  We demonstrate in simulation and experiment that the out-of-plane, far-field
scattering profile of resonance modes in photonic crystal nanobeam (PCN)
cavities can be used to identify resonance mode order. Through detection of
resonantly scattered light with an infrared camera, the overlap between optical
resonance modes and the leaky region of k-space can be measured experimentally.
Mode order dependent overlap with the leaky region enables usage of resonance
scattering as a "fingerprint" by which resonant modes in nanophotonic
structures can be identified via detection in the far-field. By selectively
observing emission near the PCN cavity region, the resonant scattering profile
of the device can be spatially isolated and the signal noise introduced by
other elements in the transmission line can be significantly reduced,
consequently improving the signal to noise ratio (SNR) of resonance detection.
This work demonstrates an increase in SNR of ~19 dB in out-of-plane scattering
measurements over in-plane transmission measurements. The capabilities
demonstrated here may be applied to improve characterization across
nanophotonic devices with mode-dependent spatial field profiles and enhance the
utility of these devices across a variety of applications.
