We report the demonstration of a cascaded configuration to realize wideband high-resolution spectrometers in silicon-nitride
for the visible range. The cascaded configuration consists of an arrayed waveguide grating followed by a set of
resonators and offers a flexibility to achieve the requirements of different sensing applications. We discuss some of the
implementation issues of these structures. A preliminary demonstration shows the capability of such spectrometer
devices to achieve ~0.1 nm wavelength resolution over 10 nm bandwidth in a compact device in a mm-scale device.
We present a semi-analytical Green's function-based technique for analyzing propagation loss in photonic crystal
waveguides (PCWGs). The method only requires the complex band structure of the PCWG to calculate the transmission
(or loss) of the structure. The plane-wave expansion method was used in this work to calculate the complex band
behavior, and the power of this technique is demonstrated by comparing the results with the brute force simulation
results for a PCWG. The possibility of extending this technique to the more practical arrangement of a random
distribution of defects using a configurational average with coherent potential approximation theory will also be
discussed.
The potentials of a nanophotonic platform, including compactness, low power consumption, integrability with other functionalities, and high sensitivity make them a suitable candidate for sensing applications. Strong light-matter interaction in such a platform enables a variety of sensing mechanisms, including refractive index change, fluorescence emission, and Raman scattering. Recent advances in nanophotonic devices include the demonstration of silicon and silicon-nitride microdisk resonators with high intrinsic Q values (0.5-2×106) for strong field enhancement and the realization of compact photonic crystal spectrometers (high spectral resolution at 100-µm length scales) for on-chip spectral analysis. These two basic building blocks, when combined with integrated fluidic channels for sample delivery, provide an efficient platform to implement different sensing mechanisms and architectures.
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.
The potentials of integrated optical systems for implementing compact and low power consumption yet highly sensitive
sensing systems have made them a viable candidate for integrated chemical and biological sensing applications. In these
integrated optical sensing systems, spectrometers have a significant role as a building block that enables on-chip
spectral analysis. Monitoring the spectral features of the signal using an on-chip spectrometer brings about a variety of
new sensing mechanisms and architectures in an integrated platform. Monitoring absorption spectra, measuring Raman
emission features, and tracking changes in spectral signatures as a result of environmental changes are some of the
schemes made possible by such spectral analysis. In this work, we implement superprism-based photonic crystal devices
in planar platforms as on-chip spectrometers. We use planar silicon platform in a silicon-on-insulator (SOI) wafers for
the infrared wavelength range. A silicon-nitride (SiN) planar platform is used for the near infrared and visible
wavelength ranges. In both SOI and SiN implementations, superprism-based spectrometers are experimentally
demonstrated and compared with grating spectrometers made in the same platform. The potentials of the demonstrated
spectrometers to meet the requirements of current and future applications in integrated optical sensing are briefly
discussed.
We present the analysis and design of three-dimensional photonic crystal demultiplexers in which the simultaneous
existence of the superprism effect and the diffraction compensation results in a compact structure. First, we report on a
diffractive index model developed to facilitate the simulation of the beam propagation in three-dimensional photonic
crystals. Then, we use tetragonal woodpile photonic crystals to design a demultiplexer.
Infiltration of planar 2D silicon photonic crystals with nanocomposites using a
simple melt processing technique is presented. The nanocomposites that were developed by
evenly dispersing functionalized TiO2 nanoparticles into a photoconducting polymer exhibit
high optical quality and tunable refractive index. The infiltrated photonic crystals show
tuning of the photonic band-gap that is controllable by the adjustment of the nanoparticle
loading level. These results may be useful in the development of tunable photonic devices,
hybrid light emitting diodes and photovoltaics.
We present methods for systematic design of couplers for efficient coupling of light into the slow group velocity modes
of photonic crystal waveguides (PCW).
Photonic crystal waveguides (PCW) on silicon-on-insulator (SOI) are considered as a promising guiding platform with
flexible guiding properties for dense photonics integration. Although SOI is a versatile material for photonics
integration, PCWs fabricated on SOI substrates suffer from small guiding bandwidth due to the coupling to leaky TM-like
modes. The purpose of this work is to present a systematic approach to increase the low-loss guiding bandwidth of
PCWs on SOI. This has been achieved by reducing the interaction of low-group-velocity modes with the surrounding
photonic crystal. By this method the low-loss bandwidth of a W1 PCW is increased from 2.5 nm to 12 nm which is the
highest reported for this type of waveguide. We also present a detailed analysis of transmission properties of W1 PCWs
and elaborate on the coupling to TM-like guided modes present in the low-loss transmission bandwidth of this device.
We present a theoretical analysis of propagation losses in photonic crystal waveguides due to
fabrication imperfections. The analysis is performed using a Green's function-based technique
with the layer Korringa-Kohn-Rostoker method. This approach requires only the calculation of
the complex mode behavior of the photonic crystal structure, from which the loss of a given
mode is directly deduced. The accuracy and applicability of the method is discussed. The
method will be demonstrated using two-dimensional photonic crystals with line-defect
waveguides having a single fabrication defect.
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 present a method for systematic design of Photonic Crystal Waveguide (PCW) bends to achieve high transmission and low dispersion over large bandwidths by identifying factors and studying their effects on transmission and dispersive properties of bends.
Compact on-chip wavelength demultiplexers and spectrometers are essential components for a variety of applications
including integrated optical information processing devices, optical communications, and integrated optical sensing.
Implementation of such devices requires strong dispersion in the optical materials, which can be realized using unique
dispersive properties of photonic crystals (PCs). Possibility of integration, compactness, and compatibility with different
host materials are the main advantages of PC based demultiplexers and spectrometers compared to other techniques.
Here, we show an implementation of superprism-based photonic crystal devices (using a diffraction compensation
scheme) that improves the performance of these devices compared to the conventional implementation. Structures
obtained through optimization have been fabricated in SOI wafers using e-beam writing and ICP etching, and spatial
separation of channels (with good isolation) in these superprism devices is experimentally demonstrated. The
performance of these superprism devices as general-purpose spectrometers and for locating spectral features in a
sensing platform will be also demonstrated and discussed. Further steps for improvement of these devices are
considered and the related implementation issues are investigated.
Wavelength demultiplexing is one of the major applications of unique dispersion properties of photonic crystals (PCs). Possibility of integration and compactness are two main advantages of PC based demultiplexers compared to other demultiplexing techniques for applications including compact spectrometers (for sensory applications) and WDM demultiplexers. Here, we show that resolution and size limitations of conventional superprism-based photonic crystal
demultiplexers are caused by the choice of configuration. We suggest an alternative implementation (combining superprism effect and focusing) that improves the performance compared to the conventional implementation in terms of being more compact and relaxing the requirement for divergence angle of the incident beam. We use effective index model to describe the beam behavior inside the photonic crystal region. Using this model, effective indices (second
order and third order) are calculated directly from the band structure and are used to find the optimal operation parameters for the demultiplexing device. Detailed calculations show that the required size of preconditioned superprism photonic crystal demultiplexers scales up as N5/2 (N being the number of channels which is proportional to the resolution of the device) which shows significant advantage over N4 dependence in conventional superprism-based devices, especially for high resolutions required in practical DWDM systems or spectroscopic applications. Structures obtained through optimization have been fabricated in SOI wafers using e-beam writing and ICP etching, and spatial separation of channels (with good isolation) in focusing superprism devices is experimentally demonstrated.
A robust scheme is proposed to adiabatically couple the light into and out of propagation modes of photonic crystal structures out of photonic bandgap. It is shown that group index plays the main role in reflection from layers with slight variations. This principle is used to design smoothly varying structures to adiabatically match the incident region to the transmission region. The smooth variation is obtained by modifying the size of holes and the aspect ratio of the unit cells in the buffer layers. It is shown that this method is insensitive to incident angle and wavelength and is therefore appropriate as general-purpose input and output buffer stages for dispersion-based applications of photonic crystal such as superprism-based demultiplexing, diffraction-free propagation, diffraction compensation, and dispersion compensation.
We show that propagation of the envelope of the optical beams inside a periodic structure can be approximately modeled using a linear transformation solely based on its band structure. Using this model a simple description of the diffraction is obtained which relies on the shape of the equal-frequency contours of the photonic crystal band structure. This model provides intuitive insight on how to design these structures for diffraction-based applications. We consider various methods to modify the band structure, or to excite appropriate regions of the band structure to realize different functionalities inside the photonic crystal. Applications like diffraction-free propagation, frequency separation in space (superprism-based devices), and diffraction compensation are discussed in more detail.
By adding a point defect to a photonic crystal structure, a microcavity can be made to trap electromagnetic energy with wavelength inside the photonic bandgap (PBG). This property together with other unique properties of photonic crystals enables us to control the propagation and spectrum of transmitted wave inside the photonic crystal waveguiding structures and to design and implement microscale optical filters. In this paper we focus on the design of notch filters in photonic crystal waveguides based on the coupling of waveguide and cavity. We discuss about the properties of a single cavity and its necessary modifications to achieve efficient coupling between cavity and waveguide and eventually obtain desired notch filters at the frequency range of interest. We also discuss the coupling of multi-cavities to a waveguide and the possibility of attaining filters with better performances is presented, and the spectrum and lineshapes of the resulting filters are characterized.
The electromagnetic interface states formed in a heterostructure composed of two semi-infinite Kronig-Penny photonic crystals have been studied. Modified transfer matrices have been used for study of Kronig-Penny photonic crystals (heterostructures with conducting interfaces) to show strong similarity between solid-state physics and electromagnetics. Our calculations are limited to TE polarization.
An improved coupled mode approach is presented for the analysis of plane-wave interaction with a spatially periodic permittivity dielectric layer. The method consists of two parts, in the first part the eigenvalues and eigenvectors of coupled mode system of equations are corrected to reduce the inherent errors, which arise by neglecting second-order derivatives. In the second part, a scattering matrix representation of cascaded systems is used to model the effect of boundaries. The problem under consideration is a 2-D planar grating structure illuminated by a TE polarized incident wave, but the method can be applied to other grating structures such as couplers, or any other structure for which the coupled mode approximation is used, similarly. Comparisons with results obtained using alternative methods are given to verify the accuracy of proposed method.
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