Revealing photonic symmetry-protected modes by the finite-difference-time-domain method.

This Letter is devoted to pointing out a specific feature of the finite-difference-time-domain (FDTD) method through the study of nano-structures supporting geometrical symmetry-protected modes that cannot be excited at certain conditions of illumination. The spatial discretization performed in the FDTD algorithm naturally leads to breaking this symmetry and allows the excitation of these modes. The quality factors of the corresponding resonances are then directly linked to the degree of symmetry breaking, i.e., the spatial grid dimension, even though the convergence criteria of the FDTD are fulfilled. This finding shows that the FDTD must be handled with great care and, more importantly, that very huge quality-factor resonances can be achieved at the cost of nanometer-scale mastered fabrication processes.

In-band pumping of Tm:LiYF4 channel waveguide: a power scaling strategy for ∼2 µm waveguide lasers.

We report on a novel power scaling strategy for thulium waveguide (WG) lasers relying on in-band pumping by high-brightness Raman fiber lasers (RFLs) and the use of liquid-phase-epitaxy-grown fluoride crystalline thin films for better thermal management. Thulium channel WGs are produced by microstructuring the Tm3+:LiYF4/LiYF4 epitaxies via diamond-saw dicing. They are pumped by a RFL based on an erbium master oscillator power amplifier and a GeO2-doped silica fiber and emit polarized output at 1679 nm. A CW in-band-pumped (H63→F43) Tm3+:LiYF4 WG laser generates up to 2.05 W of a linearly polarized single-transverse-mode output at 1881 nm with a slope efficiency of 78.3% and a laser threshold of only 12 mW (versus the absorbed pump power).

Watt-level Tm:LiYF4 channel waveguide laser produced by diamond saw dicing.

Low-loss surface channel waveguides with a cross-section of 30 × 30 µm2 are produced by diamond saw dicing of 6.2 at.% Tm3+, 3.5 at.% Gd3+:LiYF4 films grown by liquid phase epitaxy (LPE) on (001)-oriented bulk undoped LiYF4 substrates. Pumped by a Ti:Sapphire laser at 783 nm, a continuous-wave Tm:LiYF4 waveguide laser generated 1.30 W at 1880 nm (for π-polarization) with a slope efficiency of 80% with respect to the absorbed pump power. The laser threshold was at 80 mW. The waveguide morphology was studied revealing low roughness (3 ± 2 µm) as expressed by the propagation losses of <0.3 dB/cm. A combination of LPE and diamond saw dicing is a promising technology for multi-watt single-mode channel waveguide lasers and amplifiers.

Acoustic resonator based on periodically poled lithium niobate ridge.

The constant improvement of industrial needs to face modern telecommunication challenges leads to the development of novel transducer principles as alternatives to SAW and BAW solutions. The main technological limits of SAW (short-circuit between electrodes) and BAW (precise thickness control) solutions can be overcome by a new kind of transducer based on periodically poled ferroelectric substrate. The approach proposed in this paper exploits a ridge structure combined with a periodically poled transducer (PPT), allowing for the excitation of highly coupled modes unlike previously published results on planar PPTs. High-aspect-ratio ridges showing micrometer dimensions are achieved by dicing PPT plates with a diamond-tipped saw. An adapted metallization is achieved to excite acoustic modes exhibiting electromechanical coupling in excess of 15% with phase velocities up to 10 000 m·s(-1). Theoretical predictions show that these figures may reach values up to 20% and 18 000 m·s(-1), respectively, using an appropriate design.

Writing and probing light-induced waveguides thanks to an endlessly single-mode photonic crystal fiber.

We demonstrate writing and probing of light-induced waveguides in photorefractive bulk LiNbO3 crystal using an endlessly single-mode photonic crystal fiber. The optical waveguides are written at visible wavelengths by slightly raising the ferroelectric crystal temperature to benefit from the pyroelectric-driven photorefractive effect and the guiding properties are investigated at telecom wavelengths using the same photonic crystal fiber. End butt coupling with this photonic crystal fiber enables writing and probing of optical waveguides due to the self-alignment properties of spatial solitons.

Optimization of LiNbO3 photonic crystals: toward 3D LiNbO3 micro-components.

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.