Abstract

We have recently put forward a novel scheme wherein lasing is attained by a coherent superposition of two free electronic states in interfering interaction regions that can suppress stimulated absorption without hampering stimulated emission.¹In the original 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. The signal photon is delayed between wigglers to allow interaction with the same electron in both regions. If the absorption of the signal photon in regions 1 and 2 scatters the electron into a common final state $|{\overrightarrow{k}}_{\text{a}}〉$, then we have interference that can suppress absorption. The final states for emission, $|{\overrightarrow{k}}_{\text{e1}}〉\text{and}|{\overrightarrow{k}}_{\text{e2}}〉$, are then strongly orthogonal, whence there is no interference in emission. We now put forward another scheme, wherein the wigglers are replaced by Cherenkov or Smith-Purcell effects. Interference in signal absorption at frequency ω occurs in two sequential regions, if ${\overrightarrow{k}}_a\simeq \mid \overrightarrow{k}+{\overrightarrow{q}}_1(\omega )\simeq \overrightarrow{k}+{\overrightarrow{q}}_2(\omega )$, where ${\overrightarrow{q}}_1(\omega )$ and ${\overrightarrow{q}}_2(\omega )$ are the respective photon wavevectors (Fig. 1). Here too interference in emission is prevented by the orthogonality of the respective final states ${\overrightarrow{k}}_{ej}=\overrightarrow{k}-{\overrightarrow{q}}_j(\omega ),(j=1,2)$. In contrast to conventional Cherenkov lasers, in the present scheme gain does not require population inversion (bunching) in momentum space. Our present analysis of the Cherenkov and the two-wiggler schemes incorporates a new essential element, namely, corrective dispersive media for electrons and photons between the two interaction regions. The dispersive media are needed to compensate for the initial electron-beam spread. We finally discuss the possible application of the present scheme for X-ray lasing.

© 1994 IEEE