Dejun Liu

Nanoindentation is a widely used technique for localized mechanical characterization at the micro- and nanoscale, where measurement accuracy critically depends on high-resolution sensing and signal readout mechanisms. In this work, we present an improved fiber-optic nanomechanical probe (FONP) fabricated by femtosecond laser two-photon polymerization and develop a dedicated fiber-optic nanoindentation system specifically designed for automated measurements. The enhanced FONP employs Fabry–Pérot interferometric optical readout and incorporates an optical shielding layer that ensures spectral stability during surface approach without requiring a gold coating, maintaining a cross-correlation coefficient of 1 with the reference spectrum. Using the developed system, we investigate the time-dependent mechanical evolution of onion outer epidermal cells during natural dehydration, revealing a clear increase in Young’s modulus. In addition, automated array-based Young’s modulus mapping of polydimethylsiloxane (PDMS) samples shows strong agreement with a commercial nanoindenter, with deviations within 10%. These results demonstrate the accuracy, stability, and automation capability of the proposed fiber-optic interferometric nanoindentation system, highlighting its potential for compact and robust nanomechanical characterization.

We designed and fabricated a single-fiber needle probe integrated with a fiber Bragg grating (FBG) capable of simultaneously performing optical coherence tomography (OCT) imaging and temperature sensing. The Bragg wavelength of the FBG is designed to fall within the spectral bandwidth of the OCT light source, enabling dual-mode functionality with a single OCT source. The probe exhibits a lateral resolution of approximately 30 μm, operates at a working distance of about 0.9 mm in air, and features a temperature sensitivity of 10.14 pm/℃. Grating reflectance has a significant influence on OCT imaging performance; hence, multiple fiber probes with varying grating parameters were investigated. Results indicate that limiting grating reflectance to ≤24% serves as a reference threshold for balancing high-quality imaging with precise temperature sensing. Finally, experiments conducted on ex vivo mouse brains and porcine backfat further demonstrate the potential applications of this dual modal OCT needle probe.

Endoscopic technology plays a crucial role in minimally invasive diagnostic procedures and precision industrial inspections. However, producing a microsized full-color endoscope has been challenging in fabrication and aberration corrections. In this paper, we propose a miniaturized fiber endoscope featuring multiple lens elements with freeform surfaces using two-photon polymerization (TPP) 3 D nanoprinting technology to effectively correct various optical aberrations and have realized good optical performances in full-color imaging. The microlens was integrated onto an imaging fiber tip, achieving a FOV of 60° and a resolution of 7.13 line pairs per mm (lp/mm). In addition, the proposed fiber endoscope is capable of accurately reproducing full-color information while preserving image details across different image types. This research offers an innovative approach to develop high-performance endoscopes and enhance its imaging applications.

Chirped fiber Bragg gratings (CFBGs) are widely applied in telecommunications, sensing, and ultrafast laser systems due to their broadband dispersion properties. However, fabrication of CFBGs with a linear group delay remains a challenge. This study introduces a novel fabrication technique that integrates femtosecond grayscale lithography with external pulse timing control to produce CFBGs with high coupling efficiency, precise dispersion compensation, and well-controlled linear chirp rates. CFBGs with varying dispersion profiles were fabricated, achieving dispersion errors below 0.2 ps/nm. To further enhance the spectral performance, three apodization functions (Hamming, Triangular, and Gaussian) were applied to CFBGs. Both simulations and experiments demonstrate that these apodization functions effectively suppress reflection spectra side lobes and improve group delay linearity. Notably, the Gaussian apodization function yields superior results, achieving a side-lobe suppression ratio (SLSR) exceeding 15 dB and optimal spectral performance. The proposed fabrication method of CFBGs provides new possibilities for telecommunications, high power laser system and other cutting-edge scientific research.

This article presents a novel method for fabricating repeatable and uniform surface-enhanced Raman scattering (SERS) substrates. The proposed method consists of two steps: (1) the fabrication of nanohole arrays using advanced femtosecond laser-induced two-photon polymerization (TPP) technology; and (2) the deposition of 9 nm thick silver nanoparticles on the nanohole arrays. The proposed nanohole arrays were optimized at the diameter, and the thickness of the silver film at two parameters. Regarding SERS substrates, a limit of detection of 10⁻¹⁰ M (rhodamine 6G) and analytical enhancement factors up to 3.5 × 10⁴ were achieved. At 1361 cm⁻¹, the relative standard deviation (RSD) of the characteristic peak was 5.5%, demonstrating a highly reproducible SERS substrate.

We propose a high-sensitivity light sensor based on side-hole optical fiber filled with carbon tetrachloride. Using a fusion splicer, we employed a selective glue coating method to fill one of the air holes with liquid. Coupling occurs between the propagation modes of the liquid waveguide filled with carbon tetrachloride and the core propagation modes, resulting in resonant dips in the transmission spectrum. When the temperature changes, the refractive index of carbon tetrachloride changes, affecting the coupling and causing the wavelength of resonant dips to shift. In this experiment, the temperature sensitivity of the sensor was tested and reached up to 46 nm/°C. Utilizing its high-temperature sensitivity, the infrared laser light intensity was measured with a sensitivity of 0.114 nm/(mW/cm²). While maintaining a high level of sensitivity, the sensor is also easy to fabricate and cost-effective. Therefore, it holds great potential for applications requiring high-precision laser power measurement, laser manipulation, optical modulation, and optical switching.

Optical fibers are commonly employed in telecommunications for transmitting data. However, it is difficult to distinguish optical conduits from one another when servicing data transmission cables due to the large number of optical fibers and optical cables. Conventional methods of physically marking fibers are cumbersome, and the label information is accessible to any potential user. In this work, we propose and demonstrate an encrypted optical fiber tag based on an ultra-low-loss encoded cladding-type fiber Bragg grating (cladding-FBG) array. Using the femtosecond laser point-by-point (PBP) technique, binary data sequences are successfully embedded into the different positions of the optical fiber cladding in the form of Bragg gratings. The cladding-FBGs redistribute light within the cladding, enabling information retrieval via backscattered signals while having minimal impact on the core light transmission. Consequently, the insertion loss of a single encoded cladding grating is suppressed to an exceptionally low level of just 0.00155 dB, which is favorable for optical communication. The data capacity and security of the tag can be enhanced by optimizing the parameters of cladding-FBGs, such as grating periods and spatial distributions. Therefore, the proposed low-loss optical fiber tag presents significant potential for integration into network systems for optical network link identification and data storage.

On-chip micro- and nano-photonic structures are pivotal in modern industry, primarily due to their compact size, low transmission losses, and ease of integration. However, the three-dimensional (3D) bottom-up fabrication of on-chip functional structures, especially those made from inorganic materials rather than organic polymers, remains a challenge. Building inorganic devices with designed functional nanostructures on chips is critical for advancing microsystem applications. In this work, we propose a strategy for 3D nanolithography of on-chip waveguides using hybrid organic–inorganic materials through ultrafast laser-induced multiphoton absorption. The acrylate-functionalized precursors are locally cross-linked via nonlinear effects without the need for polymer additives. This process enables the direct integration of 3D photonic waveguides, resonators, and gratings onto chips, which exhibit strong stability due to densely packed silicon–oxygen molecular networks. This approach offers a promising solution for creating tailored inorganic micro- and nano-structures on chips, facilitating the development of advanced integrated functional devices.

Micro- and nano-sized optical elements have been widely explored for various applications, including telecommunications, sensing technologies, and laser processing. The “center beam-in-ring” beam configuration is promising in laser cutting and welding to avoid spatter of particles or droplets, and it holds potential applications in medical treatment. In this study we propose a dot-in-ring and a ring-in-ring beam shaping microlens fabricated on the tip of an optical fiber. These microlenses, featuring freeform light output surface designs, were fabricated on cleaved fiber ends using femtosecond laser-induced two-photon polymerization (TPP) 3D printing. Both simulations and experimental results confirm that the proposed fiber-tip microlenses effectively convert the Gaussian beam from a single-mode fiber into a dot-in-ring beam and a ring-in-ring beam, while maintaining excellent optical field propagation and energy distribution over a defined distance. This work highlights the capability of TPP 3D printing as a powerful tool for integrating micro-scale beam shaping elements onto optical fibers, enabling the development of compact devices.