Dr Patrick Salter


 I am a Senior Research Associate in the Dynamic Optics Group in Engineering Science and the W.W. Spooner Junior Research Fellow at New CollegeI conduct research into photonic engineering, particularly adaptive optics systems, laser microfabrication and diamond technology. Please see below for a brief summary of my research interests.

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Adaptive optics in Direct Laser writing

Direct Laser Writing (DLW) represents an excellent tool for microfabrication of structures in 3D inside transparent substrates. I use adaptive optical elements to control the phase of the laser beam during the DLW process. This has advantages in aberration correction, focal shaping, parallelisation and spatio-temporal pulse control.

DLW in diamond technology

DLW is particularly useful for processing inside diamond, providing many new capabilities. However, aberrations are particularly strong and the fabrication is seriously impaired without adaptive optics aberration correction.

By harnessing the aberration correction, it becomes possible to fabricate a range of complex features in 3D through laser writing:

Coherent colour centres:

NVs

Waveguides:

Conductive wires:

Wires2


For further information see:

YC Chen, PS Salter et al., "Laser writing of coherent colour centres in diamond" Nature Photonics 11, p77 (2017).

A Courvoisier, MJ Booth, PS Salter: "Inscription of 3D waveguides in diamond using an ultrafast laserApplied Physics Letters 109 (3), 031109 (2016)

 

S. A. Murphy, M. Booth, L. Li, A. Oh, P. Salter, B. Sun, D. Whitehead and A. Zadoroshnyj, Nuclear Instruments and Methods in Physics Section A, “Laser processing in 3D diamond detectorsin press doi:10.1016/j.nima.2016.04.052i (2016).

 

B Sun, PS Salter, MJ Booth:"High conductivity micro-wires in diamond following arbitrary paths" Applied Physics Letters 105 (23), 231105 (2014)


DLW in photonic waveguides

We have implemented adaptive optics strategies for beam shaping in the fabrication of photonic waveguides in glass. Using a liquid crystal spatial light modulator (SLM), it is possible to smoothly vary the cross section of a waveguide along its length, as shown in the image below, and correct for any optical aberrations.

For further information see:

L Huang, PS Salter, F Payne, MJ Booth: " Aberration correction for direct laser written waveguides in a transverse geometryOptics Express 24 (10), 10565-10574 (2016)

P.S. Salter, A. Jesacher, J.B. Spring, B.J. Metcalf, N. Thomas-Peter, R.D. Simmonds, N. K. Langford, I. A. Walmsley and M.J. Booth: “Adaptive slit beam shaping for direct laser written waveguides” Optics Letters, 37, p.470 (2012)

Parallelisation

Adaptive optic systems are also useful for the fabrication of large volume diffractive optic structures. When creating void arrays, a SLM can be used to holographically generate a large number of fabrication foci per laser pulse. Alternatively, if the laser beam is split using a microlens array, a SLM coupled to the lenslet array enables increased fabrication flexibility and uniformity.

Nanofab

Reflection image of a cubic millimetre volume optic structure comprising 40 million voids embedded in a fused silica substrate on an fcc lattice. The structure was generated using a holographically generated array of 225 foci printing 28000 spots a second.

For more information see:

P.S. Salter, M. Baum, I. Alexeev, M. Schmidt and M.J. Booth: “Exploring the depth range for three dimensional laser machining with aberration correction,” Opt. Express 22, p17644 (2014)

P.S. Salter and M.J. Booth: “An addressable microlens array for parallel laser microfabrication.” Optics Letters, 36, p.2302 (2011).

Pulse front control

Often liquid crystal spatial light modulators (SLMs) are considered as purely imposing a phase distribution onto an incident wavefront. However, since there are in fact diffractive optical elements, interesting things also happen when using ultrashort pulses. We have combined a SLM with a deformable mirror to provide control over the pulsefront of an ultrashort pulse in both space and time. 

For further information see:
B. Sun, P.S. Salter and M.J. Booth: "Pulse front adaptive optics: a new method for control of ultrashort laser pulses" Optics Express, 23 (15), 19348-19357 (2015)

B. Sun, P.S. Salter and M.J. Booth: "Pulse front adaptive optics in two-photon microscopy" Optics Letters 40 (21), 4999-5002 (2015)


Liquid Crystal Physics

Laser writing in liquid crystals

Using DLW inside a photosensitive liquid crystal (LC) mixture, it is possible to write polymer features by two photon polymerisation. The resulting structures retain the birefringence and orientation of the LC at the point of laser exposure. By applying a voltage to the LC device, the birefringence of the polymer can be tuned during laser writing. This creates interesting optical structures and controls the alignment of the neighbouring LC, to create topological networks.

LC pillars

For more information see:

CC Tartan, PS Salter, MJ Booth, S Morris and SJ Elston: "Generation of 3-dimensional polymer structures in liquid crystalline devices using direct laser writing"RSC Advances 7, p507-511 (2017)

Seeing the LC structure

By inserting an anisotropic dye into a LC mixture, it is possible to map out the local molecular alignment ("director field") in 3D using either confocal or two photon microscopy. 

 

Two photon fluorescence images showing the axial dynamics of the director structure in a liquid crystal pi cell. Using this technique we can actually see how the liquid crystal structure is reorienting with time following the application of a voltage.

For more information see:

PS Salter, G Carbone, EJ Botcherby, T Wilson, SJ Elston and EP Raynes: "Liquid crystal director dynamics imaged using two-photon fluorescence microscopy with remote focusing"Physical Review Letters 103 (25), 257803 (2009)

PS Salter, G Carbone, SA Jewell, SJ Elston and P Raynes, "Unwinding of the uniform lying helix structure in cholesteric liquid crystals next to a spatially uniform aligning surface"Physical Review E 80 (4), 041707 (2009)