Polarized micro-Raman studies of femtosecond laser written stress-induced optical waveguides in diamond

At a Glance

Metadata	Details
Publication Date	2018-01-15
Journal	Applied Physics Letters
Authors	Belén Sotillo, Andrea Chiappini, Vibhav Bharadwaj, John P. Hadden, Federico Bosia
Institutions	Istituto di Fotonica e Nanotecnologie, National Research Council
Citations	30
Analysis	Full AI Review Included

Diamond Photonic Integration: Stress Quantification via Polarized Micro-Raman Spectroscopy

Executive Summary

This documentation analyzes the application of polarized micro-Raman spectroscopy to precisely map stress distributions in single crystal diamond (SCD) waveguides fabricated via femtosecond (fs) laser writing. This technique is critical for the optimization and commercial scaling of diamond-based 3D photonic circuits and quantum devices.

Core Achievement: Validation of polarized micro-Raman spectroscopy as a non-destructive method for correlating internal stress ($\tau$) to localized refractive index change ($\Delta n$) in fs-laser-written diamond waveguides.
Material Basis: Waveguides were successfully fabricated in high-purity Type IIa Single Crystal Diamond (SCD), confirming diamond’s suitability for advanced photonic integration.
Stress Induction: The guiding mechanism relies on stress induced between two amorphized, low-density lines. This biaxial stress causes lattice deformation and lifts the phonon degeneracy, resulting in measurable Raman shifts.
Quantified Refractive Index Change: The calculated maximum refractive index increase ($\Delta n$) reached $3 \times 10^{-3}$, sufficient for stable light confinement, particularly for the TM mode (polarization parallel to [001]).
Polarization Dependence Explained: The study directly explains the polarization-sensitive guiding observed previously: Tensile stress along [001] increases the refractive index (guiding the TM mode), while stress along [110] decreases the index (preventing TE mode guiding).
Engineering Impact: This methodology provides engineers and scientists with a crucial feedback loop to tailor fs laser parameters (pulse energy, speed, line separation) for deterministic control over stress distribution and resulting waveguide properties.

Technical Specifications

The following table summarizes the key material properties, fabrication parameters, and quantified results derived from the analysis of the fs-laser-written diamond waveguides.

Parameter	Value	Unit	Context
Material	Type IIa	N/A	Synthetic CVD Single Crystal Diamond (SCD)
Substrate Orientation	{100}, {110}	Planes	Top/Bottom {100}, Side {110}
Sample Thickness	0.5	mm	Base material dimension
Unstrained Raman Frequency ($\omega_0$)	1333	cm^-1	Zone-center optical phonon ($F_{2g}$)
Laser Pulse Duration	230	fs	Yb:KGW femtosecond writing system
Laser Wavelength	515	nm	Used for creating bulk modification
Pulse Energy	100	nJ	Energy used for nonlinear absorption
Scan Speed	0.5	mm/s	Sample translation rate
Waveguide Separation	13	µm	Distance between low-density barrier lines
Measured MFD (Mode Field Diameter)	~10	µm	Observed optical mode size at $\lambda = 635$ nm
Pristine Raman Linewidth	2.1	cm^-1	Indicates initial material homogeneity
Max Refractive Index Change ($\Delta n$)	$3 \times 10^{-3}$	N/A	Calculated increase for TM mode confinement
Max Compressive Stress ($\tau_1$)	~ -0.9	GPa	Stress component parallel to [110]
Max Tensile Stress ($\tau_2$)	~ 0.6	GPa	Stress component parallel to [001]

Key Methodologies

The experiment relies on a precise combination of CVD diamond material selection, high-fidelity femtosecond laser processing, and detailed confocal Raman characterization.

Material Preparation:
- Synthetic CVD SCD (Type IIa) substrates, 5 x 5 x 0.5 mm³, were used, ensuring low defect density crucial for high-quality optical applications.
- All facets were polished, specifically ensuring high surface quality on the (110) plane used for Raman backscattering analysis.
Femtosecond Laser Writing:
- A regeneratively amplified Yb:KGW system was employed, delivering 230-fs pulses at 515 nm (500 kHz repetition rate, 100 nJ pulse energy).
- Writing was performed using a high-NA (1.25) oil immersion objective, focused along the [001] direction.
- Waveguides were formed by writing two parallel modification lines separated by 13 µm at a scan speed of 0.5 mm/s along the [110] direction.
Micro-Raman Characterization:
- A Labram Aramis microRaman system with a 532 nm DPSS laser was used in a backscattering configuration from the (110) surface.
- Confocal setup with a 100x objective (0.8 NA) provided spatial resolution below 1 µm.
- Polarized Raman measurements (using configurations like X’(Y’Y’)X’ and X’(Y’Z’)X’) were taken across the waveguide cross-section to isolate stress components parallel to [001] and [110].
Stress and Refractive Index Derivation:
- Raman peak shifts ($\Delta \omega_i$) were measured and fit to Lorentzian curves.
- Secular equations (deriving from lattice dynamical equations) were solved using known diamond deformation potential constants ($p, q, r$) and compliance constants ($S_{ij}$) to extract the internal stress components ($\tau_1, \tau_2$).
- The stress values were then related to the refractive index change ($\Delta n$) via the piezooptic tensor ($\pi_{ij}$), allowing for full mapping of the guiding region properties.

6CCVD Solutions & Capabilities

The findings in this research directly underscore the need for high-specification, custom-engineered CVD diamond materials. 6CCVD, as an expert material scientist and technical engineer, offers SCD substrates perfectly suited to replicate, extend, or scale this femtosecond laser writing technique for photonic and quantum applications.

Applicable Materials

To achieve low-loss optical waveguides and reproducible stress profiles, high-purity and minimal strain material is essential.

Material Recommendation: Optical Grade Single Crystal Diamond (SCD).
- Purity: SCD Type IIa (similar to material used in the paper) or better, minimizing native defects that interfere with color center formation (NV, SiV) or optical transmission.
- Surface Finish: 6CCVD guarantees ultra-low roughness (Ra < 1 nm on SCD surfaces) crucial for minimizing scattering losses inherent to integrated photonics.
Future Development: For electro-optic modulation or integrated sensing, 6CCVD offers Boron-Doped Diamond (BDD) films and substrates, enabling integration of electrical contacts directly adjacent to the stress-induced waveguides.

Customization Potential for Waveguide Engineers

Replicating this research requires specific crystallographic orientations and dimensions, all within 6CCVD’s core capabilities.

Custom Dimensions and Orientations: While the paper used 5x5x0.5 mm³ substrates with {100}/{110} facets, 6CCVD provides custom diamond plates and wafers up to 125mm in diameter (PCD) and large area SCD, precisely oriented to user specifications (e.g., standard [100], [110], or [111]).
Precision Thickness Control: 6CCVD provides SCD substrates in the required thickness range (0.1 µm to 500 µm), ensuring compatibility with established fs laser bulk writing depths and subsequent device handling.
Integrated Metalization Services: For researchers moving beyond passive waveguides toward active quantum devices or E-O switches, 6CCVD offers in-house custom metalization (Au, Pt, Pd, Ti, W, Cu) for electrode deposition directly onto the substrate surface or waveguide structures.

Engineering Support & Logistics

6CCVD acts as a technical partner, offering global support for advanced diamond device fabrication.

Advanced Material Consultation: 6CCVD’s in-house PhD engineering team specializes in MPCVD growth parameters, crystal strain analysis, and material selection for specific applications like fs laser processing, stress engineering, and integrated quantum photonics. We assist customers in optimizing material properties to achieve desired stress thresholds ($\tau$) for reliable refractive index modulation ($\Delta n$).
Global Supply Chain: We provide reliable Global Shipping, with DDU as default and DDP options available, ensuring secure and timely delivery of high-value, custom diamond substrates worldwide.

For custom specifications or material consultation related to advanced diamond waveguides or stress-engineered photonics, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Understanding the physical mechanisms of the refractive index modulation induced by femtosecond laser writing is crucial for tailoring the properties of the resulting optical waveguides. In this work, we apply polarized Raman spectroscopy to study the origin of stress-induced waveguides in diamond, produced by femtosecond laser writing. The change in the refractive index induced by the femtosecond laser in the crystal is derived from the measured stress in the waveguides. The results help to explain the waveguide polarization sensitive guiding mechanism, as well as provide a technique for their optimization.

Tech Support

Original Source

DOI: https://doi.org/10.1063/1.5017108