arXiv subjects

Thomas F. Krauss

Publications and source records attributed to Thomas F. Krauss.

At least 19 recordsLinked to original sources

arXiv2026

High resolution meta-stereomicroscope based on birefringent meta-optics

Achieving both high lateral and high depth resolution is a longstanding goal in stereomicroscopy. Although meta-optics have revolutionized lens design by alleviating the physical constraints of conventional optical architectures, existing metalens-assisted stereomicroscopes still suffer from field of view (FOV) mismatch between meta-optical and conventional optical components in stereomicroscopes, thereby limiting their imaging performance. Here, we show that this mismatch can be fundamentally eliminated through a fully meta-optical architecture. The integrated system enables flexible control of numerical aperture and magnification with larger depth of field (DOF) and FOV than those of the metalens-assisted stereomicroscopes. Experimentally, we achieve an integrated meta-stereomicroscope with a lateral resolution of 435 nm, surpassing the performance of previously reported stereomicroscopes. Empowered by a stereo neural network, the system enables straightforward reconstruction of high-resolution three-dimensional surface morphology with a depth resolution of 1026 nm, demonstrating the capability to simultaneously achieve high lateral and depth resolution imaging. This integrated architecture operates in both transmission and reflection modes for biomedical imaging and industrial inspection, highlighting its broad applicability for real-time observation across biomedical and industrial scenarios.

physics.optics↗

arXiv2025

Ultrasensitive Anti-Stokes Luminescence Thermometry in Transition Metal Dichalcogenide Monolayers

Accurate temperature mapping at the nanoscale is a critical challenge in modern science and technology, as conventional methods fail at these dimensions. To address this challenge, we demonstrate a highly sensitive nanothermometer using anti-Stokes photoluminescence, also known as photoluminescence upconversion (UPL), in monolayer tungsten disulfide ($\mathrm{WS_2}$). Leveraging the direct band gap and strong exciton-phonon coupling in the two-dimensional monolayers, we achieve an exceptional relative sensitivity above $4\%\,\mathrm{K}^{-1}$ across the 300 K to 425 K range, ranking it among the best-performing materials reported. A strong resonantly enhanced UPL is observed, confirming the central role of optical phonons in the upconversion mechanism. Furthermore, we introduce a new analytical model to quantitatively describe the UPL process, taking into account the interplay of phonon populations, bandgap narrowing, and substrate effects, which predicts resonant temperatures and provides a framework with broad applicability to any material exhibiting an anti-Stokes photoluminescence response. To demonstrate its use as a high-resolution optical thermometer, we map a $20\,^{\circ}\mathrm{C}$ thermal gradient across a $20\,μ\mathrm{m}$ long monolayer with a spatial resolution of $1\,μ\mathrm{m}$. With its high sensitivity, strong signal, and excellent reproducibility, our work establishes monolayer transition metal dichalcogenide as a leading platform for non-invasive thermal sensing in advanced microelectronic and biological systems.

cond-mat.mtrl-sci↗

arXiv2025

Ultrasensitive Alzheimer's Disease Biomarker Detection with Nanopillar Photonic Crystal Biosensors

The recent development of drugs able to mitigate neurodegenerative diseases has created an urgent need for novel diagnostics. Although biomarker detection directly in patients' blood is now possible, low-cost point-of-care tests remain a challenge, because relevant biomarkers, especially amyloid-β(A$β$) peptides, are small, they occur at very low concentrations, and detecting a single marker is insufficient. Here, we introduce an optical biosensor based on a nanopillar structure that employs a gold nanoparticle amplification strategy. The sensor is able to detect 20 pg/ml of A$β$42 and A$β$40 in undiluted serum, which is the clinically required level. We also show the detection of the A$β$42 and A$β$40 peptides in the same channel, which is highly relevant for assessing disease progress and opens a route towards multiplexing. Alongside their simplicity and portability, these nanotechnology innovations make a major contribution to the ability to detect and monitor the progression of neurodegenerative diseases such as Alzheimer's.

physics.optics↗

arXiv2024

High efficiency glass-based VUV metasurfaces

Most advances in metaoptics have been made at visible wavelengths and above; in contrast, the vacuum ultraviolet (VUV) has barely been explored despite numerous scientific and technological opportunities. Creating metaoptic elements at this short wavelength is challenging due to the scarcity of VUV transparent materials and the small sizes of the required nanostructures. Here, we present the first transmissive VUV (175 nm) metalens. By using UV-grade silica and trading-off the Nyquist requirement for subwavelength structures against feasibility of the fabrication process, we achieve a step-change in diffraction efficiencies for wavelengths shorter than 300 nm. Our large numerical aperture (NA = 0.5) metalens shows an average diffraction efficiency of (53.3 +- 1.4)%. This demonstration opens up new avenues for compact flat optic systems operating in the VUV range.

physics.optics↗

arXiv2023

Fano resonance-assisted all-dielectric array for enhanced near-field optical trapping of nanoparticles

Near-field optics can overcome the diffraction limit by creating strong optical gradients to enable the trapping of nanoparticles. However, it remains challenging to achieve efficient stable trapping without heating and thermal effects. Dielectric structures have been used to address this issue, but they usually offer weak trap stiffness. In this work, we exploit the Fano resonance effect in an all-dielectric quadrupole nanostructure to realize a twenty-fold enhancement of trap stiffness, compared to the off-resonance case. This enables a high effective trap stiffness of $1.19$ fN/nm for 100 nm diameter polystyrene nanoparticles with 3.5 mW/$μ$m$^{2}$ illumination. Furthermore, we demonstrate the capability of the structure to simultaneously trap two particles at distinct locations within the nanostructure array.

physics.optics↗

arXiv2022

Van der Waals Materials for Applications in Nanophotonics

Numerous optical phenomena and applications have been enabled by nanophotonic structures. Their current fabrication from high refractive index dielectrics, such as silicon or gallium phosphide, pose restricting fabrication challenges, while metals, relying on plasmons and thus exhibiting high ohmic losses, limit the achievable applications. Here, we present an emerging class of layered so-called van der Waals (vdW) crystals as a viable nanophotonics platform. We extract the dielectric response of 11 mechanically exfoliated thin-film (20-200 nm) van der Waals crystals, revealing high refractive indices up to n = 5, pronounced birefringence up to $Δ$n = 3, sharp absorption resonances, and a range of transparency windows from ultraviolet to near-infrared. We then fabricate nanoantennas on SiO$_2$ and gold utilizing the compatibility of vdW thin films with a variety of substrates. We observe pronounced Mie resonances due to the high refractive index contrast on SiO$_2$ leading to a strong exciton-photon coupling regime as well as largely unexplored high-quality-factor, hybrid Mie-plasmon modes on gold. We demonstrate further vdW-material-specific degrees of freedom in fabrication by realizing nanoantennas from stacked twisted crystalline thin-films, enabling control of nonlinear optical properties, and post-fabrication nanostructure transfer, important for nano-optics with sensitive materials.

physics.optics↗

arXiv2022

Understanding the impact of heavy ions and tailoring the optical properties of large-area Monolayer WS2 using Focused Ion Beam

Focused ion beam (FIB) has been used as an effective tool for precise nanoscale fabrication. It has recently been employed to tailor defect engineering in functional nanomaterials such as two-dimensional transition metal dichalcogenides (TMDCs), providing desirable properties in TMDC-based optoelectronic devices. However, the damage caused by the FIB irradiation and milling process to these delicate atomically thin materials, especially in the extended area, has not yet been elaboratively characterised. Understanding the correlation between lateral ion beam effects and optical properties of 2D TMDCs is crucial in designing and fabricating high-performance optoelectronic devices. In this work, we investigate lateral damage in large-area monolayer WS2 caused by the gallium focused ion beam milling process. Three distinct zones away from the milling location are identified and characterised via steady-state photoluminescence (PL) and Raman spectroscopy. An unexpected bright ring-shaped emission around the milled location has been revealed by time-resolved PL spectroscopy with high spatial resolution. Our finding opens new avenues for tailoring the optical properties of TMDCs by charge and defect engineering via focused ion beam lithography. Furthermore, our study provides evidence that while some localised damage is inevitable, distant destruction can be eliminated by reducing the ion beam current. It paves the way for the use of FIB to create nanostructures in 2D TMDCs, as well as the design and realisation of optoelectrical devices on a wafer scale.

physics.app-ph↗

arXiv2022

Correction of aberrations via polarization in single layer metalenses

The correction of multiple aberrations in an optical system requires different optical elements, which increases its cost and complexity. Metasurfaces hold great promise to providing new functionality for miniaturized and low-cost optical systems. A key advantage over their bulk counterparts is the metasurface's ability to respond to the polarization of light, which adds a new degree of freedom to the optical design. Here, we show that polarization control enables a form-birefringent metalens to correct for both spherical and off-axis aberrations using a single element only, which is not possible with bulk optics. The metalens encodes two phase profiles onto the same surface, thus allowing switching from high resolution to wide field of view operation. Such ability to obtain both high resolution and wide field of view in a single layer is an important step towards integration of miniaturized optical systems, which may find many applications, e.g., in microscopy and endoscopy.

physics.optics↗

arXiv2021

Transition metal dichalcogenide dimer nano-antennas with ultra-small gaps

Transition metal dichalcogenides have emerged as promising materials for nano-photonic resonators due to their large refractive index, low absorption within a large portion of the visible spectrum and compatibility with a wide range of substrates. Here we use these properties to fabricate WS$_2$ double-pillar nano-antennas in a variety of geometries enabled by the anisotropy in the crystal structure. Using dark field spectroscopy, we reveal multiple Mie resonances, to which we couple WSe$_2$ monolayer photoluminescence and achieve Purcell enhancement and an increased fluorescence by factors up to 240. We introduce post-fabrication atomic force microscope repositioning and rotation of dimer nano-antennas, achieving gaps as small as 10$\pm$5 nm, opening the possibility to a host of potential applications including strong Purcell enhancement of single photon emitters and optical trapping, which we study in simulations. Our findings highlight the advantages of using transition metal dichalcogenides for nano-photonics by exploring new applications enabled by their unique properties.

physics.app-ph↗

arXiv2020

Attachment and antibiotic response of early-stage biofilms studied using resonant hyperspectral imaging

Many bacterial species readily develop biofilms that act as a protective matrix against external challenge, e.g. from antimicrobial treatment. Therefore, biofilms are often responsible for persistent and recurring infections. Established methods for studying biofilms are either destructive or they focus on the biofilm surface. A non-destructive method that is sensitive to the underside of the biofilm is highly desirable, as it allows studying the penetration of antibiotics through the film. Here, we demonstrate that the high surface sensitivity of resonant hyperspectral imaging provides this capability. The method allows us to monitor the early stages of Escherichia coli biofilm formation, cell attachment and microcolony formation, in-situ and in real time. We study the response of the biofilm to a number of different antibiotics and verify our observations using confocal microscopy. Based on this ability to closely monitor the surface-bound cells, resonant hyperspectral imaging gives new insights into the antimicrobial resistance of biofilms.

physics.bio-ph↗

arXiv2019

Hybrid plasmonic waveguide coupling of photons from a single molecule

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 auto-correlation 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 coupling efficiency is found to be $β_{WG}=11.6(1.5)\%$. 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.

quant-ph↗

arXiv2017

1305 nm MoTe2-on-silicon Laser

The missing piece in the jigsaw of silicon photonics is a light source that can be easily incorporated into the standard silicon fabrication process. Recent advances in the development of atomically thin layers of semiconducting transition metal dichalogenides (TMDs), with direct bandgaps in the near-infrared region, have opened up new possibilities for addressing this need. Here, we report a unique silicon laser source that employs molybdenum ditelluride (MoTe2) as a gain material in a photonic crystal nanocavity resonator, fabricated in silicon-on-insulator. We demonstrate optically pumped MoTe2-on-silicon devices lasing at 1305 nm, i.e. in the centre of the O-band used in optical communications, operating in the continuous-wave (CW) regime, at room temperature and with a threshold power density as low as 1.5 kW/cm2. This 2D-on-silicon geometry offers the promise of an integrated low-cost electrically pumped nanoscale silicon light source, thereby adding an essential building block to the silicon photonics platform.

cond-mat.mes-hall↗

arXiv2017

Highly effective relativistic free carrier plasma mirror confined within a silicon slow light photonic crystal waveguide

Reflection at relativistically moving plasma mirrors is a well-known approach for frequency conversion as an alternative to nonlinear techniques. A key issue with plasma mirrors is the need for a high carrier concentration, of order 10^21 cm^-3, to achieve an appreciable reflectivity. To generate such high carrier concentrations, short laser pulses with extreme power densities of the order >10^15 W/cm^2 are required. Here, we introduce a novel waveguide-based method for generating relativistically moving plasma mirrors that requires much lower pump powers and much less carrier concentration. Specifically, we achieve an experimental demonstration of 35% reflection for a carrier concentration of 5*10^17/cm^3 generated by a power density of only 1.2*10^9 W/cm^2. Both the plasma mirror and the signal are confined and propagating within a solid state silicon slow light photonic crystal waveguide. This extraordinary effect only becomes possible because we exploit an indirect intraband optical transition in a dispersion engineered slow light waveguide, where the incident light cannot couple to other states beyond the moving front and has to reflect from it. The moving free carrier (FC) plasma mirror is generated by two photon absorption of 6 ps long pump pulse with a peak power of 6.2 W. The reflection was demonstrated by the interaction of a continuous wave (CW) probe wave co-propagating with the relativistic FC plasma mirror inside a 400 micro-meter long slow light waveguide. Upon interaction with the FC plasma mirror, the probe wave packets, which initially propagate slower than the plasma mirror, are bounced and accelerated, finally escaping from the front in forward direction. The forward reflection of the probe wave packets are accompanied by a frequency upshift. The reflection efficiency is estimated for the part of the CW probe interacting with the pump pulse.

physics.optics↗

arXiv2016

Lower bound for the spatial extent of localized modes in photonic-crystal waveguides with small random imperfections

Light localization due to random imperfections in periodic media is paramount in photonics research. The group index is known to be a key parameter for localization near photonic band edges, since small group velocities reinforce light interaction with imperfections. Here, we show that the size of the smallest localized mode that is formed at the band edge of a one-dimensional periodic medium is driven instead by the effective photon mass, i.e. the flatness of the dispersion curve. Our theoretical prediction is supported by numerical simulations, which reveal that photonic-crystal waveguides can exhibit surprisingly small localized modes, much smaller than those observed in Bragg stacks thanks to their larger effective photon mass. This possibility is demonstrated experimentally with a photonic-crystal waveguide fabricated without any intentional disorder, for which near-field measurements allow us to distinctly observe a wavelength-scale localized mode despite the smallness ($\sim 1/1000$ of a wavelength) of the fabrication imperfections.

physics.optics↗

arXiv2015

Pure-Quartic Solitons

Temporal optical solitons have been the subject of intense research due to their intriguing physics and applications in ultrafast optics and supercontinuum generation. Conventional bright optical solitons result from the interaction of anomalous group-velocity dispersion and self-phase modulation. Here we report the discovery of an entirely new class of bright solitons arising purely from the interaction of negative fourth-order dispersion and self-phase modulation, which can occur even for normal group-velocity dispersion. We provide experimental and numerical evidence of shape-preserving propagation and flat temporal phase for the fundamental pure-quartic soliton and periodically modulated propagation for the higher-order pure-quartic solitons. Using analytic theory, we derive the approximate shape of the fundamental pure-quartic soliton exhibiting excellent agreement with our experimental observations. Our discovery, enabled by the unique dispersion of photonic crystal waveguides, could find applications in communications and ultrafast lasers.

physics.optics↗

arXiv2014

Ultra-compact optical auto-correlator based on slow-light enhanced third harmonic generation in a silicon photonic crystal waveguide

The ability to use coherent light for material science and applications is directly linked to our ability to measure short optical pulses. While free-space optical methods are well-established, achieving this on a chip would offer the greatest benefit in footprint, performance, flexibility and cost, and allow the integration with complementary signal processing devices. A key goal is to achieve operation at sub-Watt peak power levels and on sub-picosecond timescales. Previous integrated demonstrations require either a temporally synchronized reference pulse, an off-chip spectrometer, or long tunable delay lines. We report the first device capable of achieving single-shot time-domain measurements of near-infrared picosecond pulses based on an ultra-compact integrated CMOS compatible device, with the potential to be fully integrated without any external instrumentation. It relies on optical third-harmonic generation in a slow-light silicon waveguide. Our method can also serve as a powerful in-situ diagnostic tool to directly map, at visible wavelengths, the propagation dynamics of near-infrared pulses in photonic crystals.

physics.optics↗

arXiv2013

Phase sensitive amplification in silicon photonic crystal waveguides

We experimentally demonstrate phase sensitive amplification (PSA) in a silicon photonic crystal waveguide based on pump-degenerate four-wave mixing. An 11 dB phase extinction ratio is obtained in a record compact 196 μm nanophotonic device due to broadband slow-light, in spite of the presence of two-photon absorption and free-carriers. Numerical calculations show good agreement with the experimental results.

physics.optics↗

arXiv2013

Multi-photon absorption limits to heralded single photon sources

Single photons are of paramount importance to future quantum technologies, including quantum communication and computation. Nonlinear photonic devices using parametric processes offer a straightforward route to generating photons, however additional nonlinear processes may come into play and interfere with these sources. Here we analyse these sources in the presence of multi-photon processes for the first time. We conduct experiments in silicon and gallium indium phosphide photonic crystal waveguides which display inherently different nonlinear absorption processes, namely two-photon (TPA) and three-photon absorption (ThPA), respectively. We develop a novel model capturing these diverse effects which is in excellent quantitative agreement with measurements of brightness, coincidence-to-accidental ratio (CAR) and second-order correlation function g(2)(0), showing that TPA imposes an intrinsic limit on heralded single photon sources. We devise a new figure of merit, the quantum utility (QMU), enabling direct comparison and optimisation of single photon sources.

physics.optics↗