Topography induced optical spectral shifts and finite size effect of focal spot

TL;DR

This paper investigates how surface topography causes spectral shifts in optical measurements, influenced by focal spot size, spectrometer orientation, and alignment errors, demonstrated through spectroscopic imaging of GaAs trenches.

Contribution

It reveals the impact of topography and focal spot size on spectral shifts and proposes a method to detect these shifts via dual-orientation scanning.

Findings

Spectral shifts depend on grating orientation and numerical aperture.

Topography causes variation in the grating angle across the focal spot.

Alignment errors introduce additional spectral shifts.

Abstract

We observe topography induced spectral shifts using high resolution grating spectrometers which we attribute to the fact that the focal spot has a finite size. The topography induced spectral shifts depend on spectrometer grating orientation and numerical aperture of the microscope objective. This is demonstrated by spectroscopic imaging trenches in GaAs in directions parallel and perpendicular the spectrometer entrance slit. Differences along the two directions of the LO phonon band show that the spectral shift is due to the variation of the grating angle across the non uniform illuminated focal spot caused by topography. Alignment errors of the optical axis lead to additional spectral shifts. Topography induced spectral shifts can be detected by recording spectra by scanning the sample in two perpendicular orientations with respect to the spectrometer entrance slit.