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

Abstract

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 here), the operational device lifetime, which depends mainly on the pump power (or energy) density, becomes drastically reduced. 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. 1,2Among 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 bywhere 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...

Nanoimprinted Organic Semiconductor Laser Pumped by a Light‐Emitting Diode

“…The laser threshold density is higher than the measured ASE threshold above because of the smaller excitation region [ 30 ] available from LED pumping, and slower pulse rise time of the LED. Nevertheless, this is the lowest threshold density for a UV-NIL organic laser and enables for the fi rst time a nanoimprinted laser to be LED-pumped, resulting in a compact and convenient laser source.…”

“…[ 30 ] We thereby show that it is possible to demonstrate nanoimprinted lasers that are directly powered by a single LED. The use of this simple pump source is a direct consequence of the very low lasing threshold of 0.77 kW cm − 2 , the fi rst time a UV-NIL organic laser has been demonstrated to have a threshold density below 8 kW cm − 2 .…”

Distributed feedback lasers based on perylenediimide dyes for label-free refractive index sensing

“…The excitation and collection geometry (see Fig. 1(a)) differs slightly from the one used in previous works to characterize PDI-based DFB lasers [35], since here excitation and light emission collection is done through the substrate, with the sample placed horizontally with respect to the optical table to facilitate the deposition of liquid superstrates. This geometry, which requires the use of a transparent substrate, avoids disturbing the analyte with the pump beam.…”

“…However, a problem of using a very thin active film in the DFB laser is that its threshold increases due to its lower absorption and therefore emitted light intensity, and also due to a poor confinement of the waveguide mode [34]. Also the operational lifetime would decrease, given that the device needs more pump intensity to operate [28,35]. While strategies towards sensitivity improvement have been previously explored, less attention has been devoted to the effect of these strategies on the device laser threshold and operational lifetime.…”

“…Indeed, the very large ASE photostability half-life value of 2 × 10 5 pump pulses (under excitation two times above the ASE threshold) of the 850 nm-thick film decreases by around one order of magnitude in the thinnest film. But this difference is not a material property dependent on film thickness, but a direct consequence of the higher threshold of the thin film, which requires a higher pumping intensity for operation and this parameter affects critically the photostability performance [28,35].…”