MeLPPP Microcavities in Organic Photonics

Updated 17 October 2025

MeLPPP microcavities are planar optical resonators integrating the conjugated polymer MeLPPP to support hybrid light–matter coupling and low-threshold lasing.
They employ advanced fabrication methods like micro-mechanical dry assembly and 3D nanowriting to produce high-quality Fabry–Pérot structures with tunable resonance conditions.
Their design enables systematic exploration of strong-to-weak coupling regimes and efficient vibron-mediated exciton relaxation for optimized polariton lasing.

MeLPPP microcavities are planar or microstructured optical resonators that integrate the ladder-type conjugated polymer poly(paraphenylene), known as MeLPPP, as the active medium. These systems serve as versatile platforms for studying and controlling hybrid light–matter states, supporting transitions between strong-coupling polariton lasing and conventional photon lasing regimes. The unique optical properties of MeLPPP—namely its narrow exciton linewidth, high photoluminescence quantum yield, and enhanced photostability—enable the realization of low-threshold organic lasers and advanced polaritonics. Fabrication strategies leverage both dry assembly using distributed Bragg reflectors (DBRs) and advanced lithographic nanowriting to create high-quality-factor Fabry–Pérot structures, while precise cavity engineering permits tunable resonance conditions and systematic studies of light–matter coupling phenomena.

1. Structural Configuration and Fabrication Strategies

MeLPPP microcavities are constructed by assembling high-reflectivity mirrors around thin MeLPPP films to create Fabry–Pérot resonators. The main approaches include:

Micro-mechanical dry assembly (Rupprecht et al., 2020): A bottom Bragg mirror (DBR) of alternating SiO₂ and TiO₂ layers (e.g., 129 nm/79 nm per pair, commercially available) is used as the substrate. The MeLPPP layer (or other 2D material) is transferred atop this DBR using the polydimethylsiloxane (PDMS) stamping method, thereby avoiding exposure to thermal or energetic processes.
A poly(methyl methacrylate) (PMMA) spacer is then spin-coated, with carefully tuned thickness (typically matching the λ/2 resonance condition), followed by a brief low-temperature bake for solvent removal.
The top DBR is mechanically prepared via scratching to generate fragments (10–50 µm), which are then transferred using a PDMS stamp onto the PMMA-covered stack via van-der-Waals adhesion, yielding a complete cavity without high-temperature deposition.
Alternative configurations utilize advanced 3D nanowriting in IP-DIP photoresist (Palekar et al., 2021), enabling direct lithographic fabrication of polymer/air Bragg mirrors and mechanically tunable cavity architectures. These support multimodal mechanical adjustment and integration on device substrates.

This fabrication protocol yields defect-free interfaces, preserves MeLPPP photoluminescence, and achieves state-of-the-art optical quality factors without expensive deposition or plasma equipment.

2. Microcavity Optical Principles and Resonance Tuning

The microcavity resonance is established by the Fabry–Pérot condition: $m\lambda = 2 n_{\mathrm{eff}} L_{\mathrm{eff}}$ where $m$ is the longitudinal mode number, $\lambda$ the free-space wavelength, $n_{\mathrm{eff}}$ the effective refractive index, and $L_{\mathrm{eff}}$ the cavity length (dominated by spacer thickness, penetration depth into DBRs, and MeLPPP layer thickness).

PMMA and polymer/air stack thickness precisely controls $L_{\mathrm{eff}}$ , facilitating systematic detuning and coherent paper of strong-to-weak coupling transitions.
Quarter-wavelength mirror design (layer thickness $d_i = \lambda/(4 n_i)$ ) is implemented for both conventional DBRs and lithographically defined polymer/air Bragg structures (Palekar et al., 2021).
Resonance energy and Q-factor can be fine-tuned by adjusting the PMMA concentration and mechanical compression in 3D-printed cavities.
Polarization splitting (TE/TM splitting) observed in angle-resolved spectroscopy scales parabolically with incident angle or in-plane wavevector, indicative of phase delay modulations at mirror interfaces (Rupprecht et al., 2020).

Quality factors up to Q ≈ 3827 ± 222 are routinely observed; transfer-matrix calculations suggest Q ≈ 25000 may be achievable with further minimization of material inhomogeneity.

3. Light–Matter Coupling Regimes: Strong vs. Weak Coupling

By tuning the effective cavity length, MeLPPP microcavities exhibit a continuous crossover between strong-coupling polariton lasing and conventional photon lasing regimes (Sannikov et al., 15 Oct 2025). The defining characteristics include:

Strong-coupling (polariton lasing):
- Exciton–polariton branches form via hybridization of MeLPPP Frenkel excitons and cavity photon modes.
- Lasing threshold is markedly low (e.g., $P_{\mathrm{th}}^{\mathrm{pol}}$ ≈ 62.6 μJ cm⁻² for mode $M=3$ ).
- Condensation results in $k$ -space collapse to $k_{||} = 0$ , spectral blueshift, and narrowed linewidth.
- Emission energy is "pulled" toward the polymer's gain maximum but blueshifts are limited by the hybrid state.
Weak-coupling (photon lasing):
- Occurs at increased spacer/cavity thicknesses and higher-order modes (e.g., $M=9$ ).
- Lasing threshold increases several-fold ( $P_{\mathrm{th}}^{\mathrm{ph}} \sim 245$  μJ cm⁻²).
- Conventional population inversion is necessary; emission redshifts substantially as the lasing mode is pulled toward the amplified spontaneous emission (ASE) gain peak (2.545 eV for MeLPPP).

The transition is quantitatively described by a coupled-oscillator Hamiltonian for the hybrid light–matter states: $\hat{H} = \begin{pmatrix} E_{\mathrm{cav}}(k) & \hbar\Omega_1 & \hbar\Omega_2 \ \hbar\Omega_1 & E_{X_1} & 0 \ \hbar\Omega_2 & 0 & E_{X_2} \end{pmatrix}$ where $E_{\mathrm{cav}}(k)$ is the cavity photon energy dispersion, $E_{X_1}$ and $E_{X_2}$ the energies of electronic and vibronic excitons, and $\hbar\Omega_1$ , $\hbar\Omega_2$ the respective coupling strengths.

4. Vibron-Mediated Exciton Relaxation and Threshold Behavior

MeLPPP exhibits pronounced vibron-mediated relaxation channels that critically impact polariton formation and condensation efficiency (Sannikov et al., 15 Oct 2025).

Lasing thresholds are minimized when the cavity polariton states intersect with vibrational resonances (e.g., labeled $V_2$ and $V_3$ ), due to accelerated exciton relaxation into polariton branches.
Presence of strong vibronic replicas allows rapid energy transfer from high-energy excitons into low-energy, strongly coupled polariton states, a feature absent in most inorganic semiconductor systems.
This property provides a design principle for optimizing polariton lasing efficiency in organic systems and highlights MeLPPP's suitability for low-threshold lasers at room temperature.

5. Theoretical and Experimental Characterization Methods

To fully characterize MeLPPP microcavities, integrated theoretical and experimental approaches are used:

Transfer Matrix Method (TMM): Calculation of reflectivity, transmission spectra, and electromagnetic field distribution in multilayer cavities (matrices constructed layer-by-layer, with dispersion handled through Sellmeier and Drude relations for wavelength-dependent $n_i$ ) (Rupprecht et al., 2020, Palekar et al., 2021).
Finite Element Analysis (FEA): Prediction of mechanical deformation and consequent optical path changes under external pressures in lithographically defined polymer/air systems (Palekar et al., 2021).
Angle-resolved reflectivity and photoluminescence: Mapping dispersion, polariton effective mass ( $m^* \sim 1.23\times10^{-5}\,m_e$ ), TE/TM splitting, and threshold behavior as a function of cavity length and excitation fluence (Rupprecht et al., 2020, Sannikov et al., 15 Oct 2025).

6. Applications and Future Directions

MeLPPP microcavities provide a robust and tunable platform for several advanced photonic applications:

Low-threshold polariton lasers: Room-temperature operation with energy-efficient thresholds and spectral control.
Cavity quantum electrodynamics experiments: Coherent exciton–photon interactions and quantum simulation.
Topological and spin–orbit photonics: TE/TM splitting and angle-dependent dispersion enable explorations of exotic light states.
Integrated photonic circuits: Compatibility with device substrates and chip-scale architectures fosters development of optoelectronic devices, ultrafast optical transistors, and quantum simulators.
Fundamental studies of organic light–matter coupling: Direct observation and control of vibron-assisted relaxation and gain pulling phenomenology.

Future research may involve further engineering of cavity architectures, deliberate tuning of vibrational resonances, and integration with other 2D materials, aiming to optimize polaritonic device stability and performance in organic photonics. The combination of accessible assembly methods, high Q-factors, and MeLPPP’s intrinsic properties positions these systems at the forefront of organic photonics and polaritonics.