Abstract
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.
Highlights
Outcoupling efficiency is a crucial parameter in defining the peak and average free electron laser (FEL) power available for user experiments as well as reducing the heat load deposited on the resonator structure in high power free electron lasers (FELs) systems
The use of 1D photonic crystal (PhC) as nearly 100% efficient, reflectivity tunable, broadband couplers is a novel concept towards achieving significant improvements in the performance of THz FELs and the related high-Q THz pulse stacker cavities
It is capable of addressing critical design requirements imposed by the high intracavity power levels as well as relatively large cavity-mode cross sections inherent to long wavelength THz FEL resonators
Summary
Outcoupling efficiency is a crucial parameter in defining the peak and average free electron laser (FEL) power available for user experiments as well as reducing the heat load deposited on the resonator structure in high power FEL systems. The reflection/transmission characteristics of a 1D-PhC mirror are otherwise fixed due to a few, discrete number of low-loss dielectric layers used in its assembly Overcoming this limitation, the studied scheme opens up a practical way to establish the broadband 1D-PhC mirrors for applications in THz FELs that are known to be continuously tunable devices over a large spectral range. It allows online adjustment of a well-defined coupling ratio at a given frequency within the high reflectivity band (photonic band gap) in order to optimize the FEL performance. The ability of varying the basic unit cell size (while employing the same dielectric wafers) helps to extend the operational reflectivity band of the PhC mirror, by nearly doubling it
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