Lukas Novotny

Excitons, bound electron–hole pairs, govern the optical response of semiconductors and underpin technologies ranging from photovoltaics to quantum light sources. In nanoscale systems, the spatial extent of excitons approaches atomic dimensions, yet direct imaging of excitonic processes in real space has remained elusive. Two-dimensional semiconductors such as transition metal dichalcogenides provide a platform where reduced screening, high light extraction and atomically defined defect sites enable the exploration of exciton physics at atomic length scales. However, the microscopic origin of defect-related emission and its variability across samples have remained debated. Here we show that scanning tunnelling microscopy-induced luminescence reveals the atomic and optical fingerprints of individual defects in monolayer MoS ₂ . Decoupling the material from its substrate using bilayer hexagonal boron nitride enables the detection of sharp excitonic emission and spatially localized defect emission, which we attribute to free-to-bound recombination ( e ,A ⁰ ), and map its distribution with subnanometre-resolved fluorescence imaging. We identify negatively charged carbon–hydrogen complexes as the defect species responsible for a ubiquitous defect emission band in MoS ₂ . These results provide atomically resolved images of exciton emission in a solid-state material, linking atomic defect structure to its optical fingerprint and establishing a microscopic basis for deterministic quantum emitter engineering in two-dimensional materials.

Powerful approaches to squeeze the motional state of a harmonic oscillator rely on the stepwise modulation of its resonance frequency. Such protocols can be limited by forces that vary slowly between experimental runs but are constant during a single experimental shot. Such shot-to-shot noise gives rise to a spread in experimental outcomes that masks the uncertainty intrinsic to quantum theory. Taking inspiration from spin-echo protocols, we propose a decoupling technique that, under ideal conditions, perfectly cancels shot-to-shot force noise in squeezing experiments based on parametric modulation. We implement the protocol using an optically levitated nanoparticle, where shot-to-shot force noise arises from slowly varying stray fields acting on the charge carried by the particle. Using our oscillator-echo protocol, we demonstrate shot-to-shot noise suppression to the measurement-backaction limit.

Photoinduced force microscopy enables nanoscale studies of light–matter interactions, yet photothermal effects in the probe are commonly regarded as artifacts that limit performance. Here, we show that this photothermal response can instead be exploited as a sensitive measurement principle for optical near-field mapping. Under modulated near-field illumination, partial absorption at the AFM tip apex induces a photothermal bending of the metal-coated cantilever. At slow modulation frequencies, efficient heat diffusion modulates the temperature of the entire cantilever, leading to a strongly enhanced homodyne signal. We introduce this approach as homodyne photothermal force microscopy and demonstrate its capability by imaging plasmonic silver nanocubes under evanescent illumination on a commercial AFM. Interference between the optical near-field and radiation from the induced dipole at the scatterer produces a characteristic parabolic standing-wave pattern. An analytical model based on an equivalent point dipole reproduces the observed features and enables the extraction of the complex polarizability of individual nanocubes. By reinterpreting probe photothermal motion from an artifact into a signal carrier, we establish homodyne photothermal force microscopy as a simple and powerful platform for high-resolution characterization of optical near-fields and plasmonic nanostructures.

Coherent on-chip light sources are essential components for integrated photonics, enabling interference-based logic, high-fidelity signal routing, and precision sensing. Two-dimensional (2D) materials offer a promising platform for realizing ultracompact light sources because they combine strong excitonic effects, broad material compatibility, and efficient electrical tunability. However, achieving simultaneous optical performance and stable electrical controllability is hindered by cavity geometry and electrode-induced optical losses. Here, we introduce a hybrid hexagonal boron nitride (hBN)–silicon nitride (Si3N4) photonic crystal cavity coupled to a WS2–WSe2 heterobilayer. The system features high-Q optical modes and provides robust electrical integration. We demonstrate coherent light emission from cavity-coupled WS2–WSe2 interlayer excitons, with coherence lengths exceeding 5 mm, the size of a typical photonic integrated circuit. Furthermore, by integrating MoS2 as low-loss electrodes, we show that the exciton emission can be spectrally tuned and brought in resonance with the cavity mode, without deteriorating its quality factor (Q > 1200). The combination of our hybrid cavity design with low-loss electrodes is a promising platform for on-chip light sources and 2D optoelectronics.

The sensitivity of a mechanical transducer is ultimately limited by its inherent quantum fluctuations. Here, we use an optically levitated nanoparticle to measure impulsive forces smaller than the particle's zero-point momentum uncertainty. Our approach relies on reversibly squeezing the levitated particle's center-of-mass motion to coherently amplify the perturbation. We demonstrate an impulsive-force resolution as small as 6.9 keV/c, a value 0.6 dB below the sensor's zero-point value.

An optically levitated nanoparticle in an environment of variable gas pressure is used to study stochastic processes over large parameter ranges. Feedback control is employed to exert both conservative and non-conservative forces. Additional control is provided via parametric modulation, giving rise to frequency pulling, amplification, and switching. In this article, we discuss stability criteria and show that the nanoparticle’s steady-state energy can be controlled over a range of nine orders of magnitude.

Electron tunneling is a central transport mechanism in nanoscale devices. The advent of two-dimensional (2D) van der Waals materials has greatly expanded the ability to explore unconventional tunneling phenomena in heterostructures. Conventionally, studies in these systems have focused on tunneling between a pair of electrodes separated by a barrier. Here we report a previously overlooked tunneling pathway in which electrons traverse both the barrier and the electrode to reach semiconductor layers placed outside the junction. We identify this mechanism through the observation of prominent conductance peaks in tunneling transport measurements. Combined transport and optical spectroscopy reveal that the conductance peaks are of different origin than the accompanying electroluminescence. While conductance peaks originate from electrons tunneling through the graphene-hBN-graphene barrier into empty states of the semiconductor, electroluminescence peaks originate from recombination of excitons excited via energy transfer. Our work uncovers the impact of the electronic environment adjacent to tunnel junctions and serves as a pristine probe of the material’s electronic properties. It also establishes a versatile platform for multidimensional investigations of tunneling processes, enabling new designs for on-chip optoelectronic applications.

Bistability-two distinct stable states under identical parameters-is not only a fundamental physical concept but also of importance in practical applications. While plasmon-polaritonic bistability representing history-dependent stable states within plasmonic systems has been theoretically predicted, it has yet to be demonstrated experimentally due to challenges in realizing suitable nonlinearity at feasible electric-field strengths. Here, we report the experimental observation of electrically driven plasmon-polaritonic bistability in graphene/hexagonal-boron-nitride/graphene tunneling transistors, achieved through momentum-conserving resonant tunneling of Dirac electrons. Using a small twist angle between graphene layers, we engineered devices exhibiting both electronic and plasmon-polaritonic bistability. This bistable plasmonic behavior can be precisely tuned through load resistance and electrostatic gating. Our findings open new pathways for exploring nonlinear optical and electronic phenomena in van der Waals heterostructures and mark a significant advance in nanoplasmonics, with potential applications in optical memory, sensing, and optoelectronic switching.