fuster, Aritz Retolaza, Santos Merino, María A. Díaz-García
2012 Applied Physics Letters  
It is shown that the optical pump power (or energy) density thresholds required to obtain lasing from organic second-order distributed feedback lasers, increase when the excitation area (A) is smaller than a certain value (A crit ). So, in order to obtain the minimum possible thresholds and to ensure that they constitute adequate quantities for comparison purposes, the condition A > A crit should be fulfilled. Results also indicate that when A < A crit (A crit $ 0.1 mm 2 for the devices studied
more » ... the devices studied here), the operational device lifetime, which depends mainly on the pump power (or energy) density, becomes drastically reduced. V C 2012 American Institute of Physics. [] Organic solid-state lasers (OSLs) have been widely investigated due to the advantages of easy processability, chemical versatility, wavelength tuneability, and low cost offered by organic materials. 1,2 The interest in OSLs increased with the discovery of stimulated emission in optically pumped semiconducting polymer films, 3,4 since they opened the possibility of using electrical excitation. The goal of obtaining laser diodes was initially the main motivation to decrease the laser thresholds of OSLs, so many works focused in improving the active materials and the resonators. Although diode lasers have not been demonstrated yet, thanks to all these efforts, laser thresholds have been decreased so much that today it is possible to pump with cheap inorganic diode lasers 1,5 and even with light emitting diodes. 6 Therefore, these low-cost and compact optically pumped lasers are by themselves useful for applications. 1,2 Among the various types of OSLs reported in the literature, distributed feedback (DFB) lasers have been particularly successful. 1,2 So today they are being used to develop applications in the fields of telecommunications, 2 biosensing, and chemical sensing. 7, 8 In DFB lasers, the active material is deposited as a thin film over an appropriate substrate so it constitutes a waveguide. Feedback is achieved by the incorporation of periodic nanostructures (obtained by modulating either the refractive index or the gain) that Bragg-scatter the light, thus, avoiding the need of good-quality end facets. In a one-dimensional (1D) DFB laser, the wavelength that satisfies the Bragg condition (k Bragg ) given by (1) where m is the order of diffraction, n eff is the effective refractive index of the waveguide, and K is the grating period, constitutes the resonant wavelength in the cavity, which will then be diffracted in the grating in different directions. For second-order DFBs (m ¼ 2 in Eq. (1)), light is coupled out of the film in a direction perpendicular to the waveguide film, by first-order diffraction. DFB resonators can be easily integrated into planar organic waveguides, which is a clear advantage from the fabrication point of view, as compared to other types of laser cavities. In general, OSLs demand very intense pumping conditions due to the short photoluminescence (PL) lifetimes of the active materials (typically 1 ns in the case of fluorescent materials). So, excitation is performed by tightly focusing the pump beam through the gain medium provided by a pulsed laser source. Many of the milestones in achieving low thresholds (expressed as energy per pulse) reported in the literature have implied the use of very small excitation sizes. Thresholds in the nJ/pulse 5,9,10 and even in the pJ/pulse 11,12 regimes have been obtained, with areas typically between 0.03 and 0.1 mm 2 . From a practical point of view, expressing the threshold as energy per pulse is convenient since one motivation to decrease the threshold (besides the final goal of achieving a laser diode) has been to use more compact excitation sources, which generally provide pulses of lower energy. However, in order to properly assess the threshold performance of a certain device in comparison to others, the threshold should be given as energy density (energy per pulse divided by the area of the excitation beam over the sample), or better as power density (by dividing also by the temporal pulse width). 13 It should be noted that in order to use these latter units, stationary conditions should apply, which means that the pulse duration should be larger than the PL lifetime. Very often, all these considerations have not been taken into account in the literature, partly due to the already mentioned convenience to use energy per pulse units. As a result, it is often not obvious to establish rankings of devices in terms of threshold. Fortunately, many groups provided their threshold values in various units, being aware that in order to compare their thresholds with those of other devices, power density or energy density units should be used. 6,10-14 a)
doi:10.1063/1.4768242 fatcat:gsg4ahlwgrgtbg6veuqzfnowwa