arXiv subjects

Francesco Masia

Publications and source records attributed to Francesco Masia.

11 recordsLinked to original sources

arXiv2026

Ultra-high-speed line-scan Raman imaging

Raman spectroscopic imaging has emerged as a potent tool due to its non-invasive nature and capability for chemical composition analysis. Line-scan Raman spectroscopy accelerates imaging speed by two orders of magnitude compared to point detection Raman methods. However, further enhancements in imaging speed were constrained by the readout speed of typically used charge-coupled device (CCD) spectroscopic detectors. We developed an ultra-fast line-scan Raman imaging technique based on recently available complementary metal-oxide-semiconductor (CMOS) detectors with low cost and read noise, and fast readout during exposure combined with a global shutter. Employing a high-efficiency transmissiongrating imaging spectrometer, we demonstrate imaging speeds up to two orders of magnitude faster than traditional line scan Raman imaging techniques and up to four orders of magnitude faster than point scan Raman methods, achieving Raman imaging up to 80 kHz spectral rate. We demonstrate that this technology is applicable to a variety of samples, including microplastics, biological cells, and tablets, creating images in an extremely short time frame, showcasing exceptional detection capabilities and the ability to reveal detailed information.

physics.optics↗

arXiv2025

Designing low-loss cavities across the band-gap of photonic crystal slabs

Photonic crystal cavities (PCCs) are defects in host photonic crystals (PCs) which create bound states in the PC band gap. These bound states are resonant states of the electromagnetic field with a complex resonance frequency and can have very small mode volumes. PCCs are attractive for a variety of applications, from cavity quantum electrodynamics to biosensing. A PC slab geometry is advantageous given its superior manufacturability compared to three-dimensional crystals, and the accessibility of the surface allows sensing and coupling. However, the emission into the half spaces above and below the slab limits the bound state lifetime. Controlling this emission is thus crucial for applications, most of which benefiting from a long lifetime. A range of methods to find defect geometries suppressing the emission to increase the lifetime have been demonstrated in the past. However, they do not cater for a designed resonant frequency covering a wide addressable range, as needed for multiplexed devices. Here, we demonstrate a design method controlling both resonance frequency and emission, by minimising a cost function including both losses and target frequency. We show applications on PCCs in GaAs PC slabs immersed in water, relevant for biosensing. The reduced refractive index contrast in these structures compared to previously studied PCCs embedded in vacuum renders the emission suppression more challenging. We optimize the quality factor of a standard L3 cavity from 1000 to 10^4-10^5, with an addressable resonance frequency range covering 12% relative bandwidth, spanning more than half of the band gap. We furthermore report optimised structures of H1 cavities, and provide the optimisation code for widespread use.

physics.optics↗

arXiv2022

Correlative light electron microscopy using small gold nanoparticles as single probes

Correlative light electron microscopy (CLEM) requires the availability of robust probes which are visible both in light and electron microscopy. Here we demonstrate a CLEM approach using small gold nanoparticles as a single probe. Individual gold nanoparticles bound to the epidermal growth factor protein were located with nanometric precision background-free in human cancer cells by light microscopy using resonant four-wave-mixing (FWM), and were correlatively mapped with high accuracy to the corresponding transmission electron microscopy images. We used nanoparticles of 10 nm and 5 nm radius, and show a correlation accuracy below 60 nm over an area larger than 10 um size, without the need for additional fiducial markers. Correlation accuracy was improved to below 40 nm by reducing systematic errors, while the localisation precision is below 10 nm. Polarisation-resolved FWM correlates with nanoparticle shapes, promising for multiplexing by shape recognition in future applications. Owing to the photostability of gold nanoparticles and the applicability of FWM microscopy to living cells, FWM-CLEM opens up a powerful alternative to fluorescence-based methods.

physics.optics↗

arXiv2022

uFLIM -- Unsupervised analysis of FLIM-FRET microscopy data

Despite their widespread use in cell biology, fluorescence lifetime imaging microscopy (FLIM) data-sets are challenging to analyse, because each spatial position can contain a superposition of multiple fluorescent components. Here, we present a data analysis method employing all information in the available photon budget, as well as being fast. The method, called uFLIM, determines spatial distributions and temporal dynamics of multiple fluorescent components with no prior knowledge. It goes significantly beyond current approaches which either assume the functional dependence of the dynamics, e.g. an exponential decay, or require dynamics to be known, or calibrated. Its efficient non-negative matrix factorization algorithm allows for real-time data processing. We validate in silico that uFLIM is capable to disentangle the spatial distribution and spectral properties of five fluorescing probes, from only two excitation and detection channels and a photon budget of 100 detected photons per pixel. By adapting the method to data exhibiting Forster resonant energy transfer (FRET), we retrieve the spatial and transfer rate distribution of the bound species, without constrains on donor and acceptor dynamics.

physics.bio-ph↗

arXiv2022

Low-energy electron microscopy intensity-voltage data -- factorization, sparse sampling, and classification

Low-energy electron microscopy (LEEM) taken as intensity-voltage (I-V) curves provides hyperspectral images of surfaces, which can be used to identify the surface type, but are difficult to analyze. Here, we demonstrate the use of an algorithm for factorizing the data into spectra and concentrations of characteristic components (FSC3) for identifying distinct physical surface phases. Importantly, FSC3 is an unsupervised and fast algorithm. As example data we use experiments on the growth of praseodymium oxide or ruthenium oxide on ruthenium single crystal substrates, both featuring a complex distribution of coexisting surface components, varying in both chemical composition and crystallographic structure. With the factorization result a sparse sampling method is demonstrated, reducing the measurement time by 1-2 orders of magnitude, relevant for dynamic surface studies. The FSC3 concentrations are providing the features for a support vector machine (SVM) based supervised classification of the types. Here, specific surface regions which have been identified structurally, via their diffraction pattern, as well as chemically by complementary spectro-microscopic techniques, are used as training sets. A reliable classification is demonstrated on both exemplary LEEM I-V datasets.

cond-mat.mtrl-sci↗

arXiv2018

Coherence and density dynamics of excitons in a single-layer MoS$_2$ reaching the homogeneous limit

We measure the coherent nonlinear response of excitons in a single-layer of molybdenum disulphide embedded in hexagonal boron nitride, forming a $h$-BN/MoS$_2$/$h$-BN heterostructure. Using four-wave mixing microscopy and imaging, we correlate the exciton homogeneous and inhomogeneous broadenings. We find that the exciton dynamics is governed by microscopic disorder on top of the ideal crystal properties. Analyzing the exciton ultra-fast density dynamics using amplitude and phase of the response, we investigate the relaxation pathways of the resonantly driven exciton population. The surface protection via encapsulation provides stable monolayer samples with low disorder, avoiding surface contaminations and the resulting exciton broadening and modifications of the dynamics. We identify areas localized to a few microns where the optical response is totally dominated by homogeneous broadening. Across the sample of tens of micrometers, weak inhomogeneous broadening and strain effects are observed, attributed to the remaining interaction with the $h$-BN and imperfections in the encapsulation process.

cond-mat.mes-hall↗

arXiv2018

Long Exciton Dephasing Time and Coherent Phonon Coupling in CsPbBr$_{2}$Cl Perovskite Nanocrystals

Fully-inorganic cesium lead halide perovskite nanocrystals (NCs) have shown to exhibit outstanding optical properties such as wide spectral tunability, high quantum yield, high oscillator strength as well as blinking-free single photon emission and low spectral diffusion. Here, we report measurements of the coherent and incoherent exciton dynamics on the 100 fs to 10 ns timescale, determining dephasing and density decay rates in these NCs. The experiments are performed on CsPbBr$_{2}$Cl NCs using transient resonant three-pulse four-wave mixing (FWM) in heterodyne detection at temperatures ranging from 5 K to 50 K. We found a low-temperature exciton dephasing time of 24.5$\pm$1.0 ps, inferred from the decay of the photon-echo amplitude at 5 K, corresponding to a homogeneous linewidth (FWHM) of 54$\pm$5 μeV. Furthermore, oscillations in the photon-echo signal on a picosecond timescale are observed and attributed to coherent coupling of the exciton to a quantized phonon mode with 3.45 meV energy.

cond-mat.mes-hall↗

arXiv2017

Background-free 3D nanometric localisation and sub-nm asymmetry detection of single plasmonic nanoparticles by four-wave mixing interferometry with optical vortices

Single nanoparticle tracking using optical microscopy is a powerful technique with many applications in biology, chemistry and material sciences. Despite significant advances, localising objects with nanometric position accuracy in a scattering environment remains challenging. Applied methods to achieve contrast are dominantly fluorescence based, with fundamental limits in the emitted photon fluxes arising from the excited-state lifetime as well as photobleaching. Furthermore, every localisation method reported to date requires signal acquisition from multiple spatial points, with consequent speed limitations. Here, we show a new four-wave mixing interferometry technique, whereby the position of a single non-fluorescing gold nanoparticle is determined with better than 20 nm accuracy in plane and 1 nm axially from rapid single-point acquisition measurements by exploiting optical vortices. The technique is also uniquely sensitive to particle asymmetries of only 0.5% aspect ratio, corresponding to a single atomic layer of gold, as well as particle orientation, and the detection is background-free even inside biological cells. This method opens new ways of of unraveling single-particle trafficking within complex 3D architectures.

physics.optics↗

arXiv2014

Evidence of giant oscillator strength in the exciton dephasing of CdSe nanoplatelets measured by resonant four-wave mixing

We measured the intrinsic ground-state exciton dephasing and population dynamics in colloidal quasi two-dimensional (2D) CdSe nanoplatelets at low temperature (5-50K) using transient resonant four-wave mixing in heterodyne detection. Our results indicate that below 20K the exciton dephasing is lifetime limited, with the exciton population lifetime being as fast as 1ps. This is consistent with an exciton lifetime given by a fast radiative decay due to the large in-plane coherence area of the exciton center-of-mass motion in these quasi 2D systems compared to spherical nanocrystals.

cond-mat.mes-hall↗

arXiv2012

Femtosecond phase-resolved microscopy of plasmon dynamics in individual gold nanospheres

The selective optical detection of individual metallic nanoparticles (NPs) with high spatial and temporal resolution is a challenging endeavour, yet is key to the understanding of their optical response and their exploitation in applications from miniaturised optoelectronics and sensors to medical diagnostics and therapeutics. However, only few reports on ultrafast pump-probe spectroscopy on single small metallic NPs are available to date. Here, we demonstrate a novel phase-sensitive four-wave mixing (FWM) microscopy in heterodyne detection to resolve for the first time the ultrafast changes of real and imaginary part of the dielectric function of single small (<40nm) spherical gold NPs. The results are quantitatively described via the transient electron temperature and density in gold considering both intraband and interband transitions at the surface plasmon resonance. This novel microscopy technique enables background-free detection of the complex susceptibility change even in highly scattering environments and can be readily applied to any metal nanostructure.

cond-mat.mes-hall↗

arXiv2011

Spin--flip Limited Exciton Dephasing in CdSe/ZnS Colloidal Quantum Dots

The dephasing time of the lowest bright exciton in CdSe/ZnS wurtzite quantum dots is measured from 5 K to 170 K and compared with density dynamics within the exciton fine structure using a sensitive three-beam four-wave-mixing technique unaffected by spectral diffusion. Pure dephasing via acoustic phonons dominates the initial dynamics, followed by an exponential zero-phonon line dephasing of 109 ps at 5 K, much faster than the ~10 ns exciton radiative lifetime. The zero-phonon line dephasing is explained by phonon-assisted spin-flip from the lowest bright state to dark exciton states. This is confirmed by the temperature dependence of the exciton lifetime and by direct measurements of the bright-dark exciton relaxation. Our results give an unambiguous evidence of the physical origin of the exciton dephasing in these nanocrystals.

cond-mat.mes-hall↗