Thin film absorption characterization by focus error thermal lensing.

Area of Science:

• Materials Science
• Optical Physics
• Metrology

Background:

• Accurate measurement of optical absorption in thin films is crucial for device performance.
• Existing techniques may lack sensitivity or be unsuitable for dynamic measurements.

Purpose of the Study:

• To demonstrate a simple, highly sensitive technique for measuring absorbed power in thin film dielectrics.
• To establish a method for characterizing absorption loss with high dynamic range and precision.

Main Methods:

• Utilizes thermal lensing induced by an amplitude-modulated or pulsed pump beam.
• Employs a continuous wave probe beam and a four-quadrant detector with cylindrical lenses to generate a focus error signal.
• Quantifies absorbed power by measuring changes in the probe beam profile.

Main Results:

• Demonstrates a linear dependence of the focus error signal on absorbed power over a dynamic range exceeding 10^5.
• Achieves a single-shot sensitivity of approximately 20 ppm in pulsed configuration.
• Successfully measures absorption loss in dielectric thin films on fused silica substrates.

Conclusions:

• The demonstrated thermal lensing technique provides a sensitive and robust method for measuring absorbed power in thin films.
• The technique's high sensitivity and dynamic range make it suitable for characterizing thin films, including in situ measurements of moving targets.
• This approach offers a valuable tool for materials characterization and quality control in thin film applications.