Optical Properties of Capped Metallic Nanostructures, Grown on Silicon

Reflectance anisotropy spectroscopy (RAS) is a linear optical technique that measures the difference in the reflectance of two orthogonal polarisations at normal incidence. It achieves surface and interface sensitivity when the bulk material, such as a cubic semiconductor, is optically isotropic. The penetration depth of optical radiation allows RAS to probe buried interfaces. RAS has been used to probe various one-dimensional (1-D) structures grown on vicinal Si(111) surfaces under ultra-high vacuum (UHV) conditions. The RAS system response was extended into the IR, where important optical transitions occur, for both a photoelastic modulated system and a rotating sample system using a tuneable IR laser. RAS spectra of single domain Si(111)-5x2-Au, Si(557)-Au and Si(775)-Au structures showed large minima in the region around 2 eV and, in the case of Si(111)-5x2-Au a large maximum below 1 eV. The monolayer (ML) coverage of Au required for the Si(111)-5x2-Au surface reconstruction has been extracted from the RAS response. Using the well known coverage of Au required for the Si(557)-Au reconstruction, the Au deposition rate was accurately calibrated. By analysis of the coverage required for several Si(111) vicinal off-cuts, taking into account the different step densities, a coverage for a "pure" Si(111)-5x2-Au surface was calculated. A value of 0.59 ML +/- 0.08ML was found, in agreement with the recent work. The value supports a new three chain model for the Si(111)-5x2-Au surface reconstruction. Upon deposition of small amounts of Si adatoms on the Si(111)-5x2-Au surface and subsequent annealing, the RAS spectra changed dramatically, as the adatom decorated "5x4" reconstruction was formed. Temperature dependent studies allowed 100% and a 0% adatom filled sites RAS spectra to be extracted. These spectra will be particularly useful for comparison with future ab initio optical response calculations. The optical signatures from this surface could prove to be very interesting in the study of defect induced charge density waves. A strong optical anisotropy was also seen on the Si(775)-Au and Si(557)-Au surfaces. The RAS spectra showed a minimum around 2 eV but the maximum above 1 eV, seen on the Si(111)-5x2-Au surface was not present. A possible explanation is that the chain structures on these narrower terraces are more sensitive to the presence of kinks. The average length of the Au chains is expected to be significantly shorter on the Si(775)-Au and Si(557)-Au surfaces, as the kinks will terminate the Au chains more efficiently than on the lower angle offcuts, used for the Si(111)-5x2-Au studies. The RAS response from Si(557) shows two peaks related to surface modified bulk states at 3.4 eV and 4.25 eV, and a surface state at 1.2 eV. The RAS signal was compared with preliminary ab initio optical response calculations. Reasonable results were found for a bulk terminated and relaxed Si(557) surface. However, the structure is known to consist of a triple step structure of approximately (112) orientation and the large terrace of the Si(111)-7x7 reconstruction. Calculations of the RAS spectra from Si(112) did not reproduce the features seen experimentally. Elongated Pb islands with lengths of up to 430 nm and widths of 60 nm were grown on Si(557)-Au and their RAS spectra were recorded. The wires showed a strong RAS signal with a negative peak at 1.1 eV and a positive peak at 0.47 eV. The Pb islands could be capped with a-Si and their reflection anisotropy was retained, with both peaks shifted to the IR. The results showed that capping with a-Si was largely successful. The modelling of the RAS response was less successful. A nanoantenna approach gave reasonable values of the length of the Pb islands, using the wavelength of the maximum. Other models, which attempted to predict the line shape, could reproduce either the minimum or maximum accurately but not both. However, these models neglect both quadrupolar effects and dipole-dipole interactions between islands. The sensitivity of the RAS response to the detail of the island structure indicates that RAS could be a powerful probe of plasmonic structures if a suitable theoretical model can be developed

Simultaneous electrophysiology and fiber photometry in freely behaving mice

In vivo electrophysiology is the gold standard technique used to investigate sub-second neural dynamics in freely behaving animals. However, monitoring cell-type-specific population activity is not a trivial task. Over the last decade, fiber photometry based on genetically encoded calcium indicators (GECIs) has been widely adopted as a versatile tool to monitor cell-type-specific population activity in vivo. However, this approach suffers from low temporal resolution. Here, we combine these two approaches to monitor both sub-second field potentials and cell-type-specific population activity in freely behaving mice. By developing an economical custom-made system and constructing a hybrid implant of an electrode and a fiber optic cannula, we simultaneously monitor artifact-free mesopontine field potentials and calcium transients in cholinergic neurons across the sleep-wake cycle. We find that mesopontine cholinergic activity co-occurs with sub-second pontine waves, called P-waves, during rapid eye movement sleep. Given the simplicity of our approach, simultaneous electrophysiological recording and cell-type-specific imaging provides a novel and valuable tool for interrogating state-dependent neural circuit dynamics in vivo

Depth-specific optogenetic control in vivo with a scalable, high density µLED neural probe

Controlling neural circuits is a powerful approach to uncover a causal link between neural activity and behaviour. Optogenetics has been widely adopted by the neuroscience community as it offers cell-type-specific perturbation with millisecond precision. However, these studies require light delivery in complex patterns with cellular-scale resolution, while covering a large volume of tissue at depth in vivo. Here we describe a novel high-density silicon-based microscale light-emitting diode (µLED) array, consisting of up to ninety-six 25 µm-diameter µLEDs emitting at a wavelength of 450 nm with a peak irradiance of 400 mW/mm2. A width of 100 µm, tapering to a 1 µm point, and a 40 µm thickness help minimise tissue damage during insertion. Thermal properties permit a set of optogenetic operating regimes, with ~0.5°C average temperature increase. We demonstrate depth-dependent activation of mouse neocortical neurons in vivo, offering an inexpensive novel tool for the precise manipulation of neural activity

Thermal and optical characterization of micro-LED probes for in vivo optogenetic neural stimulation

Within optogenetics there is a need for compact light sources that are capable of delivering light with excellent spatial, temporal, and spectral resolution to deep brain structures. Here, we demonstrate a custom GaN-based LED probe for such applications and the electrical, optical, and thermal properties are analyzed. The output power density and emission spectrum were found to be suitable for stimulating channelrhodopsin-2, one of the most common light-sensitive proteins currently used in optogenetics. The LED device produced high light intensities, far in excess of those required to stimulate the light-sensitive proteins within the neurons. Thermal performance was also investigated, illustrating that a broad range of operating regimes in pulsed mode are accessible while keeping a minimum increase in temperature for the brain (0.5°C). This type of custom device represents a significant step forward for the optogenetics community, allowing multiple bright excitation sites along the length of a minimally invasive neural probe

Optical fingerprints of Si honeycomb chains and atomic gold wires on the Si (111)-(5× 2)-Au surface

The intensively studied Si(111)-(5×2)-Au surface is reexamined using reflectance anisotropy spectroscopy and density functional theory simulations. We identify distinctive spectral features relating directly to local structural motifs such as Si honeycomb chains and atomic gold wires that are commonly found on Au-reconstructed vicinal Si(111) surfaces. Optical signatures of chain dimerization, responsible for the observed (×2) periodicity, are identified. The optical response, together with STM simulations and first-principles total-energy calculations, exclude the new structure proposed very recently based on the reflection high-energy electron diffraction technique analysis of Abukawa and Nishigaya [ Phys. Rev. Lett. 110 036102 (2013)] and provide strong support for the Si honeycomb chain with the triple Au chain model of Erwin et al. [ Phys. Rev. B 80 155409 (2009)]. This is a promising approach for screening possible models of complex anisotropic surface structures

Structured illumination for communications and bioscience using GaN micro-LED arrays interfaced to CMOS

Gallium-Nitride-based light-emitting diodes (LEDs) have emerged over the last two decades as highly energy-efficient, cost-effective, compact and robust light sources. While general purpose lighting has been the dominant application thus far, a variety of other applications can also exploit these advantageous properties, including optical communications, fluorescence sensing and bioscience. Micro-LEDs arrays of individually-addressable LED pixels, each pixel typically 100 µm or less, offer further advantages over conventional LEDs such as extremely high modulation bandwidths and spatio-temporally controllable illumination patterns. These arrays are also readily compatible with flip-chip integration with CMOS electronic driver arrays. Here we report how these CMOS-controlled micro-LED arrays enable “smart lighting” solutions, capable of providing services such as wireless data communication and indoor navigation in conjunction with illumination. We also demonstrate how this smart functionality opens up novel bioscience applications, including depth-specific in-vivo optical neural probes and wireless transfer of measured data

A compact integrated device for spatially selective optogenetic neural stimulation based on the Utah Optrode Array

Optogenetics is a powerful tool for neural control, but controlled light delivery beyond the superficial structures of the brain remains a challenge. For this, we have developed an optrode array, which can be used for optogenetic stimulation of the deep layers of the cortex. The device consists of a 10×10 array of penetrating optical waveguides, which are predefined using BOROFLOAT® wafer dicing. A wet etch step is then used to achieve the desired final optrode dimensions, followed by heat treatment to smoothen the edges and the surface. The major challenge that we have addressed is delivering light through individual waveguides in a controlled and efficient fashion. Simply coupling the waveguides in the optrode array to a separately-fabricated μLED array leads to low coupling efficiency and significant light scattering in the optrode backplane and crosstalk to adjacent optrodes due to the large mismatch between the μLED and waveguide numerical aperture and the working distance between them. We mitigate stray light by reducing the thickness of the glass backplane and adding a silicon interposer layer with optical vias connecting the μLEDs to the optrodes. The interposer additionally provides mechanical stability required by very thin backplanes, while restricting the unwanted spread of light. Initial testing of light output from the optrodes confirms intensity levels sufficient for optogenetic neural activation. These results pave the way for future work, which will focus on optimization of light coupling and adding recording electrodes to each optrode shank to create a bidirectional optoelectronic interface

Multisite microLED optrode array for neural interfacing

We present an electrically addressable optrode array capable of delivering light to 181 sites in the brain, each providing sufficient light to optogenetically excite thousands of neurons in vivo, developed with the aim to allow behavioral studies in large mammals. The device is a glass microneedle array directly integrated with a custom fabricated microLED device, which delivers light to 100 needle tips and 81 interstitial surface sites, giving two-level optogenetic excitation of neurons in vivo. Light delivery and thermal properties are evaluated, with the device capable of peak irradiances >80  mW  /  mm2 per needle site. The device consists of an array of 181 80  μm  ×  80  μm2 microLEDs, fabricated on a 150-μm-thick GaN-on-sapphire wafer, coupled to a glass needle array on a 150-μm thick backplane. A pinhole layer is patterned on the sapphire side of the microLED array to reduce stray light. Future designs are explored through optical and thermal modeling and benchmarked against the current device

In vivo optogenetics using a Utah Optrode Array with enhanced light output and spatial selectivity

Objective. Optogenetics allows the manipulation of neural circuits in vivo with high spatial and temporal precision. However, combining this precision with control over a significant portion of the brain is technologically challenging (especially in larger animal models). Approach. Here, we have developed, optimised, and tested in vivo, the Utah Optrode Array (UOA), an electrically addressable array of optical needles and interstitial sites illuminated by 181 μLEDs and used to optogenetically stimulate the brain. The device is specifically designed for non-human primate studies. Main results. Thinning the combined μLED and needle backplane of the device from 300 μm to 230 μm improved the efficiency of light delivery to tissue by 80%, allowing lower μLED drive currents, which improved power management and thermal performance. The spatial selectivity of each site was also improved by integrating an optical interposer to reduce stray light emission. These improvements were achieved using an innovative fabrication method to create an anodically bonded glass/silicon substrate with through-silicon vias etched, forming an optical interposer. Optical modelling was used to demonstrate that the tip structure of the device had a major influence on the illumination pattern. The thermal performance was evaluated through a combination of modelling and experiment, in order to ensure that cortical tissue temperatures did not rise by more than 1 °C. The device was tested in vivo in the visual cortex of macaque expressing ChR2-tdTomato in cortical neurons. Significance. It was shown that the UOA produced the strongest optogenetic response in the region surrounding the needle tips, and that the extent of the optogenetic response matched the predicted illumination profile based on optical modelling—demonstrating the improved spatial selectivity resulting from the optical interposer approach. Furthermore, different needle illumination sites generated different patterns of low-frequency potential activity

An optrode array for spatiotemporally precise large-scale optogenetic stimulation of deep cortical layers in non-human primates

Optogenetics has transformed studies of neural circuit function, but remains challenging to apply in large brains, such as those of non-human primates (NHPs). A major challenge is delivering intense, spatiotemporally precise, patterned photostimulation across large volumes in deep tissue. Such stimulation is critical, for example, to modulate selectively deep-layer corticocortical feedback projections. To address this unmet need, we have developed the Utah Optrode Array (UOA), a 10×10 glass needle waveguide array fabricated atop a novel opaque optical interposer then bonded to an electrically addressable μLED array. In vivo experiments with the UOA demonstrated large-scale, spatiotemporally precise, activation of deep circuits in monkey cortex. Specifically, the UOA permitted both focal (confined to single layers/columns), and widespread (multiple layers/columns) optogenetic activation of deep layer neurons, simply by varying the number of activated μLEDs and/or the irradiance. Thus, the UOA represents a powerful optoelectronic device for targeted manipulation of deep-layer circuits in NHP models.Competing Interest StatementThe authors have declared no competing interest