Flexible polymer neural probes enable minimally invasive interfacing with biological tissue. The smaller mechanical mismatch between soft polymer materials and the tissue reduces inflammation response and scarring in the tissue during chronic implantation of flexible neural probes compared to those made of rigid substrates, including Silicon, Silicon Dioxide, and Silicon Nitride. We have previously demonstrated a fully flexible Parylene photonic waveguide array platform for high-resolution targeted light delivery in tissue. Parylene photonics is a novel integrated photonic platform composed of flexible, biocompatible materials with a large refractive index contrast. The core of the photonic layer is Parylene C (n = 1.639) and the cladding is PDMS (n = 1.4), both two orders of magnitude more flexible than traditional Silicon substrates. Here, we perform optogenetic stimulation experiments using Parylene photonic waveguide arrays to deliver light to the brain in a transgenic mouse line expressing ReaChr; a red-shifted opsin. In this paper, we discuss, for the first time, the application of Parylene photonic waveguides for in vivo optogenetic stimulation of neurons in rodent models, evidenced by increased neural firing following light delivery. Spike sorting was performed to isolate neural units in the vicinity of the recording electrodes, demonstrating selective neural stimulation. Parylene photonic waveguide arrays were packaged with commercially-available single mode optical fibers and laser light sources operating at 𝜆=633 nm. Implantation of the flexible waveguide arrays was achieved via attachment to a rigid shuttle using bioresorbable polyethylene glycol (PEG) coating. Post implantation, Nissl staining was used to characterize neuronal damage following insertion. Neuroinflammation was also assessed using immunofluorescence.
Conventional optical lenses are usually used in OCT systems to perform high lateral resolution imaging. However, the
Gaussian beam profile typically used in OCT links the depth of focus (DOF) to the lateral resolution. We have
experimentally shown that using a cascade system of an ultrasonic virtual tunable optical waveguide (UVTOW) and a
short focal-length lens can provide a large DOF without severely compromising the lateral resolution compared to an
external lens with the same effective focal length. We demonstrate the tunability of the focal length that this system
offers without any need for mechanical perturbation to the imaging setup.
With the advent of optical methods for stimulation and functional recording of neuronal activity in the brain, there is a growing need for fully flexible, ultracompact photonic devices for light delivery and light collection in brain tissue. In this paper, we will discuss our recent advances in designing a flexible optoelectronic neural implant platform that integrates passive and active optical components with electrical recording functionality. We leverage the exquisite optical and electrical insulation properties Parylene C, a biocompatible and flexible polymer to realize a fully functional optoelectrical neural interface.
To use optical techniques deep in tissue, implantable microdevices which can collect and deliver light with high efficiency are needed. Flexible polymer devices can reduce tissue damage. Here, we demonstrate a fully-flexible, low-loss (3.2 dB/cm @ 680 nm), broadband (450-680 nm) integrated photonic platform composed entirely of Parylene C and PDMS. Using this platform, we demonstrate devices with an array of 6 waveguides and 1.3 cm total length. We integrate bare laser diode chips (220 x 220 μm, λ= 680 nm) to realize a light delivery system for optogenetics. Simulation, characterization, and biological demonstration will be discussed.
We discuss the characterization and analysis of ultrasonically sculpted virtual optical waveguides in scattering media using a novel physics-based renderer. Our renderer is physically accurate, unbiased, and fast. Unlike other simulators, our renderer can handle heterogeneous refractive index profiles in scattering media. Using the renderer, we characterized the effect of different parameters of virtual optical waveguides to enhance the overall light throughput in transparent and scattering media. The simulator can potentially be used for analysis-by-synthesis and design of innovative acousto-optics systems, capable of generating complex virtual elements in scattering media.
We present a novel technique to extend the focal length of an external lens without increasing the spot size using virtual optical waveguides ultrasonically sculpted in transparent and turbid media. When used in tandem with an external lens, these non-invasive virtual waveguides can further relay the focused beam of light into the medium without losing the spatial resolution. We will also show how the optical properties of this physical-virtual cascade system can be reconfigured by changing the pattern of ultrasound. This method can find intriguing applications for manipulating the trajectory of light in transparent and turbid media, including biological tissue.
We demonstrate a novel technique for in situ 3D spatial light patterning using ultrasound. By employing a customdesigned ultrasonic phased array, we form standing pressure waves that modulate the refractive index of the target medium in which light patterns are to be formed. Ultrasound pressure waves change the density of the medium locally and as a result, the refractive index is changed. Therefore, the phase front of the traveling optical waves in the medium is modulated. The interaction of light and ultrasound through the medium can be designed such that a collimated beam of light is gradually focused to multiple points deep into the medium to form arbitrary patterns of light illumination, as well as multipoint parallel imaging. These patterns can be reconfigured by changing the ultrasound interference patterns by controlling the frequency and phase of the ultrasound array elements. In this work, we demonstrate experimentally that this technique can be used to perform multi-point imaging in turbid media.
We demonstrate a hybrid integrated photonic-plasmonic platform in which photonic guided modes are used to efficiently
excite localized surface plasmon resonance (LSPR) modes of plasmonic nanoresonators. Efficient coupling of light to
the LSPR modes of plasmonic nanoresonators is demonstrated by tight integration of plasmonic nanoresonators on
silicon nitride (SiN) microresonators. It is shown that by integrating gold nanoparticles with SiN microresonators, we
can achieve high coupling efficiencies (>35%), resulting in large field enhancements. We will discuss the design,
fabrication, and characterization of the hybrid platform which consists of gold nanoparticles integrated with SiN
microring resonators.
A novel plane-wave-based approach for analytical treatment of dispersive relation is developed and applied to analyze
the behavior of electromagnetic waves in plasmonic-photonic-crystal slabs. Here Drude model is used for describing
frequency dependent permittivity of plasma rods in host dielectric medium. In the present work, dispersion relation
below and above the light line is calculated approximately by means of Maxwell-Garnett effective medium and Revised
Plane Wave Method (RPWM). The eigen-functions are then used in Revised Guided Mode Expansion (RGME) as the
set of orthonormal bases. Following this procedure, the accurate band structure is obtained. In these kind of methods
there are two main sources of error: stair-casing error due to discretization and numerical dispersion due to calculation
of frequency domain dielectric matrix elements with finite number of bases. Sub-cell averaging and harmonic inversion
methods are suggested to overcome these errors. For investigation purpose we apply this approach for calculating
photonic dispersion of dispersive and non-dispersive photonic crystal slabs. Resulted band structures are verified by
conventional FDTD method as well.
On-chip micro-interferometers are introduced in which a slab photonic crystal is used as a dispersive material system to
enhance the spectral sensitivity. The output interference pattern is observed along a detection plane. The systematic
design of these micro-interferometers is discussed. The performance of these devices as on-chip integrated micro-spectrometers
is investigated, and it is shown that by properly employing strong dispersive properties of photonic
crystals, very compact and high resolution integrated micro-interferometer/spectrometers can be realized for lab-on-a-chip
sensing applications.
Low power operation and high speed have always been desirable in applications such as data processing and
telecommunications. While achieving these two goals simultaneously, however, one encounters the well-known powerbandwidth
trade-off. This is here discussed in a typical bistable switch based on a two-dimensional photonic crystal
with Kerr type nonlinearity. The discussion is supported by the nonlinear finite difference time domain (FDTD)
simulation of a direct coupled structure with a home-developed code. Two cases of working near resonant and offresonant
are simulated to compare the power and the speed of the device in the two cases. It is shown that working nearresonance
reduces the power levels at the expense of reducing the settling time, i.e. the bandwidth limitation. The
hystersis loops for the device are also obtained with both coupled-mode theory and quasi-steady state FDTD simulation.
The impact of operating near/off resonance on the shape of the hystersis loop is discussed as a confirmation of the
previous results. Alternative ways of reducing the power while saving the bandwidth are also examined. The discussion
is general and one may investigate other optical switches to obtain similar results.
The opportunity to manipulate optical properties of materials through fabrication is the unique capability offered by
photonic crystals. Among different directions to exploit the possibilities in this field, there have been recent research
activities to engineer the dispersive properties of photonic crystals to change the propagation properties of waves
passing through these periodic structures. To provide an efficient way to implement such devices, an approximate
modeling technique will be used to simplify the analysis and design process for dispersive photonic crystal devices.
Furthermore, the issue of efficient coupling to dispersive photonic crystal modes which is crucial for practical
implementation of these devices will be addressed. Here, in particular, we will focus on employing the dispersive
properties of photonic crystals to realize compact optical spectrometers and wavelength demultiplexers. We will show
that by combining multiple dispersive properties (i.e., negative diffraction and the superprism effect) it is possible to
enhance the performance of devices targeted for such applications. The potentials of these photonic crystal devices to
meet the requirements of current and future applications in optical information processing and integrated optical sensing
will be discussed.
We have performed an analysis of harmonic contents of the optical output power for a diode laser and described the
results in details. In the first step the absolute value of power for each harmonic is obtained in terms of various diode
laser parameters, and the variations of external parameters such as modulation current, bias current and frequency are
discussed. The analysis is done by direct solution of rate equations of an arbitrary diode laser for carrier and photon
densities. We conclude that the maximum power occurs at isolated peaks and their loci have been investigated and
shown to be predictable by theory. It is known that the optical power has a nonlinear dependence on frequency, and the
maximum optical power of each harmonic attained in its resonance frequency. The resonant frequency is shown to be
tunable by bias current; thus in the next step we obtain the transfer function for different harmonic contents and have
achieved exact expression for each, allowing better optimization to gain improved results. We extend the approach to
higher harmonics and numerically calculate the THD (Total Harmonic Distortion) versus related parameters such as
frequency, bias current and modulation current. Furthermore we found an effective approach to reduce SHD (Second
Harmonic Distortion). The sequence for every arbitrary laser structure is also possible to be developed by the approach
presented in this work.
In this paper optical filters based on photonic resonant tunneling effect are analyzed by using the polynomial expansion method. Amplitude and phase response together with their dependency on the physical parameters of the filters are also investigated. These steep-edge filters show low insertion loss amplitude response, and linear phase variation in their passband, a suitable feature for WDM and DWDM applications where constant time delay and dispersion free devices are needed. Two kinds of filters, namely discrete level and continuous profile filters are introduced. These structures can be analyzed and designed by using Transfer Matrix Method. However, this approach suffers from inaccuracy and numerical instability when narrow linewidth filters are desired. Moreover, analyzing the continuous profile filters using this method calls for breaking the structure into many homogeneous sublayers. Here, a method based on Legendre expansion of electromagnetic fields is adopted to design and analyze the proposed filters. Not only the method relieves some numerical problems peculiar to conventional methods, but also can be applied for holistic analysis of filters having continuous refractive index profile and therefore eliminates the need for cumbersome multilayer analysis.
Recently, interaction of electromagnetic waves with conducting interfaces has been studied and several applications have been proposed. For instance, new type of photonic crystals similar to Kronig-Penny electronic crystals has been implemented by using these structures. In these structures a free two dimensional interface charge layer is generated at the dielectric interfaces and interesting phenomena are observed. In this manuscript, the effect of finite charge layer thickness and its asymptotic behavior toward conducting interface, where the thin charge layer is modelled via a surface conductivity σs, is numerically studied for the first time. Two different regimes are considered: first, propagation of optical waves through sub-wavelength free charge layers and its corresponding reflection and transmission coefficients for both major polarizations TE and TM; second, propagation of optical slow waves localized at the interface of two dielectrics with interface conducting layer between them..
Analytical analysis of straight single-line defect optical waveguides in two dimensional photonic crystals based on expanding electromagnetic fields in terms of Hermite polynomials is reported. This novel electromagnetic field expression is substituted in Helmholtz equation, a new set of linear ordinary differential equations with variable coefficients are obtained, and by employing differential transfer matrix method; defect modes, i.e. the guided modes propagating in the line defect waveguide, are analytically derived. The validity of the results obtained by applying the proposed approach are confirmed by comparing them to those derived by using finite difference time domain method.
A novel approach for photonic crystals devices analysis, based on perturbation theory is reported. In this method the photonic crystal device is considered as the superposition of a parent lattice and a perturbing one. Then the solution is investigated in terms of the eigensolutions of the parent lattice. This way, one can easily obtain analytic expressions within the first order perturbation, describing the effects of different parameters on the eigensolutions of the structure. The perturbation theory employed in this work is typical of what is conventionally used in quantum mechanics literature. The proposed method is explicit, works fast, and does not involve complicated numerical calculations. Although this approach can be used to obtain some rules of thumb about the eigensolutions of the device within the first order perturbation approximation, it can be further followed to higher order perturbation terms for acquiring any desired level of accuracy. Since the presented method is mostly formulated analytically, not much computational effort is required for analyzing complex structures. In this paper the approach is described in detail and some examples are given to show the usefulness of it.
Free electron lasers have been the subject of intensive interest during the recent decades. In this paper, free electron laser having sheet electron beam with arbitrary inhomogeneous profile of transverse distribution of the beam current density is studied in the linear regime, whereas a novel approach based on the Legendre polynomial expansion of eigenfunctions, already used in analyzing optical structures including stratified structures and diffraction gratings, is adapted to find the eigenfunctions and eigenvalues of the structure. As for this method is unconditionally stable, it works pretty well even in those cases in which the conventional transfer matrix method suffers from numerical instability, i.e. the detuning parameter is negative with a large absolute value. Though the used formulation, obtained by solving Maxwell's and Vlasov's equations simultaneously, is limited to the linear regime, it includes the effects of energy spread and space charge fields.
Three-dimensional vectorial diffraction analysis of phase and amplitude gratings in conical mounting is presented based on Legendre expansion of electromagnetic fields. In the so-called conical mounting, different fields components are coupled and the solution is not separable in terms of independent TE and TM cases. In contrast to conventional RCWA in which the solution is obtained using state variables representation of the coupled wave amplitudes by expanding space harmonic amplitudes of the fields in terms of the eigenfunctions and eigenvectors of the coefficient matrix defined by
rigorous coupled wave equations, here the solution of first order coupled Maxwell's equations is expanded in terms of Legendre polynomials. This approach yields well-behaved algebraic equations for deriving diffraction efficiencies and electromagnetic field profiles. It can nicely handle the cases in which conventional methods face the problem of numerical instability and inevitable round off errors; also, it yields accurate results to any desired level of accuracy. The method is applied to phase and amplitude gratings in conical mountings, comparison to other methods already reported in the literature is made, and the presented approach is justified and its usefulness in cases that other methods usually fail
is demonstrated. This general method applies well even in such cases as thick gratings, non-Bragg incidence, and cases in which higher diffracted orders are needed to be retained, or evanescent orders corresponding to real eigenvalues have to be included. The efficacy of the proposed method relies on the fact that although Legendre polynomials span a complete space, they are not eigensolutions and hence each polynomial basis function bears a weighted projection of all eigenfunctions. Thus no modal information is completely missed in the ineluctable truncation process. In deriving the formulation, a rigorous approach is followed.
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