  Absorption spectroscopy is widely used to detect samples with spectral
specificity. Here, we propose and demonstrate a method for enhancing the
sensitivity of absorption spectroscopy. Exploiting multiple light scattering
generated by a boron nitride (h-BN) scattering cavity, the optical path lengths
of light inside a diffusive reflective cavity are significantly increased,
resulting in more than ten times enhancement of sensitivity in absorption
spectroscopy. We demonstrate highly sensitive spectral measurements of low
concentrations of malachite green and crystal violet aqueous solutions. Because
this method only requires the addition of a scattering cavity to existing
absorption spectroscopy, it is expected to enable immediate and widespread
applications in various fields, from analytical chemistry to environmental
sciences.
