High-intensity focused pressure wave generation via Q-switched Er:YAG laser with a water layer formed by the coupled lens for optoacoustic conversion

Šavli, Dominik (author), Babnik, Aleš (author), Vella, Daniele (author), Jezeršek, Matija (author)

We demonstrate coating-free optoacoustic generation and focusing of ultrasound using a mechanically Q-switched (MQS) erbium-doped yttrium aluminum garnet (Er:YAG) source (~100 ns, ≤20 mJ) combined with a concave water interface that simultaneously serves as converter and acoustic lens. Axial, lateral, and focal-point measurements mapped the pressure field while varying beam diameter (2w = 5–15 mm) and pulse energy (E = 10–20 mJ). The maximum focal positive pressure (P▫$_max$▫ = 7 MPa) occurs at an intermediate diameter (~10 mm), whereas the tightest lateral/axial confinement and strongest spectral enhancement arise at larger diameters (14–15 mm) with f▫$_c$▫ = ~5 MHz and −6 dB bandwidth up to 7 MHz. Pressure increases nearly monotonically with energy. For equal fluence, larger diameters yield higher focal pressures due to greater focusing gain. Small beams (2w ≈ 5–7 mm) show shorter apparent time-of-flight (TOF) and waveform broadening, consistent with early shock-like emission from locally vaporizing region. These results provide practical rules for tuning amplitude, spectrum, and confinement, enabling sub-millimeter focusing for contamination-sensitive and therapeutic applications.

• Language: English
• Keywords: focused pressure wave, photoacoustic, Q-switch, Er:YAG
• Work type: Article
• Typology: 1.01 - Original Scientific Article
• Organization: FS - Faculty of Mechanical Engineering
• Publication status: Published
• Publication version: Version of Record
• Year: 2025
• Number of pages: 17 str.
• Numbering: Vol. 15, issue 19, [article no.] 10860
• PID: 20.500.12556/RUL-175008
• UDC: 535:534
• ISSN on article: 2076-3417
• DOI: 10.3390/app151910860
• COBISS.SI-ID: 252918019
• Publication date in RUL: 13.10.2025

Record is a part of a journal

• Title: Applied sciences
• Shortened title: Appl. sci.
• Publisher: MDPI
• ISSN: 2076-3417
• COBISS.SI-ID: 522979353

Licences

• License: CC BY 4.0, Creative Commons Attribution 4.0 International
• Link: http://creativecommons.org/licenses/by/4.0/
• Description: This is the standard Creative Commons license that gives others maximum freedom to do what they want with the work as long as they credit the author.

Secondary language

• Language: Slovenian
• Keywords: fokusirani tlačni valovi, optoakustika, preklop kvalitete, Er:YAG

Projects

• Funder: ARIS - Slovenian Research and Innovation Agency
• Project number: P2-0392-2020
• Name: Optodinamika