[Paper Review] Femtosecond laser inscription of nonlinear photonic circuits in Gallium Lanthanum Sulphide glass

Maria Ramos, Belén Sotillo+7 more|arXiv (Cornell University)|Jun 15, 2018

Laser Material Processing Techniques31 references12 citations

TL;DR

This study demonstrates femtosecond laser inscription of single-mode, low-loss waveguides in Gallium Lanthanum Sulphide (GLS) glass for nonlinear photonics at 800 nm, achieving 1.9 dB/cm insertion loss. The technique preserves the material's high nonlinear refractive index, confirmed via optical switching in directional couplers, with micro-Raman and EDX analysis revealing laser-induced bond reorganization (Ga-S and La-S) as the origin of refractive index increases without compositional changes.

ABSTRACT

We report on femtosecond laser writing of single mode optical waveguides in chalcogenide Gallium Lanthanum Sulphide (GLS) glass. A multiscan fabrication process was employed to create waveguides with symmetric single mode guidance and low insertion losses at 800 nm wavelength, compatible with Ti:Sapphire ultrafast lasers. {\mu}Raman and X-Ray microanalysis were used to elucidate the origin of the laser-induced refractive index change in GLS. Nonlinear refractive index measurements of the waveguides were performed by finding the optical switching parameters of a directional coupler, demonstrating that the nonlinear properties were preserved, evidencing that GLS is a promising platform for laser-written integrated nonlinear photonics.

Motivation & Objective

• Develop low-loss, single-mode waveguides in chalcogenide GLS glass compatible with Ti:Sapphire lasers at 800 nm for nonlinear photonics.
• Address the challenge of achieving symmetric, low-loss waveguide profiles in GLS using a multiscan femtosecond laser fabrication process.
• Elucidate the physical origin of the laser-induced refractive index change in GLS through advanced characterization.
• Demonstrate and quantify the preservation of nonlinear optical properties in laser-written waveguides for all-optical switching applications.

Proposed method

• Employed a multiscan femtosecond laser writing technique with a 230 fs, 1030 nm Yb:KGW laser at 500 kHz repetition rate, focused via a 0.42 NA objective.
• Used computer-controlled 3D motion stages and an acousto-optic modulator to precisely pattern waveguides and directional couplers in bulk GLS glass at 150 µm depth.
• Performed optical characterization using butt-coupled single-mode fibers, beam profilers, and power meters to measure insertion loss and mode profiles at 800 nm.
• Conducted nonlinear characterization via a home-built amplified Ti:Sapphire laser system (180 fs, 2 kHz, 800 nm) to measure optical switching in directional couplers.
• Applied micro-Raman spectroscopy (633 nm excitation) and energy-filtered transmission electron microscopy with EDX to analyze local structural and compositional changes.
• Correlated refractive index changes with bond reorganization by analyzing Raman band intensity ratios and BSE/EDX elemental mapping.

Experimental results

Research questions

• RQ1 Can femtosecond laser inscription produce single-mode, low-loss waveguides in GLS glass at 800 nm, compatible with Ti:Sapphire ultrafast lasers?
• RQ2 What is the physical origin of the refractive index increase induced by femtosecond laser irradiation in GLS glass?
• RQ3 To what extent are the intrinsic nonlinear optical properties of GLS preserved after laser writing?
• RQ4 How do laser parameters (pulse energy, scan speed) affect the structural integrity and optical performance of the waveguides?
• RQ5 Does the laser writing process induce significant elemental migration or compositional changes in the GLS network?

Key findings

• Single-mode waveguides with an insertion loss of 1.9 dB/cm were achieved at 800 nm using the multiscan femtosecond laser writing process.
• The nonlinear refractive index of the waveguides was measured to be on the same order of magnitude as the pristine GLS glass, confirming preservation of strong Kerr nonlinearity.
• Micro-Raman spectroscopy revealed a 25% increase in the relative intensity of the La-S bond vibration mode at 215 cm⁻¹ within the waveguide, indicating bond reorganization.
• Raman intensity maps showed a uniform increase in total Raman signal and a 15% rise in the 325 cm⁻¹ GaS₄ tetrahedral vibration band, indicating enhanced polarizability and network densification.
• EDX elemental mapping showed no significant migration of Ga, La, or S within the waveguide, ruling out ion migration as the primary mechanism for refractive index change.
• At high laser pulse energy (e.g., 400 nJ), void formation and material ejection were observed via BSE imaging and EDX, correlating with increased loss and reduced Raman intensity due to network damage.