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In this paper, waveguide mode sizes are determined by using the variational method. This approach has been used with both optical fibers and integrated optics. In the case of the integrated waveguide a comparison of theoretical and experimental results has been carried out with only limited success, although the geometric mean of the calculated orthogonal half-intensity mode sizes agreed well with experiment. The analysis used was stated to involve the reduction of the problem to a one-dimensional solution, and a more accurate form is derived for the propagation constant equation. Comparison of the new numerical results now gives closer agreement with experiment. [ABSTRACT FROM AUTHOR]

Abstract: We report on three approaches to achieve 1086nm period Bragg Gratings structures (BGs), corresponding to the third Bragg order at 1550nm, on X-cut lithium niobate (LiNbO3) substrates. The first method relies on Reactive Ionic Etching (RIE) with fluorine gases, associated with e-beam lithography and electroplating deposition. BGS with etched angles close to 60°, an aspect ratio AR=1.4 and a reflectivity of 13% have been fabricated. The second process is based on Focused Ion Beam (FIB) milling. BGs are etched in standard optical waveguides. The process has led to etched angles close to 85° with AR=6.3 and to a reflectivity of 50%. Finally, we propose an original method based on the use of lateral FIB etching on the edge of deep-etched ridge optical waveguides predefined by “optical grade dicing”. Etched angles close to verticality with AR>9 have been obtained and optical characterizations are under work. [Copyright &y& Elsevier]

This paper described a wavelength-selective couplers dedicated to drop functionality (spectral-selective couplers dedicated to wavelength demultiplexing). It is consist of two Y junctions: an input two drop channels and an output. The Y-branches distributors of optical power were carried out by a specific geometry of Ti:LiNbO3 waveguides. Each Y junction is composed by two identical bends jointed at one with a weak inclination angle to decrease losses. On each arm of the Y-branches, a Bragg grating (BG) is etched by Focused Ion Beam (FIB) in order to extract a specific wavelength band. A two-channel configuration is adopted which demonstrate a drop functionality of two spectral bands centered at 1123 nm and 1527 nm. The complementary spectrum is avalaible at the coupler output. [ABSTRACT FROM PUBLISHER]

Abstract: We report a simple process for the batch fabrication of photonic crystals in lithium niobate substrates. By means of proton-exchange (PE) followed by CHF3 reactive ion etching (RIE) we have achieved structures with a diameter of 400nm and an aspect ratio (depth-to-diameter) of 3:1. Sub-micrometric structures have been fabricated with an 85° angle between the walls and the plane of the substrate. We provide the optimized parameters – and their influence on the aspect ratio, the etching rate and the verticality of the walls. Transversal cross-section micrographs of the etched patterns, for both X-cut and Z-cut substrates, are shown as a clear evidence of the capability developed. [Copyright &y& Elsevier]

Non-Newtonian liquids are characterized by stress and velocity-dependent dynamical response. In elasticity, and in particular, in the field of phononics, reciprocity in the equations acts against obtaining a directional response for passive media. Active stimuli-responsive materials have been conceived to overcome it. Significantly, Milton and Willis have shown theoretically in 2007 that quasi-rigid bodies containing masses at resonance can display a very rich dynamical behavior, hence opening a route toward the design of non-reciprocal and non-Newtonian metamaterials. In this paper, we design a solid structure that displays unidirectional shock resistance, thus going beyond Newton's second law in analogy to non-Newtonian fluids. We design the mechanical metamaterial with finite element analysis and fabricate it using three-dimensional printing at the centimetric scale (with fused deposition modeling) and at the micrometric scale (with two-photon lithography). The non-Newtonian elastic response is measured via dynamical velocity-dependent experiments. Reversing the direction of the impact, we further highlight the intrinsic non-reciprocal response., (© 2023. Springer Nature Limited.)

In solid state physics, phase transitions can influence material functionality and alter their properties. In mechanical metamaterials, structural-phase transitions can be achieved through instability or buckling of certain structural elements. However, these fast transitions in one mechanical parameter typically affect significantly the remaining parameters, hence, limiting their applications. Here, this limitation is addressed by designing a novel 3D mechanical metamaterial that is capable of undergoing a phase transition from positive to negative Poisson's ratio under compression, without significant degradation of Young's modulus (i.e. the phase transition is elastically-stable). The metamaterial is fabricated by two-photon lithography at the micro-scale and its mechanical behavior is assessed experimentally. For another choice of structural parameters, it is then shown that the auxetic behavior of the considered 3D metamaterial class can be maintained over a wide range of applied compressive strain., (© 2022 The Authors. Advanced Science published by Wiley-VCH GmbH.)

Stepper motors and actuators are among the main constituents of control motion devices. They are complex multibody systems with rather large overall volume due to their multifunctional parts and elaborate technological assembly processes. Miniaturization of individual parts is still posing assembly problems. In this paper, a single-step lithography process to fabricate a micro-stepper engine with an accurate micrometric rotation axis and an overall sub-millimeter size is demonstrated. The device is based on the frictional contacts and chiral metamaterials to get rid of the dependence on the accuracy of parts. The functional aspects of fabricated samples are discussed for many rotation cycles and for different frictional surfaces., (© 2022 Wiley-VCH GmbH.)

Three-dimensional direct laser writing technology enables one to print polymer microstructures whose size varies from a few hundred nanometers to a few millimeters. It has been shown that, by tuning the laser power during writing, one can adjust continuously the optical and elastic properties with the same base material. This process is referred to as gray-tone lithography. In this paper, we characterize by Brillouin light scattering the complex elastic constant C11 of different reticulated isotropic polymers, at longitudinal phonon frequencies of the order of 16 GHz. We estimate the real part of the C11 constant to vary from 7 to 11 GPa as a function of laser power, whereas its imaginary part varies between 0.25 and 0.6 GPa. The linear elastic properties are further measured at a fixed laser power as a function of temperature, from 20∘C to 80∘C. Overall, we show that our 3D printed samples have a good elastic quality with high Q factors only ten times smaller than fused silica at hypersonic frequencies.

Shape morphing and the possibility of having control over mechanical properties via designed deformations have attracted a lot of attention in the materials community and led to a variety of applications with an emphasis on the space industry. However, current materials normally do not allow to have a full control over the deformation pattern and often fail to replicate such behavior at low scales which is essential in flexible electronics. Thus, in this paper, novel 2D and 3D microscopic hierarchical mechanical metamaterials using mutually-competing substructures within the system that are capable of exhibiting a broad range of the highly unusual auxetic behavior are proposed. Using experiments (3D microprinted polymers) supported by computer simulations, it is shown that such ability can be controlled through geometric design parameters. Finally it is demonstrated that the considered structure can form a composite capable of shape morphing allowing it to deform to a predefined shape., (© 2022 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

Several integrated optics solutions currently exist to develop monolithic, robust, and lightweight high-resolution spectrometers for spatial applications. An interesting option is generating a stationary wave inside a single-mode waveguide, and sampling the interference fringes using dielectric discontinuities on the surface of the waveguide. This allows the recording of the signal on a detector on top of the waveguide, and using dedicated Fourier transform methods to recover the spectrum of the source. All the difficulty is then linked to the length of the interferogram that is sampled. This determines the spectral resolution and the spacing between sampling centers, which are ultimately limited by the pixel pitch, and that will determine the spectral range of the spectrum. In addition, the dielectric discontinuities that will extract the flux from the waveguide have a relatively wide angular emission, resulting in crosstalk between pixels, and reducing the effective sampling step. Finally, the optical sensitivity of these systems is limited since the waveguides are single mode. Therefore, improving the efficiency of stationary wave Fourier transform spectrometers will require reducing the angular divergence of the sampled signal, reducing the sampling step, and increasing the optical input collection capacity. To achieve the two latest conditions, one interesting approach is spatial multiplexing. In this paper, we present the proof of concept of a multiplexed integrated optics Fourier transform spectrometer based on lithium niobate waveguides, using focused ion beam nanogrooves as sampling centers. The spatially shifted position of the antennas between consecutive waveguides will allow us to determine an unknown wavelength with tens of picometer resolution. The extraction efficiency and bandwidth of the antennas will be theoretically studied to optimize their periodicity and match a given pixel pitch. Finally, the ability to develop this concept on an electro-optic material will be of great interest to achieve further active phase modulation and increase the spectral bandwidth.

Lithium niobate (LN)-based devices are widely used in integrated and nonlinear optics. This material is robust and resistive to high temperatures, which makes the LN-based devices stable, but challenging to fabricate. In this work, we report on the design, manufacturing, and characterization of engineered dielectric media with thin-film LN (TFLN) on top for the coupling and propagation of electromagnetic surface waves at telecommunication wavelengths. The designed one-dimensional photonic crystal (1DPhC) sustains Bloch surface waves (BSWs) at the multilayer-air interface at 1550 nm wavelength with a propagation detected over a distance of 3 mm. The working wavelength and improved BSW propagation parameters open the way for exploration of nonlinear properties of BSW-based devices. It is also expected that these novel devices potentially would be able to modify BSW propagation and coupling by external thermal-electrical stimuli due to the improved quality of the TFLN top layer of 1DPhC.

The stationary wave integrated Fourier transform spectrometer (SWIFTS) is based on the sampling of a stationary wave using nano-scattering centers on the surface of a channel waveguide. Single nano-scale scattering centers above the waveguide surface will radiate the sampled signal with wide angular distribution, which is not compatible with the buried detection area of infrared (IR) detectors, resulting in crosstalk between pixels. An implementation of multiple diffraction nano-grooves (antenna) for each sampling center is proposed as an alternative solution to improve directivity towards the detector pixel by narrowing the scattering angle of the extracted light. Its efficiency is demonstrated from both simulated and measured far-field radiative patterns exhibiting a promising method to be used for the future integrated IR-SWIFTS.

We report on free-standing electro-optical LiNbO 3 waveguides with integrated tapers made by optical grade dicing. Membranes with a calibrated thickness are produced simultaneously with tapers acting as spot-size converters. Thereby, thicknesses from 450 to 500 μm can simply be achieved together with integrated tapers guaranteeing low insertion losses. These developments open the way to the low-cost production of compact and low-power-consuming electro-optical components. As an example, a 200 μm-long free-standing electro-optical Fabry-Perot is demonstrated with a figure of merit of only 0.19 V·cm in a 4.5 μm-thick membrane.

We report light-beam self-trapping triggered by the pyroelectric effect in an isolated ferroelectric thin film. Experiments are performed in an 8-μm-thick congruent undoped LiNbO(3) film bonded onto a silicon wafer. Response time two orders of magnitude faster than in bulk LiNbO(3) is reported. The original underlying physics specific of this arrangement is discussed.

In this Letter, we report a technique to etch giant aspect ratio nanostructures in lithium niobate. An 8 μm long Bragg grating on a Ti:LiNbO3 ridge waveguide was fabricated by combining optical-grade dicing and focused ion beam milling. The reflectivity was evaluated using an optical coherence tomography system: it is measured to be 53% for the TM wave and 47% for the TE wave. We study by 2D-FDTD the modeled behavior of the electromagnetic field when an angle exists between two consecutive sidewalls of the grating in order to understand the difference between ideal Bragg grating and experimental samples. These simulations allow us to optimize the parameters in order to increase the reflection of the grating up to 80%.

In this paper, temperature variations are detected thanks to an enhanced nano-optical pyroelectric sensor. Sensing is obtained with the pyroelectric effect of lithium niobate (LN) in which, a suitable air-membrane photonic crystal cavity has been fabricated. The wavelength position of the cavity mode is tuned 11.5 nm for a temperature variation of only 32 °C. These results agree quite well with 3D-FDTD simulations that predict tunability of 12.5 nm for 32 °C. This photonic crystal temperature sensor shows a sensitivity of 0.359 nm/°C for an active length of only ~5.2 μm.

Solutions for the development of compact RF passive transducers as an alternative to standard surface or bulk acoustic wave devices are receiving increasing interest. This article presents results on the development of an acoustic band-pass filter based on periodically poled ferroelectric domains in lithium niobate. The fabrication of periodically poled transducers (PPTs) operating in the range of 20 to 650 MHz has been achieved on 3-in (76.2-mm) 500-μm-thick wafers. This kind of transducer is able to excite elliptical as well as longitudinal modes, yielding phase velocities of about 3800 and 6500 ms(-1), respectively. A new type of acoustic band-pass filter is proposed, based on the use of PPTs instead of the SAWs excited by classical interdigital transducers. The design and the fabrication of such a filter are presented, as well as experimental measurements of its electrical response and transfer function. The feasibility of such a PPT-based filter is thereby demonstrated and the limitations of this method are discussed.

We report easy-to-implement techniques to improve the reflectivity of LiNbO₃ photonic crystals within the photonic bandgap. Firstly, we show that widening the channel waveguides confines the optical modes in the vertical direction, which leads to the development of the first 2D-PhCs on Ti-indiffused LiNbO₃ waveguides. We also report the first optical characterization of PhCs implemented on ridge LiNbO₃ waveguides. The reflectivity is measured using a swept-source optical coherence tomography (OCT) system, together with the transmission spectrum. Finally we report 3D-PhCs LiNbO₃ fabricated by Focused Ion Beam milling on the side of ridge waveguides.