Dynamically tunable mirrors for THz free electron laser applications

We have constructed and tested 1D-photonic crystal (PhC) mirrors with tunable reflectivity to be used as efficient, broadband outcouplers for THz free electron lasers (FELs). The test mirrors cover a spectral range between 0.5 and 1.5 THz. They are assembled by stacking up quarter-wave dielectric layers separated by vacuum. The adopted PhC mirror design enables dynamical (while lasing) adjustment of individual layer spacing. Single as well as multiple defects in the periodicity are introduced to invoke a continuous, well-defined tuning of reflectivity and outcoupling ratio. The scheme allows one to vary also the PhC period while the equidistant spacing between the layers is maintained. This feature is used to shift the photonic band gap (center) for achieving an effective extension in the reflectance spectrum. Because of the exceptional flexibility provided by the scheme in tailoring the characteristics of the PhC defects, additional features such as tunable (narrow) bandpass filtering as well as (fast) THz intensity modulation can be combined with the reflectivity/coupler properties of the proposed PhC mirrors for FELs.