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We present an overview of a new method to print 3D volumetric gradient index (GRIN) micro-optics inside a porous silica scaffold and show results for the focusing properties of an array of cylindrical GRIN lenses.

Direct laser writing (DLW) has been shown to render 3D polymeric optical components, including lenses, beam expanders, and mirrors, with submicrometer precision. However, these printed structures are limited to the refractive index and dispersive properties of the photopolymer. Here, we present the subsurface controllable refractive index via beam exposure (SCRIBE) method, a lithographic approach that enables the tuning of the refractive index over a range of greater than 0.3 by performing DLW inside photoresist-filled nanoporous silicon and silica scaffolds. Adjusting the laser exposure during printing enables 3D submicron control of the polymer infilling and thus the refractive index and chromatic dispersion. Combining SCRIBE’s unprecedented index range and 3D writing accuracy has realized the world’s smallest (15 µm diameter) spherical Luneburg lens operating at visible wavelengths. SCRIBE’s ability to tune the chromatic dispersion alongside the refractive index was leveraged to render achromatic doublets in a single printing step, eliminating the need for multiple photoresins and writing sequences. SCRIBE also has the potential to form multicomponent optics by cascading optical elements within a scaffold. As a demonstration, stacked focusing structures that generate photonic nanojets were fabricated inside porous silicon. Finally, an all-pass ring resonator was coupled to a subsurface 3D waveguide. The measured quality factor of 4600 at 1550 nm suggests the possibility of compact photonic systems with optical interconnects that traverse multiple planes. SCRIBE is uniquely suited for constructing such photonic integrated circuits due to its ability to integrate multiple optical components, including lenses and waveguides, without additional printed supports., 3D printing: SCRIBE tunes refractive index within optical elements A 3D printing technique can make a tiny structure with a varying refractive index, opening the door for manufacturing lenses and waveguides with interesting properties. Subsurface controllable refractive index via beam exposure (SCRIBE), developed by Paul V. Braun and colleagues at the University of Illinois at Urbana Champaign, involves changing a laser’s power to variably fill the nanosized pores of a silicon scaffold with photosensitive polymer. This tunes the refractive index within the final, fabricated, submicron-scale structure, causing visible light to bend differently within it according to where it passes. SCRIBE was used to fabricate a 15-micrometer diameter, spherical Luneburg lens in which the refractive index decreases from its core towards the surface. SCRIBE is particularly suited for constructing multiple components, like lenses and waveguides, for photonic integrated circuits, without the need for additionally printed supports.

Next generation mobile devices and computing architectures would benefit from ultra-high bandwidth technologies that efficiently transport and process optical signals. Subsurface fabrication can address this challenge by forming volumetric photonic integrated circuits with a more compact aerial footprint than planar on-chip circuits. These 3D optical systems may utilize densely packed low-loss, freeform optical interconnects for high volume data transfer. In this Perspective, we provide a comparative overview of the two main methods for subsurface fabrication, including our recently developed SCRIBE process, and assess the advantages and future directions of each approach. After analyzing the underlying technologies, we provide a roadmap of important steps to transition from laboratory demonstrations of individual elements to industrial-scale production of subsurface volumetric photonic integrated circuits.

Visibly transparent porous silicon dioxide (PSiO2) and PSiO2/titanium dioxide (TiO2) optical elements were fabricated by thermal oxidation, or a combination of thermal oxidation and atomic layer deposition infilling, of an electrochemically etched porous silicon (PSi) structure containing an electrochemically defined porosity profile. The thermally oxidized PSiO2 structures are transparent at visible wavelengths and can be designed to have refractive indices ranging from 1.1 to 1.4. The refractive index can be increased above 2.0 through TiO2 infilling of the pores. Applying this oxidation and TiO2 infilling methodology enabled tuning of a distributed Bragg reflector (DBR) formed from PSi across the visible spectrum. At the maximum filling, the DBR exhibited a transmission of 2% at 620 nm. Simulations match well with measured spectra. In addition to forming DBR filters, phase-shaping gradient refractive index (GRIN) elements were formed. As a demonstration, a 4 mm diameter radial GRIN PSiO2 element with a...

The effect of spatially varying birefringence on the focusing behavior of porous silicon (PSi) and porous silicon dioxide (PSiO2) gradient refractive index (GRIN) lenses is investigated. Both materials attain broad, tunable refractive indexes and birefringence profiles, with PSi having a maximum birefringence of ∼0.26 and PSiO2 a reduced maximum birefringence of ∼0.03 at 633 nm. These GRIN lenses exhibit polarization-dependent split focusing behavior, wherein the divergence angle between the twin foci increases with the birefringence gradient. PSi’s large birefringence allows the divergence angle to be tuned such that light focuses away from the center of the lens. These GRIN elements demonstrate how tunable birefringent materials can be used to engineer polarization-selective optical responses.

Understanding and controlling the self-assembly of colloidal nanostructures into ordered superstructures present scientifically interesting and technologically important research challenges. Here, we investigated the self-assembly, disordering, and reassembly of colloidal CdSe/CdS dot/rod nanorod (NR) films. We monitored the structural evolution of the NR films in real time using in situ grazing incidence small-angle and wide-angle X-ray scattering. In dry films, self-assembled from colloidal suspensions, NRs are oriented with the long axis normal to the substrate, but the preferred NR orientation is lost when dichlorobenzene vapor is introduced. Multiprobe optical and structural experiments allowed us to directly correlate the NR superlattice structure and optical absorption. We found that the optical absorption of the NR films is significantly enhanced in disordered NR films compared to NR arrays in which the rods are oriented normal to the plane of the substrate and parallel to the optical axis. Basic ...

We introduce a method to process colloidal PbSe nanocrystals (NCs) into inorganic NC thin films using chalcogenidometallate (ChaM) clusters as surface ligands, resulting in electrically coupled NC solids. NCs are first immobilized on a substrate via a self-assembled monolayer followed by chemical treatment to exchange the insulating oleate ligands with ChaM clusters. Quantum confinement in the PbSe NCs is preserved as evidenced by persistent excitonic features in the absorption spectrum. PbSe NC–ChaM composites exhibit rectification (“off” states), saturation, n-type electrical behavior, and high electron mobilities of 1.28 and 0.475 cm2 V–1 s–1 for different composite compositions.