Anton Sergeyev

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.

Core-shell potassium niobate nanowires for enhanced nonlinear optical effects.

We demonstrate the synthesis as well as the optical characterization of core-shell nanowires. The wires consist of a potassium niobate (KNbO3) core and a gold shell. The nonlinear optical properties of the core are combined with the plasmonic resonance of the shell and offer an enhanced optical signal in the near infrared spectral range. We compare two different functionalization schemes of the core material prior to the shell growth process: silanization and polyelectrolyte. We show that the latter leads to a smoother and complete core-shell nanostructure and an easier-to-use synthesis process. A Mie-theory based theoretical approach is presented to model the enhanced second-harmonic generated (SHG) signal of the core-shell wires, illustrating the influence of the fabrication-induced varying geometrical factors of wire radius and shell thickness. A spectroscopic measurement on a core-shell nanowire shows a strong localized surface plasmon resonance close to 900 nm, which matches with the SHG resonance obtained from nonlinear optical experiments with the same nanowire. According to the simulation, this corresponds to a wire radius of 35 nm and a shell thickness of 7.5 nm. By comparing SHG signals measured from an uncoated nanowire and the coated one, we obtain a 250 times enhancement factor. This is less than the calculated enhancement, which considers a cylindrical nanowire with a perfectly smooth shell. Thus, we explain this discrepancy mainly with the roughness of the synthesized gold shell.

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 heating with near infrared resonance nanodot arrays for multiplexing optofluidic applications

In this work, we show local laser-induced heating in fluids with gold nanodot arrays prepared by electron-beam lithography that cover resonances in the near infrared spectral range from 750 nm to 880 nm. We utilize two approaches to demonstrate thermal effects, solvent evaporation and flow stop, with a thermosensitive polymer solution. We show that with fluences as low as 4 μJ cm−2, significant heating of the nanostructures occurs in their immediate vicinity. We perform power and wavelength dependent measurements to determine the threshold of the thermal effects. Using wavelengths about 20 nm away from the plasmonic resonance peak, the heating drops drastically, and 30 to 40 nm away, there is mostly no thermal effect. Therefore, working close to the threshold laser power offers the possibility of multiplexed reactions or sensing without cross-talk even though a typical full width at half maximum of a plasmonic resonance spectrum can be as broad as 200 nm. Additionally, comparison with theoretical calculations of heat generation show good agreement with the experimentally determined threshold powers.

Generation and tunable enhancement of a sum-frequency signal in lithium niobate nanowires*

Recent developments in the fabrication of lithium niobate (LiNbO3) structures down to the nanoscale opens up novel applications of this versatile material in nonlinear optics. Current nonlinear optical studies in sub-micron waveguides are mainly restricted to the generation of second and third harmonics. In this work, we demonstrate the generation and waveguiding of the sum-frequency generation (SFG) signal in a single LiNbO3 nanowire with a cross-section of 517 nm × 654 nm. Furthermore, we enhance the guided SFG signal 17.9 times by means of modal phase matching. We also display tuning of the phase-matched wavelength by varying the nanowire cross-section and changing the polarization of the incident laser. The results prove that LiNbO3 nanowires can be successfully used for nonlinear wave-mixing applications and assisting the miniaturization of optical devices.

Second-harmonic generation imaging for crystal structure characterization in III-V nanowires

Semiconductor III-V nanowires are one of the key nanostructures for new generation of optoelectronic and nanophotonic devices. The most unique feature of the III-V nanowires is the possibility to engineer crystal phases that cannot be obtained at normal conditions in bulk III-V materials [1]. For example, it is possible to grow III-V nanowires with pure wurtzite, with combined wurtzite/zinc blende phases and with rotational zinc blende twins along single nanowire. The possibilities to engineer crystal structures of III-V materials make nanowires promising structures for nonlinear photonics application. The most common technique to characterize the crystal structure of nanowires is transmission electron microscopy (TEM), but this technique requires to deposit nanostructures on the special TEM substrates and it cannot be used for a non-destructive analysis of nanowires implemented, for example in lab-on-chip devices or computer components.

Nonlinear mode switching in lithium niobate nanowaveguides

Nanowaveguides (NWs), typically semiconductors, are used as nanolasers [1], as optical switching devices [2] and as imaging probes [3]. These applications can be further complemented by fully controlling the guided modes within the NW. Here, we select second-harmonic (SH) modes in the visible (VIS) and near-infrared (NIR) wavelength ranges by using SH modal phase-matching [4]. Therefore, only one SH mode is excited and waveguided within the lithium niobate (LiNbO3) NW. In this case, LiNbO3 is an appropiate material choice because of its strong nonlinear properties and its broad transparency in the VIS and NIR.

Mapping near-field of the guided second-harmonic in lithium niobate nanowaveguides with photosensitive polymer

Nanowaveguides (NWs) are valuable components for building miniaturized optical devices since they have both nanoscale and microscale dimensions. One can further extend the NW application range by exploiting nonlinear optical effects such as second-harmonic (SH) and sum-frequency (SF) generation. Lithium niobate (LiNbO3) NWs and nanopillars are demonstrated to generate and guide SH [1-4] and SF [5] signals. However, to develop nonlinear NW applications, one should know behaviour of the guided light inside the NWs. This information can be obtained by scanning near-field optical microscopy, which requires complex equipment. As an alternative, one can also use a photopolymerization technique. This technique is based on a negative photosensitive polymer, which crosslinks when irradiated by light and, thus, maps the distribution of the electric field. Previously, this technique was applied to image plasmonic standing-waves in metallic NWs through two-photon absorption (TPA) [6]. Here, we apply this technique to study the guided SH signal in LiNbO3 NWs and map its behaviour at the NW output and along the NW, which may help to maximize the output power of the nonlinear signal [7].

Generation and waveguiding of the sum-frequency signal in lithium niobate nanowires

Lithium niobate (LiNbO3) is a unique crystal that demonstrates a large range of properties such as photo-refractivity, electro-optical effect and piezoelectricity. Besides, the LiNbO3 crystal also possesses strong nonlinear optical properties, which are maintained in subwavelength structures such as nanowires. Thus, LiNbO3 nanowires and microwires are demonstrated to generate and guide the second-harmonic signal [1-4]. The guided second-harmonic signal can be applied for localized imaging [1] and light conversion. However, in order to further develop nanowire-based application, one should also demonstrate other nonlinear optical effects such as sum-frequency generation (SFG). Indeed, nanowires that generate and guide the SFG signal can be applied for all-optical switching and for obtaining unconventional spectral ranges.

Mie resonance induced enhancement of second-harmonic generation in perovskite nanoparticles

Barium titanate (BaTiO3) is a perovskite crystal, which is transparent over a wide spectral range and has a high refractive index. BaTiO3 is a suitable building block for metamaterials or photonic devices. In addition, BaTiO3 emits a strong second harmonic generation (SHG) signal due to its noncentrosymmetric crystal structure. SHG in nanostructures has potential applications in bio-imaging and lab-on-a-chip applications. BaTiO3 nanoparticles emit a strong bulk SHG signal [1] and are biocompatible. However, SHG decreases strongly with the volume of the nanostructures and means to increase the conversion efficiency are currentìy being investigated. Here, we demonstrate that the conversion efficiency to SHG is strongly enhanced by means of the Mie resonance [2] of an individual BaTiO3 nanoparticle.