arXiv subjects

Ileana-Cristina Benea-Chelmus

Publications and source records attributed to Ileana-Cristina Benea-Chelmus.

14 recordsLinked to original sources

arXiv2026

Fostering cultural change in research through innovative knowledge sharing, evaluation, and community engagement strategies

Bringing together researchers, funders, industry partners, and publishers from 14 countries across 5 continents, we advance the debate around open-science, assessment and learning. We introduce an integrative "open knowledge system" framework linking knowledge production, validation, assessment, and reuse into one ecosystem view, and translate it into actionable recommendations for each stakeholder. Shifting focus to modular, machine-readable knowledge objects, these recommendations are intended to help diagnose misaligned incentives and guide reforms that properly value all scientific contributions.

cs.SI↗

arXiv2026

Hz-resolution wide-span photonic integrated terahertz signal analyzer

Wide-span spectral and noise characterization at millimeter-wave and terahertz frequencies is increasingly important for emerging wireless, sensing, and spectroscopy systems, yet remains challenging for conventional electronic instrumentation because of the complexity and calibration burden of extender-, multiplier-, and mixer-chain architectures. Here we show that photonics can provide attractive alternatives to this highly challenging electronic instrumentation through an antenna-coupled thin-film lithium niobate electro-optic receiver. Our implementation performs fast spectral reconstruction across the widely separated WR 9.0 (80-125 GHz) and WR 2.8 (240-380 GHz) bands with a single component, with carrier-frequency errors below 3 MHz, scan speeds up to 25 THz/s, and a displayed average noise level below -104 dBm/Hz. We then extend to Hz-level resolution and demonstrate phase-noise characterisation capabilities across these ultra-wide bands by using a mode-locked femtosecond-laser frequency comb as a sampling clock, mapping high-frequency carriers to aliased intermediate-frequency tones. We validate the method by measuring the phase noise of 90.225 GHz and 270.675 GHz carriers using the same chip and system configuration. We further improve the sensitivity of the phase-noise measurement through cross-correlation between two optical probe-pulse channels. Finally, we show that the same technique can be applied synchronously to ten widely spaced frequencies distributed across more than 100 GHz, with the option to quantify their mutual coherence. These results establish TFLN integrated photonics as a scalable route toward compact millimeter-wave/terahertz instrumentation.

physics.optics↗

arXiv2026

Reconfigurable Single-Ring Photonic Molecule on Lithium Niobate

Resonant photonic structures enable optical enhancement and spectral filtering and are essential for lasers, quantum emitters, transducers, or modulators. Photonic molecules, formed by mode hybridisation in two coupled resonators, break the equidistant frequency spacing of zero-dispersion resonators and provide control over their spectrum. Reconfigurability over these devices is a key asset, allowing to align photonic resonances to target frequencies on-demand. While electro-optic materials such as thin-film lithium niobate (TFLN) have enabled frequency tuning beyond traditional thermo-optic effects, they require continuous bias, posing challenges to scalability. Here we demonstrate an optically programmable, erasable, and rewritable photonic molecule realized within a single TFLN racetrack resonator. A long-lasting photorefractive grating induced through interference of co-propagating dark and bright transverse modes promotes their hybridisation, forming a single-ring photonic molecule. We observe GHz-scale hybrid-mode splitting over a 700 GHz-wide optical bandwidth and hour-long lifetimes, and show that their coupling strength can be programmed by the optical pump used to write the grating. By selectively pumping orthogonal hybridised modes, we further demonstrate multiple reversible all-optical write-erase-rewrite cycles of these gratings. Finally, we use this technique to realize single-sideband mmWave transduction around 107 GHz with a 5 GHz tuning bandwidth. These results establish photorefraction as a reliable mechanism for reconfigurable resonances in TFLN, and suggest a route towards tunable microwave-optical functionalities within a reduced footprint.

physics.optics↗

arXiv2026

Multimode Single-Ring Photonic Molecule

Photonic molecules can mimic interactions of atomic energy levels, offering new ways to manipulate cavity eigenstates. Current methods using evanescent coupling of multiple cavities face challenges in scalability, flexibility, and coupling control, especially for complex systems. Here we introduce a new method that uses a single multimode optical ring resonator to create photonic molecules. Our design uses multiple waveguide transverse modes in one resonator, providing flexibility to engineer complex interactions without typical coupling constraints. We demonstrate arbitrary inter-mode coupling through transmissive mode converters, allowing precise tuning of resonance splitting and intrinsic losses. This approach enables selective bright-dark mode pair generation and the exploration of novel photonic phenomena such as exceptional points. This multimode photonic molecule overcomes traditional limitations and offers new possibilities for integrated photonic circuits, optical processing, and studies in non-Hermitian and nonlinear photonics.

physics.optics↗

arXiv2025

Metasurface-controlled high-speed tunable external cavity lasers

Tunable lasers are essential for optical communication, spectroscopy, and precision sensing, where flexible and fast control of the laser wavelength is needed. However, conventional tunable laser systems often rely on mechanical actuation, which limits their tuning speed, stability, and repeatability. Alternative tuning methods, such as adjusting the temperature of the gain medium or the injection current, are either slow or suffer from unwanted coupling between frequency and intensity. Electro-optic modulators implemented after the laser offer fast frequency tunability but these components are bulky and prone to high insertion losses. Here, we introduce an external cavity semiconductor laser that achieves ultrafast frequency and amplitude tunability within a compact footprint without any moving parts. The laser employs a resonant electro-optic metasurface as the external mirror, providing narrowband feedback that enables simultaneous single-mode operation and voltage-controlled tuning across a wide range of currents. Leveraging a narrow linewidth design, we demonstrate mode-hop-free linear frequency tuning with rates of up to 70 THz/s, and frequency excursions reaching 110 MHz under a 10 V peak RF drive, clearly surpassing rates attainable through mechanical actuation. Moreover, by biasing the laser near its threshold, our platform enables intensity modulation with an efficiency above 100% at a peak voltage of 4 V. These results highlight the platform's potential for applications requiring high-speed and mode-hop-free tunable lasers, such as free-space optical communication, laser ranging, and high-resolution spectroscopy.

physics.optics↗

arXiv2024

Cascaded-mode interferometers: spectral shape and linewidth engineering

Interferometers are essential tools to measure and shape optical fields, and are widely used in optical metrology, sensing, laser physics, and quantum mechanics. They superimpose waves with a mutual phase delay, resulting in a change in light intensity. A frequency-dependent phase delay then allows to shape the spectrum of light, which is essential for filtering, routing, wave shaping, or multiplexing. Simple Mach-Zehnder interferometers superimpose spatial waves and typically generate an output intensity that depends sinusoidally on frequency, limiting the capabilities for spectral engineering. Here, we present a novel framework that uses the interference of multiple transverse modes in a single multimode waveguide to achieve arbitrary spectral shapes in a compact geometry. Through the design of corrugated gratings, these modes couple to each other, allowing the exchange of energy similar to a beam splitter, facilitating easy handling of multiple modes. We theoretically and experimentally demonstrate narrow-linewidth spectra with independently tunable free spectral range and linewidth, as well as independent spectral shapes for various transverse modes. Our methodology can be applied to orthogonal optical modes of different orders, polarization, and angular momentum, and holds promise for sensing, optical metrology, calibration, and computing.

physics.optics↗

arXiv2023

Hybrid silicon-organic Huygens' metasurface for phase modulation

Spatial light modulators have desirable applications in sensing and free space communication because they create an interface between the optical and electronic realms. Electro-optic modulators allow for high-speed intensity manipulation of an electromagnetic wavefront. However, most surfaces of this sort pose limitations due to their ability to modulate intensity rather than phase. Here we investigate an electro-optic modulator formed from a silicon-organic Huygens' metasurface. In a simulation-based study, we discover a metasurface design immersed in high-performance electro-optic molecules that can achieve near-full resonant transmission with phase coverage over the full 2$π$ range. Through the electro-optic effect, we show 140$^\circ$ (0.79$π$) modulation over a range of -100 to 100 V at 1330 nm while maintaining near-constant transmitted field intensity (between 0.66 and 0.8). These results potentiate the fabrication of a high-speed spatial light modulator with the resolved parameters.

physics.optics↗

arXiv2022

Terahertz waveform synthesis from integrated lithium niobate circuits

Bridging the "terahertz (THz) gap" relies upon synthesizing arbitrary waveforms in the THz domain enabling applications that require both narrow band sources for sensing and few-cycle drives for classical and quantum objects. However, realization of custom-tailored waveforms needed for these applications is currently hindered due to limited flexibility for optical rectification of femtosecond pulses in bulk crystals. Here, we experimentally demonstrate that thin-film lithium niobate (TFLN) circuits provide a versatile solution for such waveform synthesis through combining the merits of complex integrated architectures, low-loss distribution of pump pulses on-chip, and an efficient optical rectification. Our distributed pulse phase-matching scheme grants shaping the temporal, spectral, phase, amplitude, and farfield characteristics of the emitted THz field through designer on-chip components. This strictly circumvents prior limitations caused by the phase-delay mismatch in conventional systems and relaxes the requirement for cumbersome spectral pre-engineering of the pumping light. We provide a toolbox of basic blocks that produce broadband emission up to 680 GHz with adaptable phase and coherence properties by using near-infrared pump pulse energies below 100 pJ.

physics.optics↗

arXiv2022

Resonators with tailored optical path by cascaded-mode conversions

Optical resonators enable the generation, manipulation, and storage of electromagnetic waves. They are widely used in technology and fundamental research, in telecommunications, lasers and nonlinear optics, ultra-sensitive measurements in cavity optomechanics, and the study of light-matter interactions in the context of cavity QED. The physics underlying their operation is determined by the constructive interference of electromagnetic waves at specific frequencies, giving rise to the resonance spectrum. This mechanism causes the limitations and trade-offs of resonator design, such as the difficulty of confining waves larger than the resonator and the fixed relationship between free spectral range, modal linewidth, and the resonator's refractive index and size. Here, we introduce a new class of optical resonators, generating resonances by designing the optical path through transverse mode coupling in a cascaded process created by mode-converting mirrors. The generalized round-trip phase condition leads to resonator characteristics that are markedly different from Fabry-Perot resonators and can be tailored over a wide range, such as the largest resonant wavelength, the free spectral range, the linewidth, and the quality factor. We confirm the existence of these modes experimentally in an integrated waveguide cavity with mode converters coupling two transverse modes into one supermode. The resonance signature of the cascaded-mode resonator is a spectrum resulting from the coherent superposition of the coupled transverse modes. We also demonstrate a transverse mode-independent transmission through the resonator and show that its engineered spectral properties agree with theoretical predictions. Cascaded-mode resonators introduce properties not found in traditional resonators and provide a mechanism to overcome the existing trade-offs in the design of resonators in various application areas.

physics.optics↗

arXiv2021

Gigahertz free-space electro-optic modulators based on Mie resonances

Electro-optic modulators from non-linear $χ^{(2)}$ materials are essential for sensing, metrology and telecommunications because they link the optical domain with the microwave domain. At present, most geometries are suited for fiber applications. In contrast, architectures that modulate directly free-space light at gigahertz (GHz) speeds have remained very challenging, despite their dire need for active free-space optics, in diffractive computing or for optoelectronic feedback to free-space emitters. They are typically bulky or suffer from much reduced interaction lengths. Here, we employ an ultrathin array of sub-wavelength Mie resonators that support quasi bound states in the continuum (BIC) as a key mechanism to demonstrate electro-optic modulation of free-space light with high efficiency at GHz speeds. Our geometry relies on hybrid silicon-organic nanostructures that feature low loss ($Q = $ 550 at $λ_{res} = 1594$ nm) while being integrated with GHz-compatible coplanar waveguides. We maximize the electro-optic effect by using high-performance electro-optic molecules (whose electro-optic tensor we engineer in-device to exploit $r_{33} = 100$ pm/V) and by nanoscale optimization of the optical modes. We demonstrate both DC tuning and high speed modulation up to 5~GHz ($f_{EO,-3 dB} =3$ GHz) and shift the resonant frequency of the quasi-BIC by $Δλ_{res}=$11 nm, surpassing its linewidth. We contrast the properties of quasi-BIC modulators by studying also guided mode resonances that we tune by $Δλ_{res}=$20 nm. Our approach showcases the potential for ultrathin GHz-speed free-space electro-optic modulators.

physics.optics↗

arXiv2020

Electric field correlation measurements on the electromagnetic vacuum state

Quantum mechanics ascribes to the ground state of the electromagnetic radiation zero-point electric field fluctuations that permeate empty space at all frequencies. No energy can be extracted from the ground state of a system and, therefore, these fluctuations cannot be measured directly with an intensity detector. The experimental proof of their existence came thus from more indirect evidence, such as the Lamb shift, the Casimir force between close conductors or spontaneous emission. A direct method to determine the spectral characteristics of vacuum field fluctuations has been missing so far. In this work, we perform a direct measurement of the field correlation on these fluctuations in the terahertz frequency range using electro-optic detection in a non-linear crystal placed in a cryogenic environment. We investigate their temporal and spatial coherence, which, at zero time delay and spatial distance, has a peak value of $6.2\cdot 10^{-2}~V^2/m^2$, corresponding to a fluctuating vacuum field of 0.25 V/m. With this measurement, we determine the spectral composition of the ground state of electromagnetic radiation which lies within the bandwidth of electro-optic detection.

quant-ph↗

arXiv2017

Broadband monolithic extractor for metal-metal waveguide based terahertz quantum cascade laser frequency combs

We present a monolithic solution to extract efficiently light from terahertz quantum cascade lasers with metal-metal waveguides suitable for broadband frequency comb applications. The design is optimized for a bandwidth of 500 GHz around a center frequency of 2.6 THz. A five-fold increase in total output power is observed compared to standard metal-metal waveguides. The extractor features a single-lobed far-field pattern and increases the frequency comb dynamical range to cover more than 50 % of the laser dynamic range. Frequency comb operation up to a spectral bandwidth of 670 GHz is achieved.

physics.optics↗

arXiv2017

Intensity autocorrelation measurements of quantum cascade laser frequency combs in the Terahertz range

We report on the first direct measurement of the emission character of quantum cascade laser based frequency combs, using intensity autocorrelation. The correlation technique is based on fast electro-optic sampling, with a bandwidth optimized to match the emission spectra of the comb laser, centered around 2.5~THz. We find the output light to be amplitude and frequency modulated at the same time, with intensity modulation depth as high as 90\%, occuring on timescales as short as few picoseconds. We compare these findings with intensity autocorrelation measurements of pulsed Terahertz light originating from a photoconductive antenna and find very different results. We observe no significant difference for the comb and dispersed regime. The technique will be of significant importance in future for the measurement of ultra-short pulses from quantum cascade lasers.

physics.ins-det↗

arXiv2015

Sub-cycle measurement of intensity correlations in the Terahertz range

The Terahertz frequency range bears intriguing opportunities, beyond very advanced applications in spectroscopy and matter control. Peculiar quantum phenomena are predicted to lead to light emission by non-trivial mechanisms. Typically, such emission mechanisms are unraveled by temporal correlation measurements of photon arrival times, as demonstrated in their pioneering work by Hanbury Brown and Twiss. So far, the Terahertz range misses an experimental implementation of such technique with very good temporal properties and high sensitivity. In this paper, we propose a room-temperature scheme to measure photon correlations at THz frequencies based on electro-optic sampling. The temporal resolution of 146 fs is faster than one cycle of oscillation and the sensitivity is so far limited to ~1500 photons. With this technique, we measure the photon statistics of a THz quantum cascade laser. The proposed measurement scheme allows, in principle, the measurement of ultrahigh bandwidth photons and paves the way towards THz quantum optics.

quant-ph↗