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.

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.

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.

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.

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.

Miniature optoelectronic compound eye camera.

Inspired by insect compound eyes (CEs) that feature unique optical schemes for imaging, there has recently been growing interest in developing optoelectronic CE cameras with comparable size and functions. However, considering the mismatch between the complex 3D configuration of CEs and the planar nature of available imaging sensors, it is currently challenging to reach this end. Here, we report a paradigm in miniature optoelectronic integrated CE camera by manufacturing polymer CEs with 19~160 logarithmic profile ommatidia via femtosecond laser two-photon polymerization. In contrast to µ-CEs with spherical ommatidia that suffer from defocusing problems, the as-obtained µ-CEs with logarithmic ommatidia permit direct integration with a commercial CMOS detector, because the depth-of-field and focus range of all the logarithmic ommatidia are significantly increased. The optoelectronic integrated µ-CE camera enables large field-of-view imaging (90°), spatial position identification and sensitive trajectory monitoring of moving targets. Moreover, the miniature µ-CE camera can be integrated with a microfluidic chip and serves as an on-chip camera for real-time microorganisms monitoring. The insect-scale optoelectronic µ-CE camera provides a practical route for integrating well-developed planar imaging sensors with complex micro-optics elements, holding great promise for cutting-edge applications in endoscopy and robot vision.

On-chip beam rotators, adiabatic mode converters, and waveplates through low-loss waveguides with variable cross-sections.

Photonics integrated circuitry would benefit considerably from the ability to arbitrarily control waveguide cross-sections with high precision and low loss, in order to provide more degrees of freedom in manipulating propagating light. Here, we report a new method for femtosecond laser writing of optical-fiber-compatible glass waveguides, namely spherical phase-induced multicore waveguide (SPIM-WG), which addresses this challenging task with three-dimensional on-chip light control. Fabricating in the heating regime with high scanning speed, precise deformation of cross-sections is still achievable along the waveguide, with shapes and sizes finely controllable of high resolution in both horizontal and vertical transversal directions. We observed that these waveguides have high refractive index contrast of 0.017, low propagation loss of 0.14 dB/cm, and very low coupling loss of 0.19 dB coupled from a single-mode fiber. SPIM-WG devices were easily fabricated that were able to perform on-chip beam rotation through varying angles, or manipulate the polarization state of propagating light for target wavelengths. We also demonstrated SPIM-WG mode converters that provide arbitrary adiabatic mode conversion with high efficiency between symmetric and asymmetric nonuniform modes; examples include circular, elliptical modes, and asymmetric modes from ppKTP (periodically poled potassium titanyl phosphate) waveguides which are generally applied in frequency conversion and quantum light sources. Created inside optical glass, these waveguides and devices have the capability to operate across ultra-broad bands from visible to infrared wavelengths. The compatibility with optical fiber also paves the way toward packaged photonic integrated circuitry, which usually needs input and output fiber connections.

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.

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.

A dual-functional probe for sensing pH change and ratiometric detection of Cu2.

A series of benzothiazole-based compounds were synthesized and characterized. Among them, probe Z showed significant dual-functional performance which was capable of sensing pH change and Cu2+. Probe Z displayed fluorescent turn-on under alkaline conditions due to deprotonation of the hydroxyl group along with the obviously color change from colorless to mint green. Interestingly, it further achieved in ratiometric detection of Cu2+ through absorbance or fluorescence signals in strong alkaline condition. The limit of detection was calculated correspondingly as 0.37 µM and 1.35 µM, respectively. Especially, the combination of the XNOR and INHIBIT logic gates could be used to confirm that one medium was in neutrality or alkalinity condition. Moreover, Z was successfully used in real water samples and test paper for fast identification of Cu2+, respectively.

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.

Bioinspired Zoom Compound Eyes Enable Variable-Focus Imaging.

Natural compound eyes provide the inspiration for developing artificial optical devices that feature a large field of view (FOV). However, the imaging ability of artificial compound eyes is generally based on the large number of ommatidia. The lack of a tunable imaging mechanism significantly limits the practical applications of artificial compound eyes, for instance, distinguishing targets at different distances. Herein, we reported zoom compound eyes that enable variable-focus imaging by integrating a deformable poly(dimethylsiloxane) (PDMS) microlens array (MLA) with a microfluidic chamber. The thin and soft PDMS MLA was fabricated by soft lithography using a hard template prepared by a combined technology of femtosecond laser processing and wet etching. As compared with other mechanical machining strategies, our combined technology features high flexibility, efficiency, and uniformity, as well as designable processing capability, since the size, distribution, and arrangement of the ommatidia can be well controlled during femtosecond laser processing. By tuning the volume of water injected into the chamber, the PDMS MLA can deform from a planar structure to a hemispherical shape, evolving into a tunable compound eye of variable FOV up to 180°. More importantly, the tunable chamber can functionalize as the main zoom lens for tunable imaging, which endows the compound eye with the additional capability of distinguishing targets at different distances. Its focal length can be turned from 3.03 mm to infinity with an angular resolution of 3.86 × 10-4 rad. This zoom compound eye combines the advantages of monocular eyes and compound eyes together, holding great promise for developing advanced micro-optical devices that enable large FOV and variable-focus imaging.