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.

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.

The emergence and growth of transformation optics over the past decade has revitalized interest in how a gradient refractive index (GRIN) can be used to control light propagation. Two-dimensional demonstrations with lithographically defined silicon (Si) have displayed the power of GRIN optics and also represent a promising opportunity for integrating compact optical elements within Si photonic integrated circuits. Here, we demonstrate the fabrication of three-dimensional Si-based GRIN micro-optics through the shape-defined formation of porous Si (PSi). Conventional microfabrication creates Si square microcolumns (SMCs) that can be electrochemically etched into PSi elements with nanoscale porosity along the shape-defined etching pathway, which imparts the geometry with structural birefringence. Free-space characterization of the transmitted intensity distribution through a homogeneously etched PSi SMC exhibits polarization splitting behavior resembling that of dielectric metasurfaces that require considerably more laborious fabrication. Coupled birefringence/GRIN effects are studied by way of PSi SMCs etched with a linear (increasing from edge to center) GRIN profile. The transmitted intensity distribution shows polarization-selective focusing behavior with one polarization focused to a diffraction-limited spot and the orthogonal polarization focused into two laterally displaced foci. Optical thickness-based analysis readily predicts the experimentally observed phenomena, which strongly match finite-element electromagnetic simulations.

Visibly transparent porous silicon dioxide (PSiO 2 ) and PSiO 2 /titanium dioxide (TiO 2 ) 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 PSiO 2 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 TiO 2 infilling of the pores. Applying this oxidation and TiO 2 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 PSiO 2 element with a parabolic, lens-like phase profile with a calculated focal length of 1.48 m was formed. The calculated focal length was reduced to 0.80 m upon the addition of TiO 2 . All the structures showed broad transparency in the visible and were stable to the materials conversion process.

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 processing–structure–property relationships of NR thin films demonstrate that their structure and optical properties can be reconfigured through the adjustment of solvent vapor concentration. The phase behavior and optical properties of NRs present an interesting inorganic analogue to organic liquid crystals with potential applications in emerging optoelectronic technologies.