# Dr Alex S. Clark

Faculty of Natural SciencesDepartment of Physics

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### Location

Blackett 211Blackett LaboratorySouth Kensington Campus

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## Publications

Publication Type
Year
to

67 results found

Schofield RC, Burdekin P, Fasoulakis A, Devanz L, Bogusz DP, Hoggarth RA, Major KD, Clark ASet al., 2022, Cover feature: narrow and stable single photon emission from dibenzoterrylene in para‐terphenyl nanocrystals (ChemPhysChem 4/2022), ChemPhysChem, Vol: 23, ISSN: 1439-4235

The Cover Feature illustrates the emission of photons from single dibenzoterrylene (DBT) molecules in para-terphenyl nanocrystals. The nanocrystals protect the DBT from the environment and allow for the emission of high purity single photon states which are spectrally narrow at cryogenic temperatures, making them ideal for use in quantum technologies. More information can be found in the Research Article by Ross C. Schofield, Alex S. Clark, and co-workers.

Journal article

Clark A, Clear C, Schofield R, McCutcheon D, Hoggarth R, Major Ket al., 2022, Photon indistinguishability measurements under pulsed and continuous excitation, Physical Review Research, Vol: 4, ISSN: 2643-1564

The indistinguishability of successively generated photons from a single quantum emitter is most commonly measured using two-photon interference at a beam splitter. Whilst for sources excited in the pulsed regime the measured bunching of photons reflects the full wave-packet indistinguishability of the emitted photons, for continuous wave (cw) excitation, the inevitable dependence on detector timing resolution and driving strength obscures the underlying photon interference process. Here we derive a method to extract full photon wave-packet indistinguishability from cw measurements by considering the relevant correlation functions. The equivalence of both methods is experimentally verified through a comparison of cw and pulsed excitation measurements on an archetypal source of photons, a single molecule.

Journal article

Schofield RC, Burdekin P, Fasoulakis A, Devanz L, Bogusz DP, Hoggarth RA, Major KD, Clark ASet al., 2022, Narrow and Stable Single Photon Emission from Dibenzoterrylene in para-Terphenyl Nanocrystals, CHEMPHYSCHEM, Vol: 23, ISSN: 1439-4235

Journal article

Clark AS, Blanco-Redondo A, Aharonovich I, 2021, Special Topic on Integrated Quantum Photonics, APL PHOTONICS, Vol: 6, ISSN: 2378-0967

Journal article

Schofield RC, Clear C, Hoggarth RA, Major KD, McCutcheon DPS, Clark ASet al., 2021, Photon indistinguishability measurements under pulsed and continuous excitation, Publisher: arXiv

The indistinguishability of successively generated photons from a singlequantum emitter is most commonly measured using two-photon interference at abeam splitter. Whilst for sources excited in the pulsed regime the measuredbunching of photons reflects the full wavepacket indistinguishability of theemitted photons, for continuous wave (cw) excitation the inevitable dependenceon detector timing resolution and driving strength obscures the underlyingphoton interference process. Here we derive a method to extract the photonindistinguishability from cw measurements by considering the relevantcorrelation functions. The equivalence of both methods is experimentallyverified through comparison of cw and pulsed excitation of an archetypal sourceof photons, a single molecule.

Working paper

Schofield RC, Boissier S, Jin L, Ovvyan A, Nur S, Koppens FHL, Toninelli C, Pernice WHP, Major KD, Hinds EA, Clark ASet al., 2021, Coupling a Single Molecule to an Interrupted Nanophotonic Waveguide

Single organic molecules have recently seen increased interest for use as single photon sources [1]. They emit photons with high efficiency and at favourable wavelengths for coupling to other quantum systems. While the excitation of molecules and their subsequent radiative emission is efficient [2] , the generated photons can be difficult to efficiently collect. There is therefore a large amount of ongoing work on coupling organic molecules to nanophotonic structures to modify their emission. Evanescent coupling to nanophotonic [3] , [4] and hybrid plasmonic [5] waveguides has shown promise but has limitations; the molecules must be very close to the waveguide to be in the evanescent field of the guided mode which can cause the molecules to become unstable. Here I will present our recent work on coupling organic molecules to interrupted waveguides using on chip micro-capillaries [6].

Conference paper

Toninelli C, Gerhardt I, Clark AS, Reserbat-Plantey A, Goetzinger S, Ristanovic Z, Colautti M, Lombardi P, Major KD, Deperasinska I, Pernice WH, Koppens FHL, Kozankiewicz B, Gourdon A, Sandoghdar V, Orrit Met al., 2021, Single organic molecules for photonic quantum technologies, NATURE MATERIALS, Vol: 20, Pages: 1615-1628, ISSN: 1476-1122

Journal article

Clark AS, Chekhova M, Matthews JCF, Rarity JG, Oulton RFet al., 2021, Special Topic: Quantum sensing with correlated light sources, APPLIED PHYSICS LETTERS, Vol: 118, ISSN: 0003-6951

Journal article

Boissier S, Schofield R, Jin L, Ovvyan A, Nur S, Koppens FHL, Toninelli C, Pernice WHP, Major K, Hinds E, Clark Aet al., 2021, Coherent characterisation of a single molecule in a photonic black box, Nature Communications, Vol: 12, ISSN: 2041-1723

Extinction spectroscopy is a powerful tool for demonstrating the coupling of a single quantum emitter to a photonic structure. However, it can be challenging in all but the simplest of geometries to deduce an accurate value of the coupling eﬃciency from the measured spectrum. Here we develop a theoretical framework to deduce the coupling eﬃciency from the measured transmission and reﬂection spectra without precise knowledge of the photonic environment. We then consider the case of a waveguide interrupted by a transverse cut in which an emitter is placed. We apply that theory to a silicon nitride waveguide interrupted by a gap ﬁlled with anthracene that is doped with dibenzoterrylene molecules. We describe the fabrication of these devices, and experimentally characterise the waveguide coupling of a single molecule in the gap.

Journal article

Burdekin P, Grandi S, Newbold R, Hoggarth RA, Major KD, Hinds EA, Clark ASet al., 2021, Spectroscopy of rubidium with a tuneable single photon source

Conference paper

Schofield RC, Boissier S, Jin L, Ovvyan A, Nur S, Koppens FHL, Toninelli C, Pernice WHP, Major KD, Hinds EA, Clark ASet al., 2021, Coupling a single molecule to an interrupted nanophotonic waveguide

Conference paper

Burdekin P, Grandi S, Newbold R, Hoggarth RA, Major KD, Hinds EA, Clark ASet al., 2021, Spectroscopy of Rubidium with a Tuneable Single Photon Source, Conference on Lasers and Electro-Optics Europe / European Quantum Electronics Conference (CLEO/Europe-EQEC), Publisher: IEEE

Conference paper

Burdekin P, Grandi S, Newbold R, Hoggarth RA, Major KD, Clark ASet al., 2020, Single-Photon-Level Sub-Doppler Pump-Probe Spectroscopy of Rubidium, PHYSICAL REVIEW APPLIED, Vol: 14, ISSN: 2331-7019

Journal article

Pearce E, Phillips CC, Oulton RF, Clark ASet al., 2020, Heralded spectroscopy with a fiber photon-pair source, Applied Physics Letters, Vol: 117, Pages: 1-6, ISSN: 0003-6951

The correlations between photons generated by nonlinear optical processes offer advantages for many quantum technology applications, including spectroscopy, imaging, and metrology. Here, we use spontaneous four-wave mixing in a birefringent single-mode fiber pumped by a tunable pulsed laser as a broadly tunable source of phase-matched non-degenerate photon pairs for spectroscopy. The pairs are tunable such that the idler beam measures the transmittance spectrum of a sample in the near infrared, while the visible signal beam independently reports correlation information. By the time-resolved counting of both signal and idler photons, we use photon-number correlations to remove uncorrelated noise from the probe beam. Here, we have used heralded spectroscopy to measure the absorption spectrum of gallium arsenide near its band edge, despite the idler photon spectrum being dominated by a large background from spontaneous Raman scattering.

Journal article

Schofield RC, Bogusz DP, Hoggarth RA, Nur S, Major KD, Clark ASet al., 2020, Polymer-encapsulated organic nanocrystals for single photon emission, Optical Materials Express, Vol: 10, Pages: 1586-1586, ISSN: 2159-3930

We demonstrate an emulsion-polymerisation technique to embed dibenzoterrylene-doped anthracene nanocrystals in polymethyl methacrylate (PMMA) nanocapsules. The nanocapsules require no further protection after fabrication and are resistant to sublimation compared to unprotected anthracene. The room temperature emission from single dibenzoterrylene molecules is stable and when cooled to cryogenic temperatures we see no change in their excellent optical properties compared to existing growth methods. We also show emission from nanocapsules embedded in a thin layer of titanium dioxide, highlighting their potential for integration into hybrid nanophotonic devices.

Journal article

Clear C, Schofield RC, Major KD, Iles-Smith J, Clark AS, Mccutcheon DPSet al., 2020, Phonon-induced optical dephasing in single organic molecules, Physical Review Letters, Vol: 124, Pages: 153602 – 1-153602 – 6, ISSN: 0031-9007

We present a joint experiment-theory analysis of the temperature-dependent emission spectra, zero-phonon linewidth, and second-order correlation function of light emitted from a single organic molecule. We observe spectra with a zero-phonon line together with several additional sharp peaks, broad phonon sidebands, and a strongly temperature dependent homogeneous broadening. Our model includes both localized vibrational modes of the molecule and a thermal phonon bath, which we include nonperturbatively, and is able to capture all observed features. For resonant driving we measure Rabi oscillations that become increasingly damped with temperature, which our model naturally reproduces. Our results constitute an essential characterization of the photon coherence of molecules, paving the way to their use in future quantum information applications.

Journal article

Grandi S, Nielsen MP, Cambiasso J, Boissier S, Major K, Reardon C, Krauss TF, Oulton R, Hinds E, Clark Aet al., 2019, Hybrid plasmonic waveguide coupling of photons from a single molecule, APL Photonics, Vol: 4, Pages: 086101-1-086101-6, ISSN: 2378-0967

We demonstrate the emission of photons from a single molecule into a hybrid gap plasmon waveguide (HGPW). Crystals of anthracene, doped with dibenzoterrylene (DBT), are grown on top of the waveguides. We investigate a single DBT molecule coupled to the plasmonic region of one of the guides, and determine its in-plane orientation, excited state lifetime and saturation intensity. The molecule emits light into the guide, which is remotely out-coupled by a grating. The second-order autocorrelation and cross-correlation functions show that the emitter is a single molecule and that the light emerging from the grating comes from that molecule. The couplinge fficiency is found to be βWG = 11.6(1:5)%. This type of structure is promising for building new functionality into quantum-photonic circuits, where localised regions of strong emitter-guide coupling can be interconnected by low-loss dielectric guides.

Journal article

Grandi S, Nielsen MP, Cambiasso J, Boissier S, Major KD, Reardon C, Krauss TF, Oulton RF, Hinds EA, Clark ASet al., 2019, Hybrid plasmonic waveguide coupling of photons from a single molecule

We demonstrate the emission of photons from a single molecule into a hybridgap plasmon waveguide (HGPW). Crystals of anthracene, doped withdibenzoterrylene (DBT), are grown on top of the waveguides. We investigate asingle DBT molecule coupled to the plasmonic region of one of the guides, anddetermine its in-plane orientation, excited state lifetime and saturationintensity. The molecule emits light into the guide, which is remotelyout-coupled by a grating. The second-order auto-correlation andcross-correlation functions show that the emitter is a single molecule and thatthe light emerging from the grating comes from that molecule. The couplingefficiency is found to be $\beta_{WG}=11.6(1.5)\%$. This type of structure ispromising for building new functionality into quantum-photonic circuits, wherelocalised regions of strong emitter-guide coupling can be interconnected bylow-loss dielectric guides.

Working paper

, 2019, Novel method of sub-wavelength thin film growth for single photon emission from dye molecules

Conference paper

Boissier S, Schofield R, Major K, Grandi S, Boissier S, Hinds E, Clark Aet al., 2018, Efficient excitation of dye molecules for single photon generation, Journal of Physics Communications, Vol: 2, ISSN: 2399-6528

A reliable photon source is required for many aspects of quantum technology. Organic molecules are attractive for this application because they can have high quantum yield and can be photostable, even at room temperature. To generate a photon with high probability, a laser must excite the molecule efficiently. We develop a simple model for that efficiency and discuss how to optimise it. We demonstrate the validity of our model through experiments on a single dibenzoterrylene (DBT) molecule in an anthracene crystal. We show that the excitation probability cannot exceed 75% at room temperature, but can increase to over 99% if the sample is cooled to liquid nitrogen temperature. The possibility of high photon generation efficiency with only modest cooling is a significant step towards a reliable photon source that is simple and practical.

Journal article

Grandi S, Major KD, Polisseni C, Boissier S, Clark AS, Hinds EAet al., 2016, Quantum dynamics of a driven two-level molecule with variable dephasing, Physical Review A, Vol: 94, ISSN: 1094-1622

The longitudinal (1) and transverse (2) decay rates of a two-level quantum system have a profound influenceon its evolution. Atomic systems with 2 = 121 have been studied extensively, but with the rise of solid-statequantum devices it is also important to consider the effect of stronger transverse relaxation due to interactionswith the solid environment. Here we study the quantum dynamics of a single organic dye molecule driven by alaser. We measure the variation of 2 with temperature and determine the activation energy for thermal dephasingof the optical dipole. Then we measure the second-order correlation function g(2)(τ ) of the light emitted by themolecule for various ratios 2/1 and saturation parameters S. We show that the general solution to the opticalBloch equations accurately describes the observed quantum dynamics over a wide range of these parameters,and we discuss the limitations of the various approximate expressions for g(2)(τ ) that appear in the literature.DOI:

Journal article

Polisseni C, Major KD, Boissier S, Grandi S, Clark AS, Hinds EAet al., 2016, Coupling dye molecules to a silicon nitride waveguide, Australian Conference on Optical Fibre Technology (ACOFT)

© OSA 2016. A dibenzoterrylene (DBT) molecule can emit single-photons into a waveguide. We have grown and characterised thin, DBT-doped anthracene crystals on photonic structures, including a silicon nitride ridge waveguide from which we detect single-photons.

Conference paper

Polisseni C, Major K, Boissier S, Grandi S, Clark A, Hinds EAet al., 2016, A stable, single-photon emitter in a thin organic crystal for application to quantum-photonic devices, Optics Express, ISSN: 1094-4087

Journal article

Collins MJ, Clark AS, Xiong C, Maegi E, Steel MJ, Eggleton BJet al., 2015, Random number generation from spontaneous Raman scattering, Applied Physics Letters, Vol: 107, ISSN: 1077-3118

We investigate the generation of random numbers via the quantum process of spontaneous Ramanscattering. Spontaneous Raman photons are produced by illuminating a highly nonlinearchalcogenide glass (As2S3) fiber with a CW laser at a power well below the stimulated Ramanthreshold. Single Raman photons are collected and separated into two discrete wavelengthdetuning bins of equal scattering probability. The sequence of photon detection clicks is convertedinto a random bit stream. Postprocessing is applied to remove detector bias, resulting in a final bitrate of 650 kb/s. The collected random bit-sequences pass the NIST statistical test suite for onehundred 1 Mb samples, with the significance level set to a ¼ 0:01. The fiber is stable, robust andthe high nonlinearity (compared to silica) allows for a short fiber length and low pump powerfavourable for real world application.

Journal article

He J, Bell BA, Casas-Bedoya A, Zhang Y, Clark AS, Xiong C, Eggleton BJet al., 2015, Ultracompact quantum splitter of degenerate photon pairs, Optica, Vol: 2, Pages: 779-782, ISSN: 2334-2536

Integrated sources of indistinguishable photons have attracted a lot of attention because of their applications in quantum communication and optical quantum computing. Here, we demonstrate an ultracompact quantum splitter for degenerate photon pairs based on a monolithic silicon chip. It incorporates a Sagnac loop and a microring resonator with a total footprint of 0.011  mm2, generating and deterministically splitting indistinguishable photon pairs using two-photon interference. The ring resonator provides an enhanced photon generation rate, and the Sagnac loop ensures the photons travel through equal path lengths and interfere with the correct phase to enable the reversed Hong–Ou–Mandel (HOM) effect to take place. In the experiment, we observed a HOM dip visibility of 94.5±3.3%, indicating the generated photons are in a suitable state for further integration with other components for quantum applications, such as controlled-NOT gates.

Journal article

major K, lien Y, polisseni C, grandi S, kho K, clark A, hwang J, Hinds EAet al., 2015, Growth of optical-quality anthracene crystals, doped with dibenzoterrylenefor controlled single photon production, Review of Scientific Instruments, Vol: 86, ISSN: 1089-7623

Dibenzoterrylene (DBT) molecules within a crystalline anthracene matrix show promise as quantum emitters for controlled, single photon production. We present the design and construction of a chamber in which we reproducibly grow doped anthracene crystals of optical quality that are several mm across and a few micrometres thick. We demonstrate control of the DBT concentration over the range 6 { 300 parts per trillion and show that these DBT molecules are stable single-photon emitters. We interpret our data with a simple model that provides some information on the vapour pressure of DBT.

Journal article

Jizan I, Helt LG, Xiong C, Collins MJ, Choi D-Y, Chae CJ, Liscidini M, Steel MJ, Eggleton BJ, Clark ASet al., 2015, Bi-photon spectral correlation measurements from a silicon nanowire in the quantum and classical regimes, SCIENTIFIC REPORTS, Vol: 5, ISSN: 2045-2322

The growing requirement for photon pairs with specific spectral correlations in quantum optics experiments has created a demand for fast, high resolution and accurate source characterisation. A promising tool for such characterisation uses classical stimulated processes, in which an additional seed laser stimulates photon generation yielding much higher count rates, as recently demonstrated for a χ(2) integrated source in A. Eckstein et al. Laser Photon. Rev. 8, L76 (2014). In this work we extend these results to χ(3) integrated sources, directly measuring for the first time the relation between spectral correlation measurements via stimulated and spontaneous four wave mixing in an integrated optical waveguide, a silicon nanowire. We directly confirm the speed-up due to higher count rates and demonstrate that this allows additional resolution to be gained when compared to traditional coincidence measurements without any increase in measurement time. As the pump pulse duration can influence the degree of spectral correlation, all of our measurements are taken for two different pump pulse widths. This allows us to confirm that the classical stimulated process correctly captures the degree of spectral correlation regardless of pump pulse duration, and cements its place as an essential characterisation method for the development of future quantum integrated devices.

Journal article

Zhang X, Jizan I, He J, Clark AS, Choi D-Y, Chae CJ, Eggleton BJ, Xiong Cet al., 2015, Enhancing the heralded single-photon rate from a silicon nanowire by time and wavelength division multiplexing pump pulses, OPTICS LETTERS, Vol: 40, Pages: 2489-2492, ISSN: 0146-9592

Journal article

Xiong C, Collins MJ, Steel MJ, Krauss TF, Eggleton BJ, Clark ASet al., 2015, Photonic Crystal Waveguide Sources of Photons for Quantum Communication Applications, IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, Vol: 21, ISSN: 1077-260X

Journal article

Lefrancois S, Clark AS, Eggleton BJ, 2015, Optimizing optical Bragg scattering for single-photon frequency conversion, PHYSICAL REVIEW A, Vol: 91, ISSN: 1050-2947

Journal article

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