  Toward understanding of dark energy, we propose a novel method to directly
produce a chameleon particle and force its decay under controlled gas pressure
in a laboratory-based experiment. {\it Chameleon gravity}, characterized by its
varying mass depending on its environment, could be a source of dark energy,
which is predicted in modified gravity. A remarkable finding is a
correspondence between the varying mass and a characteristic pressure
dependence of a stimulated photon-photon scattering rate in a dilute gas
surrounding a focused photon-beam spot. By observing a steep pressure
dependence in the scattering rate, we can directly extract the characteristic
feature of the chameleon mechanism. As a benchmark model of modified gravity
consistent with the present cosmological observations, a reduced $F(R)$ gravity
is introduced in the laboratory scale. We then demonstrate that the proposed
method indeed enables a wide-ranging parameter scan of such a chameleon model
with the varying mass around $(0.1-1)~[\mu \mathrm{eV}]$ by controlling
pressure values.
