Eigenstates of two-photon annihilation operators

We introduce the inverse of the harmonic-oscillator creation and annihilation operators by their actions on the number states. We then show that three of the two-photon annihilation operators, a^ ${}_{}{}^{\mathrm{°}\mathrm{-}1}{}_{}{}^{}$a^, a^a^ ${}_{}{}^{\mathrm{°}\mathrm{-}1}{}_{}{}^{}$, and a^ ${}_{}{}^2{}_{}{}^{}$, possess eigenstates that are analogous to the often-used coherent and squeezed states. A family of the eigenstates of the operator a^ ${}_{}{}^{\mathrm{°}\mathrm{-}1}{}_{}{}^{}$a^ is the customary squeezed vacuum S(σ)‖0〉 while another family of the eigenstates of a^a^ ${}_{}{}^{\mathrm{°}\mathrm{-}1}{}_{}{}^{}$ is the state S(σ)‖n=1〉. We find some interesting properties related to squeezing, bunching, and antibunching exhibited by these states. An alternative method of summing some series by using these states is also considered. It is hoped that these states will find applications in quantum optics and quantum mechanics in general.