Thermally assisted fabrication of nonlinear photonic structures in lithium niobate with femtosecond laser pulses

Abstract

Engineered domain structures play an essential role in nonlinear optics for quasi-phase-matched parametric processes. Pyroelectric field-assisted domain inversion with focused femtosecond laser pulses is a promising approach to create arbitrary two-dimensional nonlinear photonic structures in a large volume without externally applied electrical fields. We fabricate lattices of ferroelectric domains by patterning lithium niobate crystals with femtosecond laser pulses and then heating them to elevated temperatures. After cooling to room temperature, domains form below and above the laser-induced seeds. We investigate the effect of temperature and seed spacing on the number and size of inverted domains. In a temperature range of 220 °C-300 °C all domains are inverted in a two-dimensional lattice with periods of 15 µm × 6.3 µm. Smaller lattice periods result in a smaller fraction of inverted domains. Measurements with conducting, nonconducting, and short-circuited crystal surfaces reveal the influence of surface charges during the domain formation process. From the obtained domain widths and spacings, we calculate the effective nonlinear coefficient of quasi-phase-matched second-harmonic generation in two-dimensional nonlinear photonic structures.

All-Optical Domain Inversion in LiNbO3 Crystals by Visible Continuous-Wave Laser Irradiation

“…Optical domain engineering falls into two major groups: light-assisted poling and all-optical poling . While all-optical techniques purely rely on light itself, light-assisted methods still require an external electric field − or additional processing steps, such as film patterning by electron beam lithography or pyroelectric thermal treatments. , All-optical methods completely circumvent the need for additional electric fields or processing steps. All-optical poling can be subclassified as a function of the excitation wavelength: UV light (pulsed or continuous-wave CW). − Far-IR light at λ = 10.6 μm (pulsed) …”

Creation of a Periodic Domain Structure in MgOLN by Femtosecond Laser Irradiation

“…A two-step periodical poling method representing the creation of locally modified elongated volumes (tracks) and their growth by further uniform heating and cooling cycle was realized in MgOLN crystal. The tracks oriented along the polar axis were created by NIR fs-laser irradiation with 800 and 1030 nm wavelengths and 1 kHz pulse frequency [25,26]. The laser spot was moved from any polar surface to the bulk or vice versa with 80 µm/s speed.…”

“…The laser spot was moved from any polar surface to the bulk or vice versa with 80 µm/s speed. The tracks acted as the seeds for domain growth under the action of the pyroelectric field [25,26]. The domains appeared below the tracks and grew over the whole sample depth, thus forming a 2D periodical domain structure.…”

“…The pronounced backswitching of the created domains was obtained in the center of the lattices for temperatures above 300 • C. The average diameter of the central domains, about 2.4 µm, is almost independent of temperature and lattice periods. The authors pointed out the limitation of the duty cycles of the created domain patterns and suggested using a biased electric field in addition to the pyroelectric field to obtain higher conversion efficiencies [26].…”

Tilted Dirac cones and asymmetric conical diffraction in photonic Lieb-kagome lattices

“…One such system that is of particular interest due to its versatility is a photonic lattice, which consists of a periodic arrangement of weakly coupled single-mode waveguides [12]. Using fabrication techniques such as direct laser writing arbitrary lattice geometries in different host materials can be realized [13][14][15].By varying the shape of the constituting waveguides in a photonic lattice, one can simulate the influence of electrical fields, realizing phenomena such as Floquet topological insulators [16], Bloch-Zener oscillations [17] or dynamic localization [18]. Recently, even experimental evidence of type III Dirac cones has been found in a photonic lattice system [7].…”