Methods and systems for a label-free on-chip optical absorption spectrometer consisting of a photonic crystal slot waveguide are disclosed. The invention comprises an on-chip integrated optical absorption spectroscopy device that combines the slow light effect in photonic crystal waveguide and optical field enhancement in a slot waveguide and enables detection and identification of multiple analytes to be performed simultaneously using optical absorption techniques leading to a device for chemical and biological sensing, trace detection, and identification via unique analyte absorption spectral signatures. Other embodiments are described and claimed.