The heightened demand for non-mechanical approaches to beam redirection and steering has led to several electro-optical approaches. One with great potential integrates liquid crystal (LC) as a cladding layer to a planar waveguide for continuous two dimensional steering. The birefringence of LC is leveraged to tune the waveguide effective leading to refractive steering, while efficient coupling with a freespace beam is accomplished with a “tapered gap” prism coupler. The out-coupled beam can be steered by refraction in a continuous manner to follow a path or address random points with sub millisecond response times. This device architecture presents a challenge for modeling and simulation with a large parameter space. Experimental successes have motivated a custom MATLAB model that couples LC and waveguide physics. The model simulates the distortion of the nematic LC and uses the graded index profile at the cell boundary to solve the waveguide equation as a function of applied voltage. Raytracing methods are used to track the refraction of an input beam through regions of tunable waveguide index and predict the angular field of regard (FOR). Numerical simulations of the coupling region predict the coupling efficiency given the conditions of the input beam including arbitrary bandwidth. Comparison of coupling conditions and FOR measurements with empirical results allows us to rapidly prototype a device by optimizing parameters with fast algorithms that maximize the field of regard and throughput efficiency.
A non-mechanical refractive laser beam steering device has been developed to provide continuous, two-dimensional steering of infrared beams. The technology implements a dielectric slab waveguide architecture with a liquid crystal (LC) cladding. With voltage control, the birefringence of the LC can be leveraged to tune the effective index of the waveguide under an electrode. With a clever prism electrode design a beam coupled into the waveguide can be deflected continuously in two dimensions as it is coupled out into free space. The optical interaction with LC in this beamsteerer is unique from typical LC applications: only the thin layer of LC (100s of nm) near the alignment interface interacts with the beam’s evanescent field. Whereas most LC interactions take place over short path lengths (microns) in the bulk of the material, here we can interrogate the behavior of LC near the alignment interface over long path lengths (centimeters). In this work the beamsteerer is leveraged as a tool to study the behavior of LC near the alignment layer in contrast to the bulk material. We find that scattering is substantially decreased near the alignment interface due to the influence of the surface anchoring energy to suppress thermal fluctuations. By tracking the position of the deflected beam with a high speed camera, we measure response times of the LC near the interface in off-to-on switching (~ms) and on-to-off switching (~100ms). Combined, this work will provide a path for improved alignment techniques, greater optical throughput, and faster response times in this unique approach to non-mechanical beamsteering.
Beam steering is a crucial technology for a number of applications, including chemical sensing/mapping and light detection and ranging (LIDAR). Traditional beam steering approaches rely on mechanical movement, such as the realignment of mirrors in gimbal mounts. The mechanical approach to steering has several drawbacks, including large size, weight and power usage (SWAP), and frequent mechanical failures. Recently, alternative non-mechanical approaches have been proposed and developed, but these technologies do not meet the demanding requirements for many beam steering applications. Here, we highlight the development efforts into a particular non-mechanical beam steering (NMBS) approach, refractive waveguides, for application in the MWIR. These waveguides are based on an Ulrich-coupled slab waveguide with a liquid crystal (LC) top cladding; by selectively applying an electric field across the liquid crystal through a prismatic electrode, steering is achieved by creating refraction at prismatic interfaces as light propagates through the device. For applications in the MWIR, we describe a versatile waveguide architecture based on chalcogenide glasses that have a wide range of refractive indices, transmission windows, and dispersion properties. We have further developed robust shadow-masking methods to taper the subcladding layers in the coupling region. We have demonstrated devices with >10° of steering in the MWIR and a number of advantageous properties for beam steering applications, including low-power operation, compact size, and fast point-to-point steering.
The mid-wave infrared (MWIR) portion of the electromagnetic spectrum is critically important for a variety of applications such as LIDAR and chemical sensing. Concerning the latter, the MWIR is often referred to as the “molecular fingerprint” region owing to the fact that many molecules display distinctive vibrational absorptions in this region, making it useful for gas detection. To date, steering MWIR radiation typically required the use of mechanical devices such as gimbals, which are bulky, slow, power-hungry, and subject to mechanical failure. We present the first non-mechanical beam steerer capable of continuous angular tuning in the MWIR. These devices, based on refractive, electro-optic waveguides, provide angular steering in two dimensions without relying on moving parts. Previous work has demonstrated non-mechanical beam steering (NMBS) in the short-wave infrared (SWIR) and near infrared (NIR) using a waveguide in which a portion of the propagating light is evanescently coupled to a liquid crystal (LC) layer in which the refractive index is voltage-tuned. We have extended this NMBS technology into the MWIR by employing chalcogenide glass waveguides and LC materials that exhibit high MWIR transparency. As a result, we have observed continuous, 2D MWIR steering for the first time with a magnitude of 2.74° in-plane and 0.3° out-of-plane.
KEYWORDS: Nanostructures, Luminescence, Molecules, Energy transfer, Energy harvesting, Resonance energy transfer, Fluorescence resonance energy transfer, Dendrimers, Light harvesting, Molecular photonics
DNA is a biocompatible scaffold that allows for the design of a variety of nanostructures, from straightforward double stranded DNA to more complex DNA origami and 3-D structures. By modifying the structures, with dyes, nanoparticles, or enzymes, they can be used to create light harvesting and energy transfer systems. We have focused on using Förster resonance energy transfer (FRET) between organic fluorophores separated with nanometer precision based on the DNAs defined positioning. Using FRET theory we can control the direction of the energy flow and optimize the design parameters to increase the systems efficiency. The design parameters include fluorophore selection, separation, number, and orientation among others. Additionally the use of bioluminescence resonance energy transfer (BRET) allowed the use of chemical energy, as opposed to photonic, to activate the systems. Here we discuss a variety of systems, such as the longest reported DNA-based molecular photonic wires (> 30 nm), dendrimeric light harvesting systems, and semiconductor nanocrystals integrated systems where they act as both scaffold and antennae for the original excitation. Using a variety of techniques, a comparison of different types of structures as well as heterogeneous vs. homogenous FRET was realized.
In addition to maintaining the structural integrity of the cell, the plasma membrane regulates multiple important cellular processes, such as endocytosis and trafficking, apoptotic pathways and drug transport. The modulation or tracking of such cellular processes by means of controlled delivery of drugs or imaging agents <i>via</i> nanoscale delivery systems is very attractive. Nanoparticle-mediated delivery systems that mediate long-term residence (e.g., days) and controlled release of the cargoes in the plasma membrane while simultaneously not interfering with regular cellular physiology would be ideal for this purpose. Our laboratory has developed a plasma membrane-targeted liquid crystal nanoparticle (LCNP) formulation that can be loaded with dyes or drugs which can be slowly released from the particle over time. Here we highlight the utility of these nanopreparations for membrane delivery and imaging.
There is considerable research in the area of manipulating light below the diffraction limit, with potential applications ranging from information processing to light-harvesting. In such work, a common problem is a lack of efficiency associated with non-radiative losses, e.g., ohmic loss in plasmonic structures. From this point of view, one attractive method for sub-wavelength light manipulation is to use Förster resonance energy transfer (FRET) between chromophores. Although most current work does not show high efficiency, biology suggests that this approach could achieve very high efficiency. In order to achieve this goal, the geometry and spacing of the chromophores must be optimized. For this, DNA provides an easy means for the self-assembly of these complex structures. With well established ligation chemistries, it is possible to create facile hierarchical assemblies of quantum dots (QDs) and organic dyes using DNA as the platform. These nanostructures range from simple linear wires to complex 3-dimensional structures all of which can be self-assembled around a central QD. The efficiency of the system can then be tuned by changing the spacing between chromophores, changing the DNA geometry such that the donor to acceptor ratio changes, or changing the number of DNA structures that are self-assembled around the central QD. By exploring these variables we have developed a flexible optical system for which the efficiency can be both controlled and optimized.
The demonstration of fine control over nanomaterials within biological systems, particularly in live cells, is integral for
the successful implementation of nanoparticles (NPs) in biomedical applications. Here, we show the ability to
differentially label the endocytic pathway of mammalian cells in a spatiotemporal manner utilizing fluorescent
nanocolloids (NCs) doped with a perylene-based dye. EDC-based conjugation of green- and red-emitting NCs to the
iron transport protein transferrin resulted in stable bioconjugates that were efficiently endocytosed by HEK 293T/17
cells. The staggered delivery of the bioconjugates allowed for the time-resolved, differential labeling of distinct
vesicular compartments along the endocytic pathway in a nontoxic manner. We further demonstrated the ability of the
NCs to be impregnated with the anticancer therapeutic, doxorubicin. Delivery of the drug-doped nanoconjugates
resulted in the intracellular release and nuclear accumulation of doxorubicin in a time- and dose-dependent manner. We
discuss our results in the context of the utility of such materials for NP-mediated drug delivery applications.
In this paper, we present electro-chromatic switching of a cholesteric gel, which exhibits a shift in the reflection band with the application of an electric field, but preserves the intensity of the reflected light. The reflecting color of the cholesteric gel was controlled by the strength of an electric field. When optimized to reflect visible light the reflecting color of the gel reversibly shifts to shorter wavelengths with increasing voltage. The reflection intensity remains the same over a range of voltage and begins to decrease with further application of the electric field, eventually leading to homeotropic alignment. We have systematically investigated the mechanism of the switching by studying the optical spectra and measuring the dielectric constant of a cholesteric gel as a function of voltage. Using a simple model that includes tilting and untwisting of a helical structure, we show that uniform electro-chromatic switching is mostly initiated by a helix tilting mechanism, whereas decrease in the intensity of reflection is mainly caused by helical untwisting.