MXenes are two-dimensional (2D) nanomaterials whose optical, structural, and charge-transport properties can be finely tuned through composition and surface chemistry. MXetronics builds on this tunability to develop microscale electronic and photonic devices based on MXene thin films. Here, we demonstrate that imaging spectroscopic ellipsometry (ISE) and spectroscopic micro-ellipsometry (SME) provide two complementary, fully non-invasive optical approaches for characterization and process control of MXene-based comb-shaped backside electrodes used in photodetector and gas-sensing applications. SME enables sub-minute high-throughput probing of multiple representative sample sites, offering rapid assessment of fabrication quality. ISE delivers device-scale or micrometer-resolved imaging, allowing precise visualization of MXene film uniformity, structural integrity, composition, and transport-related properties throughout device processing. Leveraging SME's and ISE's high sensitivity toward these properties, we show that electrode regions altered during photoresist lithography are readily identified by their increased thickness, reduced conductivity, and distinct optical fingerprints, arising from tetramethylammonium hydroxide (TMAOH) intercalation into Ti3C2Tx. These results represent the comprehensive application of microscopic ellipsometry techniques to MXene microsystems, establishing a powerful metrology pathway for the rapidly growing fields of MXene 2D materials and MXetronics.

The next generation of photonic circuits will require programmable, subnanosecond, and energy-efficient components on a scalable platform for quantum and neuromorphic computing. Here, we present subnanosecond electrical control of highly nonlinear light-matter hybrid quasi-particles, called waveguide exciton-dipolaritons, in a highly scalable waveguide-on-chip geometry, and with extremely low power consumption. Our device performs as an optical transistor with a GHz-rate electrical modulation at a record-low total energy consumption <8 fJ/bit and a compact active area of down to 25 μm2. This work establishes waveguide-dipolariton platforms for scalable, electrically reconfigurable, ultralow power photonic circuits for both classical and quantum computing and communication.

MXenes have attracted significant attention in recent years due to their remarkable properties for electrochemical and optoelectronic applications. While the physical properties of MXene thin films, consisting of stacked delaminated flakes, have been extensively studied, the intrinsic MXene properties can only be derived from individual flakes. Indeed, flake interconnectivity, intercalated species, and film morphology introduce extrinsic factors that affect charge transport and optical properties. In this work, we quantitatively characterize the intrinsic optical, structural, and transport properties of micrometer-sized Ti3C2Tx MXene flakes by employing our non-invasive, advanced spectroscopic micro-ellipsometry (SME) technique in the visible-near-infrared spectral range. SME exploits back-focal-plane imaging in a reflection microscopy geometry to simultaneously capture the spectral and incidence-angle-dependent optical response of individual flakes with up to diffraction-limited lateral resolution. Through a comprehensive multi-flake analysis, encompassing flakes from mono- to 32 layers, we reveal thickness-dependent variations in the complex refractive index and charge transport properties of ultrathin flakes, where resistivity increases as the number of Ti3C2Tx layers (NoLs) decreases. Flake thicknesses, non-uniformities, and NoLs, determined via SME with sub-nm precision, closely match nanoscale observations from atomic force microscopy (AFM) and scanning transmission electron microscopy (STEM). Additionally, charge transport properties derived from SME agree with four-probe measurements performed on single-flake devices. Unveiling the intrinsic optical, structural, and charge transport properties of Ti3C2Tx MXene single flakes, this study establishes SME as a robust platform for quantitative MXene analyses, enabling precise optical metrology of MXene-based optoelectronic and electrochemical devices.

Recent experiments on trilayer transition-metal dichalcogenide heterostructures have revealed the rich behavior of dipolar excitons. Motivated by these experimental observations, we investigate the collective dynamics of planar quantum dipoles whose orientation fluctuates due to charge tunneling between the outer layers. Using large-scale quantum Monte Carlo simulations, we map out the low-temperature phase diagram as a function of experimentally tunable parameters. We uncover a diverse landscape of phases driven by dipolar correlations. Under strong dipole fluctuations, a quadrupolar superfluid emerges. Suppressing charge tunneling nucleates a droplet state stabilized by the attractive interaction between antiparallel dipoles. At high exciton densities, the system gives way to a partially fragmented condensate, characterized by competing quadrupolar and dipolar superfluid states. Furthermore, at a large exciton mass and high density, we find a staggered dipolar crystal. Our detailed study of the dependence of exciton energy shifts on an external electric field directly interprets existing experimental data and underscores the crucial role of the antiparallel dipolar configuration. Our results provide a guide for future experimental explorations of quantum phases of trilayer excitons.

The original proposal of quantum key distribution (QKD) was based on ideal single photon sources, which 40 years later, are still challenging to develop. Therefore, the development of decoy state protocols using weak coherent states (WCS) from lasers, set the frontier in terms of secure key rates and distances. Here, we propose and experimentally demonstrate two simple-to-implement QKD protocols that allow practical, far from ideal sub-Poissonian photon sources to outperform state-of-the-art WCS. By engineering the photon statistics of a biexciton-exciton cascade in room temperature single photon sources based on giant colloidal quantum dots coupled to nanoantennas, we show that either a truncated decoy state protocol or a heralded purification protocol can be employed to achieve a significantly increased performance in terms of the maximal allowed channel loss for secure key creation, which can exceed even that of ideal WCS by more than 3dB. We then experimentally emulate a BB84 QKD using such a quantum dot source, verifying the superiority of our protocols over the best possible BB84 WCS performance. These protocols can be utilized efficiently on a host of various quantum emitters having controllable photon statistics with a finite photon-number basis, offering a practical approach to QKD without the hindering requirements on the single photon purity of the photon source.

Efficient readout of nitrogen-vacancy (NV) centers in diamond is crucial for various quantum information technologies. However, achieving high-fidelity, single-shot readout at room temperature remains challenging due to limited photon collection efficiency (CE) and background noise. In this work, we enhance the readout efficiency of NV centers by integrating them into hybrid metal-dielectric bullseye nanoantennas using ultra-precise deterministic positioning. This approach enables highly directional emission while minimizing optical losses, resulting in a measured CE of 82% into a numerical aperture (NA) as low as 0.5, and approaching unity for NA>0.8. This marks a substantial improvement over previous realizations using nanodiamonds, highlighting the advantage of combining hybrid nanoantennas with precise positioning. Our results mark a substantial advancement towards efficient single-shot readout of NV centers by significantly improving readout fidelity and efficiency in a simple on-chip configuration.

We study the dependence of the optical response of square lattices of plasmonic nanoparticles on the mutual orientation of the lattice and the plane of incidence of illuminating light. It is shown that the lattice sums for two orientations of the plane of incidence—along the rows of the squares and along their diagonal—are different. The presented micro-ellipsometric spectra confirm that different lattice sums and different conditions for the excitation of surface plasmons in the substrate at different orientations result in a distinct dispersion of the observed modes.

The next generation of photonic circuits will require programmable, ultrafast, and energy-efficient components on a scalable platform for quantum and neuromorphic computing. Here, we present ultrafast electrical control of highly nonlinear light-matter hybrid quasi-particles, called waveguide exciton-dipolaritons, with extremely low power consumption. Our device performs as an optical transistor with a GHz-rate electrical modulation at a record-low total energy consumption ∼3 fJ/bit and a compact active area of down to 25 μm^2. This work establishes waveguide-dipolariton platforms for scalable, electrically reconfigurable, ultra-low power photonic circuits for both classical and quantum computing and communication.

To harness the potential of radially polarized single photons in applications such as high-dimensional quantum key distribution (HD-QKD) and quantum communication, we demonstrate an on-chip, room-temperature device, which generates highly directional radially polarized photons at very high rates. The photons are emitted from a giant CdSe/CdS colloidal quantum dot (gQD) accurately positioned at the tip of a metal nanocone centered inside a hybrid metal-dielectric bullseye antenna. We show that due to the large and selective Purcell enhancement specifically for the out-of-plane optical dipole of the gQD, the emitted photons can have a very high degree of radial polarization (>93%), based on a quantitative metric. Our study emphasizes the importance of accurate gQD positioning for optimal radial polarization purity through extensive experiments and simulations, which contribute to the fundamental understanding of radial polarization in nanostructured devices and pave the way for implementation of such systems in practical applications using structured quantum light.

High-dimensional quantum key distribution (HDQKD) is a promising avenue to address the inherent limitations of basic QKD protocols. However, experimental realizations of HDQKD to date have relied on indeterministic photon sources that limit the achievable key rate. In this paper, we demonstrate a full emulation of a HDQKD system using a single colloidal giant quantum dot (gQD) as a deterministic, compact and room-temperature single-photon source (SPS). We demonstrate a practical protocol by encoding information in a high-dimensional space ($d = 3$) of the orbital angular momentum of the photons. Our experimental configuration incorporates two spatial light modulators for encoding and decoding the spatial information carried by individual photons. Our experimental demonstration establishes the feasibility of utilizing high radiative quantum yield gQDs as practical SPSs for HDQKD. We also demonstrate experimentally secure qudit transmission exceeding one secure bit per photon, thus already beating the traditional d=2 QKD capacity.

We demonstrate an important step toward on-chip integration of single-photon sources at room temperature. Excellent photon directionality is achieved with a hybrid metal-dielectric bullseye antenna, while back-excitation is permitted by placement of the emitter in a subwavelength hole positioned at its center. The unique design enables a direct back-excitation and very efficient front coupling of emission either to a low numerical aperture (NA) optics or directly to an optical fiber. To show the versatility of the concept, we fabricate devices containing either a colloidal quantum dot or a nanodiamond containing silicon-vacancy centers, which are accurately positioned using two different nanopositioning methods. Both of these back-excited devices display front collection efficiencies of ∼70% at NAs as low as 0.5. The combination of back-excitation with forward directionality enables direct coupling of the emitted photons into a proximal optical fiber without any coupling optics, thereby facilitating and simplifying future integration.

An extreme yet reconfigurable nonlinear response to a single photon by a photonic system is crucial for realizing a universal two-photon gate, an elementary building block for photonic quantum computing. Yet such a response, characterized by the photon blockade effect, has only been achieved in atomic systems or solid states ones that are difficult to scale up. Here we demonstrate electrically tunable partial photon blockade in dipolar waveguide polaritons on a semiconductor chip, measured via photon-correlations. Remarkably, these "dipolar photons" display a two-orders-of-magnitude stronger nonlinearity compared to unpolarized polaritons, with an extracted dipolar blockade radius up to more than 4 μm, significantly larger than the optical wavelength, and comparable to that of atomic Rydberg polaritons. Furthermore, we show that the dipolar interaction can be electrically switched and locally configured by simply tuning the gate voltage. Finally we show that with a simple modification of the design, a full photon blockade is expected, setting a new route towards scalable, reconfigurable, chip-integrated quantum photonic circuits with strong two-photon nonlinearities.

Electrically controlled photonic circuits hold promise for information technologies with greatly improved energy efficiency and quantum information processing capabilities. However, weak nonlinearity and electrical response of typical photonic materials have been two critical challenges. Therefore, hybrid electronic-photonic systems, such as semiconductor exciton polaritons, have been intensely investigated for their potential to allow higher nonlinearity and electrical control, with limited success so far. Here we demonstrate an electrically gated waveguide architecture for field induced dipolar polaritons that allows enhanced and electrically controllable polariton nonlinearities, enabling an electrically tuned reflecting switch (mirror) and transistor of the dipolar polaritons. The polariton transistor displays blockade and antiblockade by compressing a dilute dipolar-polariton pulse exhibiting very strong dipolar interactions. The large nonlinearities are explained using a simple density-dependent dipolar polarization field that very effectively screens the external electric field. We project that a quantum blockade at the single polariton level is feasible in such a device.

As performance of van der Waals heterostructure devices is governed by the nanoscale thicknesses and homogeneity of their constituent mono- to few-layer flakes, accurate mapping of these properties with high lateral resolution becomes imperative. Spectroscopic ellipsometry is a promising optical technique for such atomically thin-film characterization due to its simplicity, noninvasive nature and high accuracy. However, the effective use of standard ellipsometry methods on exfoliated micron-scale flakes is inhibited by their tens-of-microns lateral resolution or slow data acquisition. In this work, we demonstrate a Fourier imaging spectroscopic micro-ellipsometry method with sub-5 μm lateral resolution and three orders-of-magnitude faster data acquisition than similar-resolution ellipsometers. Simultaneous recording of spectroscopic ellipsometry information at multiple angles results in a highly sensitive system, which is used for performing angstrom-level accurate and consistent thickness mapping on exfoliated mono-, bi- and trilayers of graphene, hexagonal boron nitride (hBN) and transition metal dichalcogenide (MoS2, WS2, MoSe2, WSe2) flakes. The system can successfully identify highly transparent monolayer hBN, a challenging proposition for other characterization tools. The optical microscope integrated ellipsometer can also map minute thickness variations over a micron-scale flake, revealing its lateral inhomogeneity. The prospect of adding standard optical elements to augment generic optical imaging and spectroscopy setups with accurate in situ ellipsometric mapping capability presents potential opportunities for investigation of exfoliated 2D materials.

Spectroscopic ellipsometry is a widely used optical technique in both industry and research for determining the optical properties and thickness of thin films. The effective use of spectroscopic ellipsometry on micro-structures is inhibited by technical limitations on the lateral resolution and data acquisition rate. Here, we introduce a spectroscopic micro-ellipsometer (SME), capable of recording spectrally resolved ellipsometric data simultaneously at multiple angles of incidence in a single measurement of a few seconds, with a lateral resolution down to 2 μm in the visible spectral range. The SME can be easily integrated into generic optical microscopes by the addition of a few standard optical components. We demonstrate complex refractive index and thickness measurements by using the SME, which are in excellent agreement with a commercial spectroscopic ellipsometer. The high lateral resolution is displayed by complex refractive index and thickness maps over micron-scale areas. As an application for its accuracy and high lateral resolution, the SME can characterize the optical properties and number of layers of exfoliated transition-metal dichalcogenides and graphene, for structures that are a few microns in size.

We demonstrate an important step towards on chip integration of single photon sources operating at room temperature fiber coupling of a directional quantum emitter with back-excitation. Directionality is achieved with a hybrid metal-dielectric bullseye antenna, while back-excitation is permitted by placement of the emitter at or in a sub-wavelength hole positioned at the bullseye center. Overall, the unique design enables a direct laser excitation from the back of the on-chip device and very efficient coupling of the highly collimated photon emission to either low numerical aperture (NA) free space optics or directly to an optical fiber from the front. To show the versatility of the concept, we fabricate devices containing either a colloidal quantum dot or a silicon-vacancy center containing nanodiamond, which are accurately coupled to the nano-antenna using two different nano-positioning methods. Both back-excited devices display front collection efficiencies of about 70 as low as 0.5. Moreover, the combination of back-excitation with forward low-NA directionality enables direct coupling of the emitted photons into a proximal optical fiber without the need of any coupling optics, thereby facilitating and greatly simplifying future integration.

Electrically controlled photonic circuits hold promise for information technologies with greatly improved energy efficiency and quantum information processing capabilities. However, weak nonlinearity and electrical response of typical photonic materials have been two critical challenges. Therefore hybrid electronic-photonic systems, such as semiconductor exciton-polaritons, have been intensely investigated for their potential to allow higher nonlinearity and electrical control, with limited success so far. Here we demonstrate an electrically-gated waveguide architecture for dipolar-polaritons that allows enhanced and electrically-controllable polariton nonlinearities, enabling an electrically-tuned reflecting switch (mirror) and transistor of the dipolar-polaritons. The polariton transistor displays blockade and anti-blockade by compressing a dilute dipolar-polariton pulse exhibiting very strong dipolar interactions. The large nonlinearities are well explained using a simple density-dependent polarization field that very effectively screens the external electric field. Remarkably, we show that induced dipoles have an order of magnitude enhancement of the nonlinearities as compared to fixed dipoles. We project that a quantum blockade at the single polariton level is feasible in such a device.

Layered bosonic dipolar fluids have been suggested to host a condensate of interlayer molecular bound states. However, experimental observation has remained elusive. Motivated by two recent experimental works [C. Hubert et al., Phys. Rev. X9, 021026 (2019) and D. J. Choksy et al., Phys. Rev. B 103, 045126 (2021)], we theoretically study, using numerically exact quantum Monte Carlo calculations, the experimental signatures of collective interlayer pairing in vertically stacked indirect exciton (IX) layers. We find that IX energy shifts associated with each layer evolve nontrivially as a function of density imbalance following a nonmonotonic trend with a jump discontinuity at density balance, identified with the interlayer IX molecule gap. This behavior discriminates between the superfluidity of interlayer bound pairs and independent dipole condensation in distinct layers. Considering finite temperature and finite density imbalance conditions, we find a cascade of Berezinskii-Kosterlitz-Thouless (BKT) transitions, initially into a pair superfluid and only then, at lower temperatures, into complete superfluidity of both layers. Our results may provide a theoretical interpretation of existing experimental observations in GaAs double quantum well (DQW) bilayer structures. Furthermore, to optimize the visibility of pairing dynamics in future studies, we present an analysis suggesting realistic experimental settings in GaAs and transition metal dichalcogenide (TMD) bilayer DQW heterostructures where collective interlayer pairing and pair superfluidity can be clearly observed.

Coherent exchange of single photons is at the heart of applied Quantum Optics. The negatively-charged silicon vacancy center in diamond is among most promising sources for coherent single photons. Its large DebyeWaller factor, short lifetime and extraordinary spectral stability is unique in the field of solid-state single photon sources. However, the excitation and detection of individual centers requires high numerical aperture optics which, combined with the need for cryogenic temperatures, puts technical overhead on experimental realizations. Here, we investigate a hybrid quantum photonics platform based on silicon-vacancy center in nanodiamonds and metallic bullseye antenna to realize a coherent single-photon interface that operates efficiently down to low numerical aperture optics with an inherent resistance to misalignment.