Film thickness and grating depth variation in organic second-order distributed feedback lasers

Abstract

We report on the preparation and characterization, under optical pump, of second-order one-dimensional distributed feedback (DFB) lasers based on polystyrene films doped with a perylenediimide derivative, as active media. The DFB gratings were engraved on the substrates (SiO2) by thermal nanoimprint lithography, followed by reactive ion etching. Laser emission wavelength was tuned from 554 to 583 nm by changing film thickness (h) between 240 and 1200 nm. The effect on the performance (emission wavelength, threshold, slope efficiency, number of modes, and spectral shape) of varying the grating depth (d) from 30 to 240 nm, for the whole range of h values, has been investigated. Although there is extensive work in the literature aiming to tune the emission wavelength of organic DFB lasers by h variation, the effect of changing d systematically has not been previously studied. Experimental results have been interpreted by models that take into account the presence of the grating by averaging either h or the effective refractive index. Single-mode emission (λ0) was observed for h < 1000 nm, while for thicker films lasing appeared at two different wavelengths (λ0 and λ1). Models indicate that λ0 and λ1 correspond to the TE0 and TE1 waveguide modes, respectively. It was found that d plays an important role in determining the DFB thresholds and slope efficiencies for two h regimes: (i) For h < 350 nm, lowest thresholds and highest slopes efficiencies were obtained with the shallower gratings; and (ii) for h > 1000 nm, d affects significantly the losses associated with the TE1 mode, so single mode emission was achieved at λ0 or at λ1 for deep and shallow gratings, respectively. Finally, the shape of the emission spectra, both below and above threshold, has also been analyzed in order to clarify the physical mechanisms responsible for the existence of gain. Bragg dips were observed in the spectra below threshold only for devices with d/h larger than around 0.3 and their width increased with increasing d/h. In these cases, single-mode DFB emission appeared at the long-wavelength edge of the Bragg dip, indicating that index-coupling modulation contributes significantly to the gain process. On the other hand, for smaller d/h values, Bragg dips became too small to be detected, so gain coupling becomes the dominant mechanism accounting for the presence of gain.

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

“…For ASE studies, samples consisting of active films deposited over bare fused silica (without gratings) were used. From previous film thickness dependence studies of the waveguide, ASE and DFB performance of PDI-doped PS films [34,38], it is known that the observed DFB laser peaks correspond to TE waveguide modes, and not to transverse magnetic (TM) ones. Lasing associated to TM modes occurs at a much higher threshold, so in order to be observed, devices would need to be pumped at larger intensities.…”

“…As previously mentioned, a drawback of using a very thin active film is that the DFB threshold becomes larger and the operational lifetime lower than those achievable with DFBs based on thicker films [28,34]. In order to improve the DFB threshold and operational lifetime, we have prepared devices with thicker active films.…”

“…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].…”

Tunable and flexible solvent-free liquid organic distributed feedback lasers

“…14,16,17 Particularly, since diffraction conditions for the second-order DFB gratings lead to surface emission (i.e., light is diffracted perpendicular to the waveguide plane), these DFB grating patterns have been of great interest in organic semiconductor lasers. 18 DFB structures also can provide an excellent framework for flexible and tunable lasers if proper materials are used. [19][20][21][22][23][24] Here we demonstrate tunable and flexible optically pumped liquid organic DFB lasers based on solvent-free fluidic organic semiconductors.…”

Broadly tunable (440–670 nm) solid-state organic laser with disposable capsules

“…an organic semiconductor distributed feedback (DFB) laser, will have a lowloss resonator with a very low laser threshold, but will not be so easily tuned. Continuous wavelength tunability of a solid-state organic DFB laser requires modifying the active medium shape in some way, either through a modification of the effective index of the layer (which can be done by playing on the index 16,17 , the film thickness 18,19 or the grating depth 20 ), or by mechanical means upon applying a stress to the active layer in order to induce a shift of the Bragg wavelength 21,22 . Their tuning range is limited to a few nanometers for a single grating, and at most limited by the gain bandwidth of a given material; a full coverage of the visible spectrum requires dozens of gratings with different pitches, leading to increased cost and fabrication issues.…”