Manufacturing Micro-Optics with Two-Photon Polymerization

1. Introduction

Two-Photon Polymerization (2PP) is a laser-based fabrication technique capable of producing micro- and nano-scale 3D structures with sub-micron precision. It relies on the principle of two-photon absorption, where polymerization occurs only at the exact focal point of a tightly focused femtosecond laser beam.

Unlike traditional optics manufacturing, which struggles to realize intricate structures below the micron scale, 2PP enables the direct writing of micro-optics, diffractive elements, and photonic structures with extraordinary resolution.

2. Process Overview

Photoresist Preparation – A substrate is coated with a transparent photoresist sensitive to two-photon absorption.

Laser Focusing – An ultrafast femtosecond laser is focused into the resin using a high numerical aperture objective.

Voxel Formation – Polymerization occurs only at the laser's focal point, where two photons are simultaneously absorbed.

3D Writing – By scanning the laser in 3D (x, y, z), arbitrary microstructures are written voxel by voxel.

Development – The unexposed resin is rinsed away, leaving only the solidified micro-optical structures.

3. Advantages

• Unmatched resolution: Feature sizes down to <200 nm, enabling designs beyond grinding, polishing, or molding.
• Geometric freedom: Diffractive optics, microlens arrays, and complex freeform microstructures.
• Direct 3D fabrication: No masks, molds, or preforms required.
• Functional integration: Micro-optics can be written directly onto fibers, waveguides, or other substrates.
• Isotropic properties: High structural fidelity across the full 3D volume.

4. Limitations

• Speed: Voxel-by-voxel writing is inherently slow; suited for microstructures, not bulk optics.
• Part size: Build volumes typically limited to 100 µm – 5 mm, depending on design.
• Material range: Restricted to specialized photoresists and hybrid resins.
• Cost: Requires femtosecond lasers and ultra-precise positioning systems.
• Scaling: Not practical for high-volume commodity optics compared to molding.

5. Materials & Specs

2PP Photoresists (Nanoscribe Portfolio)

IP-Dip2 (n = 1.55) — High-performance, versatile material for general-purpose high-resolution fabrication. Optimized for microlenses, diffractive optical elements, and complex 3D microstructures.

IP-S (n = 1.52) — A robust, general-purpose resist with excellent shape accuracy and mechanical stability. Used for a wide variety of micro-optical components and life-science applications.

IP-n162 (n = 1.62) — A specialized high-index resist for short focal length lenses and high-NA optics. Ideal for miniature imaging systems and fiber optics.

Optical Specifications (2PP Micro-Optics)

• Lens Aperture (Diameter): 50 µm – 5 mm
• Resolution (Lateral): 100 nm
• Minimum Feature Size: <200 nm
• Surface Roughness (Ra): <5 nm
• Shape Errors: < diameter / 1000 PTV
• Integration: Alignment-free integration of absorbing structures
• Optical Quality: Highly transparent materials with low fluorescence

6. Opdo's Role

At Opdo, Two-Photon Polymerization expands our capabilities to micro- and nano-scale optics. We collaborate with customers to develop applications and deliver parts from initial prototyping through to serial production.

This makes it possible to:

• Prototype novel micro-optical components such as diffractive optics, beam shapers, and microlens arrays.
• Deliver serially produced micro-optics where scaling is viable.
• Integrate directly into systems — including fibers, sensors, and semiconductor devices.