Terahertz Microfluidic Sensing Using a Parallel-plate Waveguide Sensor

Refractive index (RI) sensing is a powerful noninvasive and label-free sensing technique for the identification, detection and monitoring of microfluidic samples with a wide range of possible sensor designs such as interferometers and resonators ^1,2. Most of the existing RI sensing applications focus on biological materials in aqueous solutions in visible and IR frequencies, such as DNA hybridization and genome sequencing. At terahertz frequencies, applications include quality control, monitoring of industrial processes and sensing and detection applications involving nonpolar materials.Several potential designs for refractive index sensors in the terahertz regime exist, including photonic crystal waveguides ³, asymmetric split-ring resonators ⁴, and photonic band gap structures integrated into parallel-plate waveguides ⁵. Many of these designs are based on optical resonators such as rings or cavities. The resonant frequencies of these structures are dependent on the refractive index of the material in or around the resonator. By monitoring the shifts in resonant frequency the refractive index of a sample can be accurately measured and this in turn can be used to identify a material, monitor contamination or dilution, etc.The sensor design we use here is based on a simple parallel-plate waveguide ^6,7. A rectangular groove machined into one face acts as a resonant cavity (Figures 1 and 2). When terahertz radiation is coupled into the waveguide and propagates in the lowest-order transverse-electric (TE₁) mode, the result is a single strong resonant feature with a tunable resonant frequency that is dependent on the geometry of the groove ^6,8. This groove can be filled with nonpolar liquid microfluidic samples which cause a shift in the observed resonant frequency that depends on the amount of liquid in the groove and its refractive index ⁹.Our technique has an advantage over other terahertz techniques in its simplicity, both in fabrication and implementation, since the procedure can be accomplished with standard laboratory equipment without the need for a clean room or any special fabrication or experimental techniques. It can also be easily expanded to multichannel operation by the incorporation of multiple grooves ¹⁰. In this video we will describe our complete experimental procedure, from the design of the sensor to the data analysis and determination of the sample refractive index.     

Multiplexed DNA quantification by spectroscopic shift of two microsphere cavities

We have developed a novel, spectroscopic technique for high-sensitivity, label-free DNA quantification. We demonstrate that an optical resonance (whispering gallery mode) excited in a micron-sized silica sphere can be used to detect and measure nucleic acids. The surface of the silica sphere is chemically modified with oligonucleotides. We show that hybridization to the target DNA leads to a red shift of the optical resonance wavelength. The sensitivity of this resonant technique is measured as 6 pg/mm(2) mass loading, higher as compared to most optical single-pass devices such as surface plasmon resonance biosensors. Furthermore, we show that each microsphere can be identified by its unique resonance wavelength. Specific, multiplexed DNA detection is demonstrated by using two microspheres. The multiplexed signal from two microspheres allows us to discriminate a single nucleotide mismatch in an 11-mer oligonucleotide with a high signal-to-noise ratio of 54. This all-photonic whispering gallery mode biosensor can be integrated on a semiconductor chip that makes it an easy to manufacture, analytic component for a portable, robust lab-on-a-chip device.     

Light Absorption and Recycling in Hybrid Metal Halide Perovskite Photovoltaic Devices

The production of highly efficient single‐ and multijunction metal halide perovskite (MHP) solar cells requires careful optimization of the optical and electrical properties of these devices. Here, precise control of CH₃NH₃PbI₃ perovskite layers is demonstrated in solar cell devices through the use of dual source coevaporation. Light absorption and device performance are tracked for incorporated MHP films ranging from ≈67 nm to ≈1.4 µm thickness and transfer‐matrix optical modeling is utilized to quantify optical losses that arise from interference effects. Based on these results, a device with 19.2% steady‐state power conversion efficiency is achieved through incorporation of a perovskite film with near‐optimum predicted thickness (≈709 nm). Significantly, a clear signature of photon reabsorption is observed in perovskite films that have the same thickness (≈709 nm) as in the optimized device. Despite the positive effect of photon recycling associated with photon reabsorption, devices with thicker (>750 nm) MHP layers exhibit poor performance owing to competing nonradiative charge recombination in a “dead‐volume” of MHP. Overall, these findings demonstrate the need for fine control over MHP thickness to achieve the highest efficiency cells, and accurate consideration of photon reabsorption, optical interference, and charge transport properties.     

Development of Whispering Gallery Mode Polymeric Micro-optical Electric Field Sensors

Optical modes of dielectric micro-cavities have received significant attention in recent years for their potential in a broad range of applications. The optical modes are frequently referred to as "whispering gallery modes" (WGM) or "morphology dependent resonances" (MDR) and exhibit high optical quality factors. Some proposed applications of micro-cavity optical resonators are in spectroscopy¹, micro-cavity laser technology², optical communications^3-6 as well as sensor technology. The WGM-based sensor applications include those in biology⁷, trace gas detection⁸, and impurity detection in liquids⁹. Mechanical sensors based on microsphere resonators have also been proposed, including those for force^10,11, pressure¹², acceleration¹³ and wall shear stress¹⁴. In the present, we demonstrate a WGM-based electric field sensor, which builds on our previous studies^15,16. A candidate application of this sensor is in the detection of neuronal action potential.The electric field sensor is based on polymeric multi-layered dielectric microspheres. The external electric field induces surface and body forces on the spheres (electrostriction effect) leading to elastic deformation. This change in the morphology of the spheres, leads to shifts in the WGM. The electric field-induced WGM shifts are interrogated by exciting the optical modes of the spheres by laser light. Light from a distributed feedback (DFB) laser (nominal wavelength of ~ 1.3 μm) is side-coupled into the microspheres using a tapered section of a single mode optical fiber. The base material of the spheres is polydimethylsiloxane (PDMS). Three microsphere geometries are used: (1) PDMS sphere with a 60:1 volumetric ratio of base-to-curing agent mixture, (2) multi layer sphere with 60:1 PDMS core, in order to increase the dielectric constant of the sphere, a middle layer of 60:1 PDMS that is mixed with varying amounts (2% to 10% by volume) of barium titanate and an outer layer of 60:1 PDMS and (3) solid silica sphere coated with a thin layer of uncured PDMS base. In each type of sensor, laser light from the tapered fiber is coupled into the outermost layer that provides high optical quality factor WGM (Q ~ 10⁶). The microspheres are poled for several hours at electric fields of ~ 1 MV/m to increase their sensitivity to electric field.     

All-Optical Tunable Wavelength-Division Multiplexing Based on Colloidal Crystal Coated Silver Film

An all-optical tunable nanoscale wavelength-division multiplexing device is realized theoretically based on a plasmonic microstructure, which is composed of a silver film coated with a monolayer colloidal crystal made of cholesteryl iodide-doped polystyrene. The physical mechanism is attributed to the variation of surface plasmon polariton modes and guided modes caused by pump-laser-induced refractive index change of cholesteryl iodide. An up to 90-nm shift in the resonant wavelength of optical channels can be reached under excitation of a 500?mJ/cm² pump laser. The number of optical channels can be tuned by adjusting the structure parameters of the monolayer colloidal crystal. This may open a new way for the study of integrated photonic devices.     

Refractometric Sensing with Silicon Quantum Dots Coupled to a Microsphere

Quantum dots (QDs) coupled to an optical microsphere can be used as fluorescent refractometric sensors. The QD emission couples to the whispering gallery resonances of the microsphere, leading to sharp, periodic maxima in the fluorescence spectrum. Silicon QDs (Si-QDs) are especially attractive fluorophores because of their low toxicity and ease of handling. In this work, a thin layer of Si-QDs was coated onto the surface of a microsphere made by melting the end of a tapered optical fiber. Refractometric sensing experiments were conducted using two methods. First, the sphere was immersed directly into a cuvette containing methanol–water mixtures. Second, the sphere was inserted into a silica capillary and the solutions were pumped through the capillary channel. The latter method enables microfluidic operation, which is otherwise difficult to achieve with a microsphere. In both geometries, high-visibility (V?=?0.83) modes were observed with Q factors up to 1,700. Using standard signal processing methods applied to the whispering gallery mode (WGM) spectrum, sensorgram-type measurements were conducted using single Si-QD-coated microspheres. The WGM resonances shifted as a function of the refractive index of the analyte solution, giving sensitivities ranging from ~30 to 100 nm/refractive index unit (RIU) for different microspheres and a detection limit on the order of 10^?4 RIU.     

Attaching biological probes to silica optical biosensors using silane coupling agents

In order to interface with biological environments, biosensor platforms, such as the popular Biacore system (based on the Surface Plasmon Resonance (SPR) technique), make use of various surface modification techniques, that can, for example, prevent surface fouling, tune the hydrophobicity/hydrophilicity of the surface, adapt to a variety of electronic environments, and most frequently, induce specificity towards a target of interest. These techniques extend the functionality of otherwise highly sensitive biosensors to real-world applications in complex environments, such as blood, urine, and wastewater analysis. While commercial biosensing platforms, such as Biacore, have well-understood, standard techniques for performing such surface modifications, these techniques have not been translated in a standardized fashion to other label-free biosensing platforms, such as Whispering Gallery Mode (WGM) optical resonators. WGM optical resonators represent a promising technology for performing label-free detection of a wide variety of species at ultra-low concentrations. The high sensitivity of these platforms is a result of their unique geometric optics: WGM optical resonators confine circulating light at specific, integral resonance frequencies. Like the SPR platforms, the optical field is not totally confined to the sensor device, but evanesces; this "evanescent tail" can then interact with species in the surrounding environment. This interaction causes the effective refractive index of the optical field to change, resulting in a slight, but detectable, shift in the resonance frequency of the device. Because the optical field circulates, it can interact many times with the environment, resulting in an inherent amplification of the signal, and very high sensitivities to minor changes in the environment. To perform targeted detection in complex environments, these platforms must be paired with a probe molecule (usually one half of a binding pair, e.g. antibodies/antigens) through surface modification. Although WGM optical resonators can be fabricated in several geometries from a variety of material systems, the silica microsphere is the most common. These microspheres are generally fabricated on the end of an optical fiber, which provides a "stem" by which the microspheres can be handled during functionalization and detection experiments. Silica surface chemistries may be applied to attach probe molecules to their surfaces; however, traditional techniques generated for planar substrates are often not adequate for these three-dimensional structures, as any changes to the surface of the microspheres (dust, contamination, surface defects, and uneven coatings) can have severe, negative consequences on their detection capabilities. Here, we demonstrate a facile approach for the surface functionalization of silica microsphere WGM optical resonators using silane coupling agents to bridge the inorganic surface and the biological environment, by attaching biotin to the silica surface. Although we use silica microsphere WGM resonators as the sensor system in this report, the protocols are general and can be used to functionalize the surface of any silica device with biotin.     

Fabrication and Testing of Microfluidic Optomechanical Oscillators

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including microresonators. However, because of the increased acoustic radiative losses during direct liquid immersion of optomechanical devices, almost all published optomechanical experiments have been performed in solid phase. This paper discusses a recently introduced hollow microfluidic optomechanical resonator. Detailed methodology is provided to fabricate these ultra-high-Q microfluidic resonators, perform optomechanical testing, and measure radiation pressure-driven breathing mode and SBS-driven whispering gallery mode parametric vibrations. By confining liquids inside the capillary resonator, high mechanical- and optical- quality factors are simultaneously maintained.     

Internet of things-enabled photomultiplier tube- and smartphone-based electrochemiluminescence platform to detect choline and dopamine using 3D-printed closed bipolar electrodes

There is a growing demand to realize low-cost miniaturized point-of-care testing diagnostic devices capable of performing many analytical assays. To fabricate such devices, three-dimensional printing (3DP)-based fabrication techniques provide a turnkey approach with marked precision and accuracy. Here, a 3DP fabrication technique was successfully utilized to fabricate closed bipolar electrode-based electrochemiluminescence (ECL) devices using conductive graphene filament. Furthermore, using these ECL devices, Ru(bpy)₃²⁺/TPrA- and luminol/H₂O₂-based electrochemistry was leveraged to sense dopamine and choline respectively. For ECL signal capture, two distinct approaches were used, first a smartphone-based miniaturized platform and the second with a photomultiplier tube embedded with the internet of things technology. Choline sensing led to a linear range 5–700 μM and 30–700 μM with a limit of detection (LOD) of 1.25 μM (R² = 0.98, N = 3) and 3.27 μM (R² = 0.97, N = 3). Furthermore, dopamine sensing was achieved in a linear range 0.5–100 μM with an LOD = 2 μM (R² = 0.99, N = 3) and LOD = 0.33 μM (R² = 0.98, N = 3). Overall, the fabricated devices have the potential to be utilized effectively in real-time applications such as point-of-care testing.     

Flexible Pseudocapacitive Electrochromics via Inkjet Printing of Additive‐Free Tungsten Oxide Nanocrystal Ink

Direct inkjet printing of functional inks is an emerging and promising technique for the fabrication of electrochemical energy storage devices. Electrochromic energy devices combine electrochromic and energy storage functions, providing a rising and burgeoning technology for next‐generation intelligent power sources. However, printing such devices has, in the past, required additives or other second phase materials in order to create inks with suitable rheological properties, which can lower printed device performance. Here, tungsten oxide nanocrystal inks are formulated without any additives for the printing of high‐quality tungsten oxide thin films. This allows the assembly of novel electrochromic pseudocapacitive zinc‐ion devices, which exhibit a relatively high capacity (≈260 C g^?1 at 1 A g^?1) with good cycling stability, a high coloration efficiency, and fast switching response. These results validate the promising features of inkjet‐printed electrochromic zinc‐ion energy storage devices in a wide range of applications in flexible electronic devices, energy‐saving buildings, and intelligent systems.     

Degenerate Coupled Mode Division and Superposition Under Symmetry Breaking

Resonances in micrometer metal cavity structures are very important for the interactions between materials and light. Three similar cavities connected with waveguides are investigated through the finite difference time domain (FDTD) method and the coupled mode theory. Two fundamental surface resonance modes are demonstrated in the two simple cavities separately. Then two simple cavities are combined to form the third cavity. The fundamental resonant modes couple positively or oppositely to form coupled-mode resonances in the combined cavity. When the combined cavity structures are symmetric, the coupled-mode resonances lead to two transmission peaks. While the symmetry is broken with tens of nanometers displacements, the transmission peaks convert to dips. It is believed the Q value variation of coupled-mode resonances plays a key role in the conversion. When the structure is symmetric, the coupled-mode resonances in the upper and lower parts of the cavity have the same Q value and are degenerate. The superposition of them leads transmission peaks. While the symmetry is broken, the Q values of resonances in the upper and lower part of the cavity are different, leading to the degenerate coupled mode division. The superposition of the different Q-factor modes leads to the dips. The sensitive variation to the symmetry of structures can be used to control light-material interactions, optical switch, and improve the sensitivity of sensor devices.

     

Simple and Versatile 3D Printed Microfluidics Using Fused Filament Fabrication

The uptake of microfluidics by the wider scientific community has been limited by the fabrication barrier created by the skills and equipment required for the production of traditional microfluidic devices. Here we present simple 3D printed microfluidic devices using an inexpensive and readily accessible printer with commercially available printer materials. We demonstrate that previously reported limitations of transparency and fidelity have been overcome, whilst devices capable of operating at pressures in excess of 2000 kPa illustrate that leakage issues have also been resolved. The utility of the 3D printed microfluidic devices is illustrated by encapsulating dental pulp stem cells within alginate droplets; cell viability assays show the vast majority of cells remain live, and device transparency is sufficient for single cell imaging. The accessibility of these devices is further enhanced through fabrication of integrated ports and by the introduction of a Lego^®-like modular system facilitating rapid prototyping whilst offering the potential for novices to build microfluidic systems from a database of microfluidic components.     

A Tunable Slow Light Device with Multiple Channels Based on Plasmon-Induced Transparency

Slow light devices with buffering capability play a critical role in all-optical signal processing. In this paper, multiple slow light phenomena are implemented based on plasmon-induced transparency (PIT) in our device. The device mainly consists of dual tooth cavities coupled with stub resonators, respectively. Temporal coupled-mode theory model illustrates that the triple PIT phenomena can be achieved based on different formation mechanisms. The simulation results calculated by the finite-difference time-domain method reveal that significant slow light response occurs at two wavelength regions. In addition, the parameters of structure have an important influence on PIT response and slow light characteristics. Moreover, the separate manipulation of wavelength, transmission and group index at transparency peak can be achieved in different slow light channels by adjusting the structural parameters. This plasmonic device is of great significance for the design of optical networks on chips.

     

Simple Microfluidic Devices for in vivo Imaging of C. elegans,Drosophila and Zebrafish

Micro fabricated fluidic devices provide an accessible micro-environment for in vivo studies on small organisms. Simple fabrication processes are available for microfluidic devices using soft lithography techniques ^1-3. Microfluidic devices have been used for sub-cellular imaging ^4,5, in vivo laser microsurgery ^2,6 and cellular imaging ^4,7. In vivo imaging requires immobilization of organisms. This has been achieved using suction ^5,8, tapered channels ^6,7,9, deformable membranes ^2-4,10, suction with additional cooling ⁵, anesthetic gas ¹¹, temperature sensitive gels ¹², cyanoacrylate glue ¹³ and anesthetics such as levamisole ^14,15. Commonly used anesthetics influence synaptic transmission ^16,17 and are known to have detrimental effects on sub-cellular neuronal transport ⁴. In this study we demonstrate a membrane based poly-dimethyl-siloxane (PDMS) device that allows anesthetic free immobilization of intact genetic model organisms such as Caenorhabditis elegans (C. elegans), Drosophila larvae and zebrafish larvae. These model organisms are suitable for in vivo studies in microfluidic devices because of their small diameters and optically transparent or translucent bodies. Body diameters range from ~10 μm to ~800 μm for early larval stages of C. elegans and zebrafish larvae and require microfluidic devices of different sizes to achieve complete immobilization for high resolution time-lapse imaging. These organisms are immobilized using pressure applied by compressed nitrogen gas through a liquid column and imaged using an inverted microscope. Animals released from the trap return to normal locomotion within 10 min.We demonstrate four applications of time-lapse imaging in C. elegans namely, imaging mitochondrial transport in neurons, pre-synaptic vesicle transport in a transport-defective mutant, glutamate receptor transport and Q neuroblast cell division. Data obtained from such movies show that microfluidic immobilization is a useful and accurate means of acquiring in vivo data of cellular and sub-cellular events when compared to anesthetized animals (Figure 1J and 3C-F⁴).Device dimensions were altered to allow time-lapse imaging of different stages of C. elegans, first instar Drosophila larvae and zebrafish larvae. Transport of vesicles marked with synaptotagmin tagged with GFP (syt.eGFP) in sensory neurons shows directed motion of synaptic vesicle markers expressed in cholinergic sensory neurons in intact first instar Drosophila larvae. A similar device has been used to carry out time-lapse imaging of heartbeat in ~30 hr post fertilization (hpf) zebrafish larvae. These data show that the simple devices we have developed can be applied to a variety of model systems to study several cell biological and developmental phenomena in vivo.     

Fano Resonance Excited All-Optical XOR,XNOR, and NOT Gates with High Contrast Ratio

We have presented all-optical XOR, XNOR, and NOT gates using metal-insulator-metal (MIM)-coupled ring resonator. The performance of the device is evaluated by finite difference in time-domain (FDTD) method. The proposed gate utilizes a unique phenomenon of Fano resonance to excite logic OFF/ON state. Fano resonance has quite asymmetric resonance profile and the transmission spectrum of Fano profile abruptly drops to a minimum value at the resonance condition. Due to this unique resonance phenomenon, a large value of contrast ratio is obtained. The proposed XNOR gate offers a contrast ratio (C.R.) of 20.66 dB while XOR and NOT gates offer C.R. 12.8 and 18.8 dB respectively. The variation of contrast ratio is also studied against different input wavelength and it is reported that the obtained value of contrast ratio is an optimum value for the proposed structure. The device is compact sized with small dimension 0.31 λ₀², where λ₀?=?1.55 μm. The proposed device opens up the avenues for designing on-chip optical gates in the field of high-speed optical communication networks.     

Biomimetic MXene Textures with Enhanced Light‐to‐Heat Conversion for Solar Steam Generation and Wearable Thermal Management

2D materials are of particular interest in light‐to‐heat conversion, yet challenges remain in developing a facile method to suppress their light reflection. Herein, inspired by the black scales of Bitis rhinoceros, a generalized approach via sequential thermal actuations to construct biomimetic 2D‐material nanocoatings, including Ti₃C₂T_x MXene, reduced graphene oxide (rGO), and molybdenum disulfide (MoS₂) is designed. The hierarchical MXene nanocoatings result in broadband light absorption (up to 93.2%), theoretically validated by optical modeling and simulations, and realize improved light‐to‐heat performance (equilibrium temperature of 65.4 °C under one‐sun illumination). With efficient light‐to‐heat conversion, the bioinspired MXene nanocoatings are next incorporated into solar steam‐generation devices and stretchable solar/electric dual‐heaters. The MXene steam‐generation devices require much lower solar‐thermal material loading (0.32 mg cm^?2) and still guarantee high steam‐generation performance (1.33 kg m^?2 h^?1) compared with other state‐of‐the‐art devices. Additionally, the mechanically deformed MXene structures enable the fabrication of stretchable and wearable heaters dual‐powered by sunlight and electricity, which are reversibly stretched and heated above 100 °C. This simple fabrication process with effective utilization of active materials promises its practical application value for multiple solar–thermal technologies.     

Design and Fabrication of a Precious Metal‐Free Tandem Core–Shell p+n Si/W‐Doped BiVO4 Photoanode for Unassisted Water Splitting

Tandem photoelectrochemical water splitting cells utilizing crystalline Si and metal oxide photoabsorbers are promising for low‐cost solar hydrogen production. This study presents a device design and a scalable fabrication scheme for a tandem heterostructure photoanode: p⁺n black silicon (Si)/SnO₂ interface/W‐doped bismuth vanadate (BiVO₄)/cobalt phosphate (CoPi) catalyst. The black‐Si not only provides a substantial photovoltage of 550 mV, but it also serves as a conductive scaffold to decrease charge transport pathlengths within the W‐doped BiVO₄ shell. When coupled with cobalt phosphide (CoP) nanoparticles as hydrogen evolution catalysts, the device demonstrates spontaneous water splitting without employing any precious metals, achieving an average solar‐to‐hydrogen efficiency of 0.45% over the course of an hour at pH 7. This fabrication scheme offers the modularity to optimize individual cell components, e.g., Si nanowire dimensions and metal oxide film thickness, involving steps that are compatible with fabricating monolithic devices. This design is general in nature and can be readily adapted to novel, higher performance semiconducting materials beyond BiVO₄ as they become available, which will accelerate the process of device realization.     

Utilizing the Metallic Nano-Rods in Hexagonal Configuration to Enhance Sensitivity of the Plasmonic Racetrack Resonator in Sensing Application

A high sensitive plasmonic refractive index sensor based on metal-insulator-metal (MIM) waveguides with embedding metallic nano-rods in racetrack resonator has been proposed. The refractive index changes of the dielectric material inside the resonator together with temperature changes can be acquired from the detection of the resonance wavelength, based on their linear relationship. With optimum design and considering a tradeoff among detected power, structure size, and sensitivity, the finite difference time domain simulations show that the refractive index and temperature sensitivity values can be obtained as high as 2610 nm per refractive index unit (RIU) and 1.03 nm/°C, respectively. In addition, resonance wavelengths of resonator are obtained experimentally by using the resonant conditions. The effects of nano-rods radius and refractive index of racetrack resonator are studied on the sensing spectra, as well. The proposed structure with such high sensitivity will be useful in optical communications that can provide a new possibility for designing compact and high-performance plasmonic devices.     

Competition between Quercus petraea and Carpinus betulus in an ancient wood in England: seedling survivorship

Abstract. Age structures of populations of canopy trees in Wormley Wood are consistent with reports that successional change is occurring, with Carpinus betulus replacing Quercus petraea as the dominant species. TWINSPAN analysis of data from a vegetation survey identifies three communities (described in earlier work), the ‘Bracken’, ‘Bramble’, and‘Bareground’ societies, characterized by increasing prominence of C. betulus and loss of species diversity. An experiment was set up in which C. betulus, Q. petraea and Betula pendula seedlings were explanted into each of the communities. Survival of seedlings was monitored over 860 days and differences investigated. Cohort survivorship differed between species and sites. C. betulus seedlings survived longer than Q. petraea in Bracken and Bramble communities. Species- and site-specific variation in the types and effects of herbivory were found. Herbivory did not appear to be a critical factor in the survival of Q. petraea seedlings. The photosynthetic light response of the seedling species was measured in the field. The light compensation point for Q. petraea seedlings (77 μmol photon m^-2 s^-1) is higher than the maximum available light under the main tree canopies in the wood. In contrast, seedlings of C. betulus have a lower light compensation point (15 μmol photon m² s^-1) and the mature tree casts a deeper shade than Q. petraea. It is suggested that the invasion of the canopy by C. betulus, following the cessation of coppicing, is creating light levels too low for Q. petraea seedling banks to persist.     

Two-dimensional Fast Surface Imaging Using a Handheld Optical Device: In Vitro and In Vivo Fluorescence Studies

Near-infrared (NIR) optical imaging is a noninvasive and nonionizing modality that is emerging as a diagnostic tool for breast cancer. The handheld optical devices developed to date using the NIR technology are predominantly developed for spectroscopic applications. A novel handheld probe-based optical imaging device has been recently developed toward area imaging and tomography applications. The three-dimensional (3D) tomographic imaging capabilities of the device have been demonstrated from previous fluorescence studies on tissue phantoms. In the current work, fluorescence imaging studies are performed on tissue phantoms, in vitro, and in vivo tissue models to demonstrate the fast two-dimensional (2D) surface imaging capabilities of this flexible handheld-based optical imaging device, toward clinical breast imaging studies. Preliminary experiments were performed using target(s) of varying volume (0.23 and 0.45 cm³) and depth (1–2 cm), using indocyanine green as the fluorescence contrast agent in liquid phantom, in vitro, and in vivo tissue models. The feasibility of fast 2D surface imaging (∼5 seconds) over large surface areas of 36 cm² was demonstrated from various tissue models. The surface images could differentiate the target(s) from the background, allowing a rough estimate of the target''s location before extensive 3D tomographic analysis (future studies).