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

Executive Summary

Core Value Proposition: The study establishes the Numerical Aperture (NA) of the focusing lens as a critical, independent control parameter for spatially scaling laser-induced modifications (plasma channels) in bulk High Pressure High Temperature (HPHT) diamond.
Spatial Scaling Achieved: By varying the NA from 0.15 to 0.45, the length of the luminous plasma channels was controlled over a range of 5 µm to 120 µm using a fixed 1030 nm femtosecond laser setup.
Focusing Regimes: Weak focusing (low NA) significantly elongates the plasma channel, allowing for long structures (up to 120 µm) with only a slight increase in pulse power. Tight focusing (high NA) yields compact, high-intensity structures (down to 5 µm).
Quantitative Scaling Law: The range of channel length change (dL) was found to decrease nonlinearly with increasing aperture, scaling approximately as dL ~ NA^-3/2.
Filamentation Threshold: The filamentation threshold power (P_th) is lower under tight focusing (NA=0.45, P_th ≈ 0.38 MW) compared to weak focusing (NA=0.15, P_th ≈ 0.55 MW), indicating that high NA conditions favor geometric focusing over nonlinear Kerr effects.
Industrial Potential: This NA-dependent control offers a new degree of freedom for 3D processing of transparent dielectrics, enabling variation of modified region parameters (e.g., waveguide cross-section or length) without altering the laser setup or wavelength.

Technical Specifications

Parameter	Value	Unit	Context
Target Material	HPHT Synthetic Type IIA Diamond	N/A	Dimensions: 3 x 1.5 x 1.5 mm³
Laser Wavelength (λ)	1030	nm	Linearly polarized Yb⁺³ fiber laser
Pulse Duration (τ)	~300	fs	Ultrashort pulse regime
Repetition Rate (v)	100	kHz	Multi-pulse exposure
Pulse Energy Range (E)	40 - 350	nJ	Energy range studied
Peak Pulse Power Range (P)	0.35 - 1.2	MW	Power range studied
Numerical Aperture (NA) Range	0.15 - 0.45	N/A	Controlled via variable diaphragm
Focal Spot Radius (w₀)	2.2 - 0.73	µm	Corresponds to NA 0.15 - 0.45
Plasma Channel Length Range (L)	5 - 120	µm	Dependent on NA and pulse power
Channel Length Scaling	dL ~ NA^-3/2	N/A	Nonlinear dependence of length change
P_th (Weak Focusing)	0.55 ± 0.05	MW	Filamentation threshold power (NA = 0.15)
P_th (Tight Focusing)	0.38 ± 0.05	MW	Filamentation threshold power (NA = 0.45)
PL Slope (Pre-Filamentation)	0.55 to 0.75	N/A	Increases with NA (Regime #1)
PL Slope (Filamentation)	1.85 ± 0.05	N/A	Constant across all NA (Regime #2)

Key Methodologies

Laser Setup and Control: A 1030 nm femtosecond Yb⁺³ fiber laser (Satsuma) provided linearly polarized pulses (~300 fs, 100 kHz) with peak powers ranging from 0.35 MW to 1.2 MW.
Aperture Adjustment: The effective Numerical Aperture (NA) of the focusing objective (NA_max = 0.55) was dynamically adjusted between 0.15 and 0.45 using a variable diaphragm positioned before the objective lens.
Sample Irradiation: Laser pulses were focused into the bulk of a synthetic HPHT Type IIA diamond sample via the (110) face at a normal angle. The material was confirmed to be free of nitrogen impurities (<1 ppm).
Plasma Channel Visualization: Luminous plasma channels, resulting from nonlinear inter-band photoexcitation, were captured at a right angle via a second (110) face using a NA = 0.2 microscope objective and a monochromatic CMOS camera.
Filamentation Threshold Measurement: P_th was determined by observing the onset of asymmetric elongation of the luminous channel. P_th is defined as the pulse power where the “nonlinear part” (elongated due to Kerr self-focusing) exceeds the “linear part” (elongated due to geometric focusing).
Focal Shift Calculation: Experimental nonlinear focal shifts (df) were compared against theoretical calculations based on the refined expression for self-focus distance (Z_sf), which incorporates the intensity-dependent refractive index (Kerr nonlinearity).
Photoluminescence (PL) Analysis: PL peak and integrated intensity were measured as a function of pulse power and NA. Bends in the PL slope were used to identify the transition points between the geometric focusing regime (Regime #1) and the nonlinear filamentation regime (Regime #2).

Commercial Applications

3D Integrated Photonic Device Manufacturing: The ability to precisely scale the length and cross-section of modified regions (plasma channels) by simply controlling the NA allows for the fabrication of complex 3D waveguides, splitters, and couplers embedded in diamond or other wide-bandgap materials.
Variable Waveguide Fabrication: Engineers can dynamically vary the dimensions of embedded structures (e.g., changing the cross-section of a waveguide) without modifying the physical optical setup, offering high flexibility for rapid prototyping and industrial scaling.
Microfluidic and Sensing Devices: Precise control over localized material modification is essential for fabricating microfluidic channels and embedded sensing elements in robust, biocompatible materials like diamond.
High-Density Data Storage: The creation of compact, high-intensity modification structures (5 µm range) using tight focusing (high NA) is relevant for developing high-density, bulk optical data storage in transparent media.
Industrial Laser Machining Versatility: NA control provides a simple, electronic means to switch between regimes: long, low-density modification (weak focusing) for extended structures, or short, high-density modification (tight focusing) for localized micro-explosions or compact features.

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]