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Cellulose nanocrystals (CNCs) derived from native cellulose can self-assemble into liquid crystals (LCs) and preserve the LC alignment in solid films that are attractive for the preparation of optical materials and devices from bottom-up manufacturing. Birefringent aligned CNC films provide the desired phase retardation for a narrow band of wavelengths due to the intrinsic wavelength-dependent birefringence of CNCs. Here, we produce a 1/4 λ achromatic CNC-based waveplate consisting of three layers of birefringent CNC films with phase retardations and slow axis directions, which are calculated by Jones Matrix, with optimized achromatic properties. Three uniform CNC films are prepared by aligning nematic CNC LCs doped with polyethylene glycol on patterned polydimethylsiloxane substrates. The fabricated achromatic waveplate is characterized by measuring the transmission spectra, and its maximum deviation of phase retardation is around 0.06 for the wavelength range of 460–660 nm. The achromatic performance is improved by one order of magnitude compared with the single birefringent CNC films. Our CNC-based achromatic waveplate has good optical homogeneity, flexibility and can be tailored into arbitrary shape.

Collective optoelectronic phenomena such as plasmons and phonon polaritons can drive processes in many branches of nanoscale science. Classical physics predicts that a perfect thermal emitter operates at the black body limit. Numerous experiments have shown that surface phonon polaritons allow emission two orders of magnitude above the limit at a gap distance of ∼ 50 nm. This work shows that a supported multilayer graphene structure improves the state of the art by around one order of magnitude with a ∼ 1129 fold-enhancement at a gap distance of ∼ 55 nm. Coupled surface plasmon polaritons at mid- and far-infrared frequencies allow for near-unity photon tunneling across a broad swath of k-space enabling the improved result. Electric tuning of the Fermi-level allows for the detailed characterization and optimization of the colossal nanoscale heat transfer. This article is protected by copyright. All rights reserved.

Understanding the behavior of fluids with inhomogeneous structural properties is critical for many industrial and biological materials. Sulfuric acid hydrolysis of cotton with ultrasonic treatment and low-speed centrifugation during purification yields “spiky” discotic cellulose particles. Amorphous cellulose remained in solution and forms discotic units which may be nucleated by crystalline rods. Scanning electron microscope and transmission electron microscope images confirm the discotic nature of the cellulose. High speed centrifugation could recover rod-like cellulose nanocrystals from the discotic cellulose nanoparticle solution. 3.6 wt% discotic cellulose nanoparticles formed the typical nematic phase and the liquid crystal texture became more structured with increasing concentration. Our results showed that spontaneous structuring of amorphous cellulose can lead to phase behavior that cannot be achieved using just rod-like cellulose nanocrystals. Mixtures containing amorphous and crystalline cellulose can form discotic cellulose nanoparticles. These solutions can form nematic phase (A), a structured columnar-like phase (B), and a glassy phase (C). Scanning electron microscope characterization (D, E) confirms a discotic structure with some spikes and network formation

Cellulose Nanocrystals are readily produced from many plant and bacterial sources and have been studied extensively for low cost self-assembled optical elements. Nanocrystals are known to form a chiral nematic phase, which allows for production of films with chiral character. I will discuss recent advances in understanding how to control the chirality of cellulose nanocrystal solutions. The Debye length of a nanocrystal solution is typically around 4nm for pH between 1.5 and 10 and abruptly drops to around 1 nm for pH lower or higher than this range. With a Debye Length of 1nm, the solution can only form nematic phase, which is more useful for production of waveplates and other birefringent optics. Additionally I will discuss the behavior of cellulose solutions with amorphous material remaining in solution which form discotic- like systems.

Graphite, graphene, and graphene oxide suspensions can form self-assembled fibers, films, and aerogels. Producing shape-controlled colloids can enhance the control over the properties of nanostructured carbon materials. Here a lithographic exfoliation method which allows for the production of many of shape-controlled graphite platelets from a single column and millions of particles from a single HOPG chip was reported. The polydisperse thickness distribution can be varied from a few microns to tens of nanometers depending on the exfoliation conditions. Preparation of worm-like graphite from lithographic substrates creates aerogel structures with well-defined dimensions and non-trivial 3-dimensional assembly behaviors. Tubular worms and nanolantern structures were produced using ring and banana shaped masks respectively.

Anisotropic plasmonic films are a desirable material for many optoelectronic applications. Here, we propose a method to align silver nanowires (AgNWs) with the help of uniaxial nematic alignment of cellulose nanocrystal (CNC) liquid crystals (LCs) that can preserve their LC orientation in solid film. AgNWs are doped into uniaxial nematic CNC LCs, where AgNWs are oriented parallel to the director of shear- aligned CNCs without aggregation. The AgNWs orientations are determined by polarized optical and dark field microscopic images. The alignment of AgNWs is characterized by the scalar order parameter S, and the measured S around 0.59 for aligned AgNWs presents an improved anisotropy with the assistance of uniaxial nematic CNC LCs. The electrical property of aligned AgNWs is examined by the four-probe method and exhibits the maximum ratio of anisotropic sheet resistance around 5, consistent with the simulated percolation probability performance using the Monte Carlo computation. Our results show that the uniaxial nematic CNC LCs can act as an effective template in aligning AgNWs, which is compatible with 3D printing and microfluidics, and allows for the preparation of low-cost, innovative optical materials and devices.

Coupling modes between surface plasmon polaritons (SPPs) and surface phonon polaritons (SPhPs) play a vital role in enhancing near-field thermal radiation but are relatively unexplored, and no experimental result is available. Here, we consider the NFTR enhancement between two identical graphene-covered SiO

Liquid crystal elasticity forms distortions and topological defects that allow for the controlled self-assembly of inclusions within liquid crystal media. Many groups have developed this idea for dispersion, alignment, and manipulation of a collection of individual particles to create plasmonic polarizers and others have studied colloidal crystallization processes within LC hosts. Extending this idea to consider more complex superstructures where a variety of forces can produce effects on larger lengthscales can expand the space of materials design and answer first principles questions about complex fluid dynamics. I will discuss results on colloidal inclusions at liquid-liquid and liquid- air interfaces where surface tension and capillary forces augment elasticity to provide the necessary tools for such hierarchical self-assembly.

Cellulose nanocrystals produced by acid hydrolysis of native cellulose form a well-known chiral nematic liquid crystal phase. The mechanism involved in the formation of chirality has been the subject of a vigorous discussion. The pH and concentration dependence of the phase is studied using cellulose nanocrystal droplets within a silicon oil suspension, which allows for convenient real-time microscale manipulation of phase behaviors and properties. We demonstrate the existence of nematic phases at both low and high pH regions consistent with the Stroobants - Lekkerkerker - Odijk theory. Our results confirm electrostatic interactions play a critical role in controlling the strength of the chirality.

We present a controlled cascade of self-assemblings of colloidal droplets at a nematic liquid crystal--air interface into large-scale ordered structures. Changing the tilt of the droplet-induced elastic dipoles via its dependence on the nematic film thickness, we are able to control the dipole-dipole interaction and thus the self-assembling regime. For a progressively large tilt, droplets form anisotropic lattices, which then transform into arrays of repulsive chains, then to bands of half-period-shifted densely bound chains. These structures with chain order at the inner scale aggregate into different large-scale clusters that have a pronounced circular pattern and are stabilized by the many-body elastocapillary attraction.

In this paper, we show that the guided mode resonance can exist in a low-index waveguide layer on top of a high-index substrate. With the help of the interaction of diffraction from a metal grating and total internal reflection effects, we verify that the guided mode can be supported in the low-index SU8 layer on a high-index substrate. Simulation and experiment show the resonant wavelength can be simply manipulated by controlling the geometrical parameters of the metal grating and waveguide layer. This structure extends the possibilities of guided-mode resonance to a broader class of functional materials and may boost its use in applications such as field enhancement, sensing and display.

Hyperbolic metamaterials (HMs) supporting hyperbolic isofrequency curves (IFCs) provide unprecedented control on wave propagation and light‐matter interaction. However, in the microwave regime, ultrahigh wave vectors and customizable permittivity tensors cannot be supported simultaneously by current common HMs. Based on the waveguide principle, low‐loss waveguide‐type HMs (WHMs) are suggested as a new geometry format of microwave HMs, flexibly mixing positive and negative components in their effective permittivity tensors. Then, a deeply subwavelength WHM cavity, confirming the existence of giant wave vectors, is demonstrated, whose transverse size has a potential to be reduced to three orders smaller than the working free‐space wavelength. Such a WHM cavity is also demonstrated to follow the fascinating anomalous scaling law originating from the unique shape of hyperbolic IFCs. As a low‐loss platform, WHMs are predicted to produce distinctive physical phenomena based on HMs and provide novel functional devices at low frequencies.

Liquid crystal phase boundaries play an important role in self-assembly processes defining nanostructured complex fluid systems. Surface boundary conditions, director- dependent surface tension and topological defects produce elastic forces that combine to define the architecture of multiphasic anisotropic fluids. Developing a detailed understanding of these forces is necessary for a first principals understanding of anisotropic biological structures and producing the next generation of mesoscopic materials. We will discuss our recent experimental progress on thermotropic LC and colloidal LC consisting of cellulose nanocrystals and graphite.

Small molecule chiral nematic LCs contain a variety of defect lines which can cause aggregation of polymers, nanoparticles and small molecule materials such as fluorescent dyes. While normal dyes suffer aggregation induced quenching a class of propeller-like dyes display aggregation induced emission (AIE), which makes them an ideal material for cholesteric lasers. Nanoparticle dispersion is necessary for switchability, while aggregation can be used as a tool to produce assembled structures. Here we discuss strategies to promote or inhibit aggregation of various materials in chiral nematics.

Nematic-like and helicoidally orientational self-assemblies of gold nanorods co-dispersed with cellulose nanocrystals to form liquid crystalline phases are developed. Polarization-sensitive extinction spectra and two-photon luminescence imaging are used to characterize orientations and spatial distributions of gold nanorods. Cholesteric-isotropic phase coexistence and continuous domains of single-phase regions are observed and qualitatively discussed on the basis of entropic and electrostatic interactions in co-dispersions of rigid rods of different aspect ratios. Potential applications include biologically compatible plasmonic composite nanomaterials for solar biofuel production and polarization-sensitive plasmonic papers and fabrics.

Using liquid crystalline self-assembly of cellulose nanocrystals, we achieve long-range alignment of anisotropic metal nanoparticles in colloidal nanocrystal dispersions that are then used to deposit thin structured films with ordering features highly dependent on the deposition method. These hybrid films are comprised of gold nanorods unidirectionally aligned in a matrix that can be made of ordered cellulose nanocrystals or silica nanostructures obtained by using cellulose-based nanostructures as a replica. The ensuing long-range alignment of gold nanorods in both cellulose-based and nanoporous silica films results in a polarization-sensitive surface plasmon resonance. The demonstrated device-scale bulk nanoparticle alignment may enable engineering of new material properties arising from combining the orientational ordering of host nanostructures and properties of the anisotropic plasmonic metal nanoparticles. Our approach may also allow for scalable fabrication of plasmonic polarizers and nanoporous silica structures with orientationally ordered anisotropic plasmonic nanoinclusions.

Optical waveplates play an essential role in controlling polarization in various applications. We show the feasibility of facile production of optical waveplates from plant-based cellulose nanocrystal (CNC) nematics. We align CNC nematic liquid crystals doped with polyethylene glycol on periodically patterned polydimethylsiloxane substrates on large scale by shearing forces and prepare a 1/4 λ and a 1/2 λ CNC waveplates for 530 nm by bonding two slices of CNC nematic films with half-period displacement. The optical performance of the CNC waveplates is investigated by Mueller matrix analysis and is examined by measuring the transmission spectra of the waveplate between two polarizers, and optical extinction and birefringence of CNCs are extracted from the measured spectra. Our CNC waveplates of arbitrary phase difference are flexible and can be tailored into any desirable shape conveniently.

We demonstrate an improved approach to integrate various color filters on a chip-scale by using stepwise metal-insulator-metal FP cavities. The cavity is composed of a thick silver mirror, an SU8 gap layer of controlled thickness, and a thin nickel layer. Reflective colors from red to blue can be generated from these filters through a simple UV lithography process. The filters were also fabricated on a flexible substrate which could be incorporated into wearable devices. This method can realize large-scale filter arrays with simple processing and may facilitate the use of structural color filters in displays and sensing.

We demonstrate facile optical manipulation of shape of birefringent colloidal microparticles made from liquid crystal elastomers. Using soft lithography and polymerization, we fabricate elastomeric microcylinders with weakly undulating director oriented on average along their long axes. These particles are infiltrated with gold nanospheres acting as heat transducers that allow for an efficient localized transfer of heat from a focused infrared laser beam to a submicrometer region within a microparticle. Photothermal control of ordering in the liquid crystal elastomer using scanned beams allows for a robust control of colloidal particles, enabling both reversible and irreversible changes of shape. Possible applications include optomechanics, microfluidics, and reconfigurable colloidal composites with shape-dependent self-assembly.

Active elastomeric liquid crystal particles with initial cylindrical shapes are obtained by means of soft lithography and polymerization in a strong magnetic field. Gold nanocrystals infiltrated into these particles mediate energy transfer from laser light to heat, so that the inherent coupling between the temperature-dependent order and shape allows for dynamic morphing of these particles and well-controlled stable shapes. Continuous changes of particle shapes are followed by their spontaneous realignment and transformations of director structures in the surrounding cholesteric host, as well as locomotion in the case of a nonreciprocal shape morphing. These findings bridge the fields of liquid crystal solids and active colloids, may enable shape-controlled self-assembly of adaptive composites and light-driven micromachines, and can be understood by employing simple symmetry considerations along with electrostatic analogies., Comment: Main text (12 pages and 4 figures) and Supplemental Material

We report a simple and versatile technique for oriented assembly of gold nanorods on aligned single-walled carbon nanotube (SWNT) macrostructures, such as thin nanotube films and nanotube fibers. The deposition and assembly is accomplished via drop drying of dilute gold nanorod suspensions on SWNT macrostructures under ambient conditions. Guided by anisotropic interactions, gold nanorods, and polygonal platelets spontaneously align with SWNTs, resulting in macroscopic arrays of locally ordered nanorods supported on aligned SWNT substrates. SEM reveals that the scalar order parameter of rods relative to the local average SWNT alignment is 0.7 for rods on SWNT films and 0.9 for rods on SWNT fibers. This self-alignment is enabled by anisotropic gold nanoparticle-SWNT interactions and is observed for a wide range of nanoparticles, including nanorods with aspect ratios ranging from 2-35, thin gold triangular and other polygonal platelets. The plasmonic properties of aligned gold nanorods together with superior electronic, chemical and mechanical properties of SWNTs make these hybrid nanocomposites valuable for the design of self-assembled multifunctional optoelectronic materials and optical metamaterials.

We report alignment of anisotropic amphiphilic dye molecules within oblate and prolate anisotropic micelles and lamellae, the basic building blocks of surfactant-based lyotropic liquid crystals. Absorption and fluorescence transition dipole moments of these dye molecules orient either parallel or orthogonal to the liquid crystal director. This alignment enables three-dimensional visualization of director structures and defects in different lyotropic mesophases by means of fluorescence confocal polarizing microscopy and two-photon excitation fluorescence polarizing microscopy. The studied structures include nematic tactoids, Schlieren texture with disclinations in the calamitic nematic phase, oily streaks in the lamellar phase, developable domains in the columnar hexagonal phase, and various types of line defects in the discotic cholesteric phase. Orientational three-dimensional imaging of structures in the lyotropic cholesterics reveals large Burgers vector dislocations in cholesteric layering with singular disclinations in the dislocation cores that are not common for their thermotropic counterparts.

We demonstrate scaffolding of plasmonic nanoparticles by topological defects induced by colloidal microspheres to match their surface boundary conditions with a uniform far-field alignment in a liquid crystal host. Displacing energetically costly liquid crystal regions of reduced order, anisotropic nanoparticles with concave or convex shapes not only stably localize in defects but also self-orient with respect to the microsphere surface. Using laser tweezers, we manipulate the ensuing nanoparticle-microsphere colloidal dimers, probing the strength of elastic binding and demonstrating self-assembly of hierarchical colloidal superstructures such as chains and arrays.

A bendable ultra-black material consisting of graphite nanocones and nanowires is fabricated through a simple plasma etching process. The optical properties of the absorber are characterized in the wavelength range of 400-2000 nm with average specular reflectance 0.05 ± 0.03% at normal incidence and the material thickness is only around 5 μm. The reflectance of the absorber remains low at large incident angles and is relatively independent of polarization. Simulations confirm the cooperative effect of the nanowires and nanocones leading to an ultra-black thin carbon material.

Thermal microbubbles generally grow directly from the heater and are spherical to minimize surface tension. We demonstrate a novel type of microbubble indirectly generated from a graphene oxide-microheater. Graphene oxide's photothermal properties allowed for efficient generation of a thermal gradient field on the microscale. A series of approximately ellipsoidal microbubbles were generated on the smooth microwire based on heterogeneous nucleation. Other dynamic behaviors induced by the microheater such as constant growth, directional transport and coalescence were also investigated experimentally and theoretically. The results are not only helpful for understanding the bubble dynamics but also useful for developing novel photothermal bubble-based devices.

Defects in liquid crystals are used to model topological entities ranging from Skyrmions in high-energy physics to early-universe cosmic strings, as well as find practical applications in self-assembly of diffraction gratings and in scaffolding of plasmonic nanoparticles, but they are hard to control and organize into three-dimensional lattices. We laterally scan focused laser beams to produce periodic arrays of twist-stabilized defects forming either linear (fingers) or axially symmetric (torons) configurations in partially polymerizable liquid crystal films. Polymerization allows for stabilization of these structures and the formation of three-dimensional arrays of defects by stacking of the thin cholesteric films on top of each other. In the process of fabrication of such arrays, we polymerize the liquid crystal film with an array of torons or fingers and then sequentially produce and photopolymerize new liquid crystal layers on top of it, thus obtaining a three-dimensional structure of twist-stabilized defects in a layer-by-layer fashion. Templating by the polymerized layer spontaneously yields ordered organization of fingers and torons in the new cholesteric layer, thus enabling a three-dimensional ordered structure of defects. Nondestructive three-dimensional imaging of director fields by use of three-photon excitation fluorescence polarizing microscopy reveals the nature of topological singularities and physical underpinnings behind the observed templating effect. Three-dimensional patterning of defects templates the self-assembly of plasmonic nanoparticles into individual singularities and their arrays, laying the groundwork for potential applications in nanophotonics, plasmonics, metamaterial fabrication, and nanoscale energy conversion.

Manipulating large numbers of a variety of particles/wires is essential for many lab-on-a-chip technologies. Here we generate a planar array of optofluidic vortices with photothermal gradients from an easy-fabricated graphene oxide (GO) heater to achieve high-throughput and multiform manipulation at low excitation power and low loss. As a tweezer, each vortex can rapidly capture and confine particles without restrictions on shapes and materials. The stiffness of the confinement is easily tuned by adjusting the vortex dimension. As a motor, it can actuate any traps to persistently rotate/spin in clockwise or anti-clockwise mode. As a high-performance ‘workshop’, this work lays the groundwork for various self-assembly ranging from colloid-based clusters, chains, capsules, shells and ultra-thin films, through particles’ surface modification and fusion, to nanowire-based architectures. Furthermore, we can create multiple vortex arrays through fabricating an array of heaters, which enables massively parallel manipulation and distributed operations all on a chip. A lab-on-a-chip device can capture, contain and self-assemble large numbers of microparticles with innovative liquid vortex technology. Sailing He from Sweden's Royal Institute of Technology and co-workers in China developed a platform that uses graphene oxide ‘heaters’, activated by lasers, to alter the surface tension of nearby liquid droplets. This effect, known as thermocapillary convection, produces whirlpool-like spinning forces with a trapping power determined by droplet geometry. To demonstrate the potential of this approach, the team fabricated a microfluidic device featuring parallel arrays of vortices. Examples include precise control over materials, such as silver nanowires, that are difficult to handle with optical tweezers; a spinning quad-core motor providing persistent torque; and a ‘workshop’ for self-assembly that combines tweezer and motor functions to organize polystyrene particles into linear-, block- and triangle-shaped complexes. Planar arrays of optofluidic vortices are generated with photothermal gradients from an array of graphene oxide heaters to achieve multiform manipulations. As a tweezer, each vortex can rapidly capture and confine particles without any restriction on shapes or materials. As a motor, it can actuate any trapped particle to persistently rotate/spin in clockwise or anti-clockwise mode. Such a high-performance ‘workshop’ can be used for various self-assembly ranging from colloid-based clusters, chains, capsules, shells, and ultra-thin films, through particles’ surface modification and fusion, to nanowires-based architectures.

We demonstrate band edge lasing action from a cholesteric liquid crystal (CLC) containing an aggregation-induced-emission (AIE) dye as gain material. AIE materials do not suffer aggregation-caused quenching, have strong resistance to photobleaching, and can show large Stokes shift. The amplified spontaneous emission (ASE) and lasing emission of the dye-doped CLC cell have been characterized, the lasing threshold has been estimated, and its resistance to photobleaching has been measured. AIE materials with their unique properties are especially suitable for acting as gain materials in liquid crystal lasers where defect structures lower the threshold for nanoscale aggregation effects. Our studies have shown that such AIE-dye-doped CLC is capable of lasing action with unusually large Stokes shift at moderate threshold with strong resistance to photobleaching.

We experimentally demonstrate a broadband vis-NIR absorber consisting of 300-400 nm nanocone structures on highly oriented pyrolytic graphite. The nanocone structures are fabricated through simple nanoparticle lithography process and analyzed with three-dimensional finite-difference time-domain methods. The measured absorption reaches an average level of above 95% over almost the entire solar spectrum and agrees well with the simulation. Our simple process offers a promising material for solar-thermal devices.

We demonstrate the dispersion and alignment of gold nanorods in 5CB liquid crystal droplets in silicon oil with a radial director structure. This biphasic system shows that the nanorods are stabilized against phase boundary driven aggregation. The radial alignment of the droplets can be further controlled by electric field. This method is potentially useful for the fabrication of hybrid nanostructured materials, such as plasmonic polymer dispersed liquid crystals and self-assembly-based devices; it could also be extended for use with magnetic, semi-conducting, or up-conversion nanoparticles.

Graphene recently emerged as a new two-dimensional material platform with unique optical, thermal and electronic properties. Single- or few-atom-thick graphene flakes can potentially be utilized to form structured bulk composites that further enrich these properties and enable a broad range of new applications. Here we describe optical manipulation of self-aligned colloidal graphene flakes in thermotropic liquid crystals of nematic and cholesteric types. Three-dimensional rotational and translational manipulation of graphene flakes by means of holographic optical tweezers allows for non-contact spatial patterning of graphene, control of liquid crystal defects, and low-power optical realignment of the liquid crystal director using these flakes. Potential applications include optically- and electrically-controlled reconfigurable liquid crystalline dispersions of spontaneously aligning colloidal graphene flakes and new electro-optic devices with graphene-based interconnected transparent electrodes at surfaces and in the bulk of liquid crystals.

We study elasticity-mediated alignment of anisotropic goldcolloids in liquid crystals.Colloidalgold particles of controlled shapes (spheres, rods, and polygonal platelets) and sizes are prepared using well-established biosynthesis techniques with varying solvent conditions. When introduced into liquid crystalline structured solvents, these gold particles impose tangential or vertical surface boundary conditions for the liquid crystal molecules or building blocks such as chromonic molecular aggregates. This allows for multiple types of their controlled alignment in both lyotropic and thermotropic liquid crystals and is of interest for self-assembly-based fabrication of tunable nanostructured composite materials.

We explore the nanoscale colloidal self-assembly and self-alignment in liquid crystals. We use model particles with controlled shapes and sizes, including quantum dots and rods and metal nanoparticles in the form of spheres, rods, and polygonal platelets. To study these composites on the scales ranging from nanometers to millimeters and to motivate their use in metamaterial fabrication, we utilize optical microscopies, freeze-fracture transmission electron microscopy, and cryogenic transmission electron microscopy. We discuss the long-range alignment and assembly of anisotropic nanoparticles imposed by the orientational elasticity of liquid crystals, showing that these composites provide a powerful platform for self-assembly of metamaterials.

We report the observation of liquid crystals formed by giant graphene oxide flakes (aspect ratio above 10000) suspended in water. As their concentration increases, the flakes undergo transitions from an isotropic dispersion to a biphasic system and then to a discotic nematic liquid crystal. The gel-like liquid crystal displays an unusual defect-free uniform director alignment over hundreds of micrometres. We characterize the nematic order parameter, optical birefringence and elastic properties of this novel mesomorphic system.

Five additively colored alkali halides have been examined for photoelectric Hall effect by means of two different arrangements, a single cross-electrode set-up and Tartakowsky's divided electrode method. The consistently null results obtained were checked by using zincblende, a substance of known photoelectric galvanomagnetic properties, in both arrangements. From the upper limit assigned to the Hall effect, it was computed that the mean free path of photoelectrons in rocksalt is not much greater than the dimensions of a lattice cell---in agreement with Von Hippel's result for the electronic mean free path on the basis of electrical breakdown experiments. This interpretation of the null result was supported by the fact that the large initial surge of electron current in KCl and KBr at the outset of illumination also showed no Hall effect. The magnetic deflection of photoelectric current reported in rocksalt by Tartakowsky was not found, either for additively or photochemically colored specimens.