Polarization properties of solid-state organic lasers

The polarization states of lasers are crucial issues both for practical applications and fundamental research. In general, they depend in a combined manner on the properties of the gain material and on the structure of the electromagnetic modes. In this paper, we address this issue in the case of solid-state organic lasers, a technology which enables one to vary independently gain and mode properties. Different kinds of resonators are investigated: in-plane microresonators with Fabry-Perot, square, pentagon, stadium, disk, and kite shapes, and external vertical resonators. The degree of polarization $P$ is measured in each case. It is shown that although transverse electric modes prevail generally ($P$ > 0), the kite-shaped microlaser generates negative values for $P$ (i.e., a flip of the dominant polarization which becomes mostly transverse magnetic polarized). In general, we demonstrate that both the pump polarization and the resonator geometry can be used to tailor the polarization of organic lasers. With this aim in view, we, at last, investigate two other degrees of freedom, namely upon using resonant energy transfer and upon pumping the laser dye to a higher excited state. We then demonstrate that significantly lower $P$ factors can be obtained.

Figure 1

Experimental configurations of the solid-state organic lasers: (a) vertical emission (VECSOL) and (b) planar microlaser (oblique view); (c) planar microlaser geometry from a top view ($xy$ plane), indicating the $\alpha$ angle between the polarization of the pump beam and the direction of observation ($y$), illustrated with the case of a Fabry-Perot cavity. For (a)–(c) the colors are green for pumping and red for emission.

Figure 2

The molecular energy levels involved in the transitions are schematically shown on the top of the figure. Dye molecules involved in the study are as follows: DCM, PM605, RH590, Alq${}_3$, and MD7. MD7 is a homemade modified pyrromethene. For DCM, PM605, and MD7 the calculated absorption transition dipoles are indicated with a single arrow for the $S_0\to$$S_1$ transition and with a double arrow for the $S_0\to$$S_2$ transition. For PM605, two transitions with similar oscillator strengths are involved when pumped at 355 nm. One of the absorption dipoles is parallel to the absorption dipole of the $S_0\to$$S_1$ transition and is thus not represented.

Figure 3

(a) Optical microscope photos of planar microlasers, which are investigated in this paper. Scales are 0.6 $\mu$m for the thickness and about 100 $\mu$m in-plane. Colors depend on the dye molecules. (b) Typical experimental spectrum of a square microlaser. (c) Fourier transform of the spectrum in (b). The position of the first peak indicates the optical length $L$ of the periodic orbit (i.e., diamond) drawn in the inset, according to the data process described in [26]).

Figure 4

Degree of polarization $P_{\mathrm{in}\mathrm{plane}}$ versus the angle $\beta$ between the absorption and emission dipoles, following expression (2). For $\alpha =\pi /2$, $P_{\mathrm{in}\mathrm{plane}}=P_{\mathrm{VESCOL}}$.

Figure 5

Degree of polarization $P$ versus the pump intensity $I_p$ (per pulse) for ASE and Fabry-Perot in planar microresonator configuration for a DCM-PMMA layer. The intensity of emission versus $I_p$ is presented for comparison. Thin lines are drawn for guiding eyes.

Figure 6

Scheme of a sample slice in in-plane configuration (not at scale), with approximative values of bulk refractive indexes at 600 nm.

Figure 7

Normalized intensity emitted from a VECSOL with a PMMA-DCM gain material, versus the angle ${\alpha }^{\prime }$ of the pump polarization. Comparison between a 532-nm pump (green, squares) and a 355-nm pump (blue, circles), after corrections from bleaching. The curves are fitted according to Eq. (3). (Inset) Scheme of a VECSOL section. The polarization of emission is fixed and represented as a dotted red arrow, while the linear pump polarization is shown as a green continuous arrow.

Figure 8

Degree of polarization $P$ in connection with the resonator shape for a DCM-PMMA layer pumped at 532 nm (green disks) and a MD7-PMMA layer pumped at 355 nm (blue squares). (From left to right) ASE, Fabry-Perot, square (diamond modes), pentagon (inscribed pentagon), stadium (WGM), disk (WGM), disk (Fabry-Perot modes), kite (WGM), and kite (Fabry-Perot modes).

Figure 9

Pumping schemes are (a) via energy transfer from an excited molecule to the laser dye, (b) direct pumping to the $S_2$ state of the dye laser.

Figure 10

(a) Absorption spectra of Alq${}_3$ and RH590. (b) Fluorescence spectra of Alq${}_3$, RH590 and Alq${}_3$-RH590 under the same pump intensity at 355 nm. They are normalized by the maximum of Alq${}_3$-RH590 fluorescence spectrum and the variation of layer thickness is taken into account.

Figure 11

(a) Absorption spectra of PM605, DCM, Alq${}_3$, and Alq${}_3$-DCM. (b) and (c) Fluorescence spectra of PM605, DCM, and Alq${}_3$-DCM under pumping at wavelengths of 355 nm (b) and 532 nm (c), normalized by the maximum of PM605 fluorescence spectrum under 532-nm pumping wavelength.

Figure 12

Notations are as follows: absorption (${\vec{d}}_a$) and emission (${\vec{d}}_e$) transition dipoles are oriented at the angles ${\theta }_a$ and ${\theta }_e$ with respect to the $z$ axis, $\beta$, angle between the moments, $\psi$, between their projections on the $xy$ plane, $\varphi$, angle between the $x$ axis and projection of ${\vec{d}}_e$ on the $xy$ plane, $\alpha$, angle between the $y$ axis and the pump beam polarization (thick green arrow).