Laser Heterodyne Photothermal Displacement Microscopic Mapping System
MLHPDMS-633

This microscopic mapping system uses laser heterodyne photothermal displacement (LH-PD) to measure minute thermal expansion displacements at the sample surface and visualize their distribution as a two-dimensional image. The magnitude and temporal response of photothermal displacement, measured without contact or damage to the sample, provide insight into local thermal properties of materials and carrier behavior in semiconductors.

By combining microscope optics with an automated XY stage, the system measures photothermal displacement at different positions across the sample surface to visualize local differences in response and spatial variations. In semiconductors, it detects heat generated by the non-radiative recombination of photoexcited carriers, a process in which electrons and holes recombine without emitting light. This can help evaluate the distribution of defects and recombination centers.

The acquired temporal response waveforms can be used to estimate thermal diffusivity, thermal conductivity, carrier lifetime, and other properties by comparison with separately performed numerical analyses that account for thermal diffusion and carrier generation, diffusion, and recombination. The data can also be combined with separately prepared machine learning models to estimate spatial distributions of material properties across the sample surface.

■Features

• High-sensitivity LH-PD measurement: A heterodyne interferometer detects minute surface displacements with high sensitivity
• Noncontact, nondestructive absolute displacement measurement: Measures photothermal displacement in absolute terms without touching the sample
• Microscopic mapping: Visualizes photothermal displacement distributions in microscopic regions in two dimensions
• Time-resolved measurement: Acquires the rise and fall of displacement as temporal response waveforms
• Defect distribution evaluation: Detects local heating associated with non-radiative recombination to evaluate the distribution of defects and recombination centers
• Material property evaluation: Supports estimation of thermal diffusivity, thermal conductivity, carrier lifetime, and other properties through comparison with separately performed numerical analyses
• Extension to material property mapping: Can be combined with separately prepared machine learning models to estimate spatial distributions of material properties across the sample surface

■Measurement Principle: Laser Heterodyne Photothermal Displacement (LH-PD)

Laser heterodyne photothermal displacement (LH-PD) is a measurement technique that uses a heterodyne interferometer to detect thermal expansion displacement at the sample surface induced by excitation light.

The probe laser beam is split into a reference beam and a measurement beam. An acousto-optic frequency shifter (AOFS) introduces a small frequency difference between the two beams. When the measurement beam reflected from the sample surface is combined with the reference beam, a beat signal corresponding to this frequency difference is produced. Surface displacement changes the optical path length of the measurement beam and thus the phase of the beat signal. This phase change is used to determine the absolute surface displacement.

Time-resolved measurement records the phase changes over time, capturing the transient rise and fall of surface displacement as the excitation light is switched on and off.

Note: The material properties that can be evaluated and the estimation accuracy depend on the sample, measurement conditions, and analysis model.
Note: Software for numerical analysis and machine learning analysis is not included with this system.

■Specifications