A microfluidic chip that includes a substrate, a microresonator, and at least one waveguide or coupling surface. The substrate defines a microfluidic channel. The microresonator is positioned within the microfluidic channel. The at least one waveguide or coupling surface receives light having a frequency bandwidth greater than the spacing between the whispering gallery mode resonance frequencies supported by the microresonator. The at least one waveguide or coupling surface is evanescently coupled to the microresonator such that supported whispering gallery mode resonance frequencies are coupled from the at least one waveguide or coupling surface into the microresonator and light at frequencies not resonant with the microresonator are not coupled into the microresonator.