Séverine Diziain

Fabrication of nanoscale lithium niobate waveguides for second-harmonic generation

Nanoscale waveguides are basic building blocks of integrated optical devices. Especially, waveguides made from nonlinear optical materials, such as lithium niobate, allow access to a broad range of applications using second-order nonlinear frequency conversion processes. Based on a lithium niobate on insulator substrate, millimeter-long nanoscale waveguides were fabricated with widths as small as 200 nm. The fabrication was done by means of potassium hydroxide-assisted ion-beam-enhanced etching. The waveguides were optically characterized in the near infrared wavelength range showing phase-matched second-harmonic generation.

Second harmonic generation in free-standing lithium niobate photonic crystal L3 cavity

We report on second harmonic generation in a photonic crystal L3 cavity drilled in a thin self-suspended lithium niobate membrane. The cavity, resonant for the pump beam in the telecom wavelength range, exhibits a quality factor of around 500. Second harmonic generation has been measured with a low power continuous laser. A conversion efficiency of 6.4×10−9 has been estimated with an input coupled power of 53 μW.

Light propagation in a free-standing lithium niobate photonic crystal waveguide

We report on the light propagation in a one-line-defect photonic crystal waveguide (W1 PhC WG) patterned into a 450 nm thick free-standing lithium niobate membrane by ion-beam enhanced etching. The Bloch wave vectors and transmission spectrum of this PhC WG were retrieved from optical near-field images. The experimental data show good agreement with simulations performed with the three-dimensional (3D) finite-element method and the 3D finite-difference time-domain method. Those results are promising for the development of integrated optics devices operating at telecom wavelengths and based on free-standing lithium niobate PhC membranes.

Self-suspended micro-resonators patterned in Z-cut lithium niobate membranes

We report on self-suspended micro-resonators patterned in Z-cut lithium niobate on insulator substrates. The fabrication technique consists of two single steps, focused ion beam milling for the micro- and nano-structuring and subsequent SiO2 etching for the realization of thin self-suspended membranes. The fabrication process of a free-standing photonic crystal cavity and a suspended micro-disk is described and the linear and nonlinear optical properties of the micro-resonators are investigated at telecommunication wavelengths. The whispering gallery modes of the micro-disk are measured experimentally and compared to an analytical model. The fundamental transverse-electric polarized mode of the photonic crystal cavity is measured and compared to three dimensional finite difference time domain simulations. Second harmonic generation enhancement due to the field confinement in the cavity mode is demonstrated. These results are promising for the use of Z-cut lithium niobate self-suspended membranes as platforms for highly efficient miniaturized photonic devices for telecommunication applications.

Image formation properties and inverse imaging problem in aperture based scanning near field optical microscopy

Aperture based scanning near field optical microscopes are important instruments to study light at the nanoscale and to understand the optical functionality of photonic nanostructures. In general, a detected image is affected by both the transverse electric and magnetic field components of light. The discrimination of the individual field components is challenging as these four field components are contained within two signals in the case of a polarization resolved measurement. Here, we develop a methodology to solve the inverse imaging problem and to retrieve the vectorial field components from polarization and phase resolved measurements. Our methodology relies on the discussion of the image formation process in aperture based scanning near field optical microscopes. On this basis, we are also able to explain how the relative contributions of the electric and magnetic field components within detected images depend on the chosen probe. We can therefore also describe the influence of geometrical and material parameters of individual probes within the image formation process. This allows probes to be designed that are primarily sensitive either to the electric or magnetic field components of light.

Mode analysis of photonic crystal L3 cavities in self-suspended lithium niobate membranes

We report on a multimodal analysis of photonic crystal L3 cavities milled in lithium niobate free-standing membranes. The classical L3 cavity geometry is compared to an L3 cavity containing a second lattice superimposed on the primary one. Those two different geometries are investigated in terms of vertical radiation and quality (Q) factor for each mode of the cavities. Depending on the cavity geometry, some modes undergo an enhancement of their vertical radiation into small angles while other modes experience a higher Q factor. Experimental characterizations are corroborated by three-dimensional finite difference time domain simulations.

Enhancement of second harmonic generation in self-suspended lithium niobate photonic crystal cavities

Lithium niobate is a ferroelectric material widely used in nonlinear and integrated optics due to its transparency over a large wavelength range and its high optical nonlinearity. However, since the conversion efficiency of nonlinear processes is usually very weak, further enhancement mechanisms are required. Here we demonstrate the enhancement of second harmonic generation in self-suspended lithium niobate photonic crystal cavities. The structures, fabricated by ion-beam enhanced etching and focused ion beam milling, possess several resonances. The multi-modal properties of the cavities are theoretically and experimentally investigated. Second harmonic generation, measured with a low power continuous wave pump, undergoes an enhancement scaling as the square of the quality factor of the resonance.

Transmission properties of a free-standing lithium niobate photonic crystal waveguide

Transmission spectra of photonic crystal waveguides in freestanding lithium niobate membranes fabricated by means of ion-beam enhanced etching have been measured by scanning near-field optical microscopy show good agreement with 3DFDTD calculations.