Micro-Machining of Diamond, Sapphire and Fused Silica Glass Using a Pulsed Nano-Second Nd -YVO4 Laser

At a Glance

Metadata	Details
Publication Date	2021-08-23
Journal	Optics
Authors	David Waugh, C. Dale Walton
Institutions	Coventry University, University of Hull
Citations	5
Analysis	Full AI Review Included

Executive Summary

This study investigates the efficacy of a relatively inexpensive nanosecond (ns) Diode-Pumped Solid State (DPSS) Nd:YVO₄ laser (355 nm) for micrometre-scale surface engineering of optically transparent materials.

Core Value Proposition: The ns Nd:YVO₄ laser provides a cost-effective alternative to high-capital femtosecond (fs) laser systems, achieving competitive quality for specific materials and applications.
Diamond Processing Quality: Diamond exhibited the most optimized results, yielding extremely well-defined micrometre features with minimal debris formation, comparable to results achieved using more expensive fs lasers.
Threshold Fluence (F_th): The average threshold fluence for diamond and sapphire ranged significantly from 10 Jcm^-2 to 35 Jcm^-2, demonstrating a strong dependency on the cumulative effects arising from the number of incident pulses (up to 50,000).
Material Comparison: Fused silica glass processing was poor, resulting in considerable cracking and unpredictable deformation. Sapphire processing produced good features but required post-processing due to substantial deposited debris.
Mechanism: Modification occurs via multi-photon absorption, as the 3.49 eV photon energy is less than the material band gaps. Laser-induced graphitization in diamond is hypothesized to enhance absorption and improve processing quality.
Industrial Relevance: This technology is attractive for industries requiring micrometre-scale surface modification of diamond where capital cost reduction is critical.

Technical Specifications

Parameter	Value	Unit	Context
Laser Type	DPSS Nd:YVO₄	N/A	Diode-Pumped Solid State (AOT-YVO-3Q)
Wavelength (λ)	355	nm	Frequency tripled UV output
Pulse Duration	1.3	ns	Nanosecond regime
Pulse Repetition Frequency (PRF)	5	kHz	Standard processing rate used
Focused Beam Spot Diameter (ω)	~1	µm	Achieved using Geltech Aspheric Lens (f=15.29 mm, NA=0.16)
Incident Fluence Range (F_o)	11 to 74	Jcm^-2	Varied by adjusting pumping diode current
Irradiance Range	24 to 106	GWcm^-2	Calculated peak intensity
Average Threshold Fluence (F_th) Range	10 to 35	Jcm^-2	For diamond and sapphire, dependent on pulse number
Single Photon Energy (355 nm)	3.49	eV	Less than band gaps of all tested materials
Diamond Band Gap	5.5	eV	Requires multi-photon absorption
Sapphire Band Gap	~9	eV	Requires multi-photon absorption
Fused Silica Band Gap	~7	eV	Requires multi-photon absorption
Maximum Pulse Count Tested	50,000	Pulses	Used to study cumulative modification effects
Line Traverse Speed	100	mms^-1	Used for qualitative line profile assessment

Key Methodologies

The surface modification was conducted using a direct-write setup, focusing on parameter variation to assess material response and determine threshold fluences.

Laser Configuration: A 355 nm DPSS Nd:YVO₄ laser operating at a 1.3 ns pulse duration and 5 kHz PRF was utilized.
Beam Focusing: A Geltech Molded Glass Aspheric Lens (f=15.29 mm) was used to achieve a tight focus, resulting in an approximate 1 µm beam spot diameter on the material surface.
Fluence Variation: Incident fluence (F_o) was controlled between 11 Jcm^-2 and 74 Jcm^-2 by adjusting the laser pumping diode current.
Point Modification Arrays: Arrays of laser-modified sites were generated on diamond and sapphire by systematically varying the incident fluence (columns) and the number of pulses (rows, up to 50,000) to quantify the cumulative modification effect.
Threshold Fluence Calculation: The threshold fluence (F_th) for diamond and sapphire was derived by plotting the square of the modified area (D²) against the natural logarithm of the incident fluence (ln F_o), based on the Gaussian beam equation: D² = 2ω²ln(F_o/F_th).
Line Profile Generation: Qualitative assessment was performed by inducing single process lines (3 cm distance) using a fixed fluence of 50 Jcm^-2 and a sample traverse speed of 100 mms^-1.
Analysis Techniques: Surface morphology and feature quality were assessed using Optical Microscopy (Leica DM/LM) and Scanning Electron Microscopy (SEM, Zeiss Evo 60). UV/VIS spectrometry was used to approximate the optical band gaps and confirm multi-photon absorption necessity.

Commercial Applications

The successful and cost-effective micromachining of transparent materials, particularly diamond, using a ns Nd:YVO₄ laser has direct implications across several high-tech sectors:

Optical Waveguide Technology: Etching gratings and channels into fused silica and diamond substrates for integrated optics, fiber Bragg gratings, and advanced communications components.
Microfluidics and Lab-on-a-Chip: Precision fabrication of microchannels and reservoirs in glass and sapphire for biomedical diagnostics and chemical analysis devices.
High-Value Material Processing (Diamond/Sapphire):
- Jewelry/Luxury Goods: High-quality, debris-minimal engraving and surface modification of diamond materials.
- Electronics: Surface structuring of sapphire (used as a substrate for LEDs and RF components) and diamond (used in high-power electronics and heat sinks).
Cost-Optimized Manufacturing: Implementing the relatively inexpensive ns Nd:YVO₄ system reduces the capital expenditure required for micrometre-scale processing tasks, especially for diamond, which traditionally relies on costly femtosecond lasers.
Automotive and Display Technology: Surface engineering of transparent materials for enhanced durability, anti-reflective properties, or integrated sensor features in advanced automotive displays and windows.

View Original Abstract

Optically transparent materials are being found in an ever-increasing array of technological applications within industries, such as automotive and communications. These industries are beginning to realize the importance of implementing surface engineering techniques to enhance the surface properties of materials. On account of the importance of surface engineering, this paper details the use of a relatively inexpensive diode-pumped solid state (DPSS) Nd:YVO4 laser to modify the surfaces of fused silica glass, diamond, and sapphire on a micrometre scale. Using threshold fluence analysis, it was identified that, for this particular laser system, the threshold fluence for diamond and sapphire ranged between 10 Jcm−2 and 35 Jcm−2 for a laser wavelength of 355 nm, dependent on the cumulative effects arising from the number of incident pulses. Through optical microscopy and scanning electron microscopy, it was found that the quality of processing resulting from the Nd:YVO4 laser varied with each of the materials. For fused silica glass, considerable cracking and deformation occurred. For sapphire, good quality features were produced, albeit with the formation of debris, indicating the requirement for post-processing to remove the observed debris. The diamond material gave rise to the best quality results, with extremely well defined micrometre features and minimal debris formation, comparative to alternative techniques such as femtosecond laser surface engineering.

Tech Support

Original Source

DOI: https://doi.org/10.3390/opt2030016

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