Optical cavities are highly interesting elements for detecting nanoparticles, since their optical response is strongly affected - and therefore the response is easily measured - by the presence of a nanoparticle in their vicinity. Ideally, the nanoparticle should be placed in the region with maximum field confinement to maximize light-matter interaction and provide a highly sensitive response. However, in real applications, we usually do not have control over where the nanoparticle is actually placed. In this work, we consider the case of detection of non-resonant gold nanospheres using a silicon photonic crystal nanobeam cavity for the case in which the nanoparticle is not placed in the region of maximum field confinement at resonance (weak coupling). In our experiments, we use APTES (3-aminopropyltriethoxysilane) to functionalize the cavity so that gold nanoparticles can bond to it. Then we use a nanoprinter to deposit gold nanoparticles on predefined positions that, for the cavity under experimental study, turn out to be on a sidewall of the cavity. Optical characterization of the device before and after the nanoparticle deposition shows a large variation in the optical response, which we attribute to the perturbation of the optical resonance by the presence of the nanoparticle. Numerical simulations support our experimental findings, which confirm that optical cavities can perform nanoparticle detection even in cases where they are not placed in regions with maximum electric field.

Abstract Subwavelength geometrical dimension, which often originates from fabrication-induced dimensional deviations variability, can strongly influence the performance of phononic–photonic crystal optomechanical (OM) nanobeams. Here, we present a suitable variability-aware design framework for OM nanobeams by combining extraction of critical dimension variations from scanning electron microscopy images and finite-element modeling. We extracted the impact of deterministic geometrical deviations on the unit cell of the phononic and photonic crystals to identify the most critical fabrication-sensitive parameters. We extended the study to the full cavity, where SEM-informed digital-twin models are used to quantify the influence of systematic dimensional variations on the optical and mechanical resonance frequencies, the optical quality factor, the effective mode volume, and the total OM coupling rate. The results indicate that biased dimensional deviations of ±10 nm exert a minor effect on the overall device performance, whereas larger deviations have a stronger impact on the photonic, phononic, and OM response.

Optomechanical crystal cavities (OMCCs) display appealing features to function as on-chip photonic microwave oscillators (PMOs) when operated in the phonon lasing regime. Silicon OMCCs have so far demonstrated the best performance in terms of phase noise in a free-running configuration. Some preliminary experiments have also demonstrated that the generated microwave tone can be used for mixing wireless signals compliant with 5G standards. Still, more research is needed to identify the primary noise sources and determine the path towards improvement. Here, we report experiments on the realization of optomechanical PMOs operating at 4 GHz under two different driving conditions of the cavity: light coupling via a tapered fiber loop versus using an adjacent integrated waveguide illuminated by a lensed fiber. We performed measurements of the phase noise and the frequency stability in time of the detected tone for both cases and observed an improvement when the light is coupled by an adjacent integrated waveguide. This can be explained by the fact that the ambient-induced motion and changes due to perturbations in the refractive index of the medium surrounding the fiber loop result in variations of the coupling, which in turn affect the stability of the PMO. These results emphasize the relevance of a mechanically stable coupling technique when using such optomechanical devices in real applications, which can be of particular importance in environments where large vibrations are expected.

Optomechanical cavities can be used as highly sensitive mass sensors actuated by an optical field. In this work, we introduce and numerically demonstrate a new design for an optomechanical cavity consisting of a series of asymmetrically distributed rectangular silicon nanobricks, with each brick acting as an independent mechanical resonator but all coupled to the same optical field. Each silicon brick is placed on top of a thin silica pillar that ensures mechanical support whilst providing enough acoustic isolation between the individual mechanical resonances - at GHz frequencies - of each brick. The mass sensing capabilities of this cavity are studied through numerical simulations, proving that a point mass approximation can be used for silica nanoparticles with a radius smaller than 100 nm and that different nanoparticles can be measured independently but simultaneously and in real-time.

Gallium phosphide $(\text{GaP})$ is an indirect-bandgap semiconductor used for solid-state optical applications due to its optical properties with high refractive index, non-linear properties and large electronic band-gap [1], [2]. This paper investigates the properties and advantages of optomechanical cavities (OMCs) fabricated on low-cost complementary metal-oxide-semiconductor CMOS-compatible photonic wafers using GaP [3]. One particularity of this research is that the OMC design provides two resonances in the wavelength range from 1520 to 1640 nm in very compact devices (in the range of $\mu \mathrm{m}^{2}$). This enables advanced applications such as ultra-fast switching between the two resonant wavelengths of the OMC in GaP, in particular at $\lambda_{1}=1567.6$ nm and at $\lambda_{2}=1636.5$ nm. In this work, the experimental study comprises the analysis of the two optical resonances of the cavity, including its quality figure measurement (Q- factor) and the evaluation of the thermo-optical effect for different input power levels. The switch operates so that output 1 corresponds to the wavelength of the first resonance (in this case at 1567.6 nm) and output 2 to that of the second resonance (at 1636.5 nm) as depicted in Fig. 1(a). Figure 1(b) shows the experimental setup developed to test the OMC switching capability, using two lasers and an Erbium-doped fiber amplifier (EDFA) operating in the C-band with saturation power of + 13 dBm as well as the measured optical resonances at low power. Figure 1 (c) shows the variation of the center wavelength of resonance $\lambda_{2}$ when the power is fed into $\lambda_{2}$, which accounts to the thermo-optical effect (in blue) for higher power levels. In contrast, when we monitor the variation of $\lambda_{2}$ when higher input power is injected using the EDFA in $\lambda_{1}$, the second resonance results in a non-linear high-speed wavelength shift that could be used as an optical switch. As it can be observed in Fig. 1(c), the center resonance wavelength experiences a shift in the opposite direction of optical spectrum (to the left) compared with the shift due to the thermo-optical effect.

Multiband operation is a key aspect of emerging 5G-Advanced cellular systems, also named 5.5G, which target seamless provision of multi-gigabit per second connectivity employing sub-6 GHz and mm-wave overlapping coverage. All-optical frequency conversion gives flexibility due to the feasible high-speed reconfiguration in broad bands and large radio signal bandwidth. Optical frequency combs, featuring a spectrum of discrete, equally spaced coherent frequency lines, are crucial for high-precision metrology, spectroscopy, and telecommunications. Their effectiveness in all-optical frequency conversion depends on their stability in terms of frequency drift, phase noise, and power distribution across the comb lines. This paper evaluates experimentally the generation of optical frequency combs employing two distinct technologies: a dual-driven Mach-Zehnder modulator (DD-MZM) and an optomechanical crystal cavity (OMCC), and experimentally compares their performance for all-optical frequency conversion of 5G data streams. The DD-MZM implementation generates a comb with flexible line spacing and comprising several spectrum-flat lines with low phase noise ($-88.5$ dBc/Hz at 1 kHz offset and $-108.3$ dBc/Hz at 100 kHz offset). The OMCC implementation provides a reduced footprint (182 $\mu \mathrm{m}^{2}$) since it is implemented on a silicon chip and has the extra advantage of generating an optical comb without an external local oscillator, which reduces its power requirements (under 1 mW) while providing a phase noise of $-38.3$ dBc/Hz at 1 kHz offset and $-97.1$ dBc/Hz at 100 kHz offset. The polarization stability and jitter of both implementations are also evaluated. The experimental demonstration evaluates the error vector magnitude (EVM) of frequency-converted 3GPP 5G NR signals using both implementations, confirming the successful transmission with EVM smaller than 12.01% for DD-MZM up to the third harmonic and EVM smaller than 17.36% with the OMCC first harmonic.

All-optical microwave signal processing using optomechanical cavities on silicon chips in the optical domain is demonstrated. Demonstrations include low phase noise generation, frequency conversion, and enhanced stability, showing promise for nextgeneration wireless and satellite communications.

Silicon one-dimensional optomechanical cavities offer a cost-effective and highly scalable solution for the study and implementation of non-linear phenomena. By modifying the refractive index of silicon through thermal or free-carrier effects, it becomes possible to optically drive these resonators into a state of high-amplitude and coherent self-sustained mechanical oscillation. The nonlinearity stemming from this amplification mechanism provides significant adaptability in adjusting the frequency of mechanical resonators, enabling experiments such as injection locking, synchronization, and the study of chaotic dynamics. In this work, we show different novel configurations for the synchronization between mechanical flexural modes of silicon nanobeams and their locking to an external reference signal. The results hold great promise for applications in the distribution of clock signals in future photonic integrated circuits, as well as for establishing extensive networks of optomechanical resonators for studying complex non-linear dynamics.

Gallium phosphide (GaP) has recently received considerable attention as a suitable material for building photonic integrated circuits due to its remarkable optical and piezoelectric properties. Usually, GaP is grown epitaxially on III-V substrates to keep its crystallinity and later transferred to silicon wafers for further processing. Here, an alternative promising route for the fabrication of optomechanical (OM) cavities on GaP epitaxially grown on nominally (001)-oriented Si is introduced by using a two-step process consisting of a low-temperature etching of GaP followed by selective etching of the underneath silicon. The low-temperature (-30 ^oC) during the dry-etching of GaP hinders the lateral etching rate, preserving the pattern with a deviation between the design and the pattern in the GaP layer lower than 5 avoiding the complex process of transferring and bonding a GaP wafer to a silicon-on-insulator wafer. To demonstrate the quality and feasibility of the proposed fabrication route, suspended OM cavities are fabricated and experimentally characterized. The cavities show optical quality factors between 10^3 and 10^4, and localized mechanical resonances at frequencies around 3.1 GHz. Both optical and mechanical resonances are close to those previously reported on crystalline GaP structures. These results suggest a simple and low-cost way to build GaP-based photonic devices directly integrated on industry-standard Si(001) photonic wafers.

Optomechanical oscillators stand out as high-performance and versatile candidates for serving as reference clocks in sequential photonic integrated circuits. Indeed, they have the unique capability of simultaneously generating mechanical tones and optical signal modulations at frequencies determined by their geometrical design. In this context, the concept of synchronization introduces a powerful means to precisely coordinate the dynamics of multiple oscillators in a controlled manner, therefore increasing efficiency and preventing errors in signal processing photonic systems or communication interfaces. In this work, we demonstrate the cascaded injection locking of a pair of silicon-based optomechanical crystal cavities acting as optomechanical oscillators to an external reference signal that subtly modulates the laser driving only one of them. In contrast to most previous implementations, both cavities support isolated optical resonances and interact by a weak mechanical interconnection. This configuration allows one cavity to be used for probing the mechanical perturbation generated by the oscillator that receives the external forcing. The combination of the obtained results, supported by a numerical model, with remote optical injection locking schemes discussed in the literature lays the groundwork for the distribution of reference signals within large networks of processing elements in future phonon–photon hybrid circuits.

A novel design for an optomechanical cavity consisting of a series of rectangular silicon nanobricks, with each brick acting as an independent mechanical resonator but all coupled to a same optical field, is proposed, and numerically demonstrated. Each brick is placed on top of a thin silica pillar that provides mechanical support whilst isolates the individual mechanical resonances. The mass sensing capabilities of this cavity are studied through numerical simulations, proving that a point mass approximation can be used for silica nanoparticles with radius smaller than 100 nm, and that different nanoparticles can be measured independently but simultaneously.

Optomechanical crystal cavities (OMCCs) allow the interaction between localized optical and mechanical modes through the radiation-pressure force. Driving such cavities with blue-detuned lasers relative to the optical resonance can induce a phonon lasing regime where the OMCC supports self-sustained mechanical oscillations. This dynamic state results in a narrow and stable microwave tone that modulates the laser at integer multiples of the mechanical resonance frequency, ultimately creating an optomechanical (OM) frequency comb suitable for microwave photonics applications. OMCCs enable compact, low-cost power-efficient all-photonic processing of multiple microwave signals, crucial for current 5G and future beyond-5G systems, whilst being compatible with silicon integrated photonic circuits. This work reports the experimental demonstration of all-optical multi-frequency up- and down-conversion of 3GPP 5G new-radio (NR) signals from the low- to mid- and extended-mid bands using the first and second harmonics of the frequency comb generated in a silicon OMCC. The OM comb generates up to 6 harmonics in the K-band, which is suitable for microwave photonic applications. The experimental demonstration also evaluates the impact of the phase-noise and the signal-to-noise ratio (SNR) in the frequency-converted 5G NR signals when the first and second OMCC harmonics are employed for frequency conversion.

Phonon lasing is attained by driving optomechanical crystal cavities (OMCCs) with blue-detuned lasers, producing narrow and stable microwave tones at integer multiples of the mechanical frequency. We evaluate all-optical frequency upconversion employing an OM frequency comb generated with a silicon OMCC targeting the generation of 3GPP 5G new-radio signals. The performance of the upconverted 5G signal from the low-band to higher frequency bands employing the first two harmonics and their phase noises are evaluated. The proposed OMCC technology is a compact and power-efficient solution for all-photonic processing of microwave signals, a crucial aspect in 5G NR and future beyond-5G systems.

Optomechanical (OM) cavities simultaneously localize photons and phonons, thus enhancing their mutual interaction through radiation-pressure force. This acousto-optic interaction can be quantified by means of the optical-frequency shift per mechanical displacement G. The aforesaid frequency shift can also be related to the vacuum OM coupling rate g0, where only photoelastic and moving-boundary effects are commonly taken into account. However, the thermo-optic and thermal-expansion effects may also play a role since the material forming the OM cavity could be heated by the presence of photons, which should naturally affect the mechanical properties of the cavity. In this work, we introduce a theoretical approach to determine how thermal effects change the canonical OM coupling rate. To test the model, a complete set of optical-thermal-mechanical simulations was performed in two OM crystal cavities fabricated from two different materials: silicon and diamond. Our results lead us to conclude that there is a non-negligible thermal correction that is always present as a negative shift to the OM coupling rate that should be considered in order to predict more accurately the strength of the OM interaction.

Education 4.0 arises to provide citizens with the technical/digital competencies and cognitive/interpersonal skills demanded by Industry 4.0. New technologies drive this change, though time-independent learning remains a challenge, because students might face a lack of support, advice and surveillance when teachers are unavailable. This study proposes complementing presential lessons with online learning driven by ChatGPT, applied as an educational tool able to mentor K-12 students learning science at home. First, ChatGPT’s performance in the field of K-12 science is evaluated, scoring A (9.3/10 in 2023, and 9.7/10 in 2024) and providing detailed, analytic, meaningful, and human-like answers. Then, an empirical interventional study is performed to assess the impact of using ChatGPT as a virtual mentor on real K-12 students. After the intervention, the grades of students in the experimental group improved 30%, and 70% of students stated a positive perception of the AI, suggesting a positive impact of the proposed educational approach. After discussion, the study concludes ChatGPT might be a useful educational tool able to provide K-12 students learning science with the functional and social/emotional support they might require, democratizing a higher level of knowledge acquisition and promoting students’ autonomy, security and self-efficacy. The results probe ChatGPT’s remarkable capacity (and immense potential) to assist teachers in their mentoring tasks, laying the foundations of virtual mentoring and paving the way for future research aimed at obtaining a more realistic view of the AI impact on education.

Education 4.0 arises to provide citizens with the technical/digital competencies and cognitive/interpersonal skills demanded by Industry 4.0. New technologies drive this change, though time-independent learning remains a challenge, because students might face a lack of support, advice and surveillance when teachers are unavailable. This study proposes complementing presential lessons with online learning driven by ChatGPT, applied as an educational tool able to mentor K-12 students learning science at home. First, ChatGPT’s performance in the field of K-12 science is evaluated, scoring A (9.3/10 in 2023, and 9.7/10 in 2024) and providing detailed, analytic, meaningful, and human-like answers. Then, an empirical interventional study is performed to assess the impact of using ChatGPT as a virtual mentor on real K-12 students. After the intervention, the grades of students in the experimental group improved by 30%, and 70% of students stated a positive perception of the AI, suggesting a positive impact of the proposed educational approach. After discussion, the study concludes ChatGPT might be a useful educational tool able to provide K-12 students learning science with the functional and social/emotional support they might require, democratizing a higher level of knowledge acquisition and promoting students’ autonomy, security and self-efficacy. The results probe ChatGPT’s remarkable capacity (and immense potential) to assist teachers in their mentoring tasks, laying the foundations of virtual mentoring and paving the way for future research aimed at extending the study to other areas and levels, obtaining a more realistic view of AI’s impact on education.

ENHANCING K-12 STUDENTS' PERFORMANCE IN CHEMISTRY THROUGH CHATGPT-POWERED BLENDED LEARNING IN THE EDUCATION 4.0 ERA

Hybrid cavities that combine photonic and plasmonic elements can be customized to exhibit both high Q factors and extremely small mode volumes, leading to significant increases in the Purcell factor ($F_{\mathrm{P}}$). In this work, we propose a hybrid system consisting of a silicon photonic crystal cavity with a slot at its center where a gold nanoparticle is located. We demonstrate large Purcell factors ($F_{\mathrm{P}}\approx10^{7}-10^{S}$) in the telecom spectral region. The presented results can find applications in molecular optomechanics, sensing, or enhanced Raman spectroscopy.

The remote synchronization of oscillators is essential for improving the performance, efficiency, and reliability of various systems and technologies, ranging from everyday telecommunications to cutting-edge scientific research and emerging technologies. In this work, we unequivocally demonstrate a master-slave type of synchronization between two self-sustained optomechanical crystal oscillators that interact solely through an external optical feedback stage. Several pieces of experimental evidence rule out the possibility of resonant forcing, and, in contrast to previous works, indicate that synchronization is achieved in the regime of natural dynamics suppression. Our experimental results are in agreement with the predictions of a numerical model describing the specific mechanical lasing dynamics of each oscillator and the unidirectional interaction between them. The outcomes of our study pave the way toward the synchronization of clock signals corresponding to far-placed processing elements in a future synchronous photonic integrated circuit.