Reinhard Geiss

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.

Cascaded third harmonic generation in lithium niobate nanowaveguides

We predict highly efficient third harmonic generation through simultaneous phase-matching of second-harmonic generation and sum-frequency generation in lithium niobate nanowaveguides, enabled due to strong modal dispersion. We demonstrate that the waveguide size which corresponds to phase-matching is also optimal for highest mode confinement and therefore for strongly enhanced conversion efficiency.

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.

Spectral narrowing and manipulation in an optical parametric oscillator using periodically poled lithium niobate electro-optic polarization-mode converters.

We report a unique spectral narrowing and manipulation technique in an optical parametric oscillator (OPO) realized by an integrated periodically poled lithium niobate comprising an optical parametric gain medium sandwiched by two electro-optic polarization-mode converters (EO PMCs). We achieved a manipulation of the gain spectrum of the OPO via EO and/or temperature control of the EO PMCs, in which we obtained single to multiple signal spectral peaks from the OPO with a spectral width reduced by up to 10 times and peak intensity increased by up to 6 times in comparison with the original signal. Fast EO tuning of the narrowed signal spectral peak has also been demonstrated.

Fabrication of free-standing lithium niobate nanowaveguides down to 50 nm in width.

Nonlinear optical nanoscale waveguides are a compact and powerful platform for efficient wavelength conversion. The free-standing waveguide geometry opens a range of applications in microscopy for local delivery of light, where in situ wavelength conversion helps to overcome various wavelength-dependent issues, such as biological tissue damage. In this paper, we present an original patterning method for high-precision fabrication of free-standing nanoscale waveguides based on lithium niobate, a material with a strong second-order nonlinearity and a broad transparency window covering the visible and mid-infrared wavelength ranges. The fabrication process combines electron-beam lithography with ion-beam enhanced etching and produces nanowaveguides with lengths from 5 to 50 μm, widths from 50 to 1000 nm and heights from 50 to 500 nm, each with a precision of few nanometers. The fabricated nanowaveguides are tested in an optical characterization experiment showing efficient second-harmonic generation.

Plasmonic nanoparticle clusters with tunable plasmonic resonances in the visible spectral region

The seeded growth of poly(ethylene imine) – gold nanoparticle clusters enables the formation of particle assemblies with tunable optical properties. Clusters with increasing particle sizes, filling factors and assemblies consisting of PEI–gold–silver core shell particles can be synthesized in this way. Profound structural characterization is carried out via TEM imaging and FIB milling which allows visualizing the cross-section of the clusters. Determination of the optical properties was performed via UV-Vis spectroscopy and spectral dark field microscopy of individual particles. Additionally, numerical calculations were carried out based on the Mie theory. The results are in good agreement with the experimental findings and reveal the contribution of different multipoles to the spectra which cannot be resolved by UV-Vis spectroscopy in solution. The isotropic nature and adjustable properties of these clusters could render them versatile building blocks for metamaterials.

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.

Photonic microstructures in lithium niobate by potassium hydroxide-assisted ion beam-enhanced etching

Lithium niobate is treated with argon ion beam irradiation of varying energy, resulting in a homogeneously damaged surface layer. This layer is subsequently etched with potassium hydroxide (KOH), and etch rates are recorded for different experimental conditions such as varying temperature and concentration of the water-based KOH solution. The dependence of the etch rate upon the normalized fluence of the ion irradiation shows good selectivity, which makes the effect useful for nano- and microfabrication. Ultimately, photonic crystals and gratings are fabricated by KOH-assisted ion beam-enhanced etching to prove the potential of this method for applications in functional nano- and micro-optics.