Abstract

We put forward novel schemes wherein lasing is attained by coherent superpositions of two free electronic states in interfering interaction regions that suppress stimulated absorption without hampering stimulated emission. This can strongly enhance the gain of a free-electron laser. In the first scheme an electron beam with mean momentum ${\overrightarrow{k}}_1$ interacts with wiggler mode ${\overrightarrow{k}}_{w1}$ in region 1, changes its momentum (by deflection) to ${\overrightarrow{k}}_2$ and interacts with wiggler mode ${\overrightarrow{k}}_{w2}$ in region 2. The interactions in both regions add coherently, provided both wiggler beams are from the same coherent source, and the signal photon is delayed to allow interaction with the same electron in both regions. Absorption of the signal photon in regions 1 and 2 scatters the electron into a common final state $|{\overrightarrow{k}}_a>$ if ${\overrightarrow{k}}_a\approx {\overrightarrow{k}}_1-{\overrightarrow{k}}_{w1}+\overrightarrow{q}={\overrightarrow{k}}_2-{\overrightarrow{k}}_{w2}+\overrightarrow{q}$. This is the condition for interference that can suppress absorption. The final states for emission, $|{\overrightarrow{k}}_{e1}>$ and $|{\overrightarrow{k}}_{e2}$ are then strongly orthogonal, whence there is no interference in emission. In another scheme, the wigglers are replaced by Cherenkov or Smith–Purcell effects. Coherently superposed electron momenta ${\overrightarrow{k}}_1$ and ${\overrightarrow{k}}_2$ propagate in two sequential structures with different dispersion relations. Interference in signal absorption at frequency ω and direction z by both structures occurs for a common final electronic state ${|\overrightarrow{k}}_a>$ satisfying ${\overrightarrow{k}}_{az}\simeq {\overrightarrow{k}}_{1z}+q_{1z}(\omega )\simeq {\overrightarrow{k}}_{2z}+q_{2z}(\omega )$, where ${\overrightarrow{q}}_1(\omega )$ and ${\overrightarrow{q}}_2(\omega )$ are the respective wavevectors. Here too interference in emission is prevented by the orthogonality of the respective final states ${\overrightarrow{k}}_{ei}={\overrightarrow{k}}_i-{\overrightarrow{q}}_i(\omega ),(i=1,2)$.

© 1993 Optical Society of America