3D printed near-infrared high-numerical aperture achromatic metalens.

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.

Characterization and Inverse Design of Stochastic Mechanical Metamaterials Using Neural Operators.

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.

High-Q Emission from Colloidal Quantum Dots Embedded in Polymer Quasi-BIC Metasurfaces.

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.

Two-photon 3D printing diaphragm-integrated ring waveguide coupler for ultrasound detection.

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.

Continuous tuning of unidirectional emission wavelength by bending a notched-elliptical polymer microdisk.

An approach of continuously tunable unidirectional emission through bending a notched-elliptical polymer microdisk is proposed. The characteristics of the bending-dependent action are carefully analyzed, and the resonance wavelength for unidirectional emission can be tuned continuously through bending the device. Such a whispering-gallery-mode microresonator enables unidirectional emission with ultra-low divergence, of which the emission efficiency and Q factor are stabilized, demonstrating the whole structure is robust and relatively insensitive within a certain bending angle range.

Programmable Self-Regulation with Wrinkled Hydrogels and Plasmonic Nanoparticle Lattices.

This paper describes a self-regulating system that combines wrinkle-patterned hydrogels with plasmonic nanoparticle (NP) lattices. In the feedback loop, the wrinkle patterns flatten in response to moisture, which then allows light to reach the NP lattice on the bottom layer. Upon light absorption, the NP lattice produces a photothermal effect that dries the hydrogel, and the system then returns to the initial wrinkled configuration.

Highly sensitive Mach-Zehnder interferometric micromagnetic field sensor based on 3D printing technology.

A two-photon 3D printed polymer magnetic sensing device based on a Mach-Zehnder interferometer (MZI) is proposed. One arm of the MZI contains a hollow cavity and two connecting open channels that can be filled with magnetic fluids (MFs) and sealed by the UV curable adhesive, forming a magneto-optical component of the interferometer. As the magnetic field changes, the refractive index (RI) of the MF changes, and the effective RI of the guiding mode of the waveguide changes accordingly, which results in a change in the phase of the MZI.

Efficient harnessing of light from nanoscale emitters deterministically placed through polymer-pen lithography at the focus of 3D-printed ellipsoidal micro-lenses.

Collecting significant and measurable signals from the typically omnidirectional emission of nanoscale emitters is challenging. To improve the collection efficiency, it is essential to deterministically place the emitters in desired locations and design mode converters to match the modes of emission to those of the collection system. In this Letter, we propose the deterministic placement of nanoscale emitters using a pick-and-place technique called polymer-pen lithography.

Large dynamic-range fiber Bragg grating sensor system for acoustic emission detection.

A distributed feedback (DFB) fiber laser and fiber Bragg gratings (FBGs) are configured to demodulate the wavelength shifts of FBG dynamic strain sensors. The FBG sensors act as sensing units to detect the dynamic strain and the demodulators while the DFB fiber laser only acts as a narrow-linewidth light source. As the reflective spectrum of the FBG sensor changes due to dynamic strains, the output is subsequently converted into a corresponding intensity change and detected directly by a photodetector.

Inverse Design and 3D Printing of a Metalens on an Optical Fiber Tip for Direct Laser Lithography.

An inverse-designed metalens is proposed, designed, and fabricated on an optical fiber tip via a 3D direct laser-writing technique through two-photon polymerization. A computational inverse-design method based on an objective-first algorithm was used to design a thin circular grating-like structure to transform the parallel wavefront into a spherical wavefront at the near-infrared range. With a focal length about 8 μm at an operating wavelength of 980 nm and an optimized focal spot at the scale of 100 nm, our proposed metalens platform is suitable for two-photon direct laser lithography.

Multi-wavelength microresonator based on notched-elliptical polymer microdisks with unidirectional emission.

A three-dimensional notched-elliptical microdisk with a wavelength-size notch on the boundary is proposed as a multi-wavelength and unidirectional emission lasing source. The device contains multiple properly designed two-dimensional whispering gallery mode-based polymer notched microdisks with different dimensions for use as a multi-wavelength source. It can have a relatively high optical quality factor of 4000, unidirectional emission with low far-field divergence ∼4°, and the efficiency of emission is as high as 84.

Folding at the Microscale: Enabling Multifunctional 3D Origami-Architected Metamaterials.

Mechanical metamaterials inspired by the Japanese art of paper folding have gained considerable attention because of their potential to yield deployable and highly tunable assemblies. The inherent foldability of origami structures enlarges the material design space with remarkable properties such as auxeticity and high deformation recoverability and deployability, the latter being key in applications where spatial constraints are pivotal. This work integrates the results of the design, 3D direct laser writing fabrication, and in situ scanning electron microscopic mechanical characterization of microscale origami metamaterials, based on the multimodal assembly of Miura-Ori tubes.

Three-dimensional-printed Fabry-Perot interferometer on an optical fiber tip for a gas pressure sensor.

We demonstrate a three-dimensional (3D)-printed miniature optical fiber-based polymer Fabry-Perot (FP) interferometric pressure sensor based on direct femtosecond laser writing through two-photon polymerization. An unsealed cylinder column with a suspended polymer diaphragm is directly printed on a single-mode fiber tip to form an FP cavity. Here, two FP cavities with different lengths and the same diaphragm thickness (5 µm) are presented.

Adaptive fiber-ring lasers based on an optical fiber Fabry-Perot cavity for high-frequency dynamic strain sensing.

A fiber-optic Fabry-Perot (FP) interferometer integrated with an adaptive fiber-ring laser is configured as a switchable multi-wavelength fiber laser that can be utilized for ultrasound detection. Because the FP sensor acts as a wavelength filter and a reflector of the fiber-ring laser, the reflective spectrum of the FP sensor changes due to static/dynamic strains, and the wavelength of the laser output shifts accordingly, which is subsequently converted into a corresponding phase shift and demodulated by an unbalanced interferometer. By carefully controlling the polarization of the system, the lasing outputs with a side-mode suppression ratio higher than 30 dB can be obtained, and the lasing linewidth is much narrower than that of the spectrum of the FP sensor.

Miniature resonator sensor based on a hybrid plasmonic nanoring.

A miniature resonator sensor based on a hybrid plasmonic nanoring with a gold layer coated uniformly on the outer boundary is described and investigated. By using the Lumerical finite-difference-time-domain (FDTD) method, the optimized sizes of the plasmonic layer thickness and the central hole are given and insight into the dependence of spectral displacements, Q factors, sensitivity and detection limits on the ambient refractive index is presented. Simulation results reveal that the miniature resonator sensor featuring high sensitivity of 339.

Direct laser writing of a phase-shifted Bragg grating waveguide for ultrasound detection.

We demonstrate a phase-shifted Bragg grating waveguide (PS-BGW) fabricated by a direct laser writing technique via two-photon polymerization as a high-frequency ultrasonic sensor. The PS-BGW device has a cross-sectional area of 1.5  μm×2  μm, and the grating length is about 100 μm.

Polymer micro-ring resonator integrated with a fiber ring laser for ultrasound detection.

Polymer micro-ring resonators fabricated by a direct laser writing technique are presented as sensors for ultrasound detection. The optical micro-ring resonator consists of a micro-ring waveguide that acts as a wavelength selective feedback mirror to an erbium-doped fiber-ring laser (FRL). The micro-ring resonator reflection spectrum determines the lasing frequencies of the FRL.

Comparative assessment of erbium fiber ring lasers and reflective SOA linear lasers for fiber Bragg grating dynamic strain sensing.

Fiber Bragg grating (FBG) dynamic strain sensors using both an erbium-based fiber ring laser configuration and a reflective semiconductor optical amplifier (RSOA)-based linear laser configuration are investigated theoretically and experimentally. Fiber laser models are first presented to analyze the output characteristics of both fiber laser configurations when the FBG sensor is subjected to dynamic strains at high frequencies. Due to differences in the transition times of erbium and the semiconductor (InP/InGaAsP), erbium-doped fiber amplifier (EDFA)- and RSOA-based fiber lasers exhibit different responses and regimes of stability when the FBG is subjected to dynamic strains.

Fiber Bragg grating dynamic strain sensor using an adaptive reflective semiconductor optical amplifier source.

In this paper, a reflective semiconductor optical amplifier (RSOA) is configured to demodulate dynamic spectral shifts of a fiber Bragg grating (FBG) dynamic strain sensor. The FBG sensor and the RSOA source form an adaptive fiber cavity laser. As the reflective spectrum of the FBG sensor changes due to dynamic strains, the wavelength of the laser output shifts accordingly, which is subsequently converted into a corresponding phase shift and demodulated by an unbalanced Michelson interferometer.

Real-time full-field photoacoustic imaging using an ultrasonic camera.

A photoacoustic imaging system that incorporates a commercial ultrasonic camera for real-time imaging of two-dimensional (2-D) projection planes in tissue at video rate (30 Hz) is presented. The system uses a Q-switched frequency-doubled Nd:YAG pulsed laser for photoacoustic generation. The ultrasonic camera consists of a 2-D 12 x 12 mm CCD chip with 120 x 120 piezoelectric sensing elements used for detecting the photoacoustic pressure distribution radiated from the target.

Bulk-wave and guided-wave photoacoustic evaluation of the mechanical properties of aluminum/silicon nitride double-layer thin films.

The development of devices made of micro- and nano-structured thin film materials has resulted in the need for advanced measurement techniques to characterize their mechanical properties. Photoacoustic techniques, which use pulsed laser irradiation to nondestructively induce very high frequency ultrasound in a test object via rapid thermal expansion, are suitable for nondestructive and non-contact evaluation of thin films. In this paper, we compare two photoacoustic techniques to characterize the mechanical parameters of edge-supported aluminum and silicon nitride double-layer thin films.

Adaptive demodulation of dynamic signals from fiber Bragg gratings using two-wave mixing technology.

A two-wave mixing (TWM) interferometer using photorefractive (PRC) InP:Fe crystal is configured to demodulate the spectral shift of a fiber Bragg grating (FBG) sensor. The FBG is illuminated with a broadband source, and any strain in the FBG is encoded as a wavelength shift of the light reflected by the FBG. The wavelength shift is converted into phase shift by means of an unbalanced TWM interferometer.

Interaction of a scanning laser-generated ultrasonic line source with a surface-breaking flaw.

The scanning laser source (SLS) technique has been proposed recently as an effective way to investigate small surface-breaking cracks. By monitoring the amplitude and frequency changes of the ultrasound generated as the SLS scans over a defect, the SLS technique has provided enhanced signal-to-noise performance compared to the traditional pitch-catch or pulse-echo ultrasonic methods. In previous work, either a point source or a short line source was used for generation of ultrasound.

Photoacoustic probes for nondestructive testing and biomedical applications.

Fiber-optic photoacoustic sources for nondestructive testing and biomedical applications are described. The photoacoustic sources consist of a pulsed laser, a fiber-optic cable, and a generation head. The generation head is a miniature hermetically sealed chamber, which can be embedded into solid structures or immersed in liquid media.

Mass spring lattice modeling of the scanning laser source technique.

The scanning laser source (SLS) technique is a promising new laser ultrasonic tool for the detection of small surface-breaking defects. The SLS approach is based on monitoring the changes in laser generated ultrasound as a laser source is scanned over a defect. Changes in amplitude and frequency content have been observed for ultrasound generated by the laser over uniform and defective areas.