Strong- vs weak-coupling lasing in polymer-film microcavities

TL;DR

This work investigates the crossover between polariton lasing and conventional photon lasing in planar MeLPPP organic microcavities by tuning the effective cavity length to traverse strong and weak coupling. Using a single material platform, the authors observe polariton lasing with a low threshold and discernible LPB/UPB dispersions in the strongly coupled regime, followed by photon lasing with a substantially higher threshold and emission guided by the material ASE gain peak in the weakly coupled regime. Vibronic resonances play a crucial role by lowering the polariton threshold through vibron-assisted relaxation, and emission energies are pulled toward the material gain maximum in both regimes. The study demonstrates a controllable, low-threshold pathway for organic polariton devices and highlights vibronic interactions as a design knob for future polaritonic circuits.

Abstract

Organic semiconductors are particularly attractive for polaritonics due to their large exciton binding energies and oscillator strengths. Among them, the ladder-type conjugated polymer poly(paraphenylene) (MeLPPP) is distinguished by its rigid backbone, narrow exciton linewidth, high photoluminescence quantum yield, and enhanced photostability, which makes it an excellent candidate for organic polariton devices. While polariton lasing has been reported in various organic systems, systematic studies of the transition from polariton lasing to conventional photon lasing within a single, well-controlled material platform remain limited. Understanding this crossover is crucial for distinguishing polariton-specific signatures from conventional lasing. Here, we present planar organic microcavities incorporating MeLPPP as the active medium that supports polariton lasing. By tuning the effective cavity length, we track the transition from strong to weak coupling and identify its impact on the lasing behaviour. Our results reveal a many fold increase in the lasing threshold when moving from polariton to photon lasing, emission energy pulling towards the polymer gain maximum for either regimes, and an important role of vibron mediated exciton relaxation evidenced by the reduction in lasing thresholds near vibron energy resonances with respect to the S10 exciton. These findings provide fundamental insight into light-matter coupling in organics and highlight MeLPPP microcavities as a versatile platform for future applications in low-threshold lasers.

Figures (7)

• Figure 1: Material characterization. a Raman spectrum of methyl-substituted ladder-type polymer poly(paraphenylene) (MeLPPP) reference, 180-nm, non-cavity thin film with three highlighted molecular vibrational modes V1, V2, and V3. b Chemical structure of the polymer chain. c Normalized absorption (blue line) with two highlighted excitonic transitions $S_{10}$ and $S_{11}$, fluorescence (red line), amplified spontaneous emission 487.25 nm/2.545 eV (black line) spectra of the corresponding film. Gray line shows the microcavity reflectivity spectrum. d The schematic structure of an organic microcavity.
• Figure 2: Hallmarks of polariton and photon lasing. a The polariton PL intensity, linewidth, and energy as a function of pumping fluence in a strongly coupled regime (order $M=3$, cavity length $L=574\ \mathrm{nm}$) with the polariton lasing threshold indicated with the vertical gray stripe, $P^{\rm pol}_{\mathrm{th}}=62.6\ \mu\mathrm{J}\,\mathrm{cm}^{-2}$). b, Dispersion images of polariton PL below (left panel) and above (right panel) $P_{\rm th}^{\rm pol}$, for four different cavity lengths: 549, 559, 574, and 591 nm. c The PL intensity, linewidth, and energy as a function of pumping fluence in a weakly coupled regime (order $M=9$, $L=1768\ \mathrm{nm}$) with the photon lasing threshold indicated with the vertical gray stripe, $P^{\rm ph}_{\mathrm{th}}=245\ \mu\mathrm{J}\,\mathrm{cm}^{-2}$). d, Dispersion images of the PL in the weakly coupled regime below (left panel) and above (right panel) $P_{\rm th}^{\rm ph}$, for four different cavity lengths: 1723, 1749, 1768, and 1800 nm. Gray-shaded stripe indicates the ASE gain area of the material, and the right inset is a zoom-in region of the $k_{||}=0$ area of the dispersions. Black solid lines in panels b, d depict excitonic level (0-1) of MeLPPP (horizontal) and bare-cavity photon modes (parabolic), while red dashed lines in panel b represent the best fit results by a coupled oscillators model. The fitting of the weakly coupled dispersion images is done with zero interaction strength constant. The colour scale is normalized to the maximum intensity of lasing dispersion images. x50 and x10 labels in the bottom left of panels b, d are the colour scale factors.
• Figure 3: Gain pulling. Energy shift of the nonlinear PL emission at $\approx 1.2\times P_{\rm th}$ with respect to the linear, below-$P_{\rm th}$ emission from $k_{0}\equiv k_{||} =0~\rm\mu m^{-1}$, in the weakly coupled (red markers) and strongly coupled (blue markers) regimes as a function of energy offset between ASE maximum, $E_{\rm ASE}=2.545~\rm eV$, and the ground-state energy of the dispersion, $E_{\rm k_{0}}$. The normalised ASE spectrum of a 180-nm thin MeLPPP film is shown with the black solid line.
• Figure 4: Cavity length tuning dependencies. Lasing threshold fluence (squares markers, black axis), Rabi splitting constant, $\Omega_{1}$ (circle markers, red axis), and the emission energy from $k_{\parallel}=0$ of the either LPB or the cavity photon mode (triangle markers, blue axis) are plotted as a function of cavity length $L_{\mathrm{cavity}}$ for three mode orders $M=3,5,9$. Modes $M=3$ and $M=5$ correspond to a strongly coupled regime, while $M=9$ corresponds to a weakly coupled regime. Horizontal (upper panel) and vertical (bottom panel) dashed lines indicate spectral positions of V2 and V3 vibron resonances with respect to the $S_{10}$ exciton.
• Figure S1: Step-by-step chemical process of MeLPPP polymer synthesis.