  Nonlinear spectroscopy with quantum entangled photons is an emerging field of
research that holds the promise to achieve a superior signal-to-noise ratio and
effectively isolate many-body interactions. Photon sources used for this
purpose however lack the frequency tunability and spectral bandwidth demanded
by contemporary molecular materials. Here, we present design strategies for
efficient spontaneous parametric downconversion to generate biphoton states
with adequate spectral bandwidth and at visible wavelengths. Importantly, we
demonstrate, by suitable design of the nonlinear optical interaction, the scope
to engineer the degree of spectral correlations between the photons of the
pair. We also present an experimental methodology to effectively characterize
such spectral correlations. Importantly, we believe that such a
characterization tool can be effectively adapted as a spectroscopy platform to
optically probe system-bath interactions in materials.
