Vertical-Cavity Surface-Emitting Laser With Cholesteric Liquid Crystal Overlay

We study theoretically the spectral and polarization threshold characteristics of vertical-cavity surface-emitting lasers with an overlay of cholesteric liquid crystal (CLC-VCSELs). Due to the existence of CLC band gap for circularly polarized (CP) light the polarization state of the resonant modes in the compound system evolves from linear (LP) inside the VCSEL to elliptical and circular (CP) inside the CLC. As a result, the output mirror reflectivity is increased and CLC-VCSEL threshold gain is decreased. We elucidate the transition from LP to CP state of the generated light by studying the role of the CLC thickness and the top distributed Bragg reflector reflectivity. This transition takes place for 5-6 μm thick CLC and is not gradual but happens in an oscillatory fashion. When the CLC thickness is fixed and the VCSEL top mirror reflectivity is decreased a profound gradual red shift of the resonant mode wavelength and increase of threshold gain are observed.