2D Spectrometer Delay Stages

These are the three pairs of rotational stages and glass used to control the arrival times of the laser pulse sequence for the 2D spectrometer. The stages are computer controlled, and can rotate with an accuracy of one-thousandth of a degree. By rotating the glass, the effective thickness increases, delaying the laser pulse with control on the attosecond time scale. A pair of glass plates are used for each beam, rotated at opposite angles to correct for the path deviation.

Delay stages
The input side of the spectrometer, showing the three pairs of rotational stages and glass used to control the relative arrival times of the laser pulse sequence.

Pulse Compressor

Here’s the layout of the pulse compressor. It consists of both grating and prism stages.

Pulse compressor
Two stage pulse compressor, using diffraction grating and prism sections. The laser pulses from the NOPA output are highly chirped with a duration of 400 fs or more. This compensates for the chirp, producing pulses of 20 fs or less.

Non-colinear Optical Parametric Amplifier (NOPA)

Here’s the layout of the non-colinear optical parametric amplifier (NOPA). It’s a tunable light source which can produce laser light anywhere across the visible spectrum. It’s fed by the output of our titanium-sapphire regenerative amplifier (Spitfire, Spectra Physics), which provides 100 fs duration pulses with a 1 kHz repetition rate, centred at a wavelength of 800 nm. We split that to produce a white-light continuum seed and a 400 nm pump beam. The precise angle and timing of the overlap of these beams on the nonlinear crystal (beta barium borate, BBO) will change the wavelength of the NOPA output. Currently this is generating light centred around 600 nm.

Non-colinear optical parametric amplifier (NOPA)
Non-colinear optical parametric amplifier (NOPA). A tunable light source which can produce laser light anywhere across the visible range.

The output is strongly affected by the quality of the white-light seed. The white light is produced by focusing a small amount of the 800 nm laser onto a pure sapphire crystal. The produced white light radiates out and needs to be collected and collimated using a precisely placed curved mirror.

NOPA white light
Sapphire plate and off-axis parabolic mirror used to generate and collimate the white-light continuum.
NOPA White Light
Mounting for the off-axis parabolic mirror used to provide precise 4-axis positioning.

Origin of the Excited-State Absorption Spectrum of Polythiophene

This paper was a pure computational chemistry project studying the photoexcited state of a common conjugated polymer used in organic photovoltaics. It was published in the Journal of Physical Chemistry Letters, DOI: 10.1021/acs.jpclett.7b01053.

This was one of those “why hasn’t anyone done this yet?” projects which Tak gave to one of his honours students, Ras Roseli. She did a great job on the computational work, which I can claim no credit for! I provided the small amount of experimental data and wrote up the manuscript with Tak.

While I think this is a great piece of work for a (technically undergraduate) student, the field of computational chemistry moves fast. If you’re looking to replicate the techniques in this work then I’d recommend looking at some of the newer hybrid density functionals. For example, ωB97X-D, or the even newer ωB2PLYP developed by the Goerigk group.

Fun fact: There’s a spelling error on the first page which is entirely my fault. See if you can find it.

Comment on “Magnetic Field Effects on Singlet Fission and Fluorescence Decay Dynamics in Amorphous Rubrene”

This was a purely computation project based around a quantum mechanical model of molecular spin interactions in the presence of magnetic fields, for the purpose of predicting efficiency of triplet-triplet annihilation upconversion and singlet fission. This was published in the Journal of Physical Chemistry C, DOI: acs.jpcc.6b04934.

It was written in the form of a Comment on a previously published paper. The reason for this was we wanted to use the spin-Hamiltonian method as part of a chapter in my PhD thesis, however the current state of the literature was, frankly, quite obviously wrong. Hence, we took a six month detour to first work on correcting the currently accepted literature reference on the molecular spin-Hamiltonian.

It actually felt kind of bad publishing this, since the Bardeen group is well known and respected. However, it’s science, and if it’s wrong, it’s wrong… right? Funnily enough, after this correction was published, Bardeen contacted David to say “no hard feelings”, which was nice. A few weeks later they published this article. My guess is they had been sitting on that experimental data but since it didn’t match their (wrong) calculations, they didn’t want to publish it. Once the model was corrected, their experimental results actually made sense!

There’s another funny story about this paper. We got sent someone’s manuscript for review which was almost a complete copy of this work. Except they made many of the same mistakes as in the original paper and were claiming we were actually wrong. What made it really funny was they had directly copied passages of text from our article and figure captions. Yes, that paper did eventually get published, and still has some of the original mistakes in it.

Published Manuscript and Supporting Information are free to access from the publisher.

Molecular-Level Details of Morphology-Dependent Exciton Migration in Poly(3-hexylthiophene) Nanostructures

This project was primarily computational, involving the development of a hybrid classical and quantum method. It was published in the Journal of Physical Chemistry C, DOI: 10.1021/acs.jpcc.5b00705.

The morphology dependence of exciton transport in the widely used conjugated polymer poly(3-hexylthiophene) (P3HT) is elucidated by combining an accurate mesoscale coarse-grained molecular dynamics simulation model of P3HT structure with a Frenkel–Holstein exciton model. This model provides a more realistic representation than previously achieved of the molecular-level details of exciton transport on large length scales relevant to electronic applications. One hundred 300-monomer regioregular P3HT chains are simulated at room temperature for microseconds in two implicit solvents of differing solvent quality in which the polymer chains adopt contrasting morphologies: nanofiber-like aggregates or well-separated extended conformations. The model gives reasonable quantitative agreement with steady-state absorption and fluorescence and time-resolved fluorescence experiments, and provides valuable insight into the mechanism of exciton transport in conjugated polymers. In particular, exciton transfer in nanofiber aggregates is shown to occur mainly through interchain hops from chromophores on the aggregate surface toward the aggregate core, a behavior with important implications for organic electronic applications. Furthermore, the counterbalancing effects of greater orientational order and faster exciton transport in nanofiber aggregates than in extended chains are found to explain the puzzling observation of similar fluorescence anisotropy decay rates in nanofibers and free chains.

Optical Pumping of Poly(3-hexylthiophene) Singlet Excitons Induces Charge Carrier Generation

This was my first publication during my PhD. It focused on a three-pulse transient absorption technique applied to a common conjugated polymer material. It was published in the Journal of Physical Chemistry Letters, DOI: 10.1021/jz500217f.

The dynamics of high-energy excitons of poly(3-hexylthiophene) (P3HT) are shown to consist of torsional relaxation and exciton dissociation to form free carriers. In this work, we use pump–push–probe femtosecond transient absorption spectroscopy to study the highly excited states of P3HT in solution. P3HT excitons are generated using a pump pulse (400 nm) and allowed to relax to the lowest-lying excited state before re-excitation using a push pulse (900 or 1200 nm), producing high-energy excitons that decay back to the original excited state with both subpicosecond (0.16 ps) and picosecond (2.4 ps) time constants. These dynamics are consistent with P3HT torsional relaxation, with the 0.16 ps time constant assigned to ultrafast inertial torsional relaxation. Additionally, the signal exhibits an incomplete recovery, indicating dissociation of high-energy excitons to form charge carriers due to excitation by the push pulse. Our analysis indicates that charge carriers are formed with a yield of 11%.