arXiv subjects

Patrick S. Salter

Publications and source records attributed to Patrick S. Salter.

At least 19 recordsLinked to original sources

arXiv2026

Sapphire Photonic Crystal Fiber Sensor

Sapphire optical fiber shows great promise for remote sensing in extreme environments approaching 2000 degC, by using laser-processing to form a single-mode waveguide within it. However, for practical application, longer devices with high manufacturability and reliability are required. We report the design, modeling, fabrication, and optimization of an index-guiding sapphire photonic crystal fiber Bragg grating temperature sensor. The device is fabricated using femtosecond laser direct writing to inscribe both the photonic crystal waveguide and the Bragg grating. A spatial light modulator was used to compensate for the mismatch between the immersion objective and the high-index oil used. This improved the aspect ratio and suppressed cracking during fabrication, for higher reliability. The design results in a 6-fold reduction in fabrication time over an equivalent depressed cladding waveguide, significantly reducing the cost of manufacture. Devices up to 7 cm long were fabricated and spliced to standard single-mode fiber. The propagation loss was estimated to be 0.7 dB/cm and the Bragg gratings had a bandwidth of approximately 0.12 nm. Devices were tested in a furnace showing a temperature sensitivity of between 19.0-32.3 pm/degC over a range 25-1200 degC. These longer devices have the potential to enable practical high precision extreme temperature monitoring in many applications, with lower manufacturing cost and higher reliability.

physics.optics↗

arXiv2026

Straight Directional Couplers via Scan-Engineered Index Control

A novel design for straight directional waveguide couplers and interferometers is demonstrated in glass, fabricated using femtosecond laser direct writing and operating at telecommunication wavelengths (~1550 nm). The devices consisted of parallel waveguides with a spacing of 15 um, where the coupling strength was controlled by scan-engineered refractive index modulation along the length of the waveguide. Using this approach, we realized a 50:50 directional coupler formed by two identical waveguides with a footprint of < 40 um x 15 um x 6 mm, as well as a Mach-Zehnder interferometer with unbalanced arms. A waveguide array with 15 um spacing was also demonstrated, highlighting the potential for compact, high-density, and three-dimensional photonic integration.

physics.optics↗

arXiv2026

Laser written waveguides to the sample edge

A method is presented for fabrication of femtosecond laser written waveguides in glass to remove the need for polishing of substrates after processing. It is shown that by amplitude masking the fabrication laser beam near the sample edge and increasing the pulse energy it is possible to write waveguides that are not affected by edge aberrations and display mode profiles well matched to single mode fiber. Results are presented for different depths in fused silica and borosilicate glass substrates. The transmission from fiber to photonic circuit is significantly improved for situations where it is not possible to polish glass substrates after laser writing, creating new opportunities in photonic packaging.

physics.optics↗

arXiv2025

Raman Microspectroscopy for Real-Time Structure Indicator in Ultrafast Laser Writing

Femtosecond laser fabrication enables the creation of a wide range of devices, but its scalability and yield can be limited by the lack of real-time, in-situ monitoring tools. In particular, there is a strong need for metrics that directly correlate with device performance. Raman microspectroscopy provides a non-destructive route for in-situ characterization. Here, we demonstrate its potential to assess the electrical performance of laser-written graphitic electrodes in diamond. By combining hyperspectral mapping with electrical testing, we show that depletion of the 1332 cm$^{-1}$ sp3 Raman line serves as a monotonic and robust predictor of resistance, offering clear advantages over commonly used spectral features. We further introduce hyperspectral unmixing as a label-free approach to identify relevant spectral signatures in fabrication processes where Raman markers are less defined. Importantly, the methodology we present is not restricted to diamond but can be adapted to other host materials and functionalities, offering a practical path toward specification-driven fs-laser microfabrication.

physics.app-ph↗

arXiv2024

Laser-written scalable sapphire integrated photonics platform

In this paper, we demonstrate the integration of photonic devices on sapphire substrates using multi-layer depressed cladding waveguides at both 780 nm and 1550 nm. The devices are up to 10-cm long and written at depths down to 400 um. The propagation losses for single-mode guiding are ~ 0.6 dB/cm at 780 nm and ~ 0.7 dB/cm at 1550 nm. A number of structures have been fabricated with simultaneous single-mode and polarization independent operation: evanescently coupled waveguide arrays, Y-branch splitters, Mach-Zehnder interferometers, and a 2x2 directional-coupler. All the devices were fabricated using adaptive optics-assisted femtosecond laser direct writing with a customized laser writing algorithm. This work enables the integration of single-mode sapphire photonics devices in a scalable manner, enabling many applications in communications, imaging, computing, and sensing.

physics.optics↗

arXiv2024

Laser Activation of Single Group-IV Colour Centres in Diamond

Spin-photon interfaces based on group-IV colour centres in diamond offer a promising platform for quantum networks. A key challenge in the field is realizing precise single-defect positioning and activation, which is crucial for scalable device fabrication. Here we address this problem by demonstrating a two-step fabrication method for tin vacancy (SnV-) centres that uses site-controlled ion implantation followed by local femtosecond laser annealing with in-situ spectral monitoring. The ion implantation is performed with sub-50 nm resolution and a dosage that is controlled from hundreds of ions down to single ions per site, limited by Poissonian statistics. Using this approach, we successfully demonstrate site-selective creation and modification of single SnV- centres. The technique opens a window onto materials tuning at the single defect level, and provides new insight into defect structures and dynamics during the annealing process. While demonstrated for SnV- centres, this versatile approach can be readily generalised to other implanted colour centres in diamond and wide-bandgap materials.

quant-ph↗

arXiv2023

Polarization based modulation of splitting ratio in femtosecond laser direct written directional couplers

This work characterizes a phenomenon in direct laser written directional couplers where the splitting ratio for output light is dependent on the input polarization state. In general, for laser written waveguides, different coupling strengths exist for different polarization states of the input light. If the linear polarization state of the input light is not aligned with one of the symmetry axes of the system, an additional amplitude beating is imposed on the transfer of light in directional couplers of different interaction length. We present results for in-plane and out of plane directional couplers, which are supported by theoretical analysis. These new results provide insights for understanding and controlling polarization properties of directional couplers and larger photonic circuits.

physics.optics↗

arXiv2023

High-precision optical fiber sensing beyond 1000°C

Sapphire fiber can withstand around 2000°C, but it is multimoded, giving poor precision sensors. We demonstrate a single-mode sapphire fiber Bragg grating temperature sensor operating up to 1200°C. The repeatability above 1000°C is within {\pm}0.08%.

physics.optics↗

arXiv2023

High Speed Precise Refractive Index Modification for Photonic Chips through Phase Aberrated Pulsed Lasers

Integrated photonic chips have significant potential in telecommunications, classic computing, quantum systems, and topological photonics. Direct laser writing offers unique capability for creating three-dimensional photonic devices in an optical glass chip with quick prototyping. However, existing laser writing schemes cannot create index-modified structures in glass that precisely match the laser focal shape while also achieving high scanning speed and high refractive index contrast. Here, we introduce the theory of a refractive index modification scheme that combines the advantages of both traditional non-thermal and thermal regime fabrication methods. We also propose a model of waveguide formation that was verified through a thorough study on the effects of phase aberrations on the laser focus. The presented new photonic chip fabrication scheme uses a novel focal intensity distribution, where pulse energy is relocated to the bottom of a laser focus by manipulating primary and higher order spherical aberrations. The technique can produce index modifications with high scanning speed (20 mm/s or higher), high index contrast (16 x 10-3), and high precision to fabricate with arbitrary cross-sections. This method has potential to expand the capabilities of photonic chips in applications that require small-scale, high precision, or high contrast refractive index control.

physics.optics↗

arXiv2023

Additive GaN solid immersion lenses for enhanced photon extraction efficiency from diamond color centers

Effective light extraction from optically active solid-state spin centres inside high-index semiconductor host crystals is an important factor in integrating these pseudo-atomic centres in wider quantum systems. Here we report increased fluorescent light collection efficiency from laser-written nitrogen vacancy centers (NV) in bulk diamond facilitated by micro-transfer printed GaN solid immersion lenses. Both laser-writing of NV centres and transfer printing of micro-lens structures are compatible with high spatial resolution, enabling deterministic fabrication routes towards future scalable systems development. The micro-lenses are integrated in a non-invasive manner, as they are added on top of the unstructured diamond surface and bond by Van-der-Waals forces. For emitters at 5 micrometer depth, we find approximately 2x improvement of fluorescent light collection using an air objective with a numerical aperture of NA = 0.95 in good agreement with simulations. Similarly, the solid immersion lenses strongly enhance light collection when using an objective with NA = 0.5, significantly improving the signal-to-noise ratio of the NV center emission while maintaining the NV's quantum properties after integration.

physics.optics↗

arXiv2022

On-chip beam rotators, adiabatic mode converters, and waveplates through low-loss waveguides with variable cross-sections

Photonics integrated circuitry would benefit considerably from the ability to arbitrarily control waveguide cross-sections with high precision and low loss, in order to provide more degrees of freedom in manipulating propagating light. Here, we report a new method for femtosecond laser writing of optical-fibre-compatible glass waveguides, namely spherical phase induced multi-core waveguide (SPIM-WG), which addresses this challenging task with three dimensional on-chip light control. Fabricating in the heating regime with high scanning speed, precise deformation of cross-sections is still achievable along the waveguide, with shapes and sizes finely controllable of high resolution in both horizontal and vertical transversal directions. We observed that these waveguides have high refractive index contrast of 0.017, low propagation loss of 0.14 dB/cm, and very low coupling loss of 0.19 dB coupled from a single mode fibre. SPIM-WG devices were easily fabricated that were able to perform on-chip beam rotation through varying angles, or manipulate polarization state of propagating light for target wavelengths. We also demonstrated SPIM-WG mode converters that provide arbitrary adiabatic mode conversion with high efficiency between symmetric and asymmetric non-uniform modes; examples include circular, elliptical modes and asymmetric modes from ppKTP (periodically-poled potassium titanyl phosphate) waveguides which are generally applied in frequency conversion and quantum light sources. Created inside optical glass, these waveguides and devices have the capability to operate across ultra-broad bands from visible to infrared wavelengths. The compatibility with optical fibre also paves the way toward packaged photonic integrated circuitry, which usually needs input and output fibre connections.

physics.optics↗

arXiv2021

Single-mode sapphire fiber Bragg grating

We present here the inscription of single-mode waveguides with Bragg gratings in sapphire. The waveguide Bragg gratings have a novel multi-layer depressed cladding design in the 1550 nm telecommunications waveband. The Bragg gratings have a narrow bandwidth (<0.5 nm) and have survived annealing at 1000°C. The structures are inscribed with femtosecond laser direct writing, using adaptive beam shaping with a non-immersion objective. A single-mode sapphire fiber Bragg grating is created by writing a waveguide with a Bragg grating within a 425 μm diameter sapphire optical fiber, providing significant potential for accurate remote sensing in ultra-extreme environments.

physics.optics↗

arXiv2021

Revealing complex optical phenomena through vectorial metrics

Advances in vectorial polarisation-resolved imaging are bringing new capabilities to applications ranging from fundamental physics through to clinical diagnosis. Imaging polarimetry requires determination of the Mueller matrix (MM) at every point, providing a complete description of an object's vectorial properties. Despite forming a comprehensive representation, the MM does not usually provide easily-interpretable information about the object's internal structure. Certain simpler vectorial metrics are derived from subsets of the MM elements. These metrics permit extraction of signatures that provide direct indicators of hidden optical properties of complex systems, while featuring an intriguing asymmetry about what information can or cannot be inferred via these metrics. We harness such characteristics to reveal the spin-Hall effect of light, infer microscopic structure within laser-written photonic waveguides, and conduct rapid pathological diagnosis through analysis of healthy and cancerous tissue. This provides new insight for the broader usage of such asymmetric inferred vectorial information.

physics.optics↗

arXiv2021

Microscopic processes during ultra-fast laser generation of Frenkel defects in diamond

Engineering single atomic defects into wide bandgap materials has become an attractive field in recent years due to emerging applications such as solid-state quantum bits and sensors. The simplest atomic-scale defect is the lattice vacancy which is often a constituent part of more complex defects such as the nitrogen-vacancy (NV) centre in diamond, therefore an understanding of the formation mechanisms and precision engineering of vacancies is desirable. We present a theoretical and experimental study into the ultra-fast laser generation of vacancy-interstitial pairs (Frenkel defects) in diamond. The process is described by a set of coupled rate equations of the pulsed laser interaction with the material and of the non-equilibrium dynamics of charge carriers during and in the wake of the pulse. We find that a model for Frenkel defect generation via the recombination of a bound biexciton as the electron plasma cools provides good agreement with experimental data, reproducing an effective non-linearity of $\sim$ 40 for Frenkel defect generation with respect to laser pulse energy.

cond-mat.mtrl-sci↗

arXiv2018

Laser writing of scalable single colour centre in silicon carbide

Single photon emitters in silicon carbide (SiC) are attracting attention as quantum photonic systems. However, to achieve scalable devices it is essential to generate single photon emitters at desired locations on demand. Here we report the controlled creation of single silicon vacancy ($V_{Si}$) centres in 4H-SiC using laser writing without any post-annealing process. Due to the aberration correction in the writing apparatus and the non-annealing process, we generate single $V_{Si}$ centres with yields up to 30%, located within about 80 nm of the desired position in the transverse plane. We also investigated the photophysics of the laser writing $V_{Si}$ centres and conclude that there are about 16 photons involved in the laser writing $V_{Si}$ centres process. Our results represent a powerful tool in fabrication of single $V_{Si}$ centres in SiC for quantum technologies and provide further insights into laser writing defects in dielectric materials.

cond-mat.mes-hall↗

arXiv2018

Laser writing of individual atomic defects in a crystal with near-unity yield

Atomic defects in wide band gap materials show great promise for development of a new generation of quantum information technologies, but have been hampered by the inability to produce and engineer the defects in a controlled way. The nitrogen-vacancy (NV) color center in diamond is one of the foremost candidates, with single defects allowing optical addressing of electron spin and nuclear spin degrees of freedom with potential for applications in advanced sensing and computing. Here we demonstrate a method for the deterministic writing of individual NV centers at selected locations with high positioning accuracy using laser processing with online fluorescence feedback. This method provides a new tool for the fabrication of engineered materials and devices for quantum technologies and offers insight into the diffusion dynamics of point defects in solids.

quant-ph↗

arXiv2018

Microscope calibration using laser written fluorescence

There is currently no widely adopted standard for the optical characterisation of fluorescence microscopes. We used laser written fluorescence to generate two- and three-dimensional patterns to deliver a quick and quantitative measure of imaging performance. We report on the use of two laser written patterns to measure the lateral resolution, illumination uniformity, lens distortion and colour plane alignment using confocal and structured illumination fluorescence microscopes.

physics.optics↗

arXiv2017

High resolution structural characterisation of laser-induced defect clusters inside diamond

Laser writing with ultrashort pulses provides a potential route for the manufacture of three-dimensional wires, waveguides and defects within diamond. We present a transmission electron microscopy (TEM) study of the intrinsic structure of the laser modifications and reveal a complex distribution of defects. Electron energy loss spectroscopy (EELS) indicates that the majority of the irradiated region remains as $sp^3$ bonded diamond. Electrically-conductive paths are attributed to the formation of multiple nano-scale, $sp^2$-bonded graphitic wires and a network of strain-relieving micro-cracks.

physics.app-ph↗