Three-dimensional laser micromachining system with integrated sub-100†nm resolution in-situ measurement.

In this study, a three-dimensional (3D) laser micromachining system with an integrated sub-100 nm resolution in-situ measurement system was proposed. The system used the same femtosecond laser source for in-situ measurement and machining, avoiding errors between the measurement and the machining positions. It could measure the profile of surfaces with an inclination angle of less than 10°, and the measurement resolution was greater than 100 nm. Consequently, the precise and stable movement of the laser focus could be controlled, enabling highly stable 3D micromachining. The results showed that needed patterns could be machined on continuous surfaces using the proposed system. The proposed machining system is of great significance for broadening the application scenarios of laser machining.

Observation of delocalization transition in topological waveguide arrays with long-range interactions.

Topological edge states are a generic feature of topological insulators, and the long-range interactions, which break certain properties of topological edge states, are always non-negligible in real physical systems. In this Letter, we investigate the influence of next-nearest-neighbor (NNN) interactions on the topological properties of the Su-Schrieffer-Heeger (SSH) model by extracting the survival probabilities at the boundary of the photonic lattices. By introducing a series of integrated photonic waveguide arrays with different strengths of long-range interactions, we experimentally observe delocalization transition of light in SSH lattices with nontrivial phase, which is in good agreement with our theoretical predictions. The results indicate that the NNN interactions can significantly affect the edge states, and that the localization of these states can be absent in topologically nontrivial phase. Our work provides an alternative way to investigate the interplay between long-range interactions and localized states, which may stimulate further interest in topological properties in relevant structures.

Three-dimensional on-chip mode converter.

The implementation of transverse mode, polarization, frequency, and other degrees of freedom (d.o.f.s) of photons is an important way to improve the capability of photonic circuits. Here, a three-dimensional (3D) linear polarized (LP) LP11 mode converter was designed and fabricated using a femtosecond laser direct writing (FsLDW) technique. The converter included multi-mode waveguides, symmetric Y splitters, and phase delaying waveguides, which were constructed as different numbers and arrangements of circular cross section waveguides. Finally, the modes (LP11a and LP11b) were generated on-chip with a relatively low insertion loss (IL). The mode converter lays a foundation for on-chip high-order mode generation and conversion between different modes, and will play a significant role in mode coding and decoding of 3D photonic circuits.

Arbitrarily rotated optical axis waveguide induced by a trimming line.

Rotated optical axis waveguides can facilitate on-chip arbitrary wave-plate operations, which are crucial tools for developing integrated universal quantum computing algorithms. In this paper, we propose a unique technique based on femtosecond laser direct writing technology to fabricate arbitrarily rotated optical axis waveguides. First, a circular isotropic main waveguide with a non-optical axis was fabricated using a beam shaping method. Thereafter, a trimming line was used to create an artificial stress field near the main waveguide to induce a rotated optical axis. Using this technique, we fabricated high-performance half- and quarter-wave plates. Subsequently, high-fidelity (97.1%) Pauli-X gate operation was demonstrated via quantum process tomography, which constitutes the basis for the full manipulation of on-chip polarization-encoded qubits. In the future, this work is expected to lead to new prospects for polarization-encoded information in photonic integrated circuits.

Single NV centers array preparation and static magnetic field detection.

To solve the problem of static magnetic field detection accuracy and consistency, we prepared an array of single NV centers for static magnetic field vector and gradient detection using the femtosecond laser direct writing method. The prepared single NV centers are characterized by fewer impurity defects and good stress uniformity, with an average spatial positioning error of only 0.2 µm. This array of single NV centers can achieve high accuracy magnetic field vector and gradient measurement with GBZ≈-0.047 µT/µm in the Z-axis. This result provides a new idea for large-range, high-precision magnetic field vector and gradient measurements.

Phase customization in photonic integrated circuits with trimmed waveguides.

Accurate photon phase control on a chip is essential to improve the expandability and stability of photonic integrated circuits (PICs). Here, we propose a novel, to the best of our knowledge, on-chip static phase control method in which a modified line is added close to the normal waveguide with a lower-energy laser. By controlling the laser energy and the position and length of the modified line, the optical phase can be precisely controlled with low loss and a three-dimensional (3D) path. Customizable phase modulation ranging from 0 to 2π is performed with a precision of λ/70 in a Mach-Zehnder interferometer. The proposed method can customize high-precision control phases without changing the waveguide's original spatial path, which is expected to control the phase and solve the phase error correction problem during processing of large-scale 3D-path PICs.

Edge State, Localization Length, and Critical Exponent from Survival Probability in Topological Waveguides.

Edge states in topological phase transitions have been observed in various platforms. To date, verification of the edge states and the associated topological invariant are mostly studied, and yet a quantitative measurement of topological phase transitions is still lacking. Here, we show the direct measurement of edge states and their localization lengths from survival probability. We employ photonic waveguide arrays to demonstrate the topological phase transitions based on the Su-Schrieffer-Heeger model. By measuring the survival probability at the lattice boundary, we show that in the long-time limit, the survival probability is P=(1-e^{-2/ξ_{loc}})^{2}, where ξ_{loc} is the localization length. This length derived from the survival probability is compared with the distance from the transition point, yielding a critical exponent of ν=0.94±0.04 at the phase boundary. Our experiment provides an alternative route to characterizing topological phase transitions and extracting their key physical quantities.

Machine vision-based high-precision and robust focus detection for femtosecond laser machining.

We propose a machine vision-based focus detection method (MVFD) for femtosecond laser machining. By analyzing the laser focus pattern, the defocus direction and distance are obtained simultaneously. The proposed technique presents high precision with an average error of 0.047 µm and a root mean square error (RMSE) of 0.055 µm. Moreover, the method is robust and is less affected by the tilted sample. For the curved surface sample, the average error and RMSE are 0.093 and 0.145 µm, respectively. Thus, the proposed focus detection method can be easily combined with laser processing equipment, which is widely used in large-range and high-precision femtosecond laser processing.

Resetting directional couplers for high-fidelity quantum photonic integrated chips.

In this Letter, we propose a fabrication technique based on femtosecond laser secondary direct writing (FsLSDW) that allows us to statically reset the beam-splitting ratio of directional couplers. By modifying the interaction region with a second inscription, the coupling coefficient of the reconstructed devices can be indeed changed continuously within the range of 0.49-2.1 rad/mm, thus enabling a complete tunability of the reconstructed splitting ratio from zero to full power transfer between the waveguides. This powerful reconstruction capability facilitates the arbitrary reset of an imperfect device, from any initial splitting ratio to the correct one. In the future, such static control method could potentially solve the fabrication error problem in the manufacturing of high-fidelity large-scale integrated photonic quantum chips.

Deep diamond single-photon sources prepared by a femtosecond laser.

Nitrogen-vacancy color centers (NVs) in diamond have several potential applications ranging from quantum computing to data storage. However, artificial NVs are often close to the surface, which limits their spatial density and applicability. Here we demonstrate an effective and precise method for preparing deep single NVs in diamond. The method is based on a spatial-shaped femtosecond laser to overcome laser defocus in high-refractive materials, and realizes the preparation of single NVs at 95 µm. In addition, owing to the good energy distribution of the shaped laser focus, the single NVs exhibit a statistic yield of 56%±11% with excellent qualities. This processing method will contribute to the integration of color centers with emerging optical elements and high-density data storage.

Circular cross section waveguides processed by multi-foci-shaped femtosecond pulses.

We developed a simple multi-foci-shaped femtosecond pulsed (MFSFP) method for processing circular cross section waveguides in transparent materials. With this flexible processing method, the focus energy distribution can be designed freely and arbitrarily, and single-mode waveguides with cross section circularity better than 96.0% were achieved. The mode shape difference (1.93%) of circular waveguides is smaller than the difference (7.01%) of normal elliptical waveguides. The coupling abilities of the two kinds of waveguides were investigated with three-dimensional (3D) directional couplers in both experiments and theoretical simulations. The coupling coefficient difference of circular waveguides in vertical and horizontal coupling directions was ∼0.01mm-1, which was smaller than 0.33mm-1 of normal waveguides. The circular symmetric waveguides will play an important role in large-scale high-intensity 3D photonic integrated circuits.

General Rules Governing the Dynamical Encircling of an Arbitrary Number of Exceptional Points.

Dynamically encircling an exceptional point in non-Hermitian systems has drawn great attention recently, since a nonadiabatic transition process can occur and lead to intriguing phenomena and applications such as the asymmetric switching of modes. While all previous experiments have been restricted to two-state systems, the dynamics in multistate systems where more complex topology can be formed by exceptional points, is still unknown and associated experiments remain elusive. Here, we propose an on-chip photonic system in which an arbitrary number of exceptional points can be encircled dynamically. We reveal in experiment a robust state-switching rule for multistate systems, and extend it to an infinite-period system in which an exceptional line is encircled with outcomes being located at the Brillouin-zone boundary. The proposed versatile platform is expected to reveal more physics related to multiple exceptional points and exceptional lines, and give rise to applications in multistate non-Hermitian systems.

Long focusing range and self-healing Bessel vortex beam generator: publisher's note.

This publisher's note contains corrections to Opt. Lett.45, 2580 (2020).OPLEDP0146-959210.1364/OL.391232.

Long focusing range and self-healing Bessel vortex beam generator.

Here a continuous axial-spiral phase microplate (CAsPP), based on combining a logarithmic axicon and a spiral phase plate, was proposed for generating high-quality higher-order Bessel vortex beams. The novel optical component implemented via femtosecond laser direct writing possesses compact geometry and unique optical properties. The CAsPP with a diameter of 80 µm possesses a controllable long focus ranging from 50 to 600 µm and exhibits a good self-healing ability after free transmission of about 45 µm. Unique optical properties were demonstrated in both experiments and simulations, which were well matched to each other. This Letter provides new opportunities for applications in integrated optics, optical trapping, laser machining, and information reconstruction.

Diamond optical vortex generator processed by ultraviolet femtosecond laser.

We propose a precise diamond micromachining method based on ultraviolet femtosecond laser direct writing and a mixed acid heating chemical treatment. The chemical composition of the attached clusters generated during laser ablation and their effects on morphologies were investigated in experiments. The averaged roughness of pristine and processed regions reduced to 0.64 nm and 9.4 nm from 20.5 nm and 37.4 nm, respectively. With this method, spiral zone plates (SZPs) were inscribed on a high-pressure high-temperature diamond surface as micro-optical vortex generators. The optical performances of the diamond SZPs were characterized in both experiments and simulations, which were very consistent with each other. This chemical auxiliary processing method will contribute greatly to the wide application of integration and miniaturization of diamond surface optical components.

Convex silica microlens arrays via femtosecond laser writing.

We report fabrication of silica convex microlens arrays with controlled shape, size, and curvature by femtosecond laser direct writing. A backside etching in dye solution was utilized for laser machining high-fidelity control of material removal and real-time surface cleaning from ablation debris. Thermal annealing was applied to reduce surface roughness to 3 nm (rms). The good optical performance of the arrays was confirmed by focusing and imaging tests. Complex 3D micro-optical elements over a footprint of $ 100 \times 100\;\unicode{x00B5}{{\rm m}^2} $100×100µm2 were ablated within 1 h (required for practical applications). A material removal speed of $ 120\;\unicode{x00B5}{{\rm m}^3}/{\rm s} $120µm3/s ($ 6 \times {10^5} \;{{\rm nm}^3}/{\rm pulse} $6×105nm3/pulse) was used, which is more than an order of magnitude higher compared to backside etching using a mask projection method. The method is applicable for fabrication of micro-optical components on transparent hard materials.

UV-NIR femtosecond laser hybrid lithography for efficient printing of complex on-chip waveguides.

We propose UV-IR femtosecond laser hybrid lithography for the efficient printing of complex on-chip waveguides, which offers good performance in terms of processing efficiency and accuracy. With this three-dimensional printing technology, waveguides with complex cross-section shapes, such as owls and kittens, can be easily fabricated with an efficiency increased by 1500% (for ${6}\;\unicode{x00B5} {\rm m}\; \times \;{6}\;\unicode{x00B5} {\rm m}$6µm×6µm). In addition, a circular cross-section waveguide with an extremely low birefringence and complex ${8} \times {8}$8×8 random walk networks were quickly customized, which implies that in the design and preparation of the large-scale optical chips, the proposed maskless method allows for the preparation of highly customized devices.

A Benzothiazole-Based Fluorescent Probe for Ratiometric Detection of Al3+ and Its Application in Water Samples and Cell Imaging.

An easily prepared benzothiazole-based probe (BHM) was prepared and characterized by general spectra, including 1H NMR, 13C NMR, HRMS, and single-crystal X-ray diffraction. Based on the synergistic mechanism of the inhabitation of intramolecular charge transfer (ICT), the BHM displayed high selectivity and sensitivity for Al3+ in DMF/H2O (v/v, 1/1) through an obvious blue-shift in the fluorescent spectrum and significant color change detected by the naked eye, respectively. The binding ratio of BHM with Al3+ was 1:1, as determined by the Job plot, and the binding details were investigated using FT-IR, 1H NMR titration, and ESI-MS analysis. Furthermore, the BHM was successfully applied in the detection of Al3+ in the Songhua River and on a test stripe. Fluorescence imaging experiments confirmed that the BHM could be used to monitor Al3+ in human stromal cells (HSC).

Mirror-rotation-symmetrical single-focus spiral zone plates.

In this Letter, we report mirror-rotation-symmetrical single-focus spiral zone plates (MS-SZPs) fabricated by femtosecond laser direct writing. The novel optical element can generate a single-focus vortex beam, owing to the element's complicated continuous surface. The MS-SZP surface possesses reverse mirror-rotation symmetry, which ensures that the transfer element has the same surface morphology as the original element. Both the transfer element and original element have good optical properties. The single-focus behavior was investigated by a microscopic imaging system and found to be in good agreement with theoretical simulation results. The innovative optical component is expected to be widely used in optical communication, quantum computation, optical manipulation, and other fields.

Single-pulse writing of a concave microlens array.

This work developed a method of femtosecond laser (fs-laser) parallel processing assisted by wet etching to fabricate 3D micro-optical components. A 2D fs-laser spot array with designed spatial distribution was generated by a spatial light modulator. A single-pulse exposure of the entire array was used for parallel processing. By subsequent wet etching, a close-packed hexagonal arrangement, 3D concave microlens array on a curved surface with a radius of approximately 120 µm was fabricated, each unit lens of which has designable spatial distribution. Characterization of imaging was carried out by a microscope and showed a unique imaging property in multi-planes. This method provides a parallel and efficient technique to fabricate 3D micro-optical devices for applications in optofluidics, optical communication, and integrated optics.