Cavity modes of exciton–polaritons in an organic crystal with distributed Bragg reflectors

We study optical spectra of cavity modes of exciton–polaritons in an organic crystal. The cavity is composed of distributed Bragg reflectors and anthracene crystals occupy the gap of the cavity. Many transmittance peaks corresponding to the cavity modes appear in the photonic band gap region of the transmittance spectra. The spectral positions of the cavity modes converge to the exciton resonance in anthracene. Dispersion of the cavity modes is obtained from the spectral peaks. It looks like a dispersion of the exciton–polariton in anthracene; however it differs slightly from the dispersion curve of the polariton. This discrepancy between the dispersion of the modes and the polariton curve is ascribed to the effective cavity length varying with the refractive index of anthracene.