Iridescence from Total Internal Reflection at 3D Microscale Interfaces: Mechanistic Insights and Spectral Analysis

Abstract

An experimental investigation and the optical modeling of the structural coloration produced from total internal reflection interference within 3D microstructures are described. Ray‐tracing simulations coupled with color visualization and spectral analysis techniques are used to model, examine, and rationalize the iridescence generated for a range of microgeometries, including hemicylinders and truncated hemispheres, under varying illumination conditions. An approach to deconstruct the observed iridescence and complex far‐field spectral features into its elementary components and systematically link them to ray trajectories that emanate from the illuminated microstructures is demonstrated. The results are compared with experiments, wherein microstructures are fabricated with methods such as chemical etching, multiphoton lithography, and grayscale lithography. Microstructure arrays patterned on surfaces with varying orientation and size lead to unique color‐traveling optical effects and highlight opportunities for how total internal reflection interference can be used to create customizable reflective iridescence. The findings herein provide a robust conceptual framework for rationalizing this multibounce interference mechanism and establish approaches for characterizing and tailoring the optical and iridescent properties of microstructured surfaces.

Tuning structural coloration through evanescent wave absorption at microscale concave interfaces

“…Half of the structures were masked during the dye evaporation to create a sample that was half-coated with the dye to facilitate direct comparison. Black pigmented silicone (n2=1.41) was then coated onto the back of the microstructures to reduce the refractive index contrast at the TIR interface, which enhances the color vibrancy; the black pigmentation also helps to reduce stray light scattered from behind the structures 12 . For the refractive indices used (n1=1.58, n2=1.41) and the range of incidence angles (θ) we anticipate for the structure shown in Figure 1b with illumination from the normal, we estimate dp to be on the scale of 100 to 500 nm for visible wavelengths (λ=400 to 700 nm).…”

“…Recently, we reported a design principle for generating iridescent structural color from interference which occurs between light rays undergoing different multibounce trajectories of total internal reflection (TIR) in a microstructure 11 . Light that interferes by this mechanism can reflect by TIR many times within the microstructured cavity, with the specific ray trajectories determined by the interface geometry, refractive index, and illumination angle [11][12][13] . Previous research has thus shown that the coloration from TIR interference can be tuned by changing parameters such as the microstructure's geometry (e.g.…”

“…Previous research has thus shown that the coloration from TIR interference can be tuned by changing parameters such as the microstructure's geometry (e.g. radius of curvature, contact angle, symmetry) and refractive index contrast [11][12][13][14] , with demonstrated use in applications such as sensors 14 , decorative films 12,13 printing 15 , and pigments 16 . The structural color generated by TIR interference is unique in that the coloration results from additive effects of multiple different interference events, providing opportunities for customizing the reflective colors in ways not accessible by other mechanisms.…”

“…Reflection spectra were collected by directing the iridescent projection into a macro lens attached to a Kymera 193i spectrograph (Andor) with an attached Zyla 4.2 sCMOS camera (Figure S2a-c). The procedure and custom normalization process were detailed previously for collection of angularly-resolved spectra 12 . All spectra collected along the length of the slit of the imaging spectrograph could be analyzed to compare the non-dyed and dyed portions of the masked microstructure sample (Figure S3a-c).…”

“…Ray tracing simulations were conducted using Zemax OpticStudio to simulate the far-field caustic of the illuminated non-dyed hemicylinder sample. A setup similar to previously described was used 12 . The IR-813 toluenesulfonate coated sample was illuminated under room lighting (approximately 400-700 nm), where the incident light angle was held constant at 0°, or illuminated with a near-infrared LED flashlight (850 nm) where the sample stage was held constant at 0° (horizontal).…”