Mohit Diwekar

Optical and magneto-optical studies of two-dimensional metallodielectric photonic crystals on cobalt films

We studied the optical transmission and magneto-optical effect through a subwavelength hole array fabricated on a ferromagnetic cobalt (Co) thin film in comparison to a control unperforated Co film having the same thickness. We found that the perforated film sustains extraordinary transmission bands through the hole array, which can be well explained as due to light coupling to surface plasmons on the two film interfaces. We also found that due to resonant coupling to the surface plasmons, the magneto-optical Kerr effect in the spectral range of the anomalous transmission bands of the perforated Co film is much smaller than that in the control Co film.

Long-Term Bilayer Encapsulation Performance of Atomic Layer Deposited Al

We present an encapsulation scheme that combines atomic layer deposited (ALD) Al2O3 and Parylene C for the encapsulation of implantable devices. The encapsulation performances of combining alumina and Parylene C was compared to individual layers of Parylene C or alumina and the bilayer coating had superior encapsulation properties. The alumina-Parylene coated interdigitated electrodes (IDEs) soaked in PBS for up to nine months at temperatures from 37 to 80 °C for accelerated lifetime testing. For 52-nm alumina and 6-μm Parylene C, leakage current was ~20 pA at 5 VDC, and the impedance was about 3.5 MΩ at 1 kHz with a phase near -87° from electrochemical impedance spectroscopy for samples soaked at 67 °C for equivalent lifetime of 72 months at 37 °C. The change of impedance during the whole soaking period (up to 70 months of equivalent soaking time at 37 °C) over 1 to 10 6 Hz was within 5%. The stability of impedance indicated almost no degradation of the encapsulation. Bias voltage effect was studied by continuously applying 5 VDC, and it reduced the lifetime of Parylene coating by ~75% while it showed no measurable effect on the bilayer coating. Lifetime of encapsulation of IDEs with topography generated by attaching a coil and surface mount device (SMD) capacitor was about half of that of planer IDEs. The stable long-term insulation impedance, low leakage current, and better lifetime under bias voltage and topography made this double-layer encapsulation very promising for chronic implantable devices.

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This paper characterizes the Utah Slant Optrode Array (USOA) as a means to deliver infrared light deep into tissue. An undoped crystalline silicon (100) substrate was used to fabricate 10 × 10 arrays of optrodes with rows of varying lengths from 0.5 mm to 1.5 mm on a 400-μm pitch. Light delivery from optical fibers and loss mechanisms through these Si optrodes were characterized, with the primary loss mechanisms being Fresnel reflection, coupling, radiation losses from the tapered shank and total internal reflection in the tips. Transmission at the optrode tips with different optical fiber core diameters and light in-coupling interfaces was investigated. At λ = 1.55μm, the highest optrode transmittance of 34.7%, relative to the optical fiber output power, was obtained with a 50-μm multi-mode fiber butt-coupled to the optrode through an intervening medium of index n = 1.66. Maximum power is directed into the optrodes when using fibers with core diameters of 200 μm or less. In addition, the output power varied with the optrode length/taper such that longer and less tapered optrodes exhibited higher light transmission efficiency. Output beam profiles and potential impacts on physiological tests were also examined. Future work is expected to improve USOA efficiency to greater than 64%.

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Abstract. We establish performance characteristics of needle-type waveguides in three-dimensional array architectures as light delivery interfaces into deep tissue for applications, such as optogenetic and infrared (IR) neural stimulation. A single optrode waveguide achieves as high as 90% transmission efficiency, even at tissue depths >1  mm. Throughout the visible and near-IR spectrum, the effective light attenuation through the waveguide is ∼3 orders of magnitude smaller than attenuation in tissue/water, as confirmed by both simulation and experimental results. Light emission profiles from the optrode tips into tissue were also measured. Beam widths of 70 to 150 μm and full-angle divergence ranging from 13 to 40 deg in tissue can be achieved. These beam characteristics satisfy a wide range of requirements for targeted illumination in neural stimulation.

_{\bf 3}

We demonstrate a photoactivated surface coupling scheme for achieving spatial overlap between biomolecules of interest and optical near field excitation. Using aluminium nanoapertures, we obtained increased coupling efficiency of biotinylated capture probe oligos to the photoactivated surface due to ~3× nanoaperture enhancement of UV light. We further validate DNA sensor functionality via the hybridization of Cy-5 labeled target oligos, with up to 8× fluorescence enhancement obtained from a commercial microarray scanner. This generic photoimmobilization strategy is an essential step to realizing miniaturized plasmon enhanced detection arrays by virtue of localizing capture molecules to the region of plasmonic enhancement.

and Parylene C for Biomedical Implantable Devices

We study light transmission through individual and arrays of sub-wavelength metallic apertures of conical shape as a function of their taper angle. With a fixed aperture size at the substrate, there is a dramatic increase in light throughput with increasing taper angle. Similar behaviors hold when considering the intensity enhancement inside the aperture near the substrate interface, where more than four-fold increase can be obtained. When arranged in regular arrays, apertures can interact via surface waves, creating distinct minima and maxima in light transmission and intensity enhancement. The effect of aperture taper is also to dramatically increase light transmission and intra-aperture intensity, but with a red shift in the maxima and negligible shift in the minima. The use of conical apertures should improve the efficiency of nonlinear optical processes as well as applications of light harvesting, such as biomolecule detection.

Characterization of a 3D optrode array for infrared neural stimulation

We present an early-generation Utah Slant Optrode Array (USOA) for infrared (IR) neural stimulation. Intrafascicular IR stimulation with the early prototype in the cat sciatic nerve produced highly selective and artifact-free responses, which outperformed extraneural IR stimulation. We characterized the light delivery and loss mechanisms of the device in order to facilitate design optimization. Fabrication of the USOA takes advantage of the extensive research in the development of the Utah Slant Electrode Array (USEA). An undoped (ρ > 20 ω ρ cm) c-Si (100) substrate was used to produce a 10 x 10 array of optrodes with lengths from 0.5 mm to 1.5 mm in a 400-μm pitch. This substrate is able to transmit IR (λ > 1.1μm) with negligible absorption losses. The optrodes were coupled to the laser source via fibers of different core diameters through in-coupling interfaces of varying refractive indices. The effect of these factors on optrode transmission efficiency was investigated. At 1550nm, transmittance for a butt-coupled 50-μm multimode fiber through a medium of index n = 1.66 was measured as 34.7%, which was the maximum value obtained. When the refractive index of the intervening medium was lowered, transmission decreased according to Fresnel reflection theory. Above 100-μm core size, transmitted power decreases by 40% with each doubling of the fiber core diameter. Transmission was also found to be dependent on the optrode length, where shorter and more tapered optrodes provided less output power. The results suggest that Fresnel, coupling, and radiation losses are the primary loss mechanisms.

Deep-tissue light delivery via optrode arrays

Early-generation waveguide arrays made of silicon (Si) and fused silica (SiO2) were micromachined for infrared (IR) neural stimulation and optogenetics applications in able to provide comprehensive and selective access to distributed targets. Edge-coupled multimode fibers delivered light through the optrodes. Maximum normalized output power from the tips, with respect to the fiber output, was determined as 0.33 and 0.71 for silicon and glass optrodes, respectively.

Photoactivated capture molecule immobilization in plasmonic nanoapertures in the ultraviolet

3D needle-type glass waveguide arrays were developed as potentially compact neural interfaces for light delivery in deep-tissue. As much as 90% of input light is transmitted via a single optrode to depths >1mm in tissue. Light emission profiles from the optrode tips into tissue can exhibit beam widths of 70-150 μm and full-angle divergence ranging from 13-40°. These beam characteristics may be able to satisfy a wide range of requirements for targeted illumination in neural stimulation.

Increased light gathering capacity of sub-wavelength conical metallic apertures

Penetrating waveguide arrays made of glass (SiOO) and silicon were fabricated for infared (IR) neural stimulation to provide 3D access to the brain or peripheral nerves for selective deep-tissue stimulation with different spatiotemporal patterns. Comprehensive bench characterization was performed to determine light delivery and loss mechanisms. Fused silica/quartz arrays have optrodes of constant geometry with a pyramidal tip at the end of a straight-edge shank; length, width, and tip angle of each optrode can be varied independently from array to array. Undoped silicon arrays are similar in form to the Utah Slant Electrode Array, which has tapered microneedles of varying length in one direction. Light transmission efficiency was investigated with input from different optical fibers. With a 120-μmm wide and 1.5-mm long glass optrode having a tip taper angle of 45° with respect to the optical axis, 70% of IR light from a butt-coupled 50-μm fiber is transmitted out of the tips; shank length and tip taper does not affect the output power. However, transmission is only 39% for a 1.5- mm long Si optrode, and less for shorter more tapered optrodes. Similar beam profiles were obtained for both arrays when glass optrodes have a 45° tip taper; decreasing the glass optrode tip angle to 30° increases the full-angle divergence from 15° to 55°, which leads to a wider yet shallower illumination volume. Results reveal that the dominant source of loss in both devices is from total internal reflection within the tips. Additional losses in silicon include tapered shank radiation and reflection from its high refractive index.