Organic microcavity laser based on high gain monolayer

Microcavity lasers in which the emission occurs perpendicular to the device plane such as in a Vertical Cavity Surface Emitting Laser (VCSEL) are used in many applications including telecommunications and large-scale photonics... [ view full abstract ]

Microcavity lasers in which the emission occurs perpendicular to the device plane such as in a Vertical Cavity Surface Emitting Laser (VCSEL) are used in many applications including telecommunications and large-scale photonics integration, however no coherent emission from monolayer based microcavity devices have been observed so far. Commonly, in Organic VCSELs, the active media which provides optical gain is distributed between two mirrors of the microcavity.  In this work fluorescent molecules are confined into a single molecular layer providing surprisingly high optical gain of more than 1000 cm^-1 [1]. This is the first demonstration of lasing using an organic monolayer and the first demonstration of lasing with any monolayer in planar microcavity configuration.

The optical gain is provided by amphiphilic fluorescent dye, Lissamine Rhodamine B sulfonyl didodecyl amine (LRSD). LRSD was assembled into a monolayer via Langmuir-Blodgett deposition. Despite the relatively high concentration of molecules in the monolayer, ~0.3/nm², the dye shows high quantum yield. The device was assembled from two separately grown highly reflective mirrors with distance between them λ/2 using spacers of Poly(methyl methacrylate)  (Fig. 1a). With such a design, we were able to achieve microcavity quality factor of Q > 6000.

Lasing was observed upon excitation by nanosecond pulses at a threshold absorbed energy density of 4.4 μJ/cm², when 5% of the fluorescent molecules were excited. Lasing was accompanied by a change in slope of the output intensity curve, the appearance of polarized emission, and a narrow spectral line above the threshold (Fig. 1 b-c). Tuning the microcavity, we observed lasing in the spectral region between 585 nm and 610 nm.

Monolayer based Organic VCSELs have potential for gain layer optimization at the molecular level and can be useful in engineering energy transfer dynamics for fluorescence based chemical sensing applications. By localizing the gain to a single nanometer thick slice of the electromagnetic field, such devices can enable coherent coupling between excitons and thus ultimately lead to superradiant lasing and other fundamental studies on quantum coupling between light and molecules in monolayers.

[1] A. Palatnik et al., ACS Nano, 11(5), 4514-4520 (2017).