Optical components deep inside transparent plastics

Polymers are currently of huge interest for 3D printing and additive manufacture, in which lasers build up nanosized structures using two-photon polymerization or multiphoton photolithography around spaces that are then infilled. However, the polymer and photoresist properties must be tailored to the laser wavelength, which involves pre- and post-processing steps, including doping the material to sensitize it to light or heat treatment. We have developed a single-stage manufacturing procedure using commonly available and low-cost raw materials.

Only femtosecond laser pulses can alter the refractive index of a transparent material: nonlinear absorption at high intensities permits the formation of 3D nanoscale structures. Tight focusing, using lenses with large numerical aperture (NA ≥ 0.5), can create highly localized nanoscale modifications near the focus, but aberration at the air-material interface distorts these with depth. On the other hand, low-NA inscription is accompanied by nonlinear guiding useful for generating micron-wide deep structures. Therefore, precise focusing with a range of NAs is required to create flexible 3D photonic devices in transparent optical materials (see Figure 1).

Figure 1. Subsurface diffractive photonic structures inscribed using femtosecond lasers. (Credit: Alexandra Baum.)

The specific optical material we have chosen is Perspex—poly(methyl methacrylate), PMMA—a widely available, low-cost, biocompatible polymer used in applications ranging from contact lenses, to lab-on-a-chip, microfluidics, and optical fiber applications. Choosing a polymer in preference to glasses and semiconductors cuts costs and contributes to advances in organic or carbon-based optoelectronic devices. However, polymers can be chemically and thermally unstable compared with glasses, and photonic structures may fade with time. We have tailored the inscription process to surpass the limitations of materials, dimensions, and chemistry, to facilitate unique structures entirely formed by laser-polymeric interactions. Parameters studied include NA, fluence, wavelength, and incident polarization.¹

We generated a permanent, positive change in refractive index n up to +Δn_max=4×10⁻³ (comparable to doped material/glass) at 1kHz repetition rate with 800nm, 100fs pulses. New insights into the photochemical mechanism of the laser writing process indicate that the refractive index change is created by a combination of depolymerization—including accumulation of photodegradation products—absorption center formation, and crosslinking, depending on the exact writing conditions. We used microscanning Raman spectroscopy to map the chemical bond changes in photonic structures up to 500μm below the surface. We also explored inscription at IR (775nm) and UV (387nm) wavelengths with pulse durations of 45–160fs and different polarizations to control the nonlinear interaction and photochemistries.²

Writing below the surface at high NA causes spherical aberration such that written structures become elongated in the beam propagation direction, creating asymmetric structures such as elliptical cross-section waveguides. We used a spatial light modulator (SLM) to control and correct these effects with depth. By studying the nonlinear aspects of multiphoton absorption and plasma generation, we clarified the nonlinear effects of polarization on writing conditions.³ Linear polarization couples more strongly than circular. The SLM allowed diffractive, multibeam inscription when combined with a low-NA lens, enabling fast fabrication of very efficient volume Bragg gratings (VBGs).⁴

Inscription was critically dependent on the duration of the femtosecond pulse.⁵ There was more impact ionization relative to multiphoton ionization with longer pulses, causing heating and catastrophic breakdown. At 387nm, structures as small as 430nm were inscribed with a holographic setup, to form a thin VBG with ∼2300lines/mm. With higher NA (∼1) and when using dynamic phase correction with an SLM used for depth control, we could create photonic nanostructures with dimensions as small as 200nm. Such 3D nanostructures can create photonic crystal structures, shrinking integrated optical circuits and incorporating optical interconnects and highly selective filters.

In summary, we have shown that laser inscription enables the building blocks for optical devices to be created by engineering the refractive index within optical transparent materials. The limitations of materials, dimensions, and chemistry can be circumvented by creating 3D structures with dimensions of the order of 10²nm, enabling light to be guided and suppressed in designed wavelength ranges. Careful choice of laser pulse focal volume, duration, repetition rate, writing speed, and total exposure controls time- and temperature-dependent chemical reactions such as depolymerization and chain scission, leading to refractive index changes. The technique can be used to create high-quality waveguides, splitters, and diffractive photonic structures within a single substrate and will deliver flexible, low-cost photonic technology. Our future work will involve SLM-based dynamic wavefront/polarization control, synchronized with a motion control system to generate complete, integrated optical circuits at speed.

The authors acknowledge support from the UK Engineering and Physical Sciences Research Council, the Unilever-Manchester Advanced Measurement Partnership, Vista Optics, Rinck Elektronik, Jena, and the North West Science Fund (N0003200). The University of Manchester provided studentships for postgraduate research students Alexandra Baum and Anca Taranu, and an Overseas Research Studentship for postgraduate research student Shijie Liang.