Causes and Effects of Low Absorption of Infrared Lasers on Highly Reflective Materials
Infrared lasers are widely used in metal processing, but for highly reflective materials such as copper, aluminum, gold, and silver, the absorption rate is generally low. Low absorption directly affects laser energy coupling efficiency, processing stability, and the overall process window. The following explains the phenomena based on material characteristics, laser parameters, and optical interaction mechanisms.
I. Optical Properties of Highly Reflective Materials
Highly reflective materials have high free-electron concentration, and their surfaces exhibit high reflectivity in the infrared wavelength range. Surface reflectivity is mainly determined by the complex refractive index of the material, in which the real and imaginary parts related to electrical conductivity dictate reflection and absorption behavior at the interface. For infrared wavelengths (such as 1064 nm or near-infrared), the reflectivity of copper and aluminum can exceed 90%, with absorption rates only around 3%–7%. Therefore, laser energy cannot be effectively deposited into the material.
II. Wavelength Characteristics of Infrared Lasers
The wavelength of infrared lasers lies in the weak absorption response zone of metallic free electrons. In this wavelength range, metals have a small skin depth, meaning optical energy decays electromagnetically within a very shallow surface layer, resulting in limited effective absorption. Meanwhile, the photon energy of infrared light is low and cannot induce strong electromagnetic coupling, reducing the interaction efficiency between the laser and the metal surface.
III. Influence of Laser Incidence Angle and Polarization on Absorption
The incidence angle and polarization state alter the reflection behavior at the interface. S-polarized light has higher reflectivity on metal surfaces, while P-polarized light can achieve lower reflectivity at specific angles. However, in practical welding, cleaning, or marking applications, maintaining a stable polarization direction is difficult, so overall absorption remains low.
IV. Influence of Material Surface Conditions on Absorption
Surface roughness, oxide-film thickness, and contamination affect scattering and absorption of the laser energy. Examples include:
Oxide layers can increase the absorption of copper in the infrared range.
Rough surfaces allow multiple scattering, increasing effective absorption.
However, in the initial processing stage of smooth highly reflective materials, absorption remains significantly low.
V. Effects of Low Absorption on Processing
Energy coupling difficulty: Laser energy cannot be effectively deposited, resulting in insufficient weld penetration or low marking efficiency.
Increased risk of back-reflection: High reflectivity may cause laser return, potentially damaging internal optical components of the laser source.
Narrower process window: Processing becomes highly sensitive to power, focus position, and scanning speed, leading to unstable results.
Difficult initial melting: At the start of processing, low absorption prevents stable melt-pool formation, requiring higher energy density.
VI. Methods to Improve Infrared Laser Absorption on Highly Reflective Materials
Increase power density: Reduce spot size or increase peak power to strengthen initial energy coupling.
Use modulated laser processes (e.g., MOPA pulses): High peak power in pulses can rapidly heat the material surface and reduce reflectivity.
Surface pretreatment: Roughening, sandblasting, cleaning, or controlled oxidation can improve absorption.
Use variable waveforms or multi-mode lasers: Different pulse widths and frequencies improve absorption stability.
Use blue or green lasers instead of infrared: Visible-light lasers have significantly higher absorption on copper, aluminum, and similar materials, depending on equipment requirements.