Optical Modulation Due to Energy Exchange between Photonic and Exciton Modes in the Intermediate Coupling Regime.

Actively tunable photonic devices are vital for next-generation optoelectronics, requiring rapid switching and high bandwidth. Although organic optoelectronic devices have found wide applications, their use as optical modulators has been limited by low absorption in the critical near-infrared (NIR) region, a slow response time, and weak nonlinearities. To address these limitations, we developed a scheme based on intermediate exciton-photon coupling in an NIR-absorbing squaraine dye-based photonic structure. Using energy-momentum-resolved pump-probe spectroscopy, we show that the sign and magnitude of the optical response of our system depend strongly on the energy detuning between the excitonic and photonic modes. These data are analyzed using temporal coupled-mode theory to show that near resonance, a distinct energy exchange process emerges in the crossover regime between strong and weak light-matter coupling. This effect enables dynamic control over the photoinduced response, providing a pathway for broadband optical signal modulation extending into the NIR spectral region.