Numerical Aperture-Dependent Spatial Scaling of Plasma Channels in HPHT Diamond

At a Glance

Metadata	Details
Publication Date	2023-10-23
Journal	Photonics
Authors	Yulia Gulina, Jiaqi Zhu, George Krasin, Evgeny V. Kuzmin, S. I. Kudryashov
Institutions	P.N. Lebedev Physical Institute of the Russian Academy of Sciences
Citations	6
Analysis	Full AI Review Included

Technical Documentation & Analysis: Numerical Aperture-Dependent Spatial Scaling of Plasma Channels in HPHT Diamond

This document analyzes the research on femtosecond laser-induced plasma channel formation in diamond, focusing on the critical role of Numerical Aperture (NA) in controlling spatial scaling. This application is highly relevant to 6CCVD’s core mission of providing high-purity, precision-engineered diamond materials for advanced photonics and 3D microfabrication.

Executive Summary

Core Achievement: Demonstrated precise spatial scaling (length and position) of femtosecond laser-induced plasma channels in bulk synthetic diamond by varying only the Numerical Aperture (NA) of the focusing lens (0.15-0.45).
Material Platform: The study utilized high-purity synthetic High Pressure High Temperature (HPHT) Type IIA diamond, confirming diamond’s robust optical nonlinearity and suitability for ultrafast laser processing.
Scaling Mechanism: Weak focusing (low NA) significantly elongates the plasma channel (up to 120 µm) with minimal power increase, dominated by the nonlinear Kerr effect.
Compaction Mechanism: Tight focusing (high NA) yields compact, high-intensity plasma structures (down to 5 µm), where geometric focusing dominates over nonlinear effects.
Threshold Control: The filamentation threshold power (P_th) was shown to decrease significantly with increasing NA, dropping from 0.55 MW (NA=0.15) to 0.38 MW (NA=0.45).
Industrial Value: This NA-dependent control provides a new, non-invasive degree of freedom for 3D processing of transparent dielectrics, enabling the fabrication of structures with varying cross-sections or lengths without altering the laser setup.

Technical Specifications

The following hard data points were extracted from the experimental results regarding the laser parameters and resulting plasma channel characteristics:

Parameter	Value	Unit	Context
Material Studied	HPHT Type IIA Diamond	N/A	High purity, synthetic, 3 x 1.5 x 1.5 mm³
Laser Wavelength (λ)	1030	nm	Yb⁺³ ion fiber laser source
Pulse Duration (τ)	~300	fs	Ultra-short pulse regime
Repetition Rate (ν)	100	kHz	Multi-pulse exposure
Pulse Energy Range (E)	40-350	nJ	Energy used for channel formation
Peak Pulse Power (P)	0.35-1.2	MW	Range studied for channel formation
Numerical Aperture (NA) Range	0.15-0.45	N/A	Controlled via variable diaphragm
Focal Spot Size (w₀)	2.2-0.73	µm	Corresponding 1/e² radius
Plasma Channel Length Range	5-120	µm	Dependent on NA and Pulse Power
Filamentation Threshold Power (P_th)	0.55 ± 0.05	MW	Weak focusing (NA = 0.15)
Filamentation Threshold Power (P_th)	0.38 ± 0.05	MW	Tight focusing (NA = 0.45)
Channel Length Scaling	dL < NA^-3/2	N/A	Nonlinear decrease with increasing aperture

Key Methodologies

The experiment focused on controlling the focusing geometry to investigate nonlinear light-matter interaction in bulk diamond:

Material Preparation: A high-purity synthetic HPHT Type IIA diamond (3 x 1.5 x 1.5 mm³) was selected, confirmed via IR spectroscopy to have extremely low nitrogen impurity absorption bands (< 1 ppm).
Laser Setup: A femtosecond Yb⁺³ ion fiber laser (1030 nm, ~300 fs, 100 kHz) provided linearly polarized radiation.
NA Control: The Numerical Aperture was dynamically adjusted in the range NA = 0.15-0.45 using a variable diaphragm mounted in front of the focusing objective.
Irradiation Geometry: The laser was focused into the bulk of the sample via the (110) face at normal incidence.
Plasma Channel Registration: Luminous plasma channels were captured at a right angle via another (110) face using a NA = 0.2 objective and a monochromatic CMOS camera.
Filamentation Threshold Determination: The onset of filamentation (P_th) was identified by observing the visible asymmetric elongation of the luminous channel, where the “nonlinear part” exceeded the “linear part.”
Damage Assessment: Raman spectroscopy and optical microscopy confirmed that in the studied pulse energy range, there was no permanent material modification (no graphitization or aggregation of vacancy centers).

6CCVD Solutions & Capabilities

The research highlights the critical need for high-quality, low-defect diamond substrates to enable precise nonlinear optical processing. 6CCVD’s expertise in MPCVD diamond growth and precision fabrication directly addresses the requirements for replicating and advancing this research into industrial applications.

Applicable Materials

To replicate or extend this research, 6CCVD recommends the following materials, which offer superior control over purity and geometry compared to standard HPHT:

Optical Grade Single Crystal Diamond (SCD): Ideal for replicating the high-purity Type IIA environment. Our MPCVD SCD offers extremely low nitrogen content (< 1 ppm) and controlled crystalline orientation, ensuring minimal linear absorption and maximizing the fidelity of nonlinear Kerr effects and plasma generation.
Heavy Boron Doped Diamond (BDD): For extending the research into applications requiring controlled conductivity or enhanced plasma dynamics. BDD allows for the study of how free carrier density, independent of photoionization, influences self-focusing and plasma defocusing effects.

Customization Potential

The ability to precisely control the spatial dimensions of modified regions (5 µm to 120 µm) is crucial for 3D photonic device fabrication (e.g., waveguides). 6CCVD provides the necessary material foundation and post-processing services for industrial implementation:

Research Requirement/Challenge	6CCVD Solution & Capability	Technical Advantage
Substrate Size & Thickness	SCD/PCD Substrates up to 10 mm thick	Enables deeper 3D processing volumes and larger device footprints than the 1.5 mm thick samples used in the study.
Industrial Scaling	PCD Wafers up to 125 mm diameter	Provides a cost-effective path for scaling up 3D microfabrication of large-area photonic arrays or sensing platforms.
Surface Quality	Precision Polishing (SCD: Ra < 1 nm)	Ultra-smooth surfaces are essential for high-NA focusing and minimizing scattering losses, ensuring accurate control over the focal shift (df) and plasma channel initiation.
Device Integration	Custom Metalization Services	We offer in-house deposition of standard contacts (Au, Pt, Ti, W, Cu) for integrating electrodes or thermal management layers onto the processed diamond, critical for creating functional devices from the modified bulk material.
Specific Geometries	Laser Cutting and Shaping	Custom dimensions, specific crystal orientations, and complex shapes can be achieved via our advanced laser cutting capabilities, ensuring optimal alignment for focusing experiments.

Engineering Support

6CCVD’s in-house PhD team specializes in the material science of diamond for extreme optical and electronic applications. We offer comprehensive engineering support for projects involving femtosecond laser processing and 3D microfabrication. Our experts can assist in selecting the optimal SCD or PCD grade, determining appropriate thickness and orientation, and designing metalization schemes to maximize device performance and yield.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

The investigation of plasma channels induced by focused ultra-short 1030-nm laser pulses in bulk of synthetic High Pressure High Temperature (HPHT) diamond revealed strong dependence of their spatial parameters on the used numerical aperture of the lens (NA = 0.15-0.45). It was shown that at weak focusing conditions it is possible to significantly increase the length of the plasma channel with a slight increase in pulse power, while tight focusing allows one to obtain more compact structures in the same range of used powers. Such a dependence paves the way to new possibilities in 3D processing of transparent dielectrics, allowing one, for example, to vary the spatial parameters of modified regions without changing the setup, but only by controlling the lens aperture, which seems very promising for industrial applications.

Tech Support

Original Source

DOI: https://doi.org/10.3390/photonics10101177

References

2014 - Ultrafast Lasers—Reliable Tools for Advanced Materials Processing [Crossref]
2023 - CMOS-compatible direct laser writing of sulfur-ultrahyperdoped silicon: Breakthrough pre-requisite for UV-THz optoelectronic nano/microintegration [Crossref]
2015 - Overcoming Kerr-Induced Capacity Limit in Optical Fiber Transmission [Crossref]
2013 - In Vivo Three-Photon Microscopy of Subcortical Structures within an Intact Mouse Brain [Crossref]
2008 - Micromachining of photonic devices by femtosecond laser pulses
2009 - Femtosecond laser-induced formation of nanometer-width grooves on synthetic single-crystal diamond surfaces [Crossref]
2013 - Femtosecond laser-induced microstructures on diamond for microfluidic sensing device applications [Crossref]
2013 - From Self-Focusing Light Beams to Femtosecond Laser Pulse Filamentation [Crossref]
1975 - Self-Focusing: Theory [Crossref]
2004 - Dynamics of Femtosecond Laser Interactions with Dielectrics [Crossref]