arXiv subjects

Lukas Novotny

Publications and source records attributed to Lukas Novotny.

At least 73 records · Page 4Linked to original sources

arXiv2019

Beam steering with a nonlinear optical phased array antenna

Transition metal dichalcogenides (TMDCs) exhibit high second harmonic (SH) generation in the visible due to their non-centrosymmetric crystal structure in odd-layered form and direct bandgap transition when thinned down to a monolayer. In order to emit the SH radiation into a desired direction one requires a means to control the phase of the in-plane nonlinear polarization. Here, we couple the SH response of a monolayer MoS$_{\text{2}}$ to an optical phased array antenna and demonstrate controllable steering of the nonlinear emission. By exploiting the intrinsic SH generation by the phased array antenna we achieve uniform emission efficiency into a broad angular range. Our work has relevance for novel optoelectronic applications, such as programmable optical interconnects and on-chip LIDAR.

physics.optics↗

arXiv2019

Waveguide-integrated van der Waals heterostructure photodetector at telecom band with high speed and high responsivity

Intensive efforts have been devoted to exploit novel optoelectronic devices based on two-dimensional (2D) transition-metal dichalcogenides (TMDCs) owing to their strong light-matter interaction and distinctive material properties. In particular, photodetectors featuring both high-speed and high-responsivity performance are of great interest for a vast number of applications such as high-data-rate interconnects operated at standardized telecom wavelengths. Yet, the intrinsically small carrier mobilities of TMDCs become a bottleneck for high-speed application use. Here, we present high-performance vertical van der Waals heterostructure-based photodetectors integrated on a silicon photonics platform. Our vertical MoTe2/graphene heterostructure design minimizes the carrier transit path length in TMDCs and enables a record-high measured bandwidth of at least 24GHz under a moderate bias voltage of -3 volts. Applying a higher bias or employing thinner MoTe2 flakes boosts the bandwidth even to 50GHz. Simultaneously, our device reaches a high external responsivity of 0.2A/W for incident light at 1300nm, benefiting from the integrated waveguide design. Our studies shed light on performance trade-offs and present design guidelines for fast and efficient devices. The combination of 2D heterostructures and integrated guided-wave nano photonics defines an attractive platform to realize high-performance optoelectronic devices, such as photodetectors, light-emitting devices and electro-optic modulators.

cond-mat.mes-hall↗

arXiv2019

Cavity-Based 3D Cooling of a Levitated Nanoparticle via Coherent Scattering

We experimentally realize cavity cooling of all three translational degrees of motion of a levitated nanoparticle in vacuum. The particle is trapped by a cavity-independent optical tweezer and coherently scatters tweezer light into the blue detuned cavity mode. For vacuum pressures around $10^{-5}\,{\rm mbar}$, minimal temperatures along the cavity axis in the mK regime are observed. Simultaneously, the center-of-mass (COM) motion along the other two spatial directions is cooled to minimal temperatures of a few hundred $\rm mK$. Measuring temperatures and damping rates as the pressure is varied, we find that the cooling efficiencies depend on the particle position within the intracavity standing wave. This data and the behaviour of the COM temperatures as functions of cavity detuning and tweezer power are consistent with a theoretical analysis of the experiment. Experimental limits and opportunities of our approach are outlined.

quant-ph↗

arXiv2019

Light from van der Waals quantum tunneling devices

The understanding of and control over light emission from quantum tunneling has challenged researchers for more than four decades due to the intricate interplay of electrical and optical properties in atomic scale volumes. Here we introduce a device architecture that allows for the disentanglement of electronic and photonic pathways - van der Waals quantum tunneling devices. The electronic properties are defined by a stack of two-dimensional atomic crystals whereas the optical properties are controlled via an external photonic architecture. In van der Waals heterostructure made of gold, hexagonal boron nitride and graphene we find that inelastic tunneling results in the emission of photons and surface plasmon polaritons. By coupling these heterostructures to optical nanocube antennas we achieve resonant enhancement of the photon emission rate in narrow frequency bands by four orders of magnitude. Our results lead the way towards a new generation of nanophotonic devices that are driven by quantum tunneling.

cond-mat.mes-hall↗

arXiv2018

Cold Damping of an Optically Levitated Nanoparticle to micro-Kelvin Temperatures

We implement a cold damping scheme to cool one mode of the center-of-mass motion of an optically levitated nanoparticle in ultrahigh vacuum from room temperature to a record-low temperature of 100 micro-Kelvin. The measured temperature dependence on feedback gain and thermal decoherence rate is in excellent agreement with a parameter-free model. We determine the imprecision-backaction product for our system and provide a roadmap towards ground-state cooling of optically levitated nanoparticles.

physics.optics↗

arXiv2018

Quantized thermal conductance in metallic heterojunctions

To develop next-generation electronics and high efficiency energy-harvesting devices, it is crucial to understand how charge and heat are transported at the nanoscale. Metallic atomic-size contacts are ideal systems to probe the quantum limits of transport. The thermal and electrical conductance of gold atomic contacts has been recently proven to be quantized at room temperature. However, a big experimental challenge in such measurements is represented by the fast breaking dynamics of metallic junctions at room temperature, which can exceed the typical response time of the thermal measurement. Here we use a break-junction setup that combines Scanning Tunneling Microscopy with suspended micro electro-mechanical systems with a gold-covered membrane and an integrated heater acting also as thermometer. By using other metals as tip materials, namely Pt, PtIr and W, we show heat transport measurements through single gold atomic contacts. The dependence of the thermal conductance is analysed as function of contact size and material used. We find that by using Pt and Pt-Ir tips we can maximize the mechanical stability and probability of forming single Au atomic contacts. We then show the quantization of the electrical and thermal conductance with the verification of the Wiedemann-Franz law at the atomic scale. We expect these findings to increase the flexibility of experimental techniques probing heat transport in metallic quantum point contacts and to enable the investigation of thermal properties of molecular junctions.

cond-mat.mes-hall↗

arXiv2018

GHz Rotation of an Optically Trapped Nanoparticle in Vacuum

We report on rotating an optically trapped silica nanoparticle in vacuum by transferring spin angular momentum of light to the particle's mechanical angular momentum. At sufficiently low damping, realized at pressures below $10^{-5}$ mbar, we observe rotation frequencies of single 100 nm particles exceeding 1 GHz. We find that the steady-state rotation frequency scales linearly with the optical trapping power and inversely with pressure, consistent with theoretical considerations based on conservation of angular momentum. Rapidly changing the polarization of the trapping light allows us to extract the pressure-dependent response time of the particle's rotational degree of freedom.

physics.optics↗

arXiv2018

Quantized optical near-field interactions measured with a superconducting nanowire detector

The quantum nature of optical near-fields is interesting from a theoretical perspective and of importance for practical applications, such as high-resolution imaging, sensing and antenna-coupled quantum light sources. In this work we use a custom-designed superconducting nanowire single-photon detector (SNSPD) to directly read out the near-field interaction between source and detector. We use a subwavelength-sized aperture at the end of an optical fiber to record spatial near-field maps and to measure the distance dependence of the optical near-field interaction. Our measurements can be well described by a superposition of evanescent source fields with no noticeable probe-sample coupling. Our approach can be used for further studies of quantum near-field interactions and for the development of near-field imaging techniques with single quantum sensitivity.

physics.app-ph↗

arXiv2018

Measuring the Internal Temperature of a Levitated Nanoparticle in High Vacuum

The interaction of an object with its surrounding bath can lead to a coupling between the object's internal degrees of freedom and its center-of-mass motion. This coupling is especially important for nanomechanical oscillators, which are amongst the most promising systems for preparing macroscopic objects in quantum mechanical states. Here we exploit this coupling to derive the internal temperature of a levitated nanoparticle from measurements of its center-of-mass dynamics. For a laser-trapped silica particle in high vacuum we find an internal temperature of $1000(60)\,\mathrm{K}$. The measurement and control of the internal temperature of nanomechanical oscillators is of fundamental importance because blackbody emission sets limits to the coherence of macroscopic quantum states.

physics.ins-det↗

arXiv2018

Minimizing Residues and Strain in 2D Materials Transferred from PDMS

Integrating layered two-dimensional (2D) materials into 3D heterostructures offers opportunities for novel material functionalities and applications in electronics and photonics. In order to build the highest quality heterostructures, it is crucial to preserve the cleanliness and morphology of 2D material surfaces that come in contact with polymers such as PDMS during transfer. Here we report that substantial residues and up to ~0.22% compressive strain can be present in monolayer MoS$_{2}$ flakes transferred using PDMS. We show that a UV-ozone pre-cleaning of the PDMS surface before exfoliation significantly reduces organic residues on transferred MoS$_{2}$ flakes. An additional 200$^{\circ}$C vacuum anneal after transfer efficiently removes interfacial bubbles and wrinkles as well as accumulated strain, thereby restoring the surface morphology of transferred flakes to their native state. Our recipe is important for building clean heterostructures of 2D materials and increasing the reproducibility and reliability of devices based on them.

cond-mat.mtrl-sci↗

arXiv2018

Calibration and temperature measurement of levitated optomechanical sensors

Optically levitated nanoparticles offer enormous potential for precision sensing. However, as for any other metrology device, the absolute measurement performance of a levitated-particle sensor is limited by the accuracy of the calibration relating the measured signal to an absolute displacement of the particle. Here, we suggest and demonstrate calibration protocols for levitated-nanoparticle sensors. Our calibration procedures include the treatment of anharmonicities in the trapping potential, as well as a protocol using a harmonic driving force, which is applicable if the sensor is coupled to a heat bath of unknown temperature. Finally, using the calibration, we determine the center-of-mass temperature of an optically levitated particle in thermal equilibrium from its motion, and discuss the optimal measurement time required to determine said temperature.

physics.optics↗

arXiv2018

Optical potential mapping with a levitated nanoparticle at sub-wavelength distances from a membrane

The controllable positioning of a vacuum-levitated object near a material surface is of importance for studying short-range forces, such as Casimir forces, interfacial friction forces, or gravity in yet unexplored parameter regimes. Here we optically levitate a nanoparticle in a laser beam strongly focused on a dielectric membrane. By investigating the motion of the trapped particle in vacuum, we map the position-dependent optical potential of the particle. We interferometrically measure the distance between the particle and the surface and demonstrate stable trapping in sub-wavelength proximity of the dielectric surface. Our work is important for the development of on-chip levitated optomechanics and for measuring short-range forces at sub-wavelength distances.

physics.optics↗

arXiv2018

Carbon nanotube chirality determines properties of encapsulated linear carbon chain

Long linear carbon chains encapsulated inside carbon nanotubes are a very close realization of carbyne, the truly one-dimensional allotrope of carbon. Here we study individual pairs of double-walled carbon nanotubes and encapsulated linear carbon chains by tip-enhanced Raman scattering. We observe that the radial breathing mode of the inner nanotube correlates with the frequency of the carbon chain's Raman mode, revealing that the nanotube chirality determines the vibronic and electronic properties of the encapsulated carbon chain. We provide the missing link that connects the properties of the encapsulated long linear carbon chain with the structure of the host nanotube.

cond-mat.mes-hall↗

arXiv2018

Defect-Free Carbon Nanotube Coils

Carbon nanotubes are promising building blocks for various nanoelectronic components. A highly desirable geometry for such applications is a coil. However, coiled nanotube structures reported so far were inherently defective or had no free ends accessible for contacting. Here we demonstrate the spontaneous self-coiling of single-wall carbon nanotubes into defect-free coils of up to more than 70 turns with identical diameter and chirality, and free ends. We characterize the structure, formation mechanism, and electrical properties of these coils by different microscopies, molecular dynamics simulations, Raman spectroscopy, and electrical and magnetic measurements. The coils are highly conductive, as expected for defect-free carbon nanotubes, but adjacent nanotube segments in the coil are more highly coupled than in regular bundles of single-wall carbon nanotubes, owing to their perfect crystal momentum matching, which enables tunneling between the turns. Although this behavior does not yet enable the performance of these nanotube coils as inductive devices, it does point a clear path for their realization. Hence, this study represents a major step toward the production of many different nanotube coil devices, including inductors, electromagnets, transformers, and dynamos.

cond-mat.mes-hall↗

arXiv2017

Sensing of Static Forces with Free-Falling Nanoparticles

Miniaturized mechanical resonators have proven to be excellent force sensors. However, they usually rely on resonant sensing schemes, and their excellent performance cannot be utilized for the detection of static forces. Here, we report on a novel static-force sensing scheme and demonstrate it using optically levitated nanoparticles in vacuum. Our technique relies on an off-resonant interaction of the particle with a weak static force, and a resonant read-out of the displacement caused by this interaction. We demonstrate a force sensitivity of $10\,\mathrm{aN}$ to static gravitational and electric forces acting on the particle. Our work not only provides a tool for the closer investigation of short-range forces, but also marks an important step towards the realization of matter-wave interferometry with macroscopic objects.

physics.ins-det↗

arXiv2017

Radiation Damping of a Polarizable Particle

A polarizable body moving in an external electromagnetic field will slow down. This effect is referred to as radiation damping and is analogous to Doppler cooling in atomic physics. Using the principles of special relativity we derive an expression for the radiation damping force and find that it solely depends on the scattered power. The cooling of the particle's center-of-mass motion is balanced by heating due to radiation pressure shot noise, giving rise to an equilibrium that depends on the ratio of the field's frequency and the particle's mass. While damping is of relativistic nature heating has it's roots in quantum mechanics.

physics.optics↗

arXiv2017

Controlling the net charge on a nanoparticle optically levitated in vacuum

Optically levitated nanoparticles in vacuum are a promising model system to test physics beyond our current understanding of quantum mechanics. Such experimental tests require extreme control over the dephasing of the levitated particle's motion. If the nanoparticle carries a finite net charge, it experiences a random Coulomb force due to fluctuating electric fields. This dephasing mechanism can be fully excluded by discharging the levitated particle. Here, we present a simple and reliable technique to control the charge on an optically levitated nanoparticle in vacuum. Our method is based on the generation of charges in an electric discharge and does not require additional optics or mechanics close to the optical trap.

physics.optics↗

arXiv2017

Optically levitated nanoparticle as a model system for stochastic bistable dynamics

Nano-mechanical resonators have gained an increasing importance in nanotechnology owing to their contributions to both fundamental and applied science. Yet, their small dimensions and mass raises some challenges as their dynamics gets dominated by nonlinearities that degrade their performance, for instance in sensing applications. Here, we report on the precise control of the nonlinear and stochastic bistable dynamics of a levitated nanoparticle in high vacuum. We demonstrate how it can lead to efficient signal amplification schemes, including stochastic resonance. This work paves the way for the use of levitated nanoparticles as a model system for stochastic bistable dynamics, with applications to a wide variety of fields.

physics.optics↗