Low-order non-linear spectroscopy of chiral molecules
File(s)
Author(s)
Vogwell, Joshua
Type
Thesis
Abstract
We present a computational and theoretical study of low-order non-linear spec
troscopy in chiral molecules. In particular, we present a novel technique for ultrafast
sum-frequency generation spectroscopy in chiral molecules, and a discussion of the
implications of molecular conformers upon our technique. First, we present the
results of time-dependent density-functional-theory simulations of enantiosensitive
sum-frequency generation in the chiral molecule propylene oxide, using the software
package OCTOPUS combined with signal processing and propagation codes. Us
ing these tools, we demonstrate how sum-frequency generation can be used in an
ensemble of randomly oriented chiral molecules to produce a far-field signal which
is enantiosensitive on the level of total intensity. This enantiosensitivity is much
more efficient than older optical-activity based methods, since it eschews magnetic
effects in favour of stronger electric-dipole interactions, whilst remaining fully con
trollable using the phases of input components. This technique presents exciting
opportunities for ultrafast chiral measurement, using relatively low laser intensities
which may be applicable to general laboratory settings. Secondly, we use a bespoke
spectral Schr¨odinger-equation solver in a basis set of field-free states obtained using
Density Functional Theory (DFT) to simulate the polarisation response of randomly
oriented carvone to the driving field used in chiral sum-frequency generation. We
repeat this for multiple sets of input frequencies and for the three most common
conformers of carvone. In doing so, we demonstrate that our chiroptical technique
is robust against conformational effects. We further demonstrate the existence of
interconformer interferences, as well as enantiosensitive interference between con
formers. This presents further opportunities to use solvents or companion molecules
to generate reference signals for ultrafast heterodyne measurements of chirality, and
to study conformational geometry itself in an ultrafast-optics setting.
troscopy in chiral molecules. In particular, we present a novel technique for ultrafast
sum-frequency generation spectroscopy in chiral molecules, and a discussion of the
implications of molecular conformers upon our technique. First, we present the
results of time-dependent density-functional-theory simulations of enantiosensitive
sum-frequency generation in the chiral molecule propylene oxide, using the software
package OCTOPUS combined with signal processing and propagation codes. Us
ing these tools, we demonstrate how sum-frequency generation can be used in an
ensemble of randomly oriented chiral molecules to produce a far-field signal which
is enantiosensitive on the level of total intensity. This enantiosensitivity is much
more efficient than older optical-activity based methods, since it eschews magnetic
effects in favour of stronger electric-dipole interactions, whilst remaining fully con
trollable using the phases of input components. This technique presents exciting
opportunities for ultrafast chiral measurement, using relatively low laser intensities
which may be applicable to general laboratory settings. Secondly, we use a bespoke
spectral Schr¨odinger-equation solver in a basis set of field-free states obtained using
Density Functional Theory (DFT) to simulate the polarisation response of randomly
oriented carvone to the driving field used in chiral sum-frequency generation. We
repeat this for multiple sets of input frequencies and for the three most common
conformers of carvone. In doing so, we demonstrate that our chiroptical technique
is robust against conformational effects. We further demonstrate the existence of
interconformer interferences, as well as enantiosensitive interference between con
formers. This presents further opportunities to use solvents or companion molecules
to generate reference signals for ultrafast heterodyne measurements of chirality, and
to study conformational geometry itself in an ultrafast-optics setting.
Version
Open Access
Date Issued
2025-07-03
Date Awarded
2026-06-01
Copyright Statement
Attribution-NonCommercial 4.0 International Licence (CC BY-NC)
License URL
Advisor
Matthews, Mary
Ayuso , David
Sponsor
Royal Society (Great Britain)
Grant Number
URF\R1\201333
URF\ERE\210358
URF\R1\191759
Publisher Department
Department of Physics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)