Crystals Materials

Optical Coating Processes

Introduction

There are various methods for manufacturing optical thin films, broadly categorized into three types: Physical Vapor Deposition (PVD), Chemical Vapor Deposition (CVD), and Chemical Solution Deposition(CSD).

1. Physical Vapor Deposition (PVD) Technology

Optical coating processes

Almost every laser crystal and optic is finished with a thin-film coating: anti-reflection (AR) to pass light at a wavelength, high-reflection (HR) mirrors, or partial-reflection (PR) output couplers. These coatings are deposited as multilayer dielectric stacks, and their quality decides transmission, damage threshold and lifetime — especially under high laser fluence.

Common deposition processes

Process	Typical use
E-beam / thermal evaporation	Standard dielectric stacks; cost-effective
Ion-assisted deposition (IAD)	Denser films, lower shift, better durability
Ion-beam sputtering (IBS)	Very dense, low-loss, high-damage-threshold films
Sputtering	Hard, stable coatings for demanding environments

Design goals

A coating is designed (often with thin-film design software) to give the target reflectance/transmittance at the specified wavelength(s) and angle, while maximising laser-damage threshold and minimising absorption and stress. For laser crystals, AR coatings are applied at both the pump and lasing wavelengths. See our thin-film principles and design article for the underlying theory.

Frequently asked questions

AR vs HR vs PR? AR minimises reflection (max transmission); HR maximises reflection (mirrors); PR gives a chosen partial reflectance (output couplers).

Which process for high damage threshold? Dense, low-defect processes like IBS and ion-assisted deposition typically give the highest laser-damage threshold.

DIBS-PVD-Coating — Dual Ion Beam Sputtering (DIBS) / Dual Ion Beam (Sputter) Deposition (DIBD)

By adding an assist ion beam source an extra ion bombardment at the substrate can be introduced. This is used to influence the growing film or pre-clean the substrate.

In PVD technology, the deposition material comes directly from the source material (referred to as the coated material). It involves three basic processes: generation of vapor-phase deposition material, transportation of vapor-phase deposition material, and deposition of vapor-phase deposition material.

(1) Generation of Vapor-Phase Deposition Material
There are two ways to generate vapor-phase deposition material:

a. Evaporation Coating: The deposition material is heated and evaporated to form vapor-phase deposition material, known as evaporation coating.

b. Sputtering Coating: High-energy particles bombard the target material, causing atoms of the target material to be ejected and become vapor-phase deposition material, known as sputtering coating.

(2) Transportation of Vapor-Phase Deposition Material
The transportation of vapor-phase deposition material must be carried out in a vacuum of less than 1.33x10^-2Pa. This is done to:

a. Reduce collisions and chemical reactions between vapor-phase deposition material and other gas molecules during its flight towards the substrate (referred to as the substrate).

b. Minimize chemical reactions between the deposition material, vapor-phase deposition material, and the substrate with other reactive gas molecules.

c. Prevent other gas molecules from entering the film layer and becoming impurities during the deposition process.

d. Improve the deposition speed, purity, density, and adhesion of the thin film to the substrate.

(3) Deposition of Vapor-Phase Deposition Material
Deposition of vapor-phase deposition material is the process of vapor-phase deposition material atoms condensing on the substrate. This can occur through natural condensation or by introducing other reactive gas atoms to form compound films through chemical reactions, known as reactive deposition. Additionally, during the process of vapor-phase deposition material atoms condensing into a film on the substrate, high-energy ions can be simultaneously used to bombard the film layer to improve its structure, properties, and adhesion, known as ion deposition.