Carleman estimates and boundedness of associated multiplier operators

Let $P(D)$ be the Laplacian $\Delta,$ or the wave operator $\square$. The following type of Carleman estimate is known to be true on a certain range of $p,q$: $$\|e^{v\cdot x}u\|_{L^q(\mathbb{R}^d)} \le C\|e^{v\cdot x}P(D)u\|_{L^p(\mathbb{R}^d)}$$ with $C$ independent of $v\in \mathbb{R}^d$. The estimates are consequences of the uniform Sobolev type estimates for second order differential operators due to Kenig-Ruiz-Sogge \cite{KRS} and Jeong-Kwon-Lee \cite{JKL}. The range of $p,q$ for which the uniform Sobolev type estimates hold was completely characterized for the second order differential operators with nondegenerate principal part. But the optimal range of $p,q$ for which the Carleman estimate holds has not been clarified before. When $P(D)=\Delta$, $\square$, or the heat operator, we obtain a complete characterization of the admissible $p,q$ for the aforementioned type of Carleman estimate. For this purpose we investigate $L^p$-$L^q$ boundedness of related multiplier operators. As applications, we also obtain some unique continuation results.