Sridhar Krishnaswamy

Polymers with gain media can enable 3D printing of chip-scale optical amplifiers and lasers. In this work, a 3D-printable resin containing uniform dispersion of core/shell erbium–ytterbium–cerium codoped rare-earth nanoparticles (RENPs) is designed, synthesized, and characterized. The core/shell RENPs (NaYF 4:Er 3+,Yb 3+,Ce 3+ @NaYF 4 ) are synthesized by a high-temperature thermal decomposition method. Uniform dispersion at high concentrations of the RENPs in a 3D printable resin is achieved by selectively polymerizing methyl methacrylate on the surface of the RENPs, which prevents nanoparticle agglomeration. It is observed that codoping with Ce 3 + ions facilitates nonradiative energy transfer between the upper excited states of Er 3 + ions to the Ce 3 + ions, significantly suppressing up-conversion pathways and leading to an order of magnitude increase in the down-conversion (1.53 μm) to up-conversion (0.536 μm) ratio using a 980 nm pump. The resin is used in a two-photon lithography 3D printer to fabricate ultrashort chip-scale spiral optical waveguide amplifiers in the telecommunication C-band. The fabricated amplifier exhibited a relative gain of 7.7 dB for a 1.5 mm long spiral waveguide, with a pump power operating below 130 mW. The subcentimeter length of the amplifiers enables high small-signal gain to be achieved with low pump power for amplifying signals in the microwatt range, making them suitable for integration into chip-scale devices.

Mechanical Metamaterials In article number 2420063, Horacio D. Espinosa and co-workers introduce a data-efficient inverse design framework using neural operators trained on sparse experimental data to predict and tailor the nonlinear mechanical behavior of spinodal metamaterials. The approach enables accurate, application-driven design of architected materials for extreme environments—paving the way for next-generation aerospace and multifunctional systems.

Traditional optical Fresnel microlenses have limitations such as large size, limited optical quality for imaging, and low focusing efficiency in achromatic lenses with high NA. In contrast, metalenses rely on their subwavelength structure to modulate the phase distribution, resulting in smaller volumes and superior focusing performance. In this work, we inverse designed and fabricated an achromatic metalens with high-NA and broad wavelength range through direct laser writing using the two-photon polymerization technique. With a focal length of 19 μm, a thickness of 3.6 μm, and a numerical aperture of 0.8, the metalens exhibits an average focusing efficiency of 53.6% and an average half maximum width of 1.27 μm at the working wavelength. The measured average focusing efficiency is 50.4% within the bandwidth range of 1510 nm-1610 nm. The presented work demonstrates the great potential of 3D printing and inverse design for realizing functional meta-devices for aerospace sector.

Machine learning (ML) is emerging as a transformative tool for the design of mechanical metamaterials, offering properties that far surpass those achievable through lab-based trial-and-error methods. However, a major challenge in current inverse design strategies is their reliance on extensive computational and/or experimental datasets, which becomes particularly problematic for designing micro-scale stochastic architected materials that exhibit nonlinear mechanical behaviors. Here, a comprehensive end-to-end scientific ML framework, leveraging deep neural operators (including DeepONet and its variants) is introduced, to directly learn the relationship between the complete microstructure and mechanical response of architected metamaterials from sparse but high-quality in situ experimental data. Various neural operators and standard neural networks are systematically compared to identify the model that offers better interpretability and accuracy. The approach facilitates the efficient inverse design of structures tailored to specific nonlinear mechanical behaviors. Results obtained from stochastic spinodal microstructures, printed using two-photon lithography, reveal that the prediction error for mechanical responses is within a range of 5 - 10%. This work underscores that by employing neural operators with advanced nano- and micro-mechanical experiments, the design of complex micro-architected materials with desired properties becomes feasible, even in scenarios constrained by data scarcity. This work marks a significant advancement in the field of materials-by-design, potentially heralding a new era in the discovery and development of next-generation metamaterials with unparalleled mechanical characteristics derived directly from experimental insights.

Metasurfaces supporting narrowband resonances are of significant interest in photonics for molecular sensing, quantum light source engineering, and nonlinear photonics. However, many device architectures rely on large refractive index dielectric materials and lengthy fabrication processes. In this work, we demonstrate quasi-bound states in the continuum (quasi-BICs) using a polymer metasurface exhibiting experimental quality factors of 305 at visible wavelengths. Our fabrication process only consists of electron-beam lithography and resist development, making it compatible with large-scale fabrication techniques. Additionally, we address the challenges of integrating colloidal quantum dots (CQDs) into the nanopillars, such as depletion-induced aggregation and excess nanoparticle removal, by leveraging our previously reported nanoparticle functionalization method and modified development procedures. We demonstrate both narrowband and polarized emission from our CQD-integrated quasi-BIC metasurfaces. Our proposed metasurface platform is broadly applicable across various quantum emitters and fabrication methods and could enable advancements in scalable manufacturing of resonant optical devices.

We report a dual-resonant cavity-based fiber-optic Fabry-Perot (FP) interferometer integrated on an optical fiber tip for ultrasound measurement, that is immediately printed through the utilization of two-photon polymerization technology employing direct laser writing. One of the resonant cavities is utilized for acoustic wave pressure amplification, while the other FP cavity is functionalized as an optical resonator. The combination of the two cavities can significantly enhance the vibration of the diaphragm, which can be used for the detection of weak acoustic waves. To improve its responsivity to acoustic waves, the designed structure is optimized by adjusting the parameters thickness of the upper diaphragm and the distance between the double diaphragms. The results show that the fabricated device can achieve low noise equivalent acoustic signal level of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$26.9~\mu $ </tex-math></inline-formula>Pa/Hz<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\mathrm {1/2}}$ </tex-math></inline-formula> at 100 kHz. This value is much higher than that of FP devices with single uniform diaphragm. Such miniature sensor has broad prospects for development applicable to fields of structural health monitoring, photoacoustic imaging, and underwater acoustic sensing.

Abstract Two‐photon polymerization 3D‐printing is a well‐known technique for fabricating passive micro/nanoscale structures, such as microlenses and inversely designed polarization splitters. The integration of light emitting nanoparticle (NP) dopants, such as quantum dots (QDs) and rare‐earth doped nanoparticles (RENPs), into a polymer resist would enable 3D printing of active polymer micro‐photonic devices, including sensors, lasers, and solid‐state displays. Many NPs contain oleic acid ligands to prevent degradation, but oleate‐capped NPs (oc‐NPs) tend to agglomerate in nonpolar media despite the hydrophobicity of the ligand. This results in an uneven nanoparticle distribution and increased optical extinction. In this work, a general approach is proposed for dispersing oc‐NPs in commercial 3D printable polymers. Controlled growth of small carbon chains around the oc‐NPs is achieved by functionalizing them with methyl‐methacrylate monomers. The proposed approach is validated on RENPs (≈65 nm) and CdSe/ZnS quantum dots (≈12 nm) using commercial polymer resists (IP‐Dip and IP‐Visio). Dispersions of functionalized NPs (f‐NPs) have improved the NP density by an order of magnitude and are shown to be stable for several weeks with minimal impact on printing quality. The approach is generalizable to other oc‐NPs, enabling synthesis of functional resins for high‐quality polymer‐based optical and electronic devices.

Two-photon polymerization (2PP) 3D printing is a well-known technique for fabricating passive micro/nanoscale structures, such as microlenses and inversely designed polarization splitters. The integration of light emitting nanoparticle (NP) dopants, such as quantum dots (QDs) and rare-earth doped nanoparticles (RENPs), into a polymer resist would enable 3D printing of active polymer micro-photonic devices, including sensors, lasers, and solid-state displays. Many NPs are stabilized with oleic acid ligands to prevent degradation, but oleate-capped NPs (oc-NPs) tend to agglomerate in nonpolar media despite the hydrophobicity of the ligand. This results in an uneven distribution of NPs in polymers and increased optical extinction properties. In this work, we propose a general approach for dispersing various oc-NPs in commercial 3D printable polymers. We achieve controlled growth of small carbon chains around the oc-NPs by functionalizing the NPs with methyl-methacrylate monomers. The proposed approach is validated on RENPs (~65 nm) and CdSe/ZnS quantum dots (~12 nm) using different commercial polymer resists (IP-Dip and IP-Visio). Dispersions of functionalized NPs (f-NPs) have improved NP density by an order of magnitude and are shown to be stable for several weeks with minimal impact on printing quality. Our approach is generalizable to a variety of oc-NPs and ultimately leads to higher quality polymer-based optical and electronic devices.

We demonstrate a novel optically active ring resonator that is 3D printed on the tip of a single-mode optical fiber. The ring resonator is printed using two-photon polymerization of a resin that has been doped with core-shell NaYF₄: Yb³⁺, Er³⁺ nanoparticles. The integration of these optically active nanoparticles into the resonator allows the exploitation of their upconversion luminescence properties, making it sensitive to temperature and strain variations. The sensing performance of the device is based on the change in the luminescence intensity corresponding to the variation in surrounding temperature. As the ambient conditions change, variations in the refractive index and geometry affect the change in the intensity of the ring resonator which can be remotely demodulated from the light emissions collected by the optical fiber itself.

In this work, we report the fabrication and characterization of a fiber-optic temperature sensor based on embedding rare-earth nanoparticle emitters (NaYF4: Yb³⁺, Er³⁺) inside an optical fiber. A micro channel passing through the core of Single Mode Fiber (SMF) is fabricated using a femtosecond laser, and the microcavity is subsequently filled with optically active medium. In this work, the NaYF4: Yb³⁺, Er³⁺ works as an active medium with temperature-sensitive upconversion luminescence properties to provide temperature measurements allowing for real-time temperature monitoring. The obtained results show promise for addressing temperature-related challenges in various fields, reaffirming the crucial role of optical sensors for a wide range of applications including industrial, medical, and environmental monitoring.

Machine learning (ML) is emerging as a transformative tool for the design of architected materials, offering properties that far surpass those achievable through lab-based trial-and-error methods. However, a major challenge in current inverse design strategies is their reliance on extensive computational and/or experimental datasets, which becomes particularly problematic for designing micro-scale stochastic architected materials that exhibit nonlinear mechanical behaviors. Here, we introduce a new end-to-end scientific ML framework, leveraging deep neural operators (DeepONet), to directly learn the relationship between the complete microstructure and mechanical response of architected metamaterials from sparse but high-quality in situ experimental data. The approach facilitates the inverse design of structures tailored to specific nonlinear mechanical behaviors. Results obtained from spinodal microstructures, printed using two-photon lithography, reveal that the prediction error for mechanical responses is within a range of 5 - 10%. Our work underscores that by employing neural operators with advanced micro-mechanics experimental techniques, the design of complex micro-architected materials with desired properties becomes feasible, even in scenarios constrained by data scarcity. Our work marks a significant advancement in the field of materials-by-design, potentially heralding a new era in the discovery and development of next-generation metamaterials with unparalleled mechanical characteristics derived directly from experimental insights.

We demonstrate a diaphragm-integrated ring waveguide coupler fabricated by the two-photon direct laser wring technique as an ultrasonic sensor, which is integrated on an optical fiber tip. The device consists of a micro-ring waveguide with a diameter of 5 µm functionalized as an optical fiber tip light reflection mirror and a straight waveguide connecting a diaphragm. The evanescent field coupling can be realized between the two waveguides, and the coupling efficiency can be changed due to the variation of the coupling gap induced by ultrasound. Accordingly, the light reflection can be changed. Based on the plate vibration theory, the vibration frequency can be changed through optimizing the diaphragm size. The experiments show that the device exhibits a high sensitivity and low noise equivalent acoustic signal level of 1.07 mPa/Hz 1/2 at 100 kHz, which has great potential in various acoustic wave sensing applications.

The corrugated rolling process is a new type of metal clad plate production method, and the high-precision testing of the corrugated interface is of great significance to the improvement of product quality. This paper integrates laser ultrasonics and Synthetic Aperture Focusing Technique (SAFT) to characterize the corrugated interface accurately. A photoacoustic platform is developed for the non-contact automatic acquisition of laser ultrasonic signals, and these signals are filtered and gated to achieve high-quality imaging. Meanwhile the effect of signal acquisition efficiency on imaging quality is also discussed. Since the ultrasonic excitation point is separated from the detection point in this experiment, an interface profile calculation method based on the ellipses’ common tangent line is developed, and the double-layer imaging of the corrugated clad plate is realized using the fitting curve of the interface profile and Fermat’s principle. The results show that the image obtained by this method are consistent with the results obtained by the 3D surface profiler and Energy Dispersive Spectroscopy (EDS), which indicates that this method can not only provides an essential reference for the research of corrugated rolling process, but also provides new approaches for non-contact non-destructive-testing applications in the materials processing industry.

In this paper, we will discuss the development of 3D printing strategies which enable rapid 3D fabrication of polymer photonic devices and sensors with applications to system health monitoring (SHM). Two-photon polymerization (2PP) 3D printers with 100 nm spatial resolution are commercially available and have enabled the design and fabrication of integrated nano-to microscale polymer photonic devices. Such 3D printing approaches allow us to design truly 3D photonic devices, and this opens the door to fabrication of complex shaped devices that are often produced by methods such as inverse design. Specifically, we report the development of optically active resins that are compatible with two-photon polymerization. We will discuss multiscale 3D printing of photonic devices and sensors with both passive and optically active resins that exhibit both up and down conversion emission when pumped at 980 nm.

Optical fiber microresonators have attracted considerable interest for acoustic detection because of their compact size and high optical quality. Here, we have proposed, designed, and fabricated a spring-based Fabry–Pérot cavity microresonator for highly sensitive acoustic detection. We observed two resonator vibration modes: one relating to the spring vibration state and the other determined by the point-clamped circular plate vibration mode. We found that the vibration modes can be coupled and optimized by changing the structure size. The proposed resonator is directly 3D printed on an optical fiber tip through two-photon polymerization and is used for acoustic detection and imaging. The experiments show that the device exhibits a high sensitivity and low noise equivalent acoustic signal level of 2.39 mPa / Hz 1/2 at 75 kHz that can detect weak acoustic waves, which can be used for underwater object imaging. The results demonstrate that the proposed work has great potential in acoustic detection and biomedical imaging applications.