arXiv subjects

Lukas Novotny

Publications and source records attributed to Lukas Novotny.

At least 55 records · Page 3Linked to original sources

arXiv2021

Transverse Confinement of Electron Beams in a 2D Optical Lattice for Compact Coherent X-Ray Sources

Compact coherent x-ray sources have been the focus of extensive research efforts over the past decades. As a result, several novel schemes like optical and nano-undulators for generating x-ray emissions in "table-top" setups are proposed, developed, and assessed. Despite the extensive efforts in the past decades, there exists no operational FEL based on optical or electromagnetic undulators. By combining the particle confinement capability of optical cavities with wiggling motion inside an optical undulator, this paper proposes a new concept for making a compact coherent x-ray source. The full-wave solution of first-principle equations based on finite-difference time-domain and particle-in-cell (FDTD/PIC) is performed to simulate inverse-Compton scattering (ICS) off both free and confined electrons. It is shown that the strong space-charge effect in a low-energy electron beam (5 MeV) is the main obstacle in acquiring coherent gain through the ICS mechanism with a 10-micrometer laser. Subsequently, it is shown that by confining the electron beam at the field nodes of an optical cavity, the space-charge effect is compensated, and additionally, the ultrahigh charge density enables high FEL-gain at confinement spots. The full-wave numerical simulations predict enhancement of about three orders of magnitude in the radiation efficiency when ICS is carried out with confined electrons compared to free electrons. These theoretical results show promising potential as a new scheme for implementing a compact coherent x-ray source.

physics.acc-ph↗

arXiv2021

Quantum control of a nanoparticle optically levitated in cryogenic free space

Tests of quantum mechanics on a macroscopic scale require extreme control over mechanical motion and its decoherence. Quantum control of mechanical motion has been achieved by engineering the radiation-pressure coupling between a micromechanical oscillator and the electromagnetic field in a resonator. Furthermore, measurement-based feedback control relying on cavity-enhanced detection schemes has been used to cool micromechanical oscillators to their quantum ground states. In contrast to mechanically tethered systems, optically levitated nanoparticles are particularly promising candidates for matter-wave experiments with massive objects, since their trapping potential is fully controllable. In this work, we optically levitate a femto-gram dielectric particle in cryogenic free space, which suppresses thermal effects sufficiently to make the measurement backaction the dominant decoherence mechanism. With an efficient quantum measurement, we exert quantum control over the dynamics of the particle. We cool its center-of-mass motion by measurement-based feedback to an average occupancy of 0.65 motional quanta, corresponding to a state purity of 43%. The absence of an optical resonator and its bandwidth limitations holds promise to transfer the full quantum control available for electromagnetic fields to a mechanical system. Together with the fact that the optical trapping potential is highly controllable, our experimental platform offers a route to investigating quantum mechanics at macroscopic scales.

quant-ph↗

arXiv2021

Raman Sideband Thermometry of Single Carbyne Chains

We demonstrate Raman sideband thermometry of single carbyne chains confined in double-walled carbon nanotubes. Our results show that carbyne's record-high Raman scattering cross section enables anti-Stokes Raman measurements at the single chain level. Using laser irradiation as a heating source, we exploit the temperature dependence of the anti-Stokes/Stokes ratio for local temperature sensing. Due to its molecular size and its large Raman cross section carbyne is an efficient probe for local temperature monitoring, with applications ranging from nanoelectronics to biology.

cond-mat.mes-hall↗

arXiv2021

Kovacs memory effect with an optically levitated nanoparticle

The understanding of the dynamics of nonequilibrium cooling and heating processes at the nanoscale is still an open problem. These processes can follow surprising relaxation paths due to, e.g., memory effects, which significantly alter the expected equilibration routes. The Kovacs effect can take place when a thermalization process is suddenly interrupted by a change of the bath temperature, leading to a non-monotonic dependence of the energy of the system. Here, we demonstrate that the Kovacs effect can be observed in the thermalization of the center of mass motion of a levitated nanoparticle. The temperature is controlled during the experiment through an external source of white gaussian noise. We describe our experiments in terms of the dynamics of a Brownian particle in a harmonic trap without any fitting parameter, suggesting that the Kovacs effect canappear in a large variety of systems.

cond-mat.mes-hall↗

arXiv2021

Observation of radiation torque shot noise on an optically levitated nanodumbbell

According to quantum theory, measurement and backaction are inextricably linked. In optical position measurements, this backaction is known as radiation pressure shot noise. In analogy, a measurement of the orientation of a mechanical rotor must disturb its angular momentum by radiation torque shot noise. In this work, we observe the shot-noise torque fluctuations arising in a measurement of the angular orientation of an optically levitated nanodumbbell. We feedback cool the dumbbell's rotational motion and investigate its reheating behavior when released from feedback. In high vacuum, the heating rate due to radiation torque shot noise dominates over the thermal and technical heating rates in the system.

physics.optics↗

arXiv2020

Escape dynamics of active particles in multistable potentials

Rare transitions between long-lived metastable states underlie a great variety of physical, chemical and biological processes. Our quantitative understanding of reactive mechanisms has been driven forward by the insights of transition state theory. In particular, the dynamic framework developed by Kramers marks an outstanding milestone for the field. Its predictions, however, do not apply to systems driven by non-conservative forces or correlated noise histories. An important class of such systems are active particles, prominent in both biology and nanotechnology. Here, we trap a silica nanoparticle in a bistable potential. To emulate an active particle, we subject the particle to an engineered external force that mimics self-propulsion. We investigate the active particle's transition rate between metastable states as a function of friction and correlation time of the active force. Our experiments reveal the existence of an optimal correlation time where the transition rate is maximized. This novel \emph{active turnover} is reminiscent of the much celebrated Kramers turnover despite its fundamentally different origin. Our observations are quantitatively supported by a theoretical analysis of a one-dimensional model. Besides providing a deeper understanding of the escape dynamics of active particles in multistable potentials, our work establishes a new, versatile experimental platform to study particle dynamics in non-equilibrium settings.

cond-mat.soft↗

arXiv2020

Coupling interlayer excitons to whispering gallery modes in van der Waals heterostructures

Van der Waals heterostructures assembled from two-dimensional materials offer a promising platform to engineer structures with desired optoelectronic characteristics. Here we use waveguide-coupled disk resonators made of hexagonal boron nitride (h-BN) to demonstrate cavity-coupled emission from interlayer excitons of a heterobilayer of two monolayer transition metal dichalcogonides. We sandwich a MoSe$_{\rm 2}$ - WSe$_{\rm 2}$ heterobilayer between two slabs of h-BN and directly pattern the resulting stack into waveguide-coupled disk resonators. This enables us to position the active materials into regions of highest optical field intensity, thereby maximizing the mode overlap and the coupling strength. Since the interlayer exciton emission energy is lower than the optical band gaps of the individual monolayers and since the interlayer transition itself has a weak oscillator strength, the circulating light is only weakly reabsorbed, which results in an unaffected quality factor. Our devices are fully waveguide-coupled and represent a promising platform for on-chip van der Waals photonics.

cond-mat.mtrl-sci↗

arXiv2020

Cooling mechanical oscillators by coherent control

In optomechanics, electromagnetic fields are harnessed to control a single mode of a mechanically compliant system, while other mechanical degrees of freedom remain unaffected due to the modes' mutual orthogonality and high quality factor. Extension of the optical control beyond the directly addressed mode would require a controlled coupling between mechanical modes. Here, we introduce an optically controlled coupling between two oscillation modes of an optically levitated nanoparticle. We sympathetically cool one oscillation mode by coupling it coherently to the second mode, which is feedback cooled. Furthermore, we demonstrate coherent energy transfer between mechanical modes and discuss its application for ground-state cooling.

quant-ph↗

arXiv2020

Optically levitated rotor at its thermal limit of frequency stability

Optically levitated rotors are prime candidates for torque sensors whose precision is limited by the fluctuationsof the rotation frequency. In this work we investigate an optically levitated rotor at its fundamental thermal limitof frequency stability, where rotation-frequency fluctuations arise solely due to coupling to the thermal bath.

physics.optics↗

arXiv2020

Raman Scattering Cross Section of Confined Carbyne

We experimentally quantify the Raman scattering from individual carbyne chains confined in double-walled carbon nanotubes. We find that the resonant differential Raman cross section of confined carbyne is on the order of $10^{-22}$ cm$^{2}\,$sr$^{-1}$ per atom, making it the strongest Raman scatterer ever reported.

cond-mat.mes-hall↗

arXiv2020

TMDC Resonators for Second Harmonic Signal Enhancement

In addition to their strong nonlinear optical response, transition metal dichalcogenides (TMDCs) possess a high refractive index in the visible and infrared regime. Therefore, by patterning those TMDCs into dielectric nanoresonators, one can generate highly confined electromagnetic modes. Controlled fabrication of TMDC nanoresonators does not only enhance the material's intrinsic nonlinear response, but also allows for spatially shaping the emission via nanoresonator arrays. Here we fabricate patterned WS2 disks that support a high internal resonant electric field and show strong enhancement of second harmonic (SH) generation in the visible regime. In addition, we assemble the WS2 disks in arrays to spatially direct the coherent SH emission, in analogy to phased array antennas. Finally, we investigate and discuss drastic differences in the areal emission origin and intensity of the measured SH signals, which we find to depend on material variations of the used bulk WS2.

physics.optics↗

arXiv2019

Rapid flipping of parametric phase states

Since the invention of the solid-state transistor, the overwhelming majority of computers followed the von Neumann architecture that strictly separates logic operations and memory. Today, there is a revived interest in alternative computation models accompanied by the necessity to develop corresponding hardware architectures. The Ising machine, for example, is a variant of the celebrated Hopfield network based on the Ising model. It can be realized with artifcial spins such as the `parametron' that arises in driven nonlinear resonators. The parametron encodes binary information in the phase state of its oscillation. It enables, in principle, logic operations without energy transfer and the corresponding speed limitations. In this work, we experimentally demonstrate flipping of parametron phase states on a timescale of an oscillation period, much faster than the ringdown time τthat is often (erroneously) deemed a fundamental limit for resonator operations. Our work establishes a new paradigm for resonator-based logic architectures.

physics.app-ph↗

arXiv2019

Motional Sideband Asymmetry of a Nanoparticle Optically Levitated in Free Space

The hallmark of quantum physics is Planck's constant $h$, whose finite value entails the quantization that gave the theory its name. The finite value of $h$ gives rise to inevitable zero-point fluctuations even at vanishing temperature. The zero-point fluctuation of mechanical motion becomes smaller with growing mass of an object, making it challenging to observe at macroscopic scales. Here, we transition a dielectric particle with a diameter of 136 nm from the classical realm to the regime where its zero-point motion emerges as a sizeable contribution to its energy. To this end, we optically trap the particle at ambient temperature in ultrahigh vacuum and apply active feedback cooling to its center-of-mass motion. We measure an asymmetry between the Stokes and anti-Stokes sidebands of photons scattered by the levitated particle, which is a signature of the particle's quantum ground state of motion.

physics.optics↗

arXiv2019

Electron transport through metal/MoS2 interfaces: edge- or area-dependent process?

In ultra-thin two-dimensional (2-D) materials, the formation of ohmic contacts with top metallic layers is a challenging task that involves different processes than in bulk-like structures. Besides the Schottky barrier height, the transfer length of electrons between metals and 2-D monolayers is a highly relevant parameter. For MoS$_2$, both short ($\le$30 nm) and long ($\ge$0.5 $μ$m) values have been reported, corresponding to either an abrupt carrier injection at the contact edge or a more gradual transfer of electrons over a large contact area. Here we use \textit{ab initio} quantum transport simulations to demonstrate that the presence of an oxide layer between a metallic contact and a MoS$_2$ monolayer, for example TiO$_2$ in case of titanium electrodes, favors an area-dependent process with a long transfer length, while a perfectly clean metal-semiconductor interface would lead to an edge process. These findings reconcile several theories that have been postulated about the physics of metal/MoS$_2$ interfaces and provide a framework to design future devices with lower contact resistances.

cond-mat.mes-hall↗

arXiv2019

One-dimensional edge contacts to a monolayer semiconductor

Integration of electrical contacts into van der Waals (vdW) heterostructures is critical for realizing electronic and optoelectronic functionalities. However, to date no scalable methodology for gaining electrical access to buried monolayer two-dimensional (2D) semiconductors exists. Here we report viable edge contact formation to hexagonal boron nitride (hBN) encapsulated monolayer MoS$_2$. By combining reactive ion etching, in situ Ar$^+$ sputtering and annealing, we achieve a relatively low edge contact resistance, high mobility (up to ~30 cm$^2$/Vs) and high on-current density (>50 uA/um at V$_{\rm DS}$ = 3V), comparable to top contacts. Furthermore, the atomically smooth hBN environment also preserves the intrinsic MoS$_2$ channel quality during fabrication, leading to a steep subthreshold swing of 116 mV/dec with a negligible hysteresis. Hence, edge contacts are highly promising for large-scale practical implementation of encapsulated heterostructure devices, especially those involving air sensitive materials, and can be arbitrarily narrow, which opens the door to further shrinkage of 2D device footprint.

cond-mat.mtrl-sci↗

arXiv2019

Resolved Sideband Cooling of a Levitated Nanoparticle in the Presence of Laser Phase Noise

We investigate the influence of laser phase noise heating on resolved sideband cooling in the context of cooling the center-of-mass motion of a levitated nanoparticle in a high-finesse cavity. Although phase noise heating is not a fundamental physical constraint, the regime where it becomes the main limitation in Levitodynamics has so far been unexplored and hence embodies from this point forward the main obstacle in reaching the motional ground state of levitated mesoscopic objects with resolved sideband cooling. We reach minimal center-of-mass temperatures comparable to $T_{min}=10$mK at a pressure of $p = 3\times 10^{-7}$mbar, solely limited by phase noise. Finally we present possible strategies towards motional ground state cooling in the presence of phase noise.

quant-ph↗

arXiv2019

Optical antennas driven by quantum tunneling: a key issues review

Analogous to radio- and microwave antennas, optical nanoantennas are devices that receive and emit radiation at optical frequencies. Until recently, the realization of electrically driven optical antennas was an outstanding challenge in nanophotonics. In this review we discuss and analyze recent reports in which quantum tunneling-specifically inelastic electron tunneling-is harnessed as a means to convert electrical energy into photons, mediated by optical antennas. To aid this analysis we introduce the fundamentals of optical antennas and inelastic electron tunneling. Our discussion is focused on recent progress in the field and on future directions and opportunities.

cond-mat.mes-hall↗

arXiv2019

Optimal Position Detection of a Dipolar Scatterer in a Focused Field

We theoretically analyze the problem of detecting the position of a classical dipolar scatterer in a strongly focused optical field. We suggest an optimal measurement scheme and show that it resolves the scatterer's position in three dimensions at the Heisenberg limit of the imprecision-backaction product. We apply our formalism to levitated-optomechanics experiments and show that backscattering detection provides sufficient information to feedback-cool the particle's motion along the optical axis to a phonon occupancy below unity under realistic experimental conditions.

physics.optics↗