Nonlinear Interferometer: Design, implementation and phase-sensitive sum frequency generation measurement
Abstract: Sum frequency generation (SFG) spectroscopy is a unique tool for
probing the vibrational structure of numerous interfaces. Since SFG is a nonlinear
spectroscopy, it has long been recognized that measuring only the intensity - the absolute
square of the surface response - limits the potential of SFG for examining interfacial
interactions and dynamics. The potential is unlocked by meas... read moreuring the phase-sensitive or
imaginary response. As with any phase, the phase-sensitive SFG response is measured
relative to a reference; the spatial relationship between the phase reference and the
sample modulates the observed interference intensity and impacts sensitivity and accuracy.
We have designed and implemented a nonlinear interferometer to directly measure the
phase-sensitive response. If the phase of the reference is known, then the interferometer
produces an absolute phase of the surface. Compared to current configurations, phase
accuracy and stability are greatly improved due to active stabilization of the
sample-reference position. The design is versatile thus, can be used for any system that
can be probed with SFG including buried interfaces and those with high vapor pressure.
Feasibility and advantages of the interferometer are demonstrated using an
octadecyltrichlorosilane (OTS) film on fused silica. Chapter 1 describes the theory of
scalar sum frequency generation (SFG) spectroscopy and fundamental work on ammonia water,
trimethylamine (TriMA) water and tetramethylammonium hydroxide (TMAH) water using scalar
SFG. Analyzing and understanding these spectra requires phase-sensitive spectra; data left
for a future student using the nonlinear interferometer. Indeed, understanding the impact
of ammonia and amines on the aqueous surface was a prime motivator for developing the
nonlinear interferometer. Chapter 2 is the heart of the thesis: the building, and
implementation of the nonlinear interferometer. Octadecyltrichlorosilane (OTS) on silica
window is used as a demonstration surface to test the validity of the nonlinear
interferometer. Aqueous amines are just the starting point for applications using this
nonlinear interferometer to understand chemical systems at interfaces. Detailed
step-by-step alignment procedure of the nonlinear interferometer, data processing
procedures, Z-scan of the non-resonance, sample preparation, dual channel approach, scalar
SFG of z-cut quartz, and other phase measurement can be found in the
Thesis (Ph.D.)--Tufts University, 2017.
Submitted to the Dept. of Chemistry.
Advisor: Mary Shultz.
Committee: Charles Sykes, Arthur Utz, and Michelle Foster.
Keyword: Physical chemistry.read less