Si-Ge lateral avalanche photodiodes (Si-Ge LAPDs) are promising devices for single photon detection, but they also have technology challenges. Si-Ge LAPDs are CMOS compatible and capable of detecting photons near the 1550 nm telecommunications bands. However, the Si-Ge LAPD exhibits a unique avalanche multiplication process in silicon, where the electrons and holes follow curved paths in three-dimensional space. Traditional models for the analysis of the avalanche multiplication process assume one-dimensional paths for the carriers that undergo the chains of impact ionizations; therefore, they are not suitable for analyzing the avalanche properties of Si-Ge LAPDs. In this paper, the statistics of the avalanche process in the Si-Ge LAPD are modeled analytically using a method that was recently developed by our group for understanding the avalanche multiplication in nanopillar, core-shell GaAs avalanche photodiodes, for which the electric field is non-uniform in magnitude and direction. Specifically, the calculated mean avalanche gain and the excess noise are presented for the Si-Ge LAPD device. It is also shown that the avalanche characteristics depend upon the specific avalanche path taken by the carrier, which depends, in turn, on the lateral location where each photon is absorbed in the Ge absorber. This property can be exploited to achieve reduced excess noise as well as wavelength-sensitive single-photon detection.
Conversion of plane waves to surface waves prior to detection allows key advantages in changes to the architecture of the detector pixels in a focal plane array. We have integrated subwavelength patterned metal nanoantennas with various detector materials to incorporate these advantages: midwave infrared indium gallium arsenide antimonide detectors and longwave infrared graphene detectors.
Nanoantennas offer a means to make infrared detectors much thinner by converting incoming plane waves to more tightly bound and concentrated surface waves. Thinner architectures reduce both dark current and crosstalk for improved performance. For graphene detectors, which are only one or two atomic layers thick, such field concentration is a necessity for usable device performance, as single pass plane wave absorption is insufficient. Using III-V detector material, we reduced thickness by over an order of magnitude compared to traditional devices.
We will discuss Sandia’s motivation for these devices, which go beyond simple improvement in traditional performance metrics. The simulation methodology and design rules will be discussed in detail. We will also offer an overview of the fabrication processes required to make these subwavelength structures on at times complex underlying devices based on III-V detector material or graphene on silicon or silicon carbide. Finally, we will present our latest infrared detector characterization results for both III-V and graphene structures.
Nanoantennas are an enabling technology for visible to terahertz components and may be used with a variety of detector materials. We have integrated subwavelength patterned metal nanoantennas with various detector materials for infrared detection: midwave infrared indium gallium arsenide antimonide detectors, longwave infrared graphene detectors, and shortwave infrared germanium detectors. Nanoantennas offer a means to make infrared detectors much thinner, thus lowering the dark current and improving performance. The nanoantenna converts incoming plane waves to more tightly bound and concentrated surface waves. The active material only needs to extend as far as these bound fields. In the case of graphene detectors, which are only one or two atomic layers thick, such field concentration is a necessity for usable device performance, as single pass absorption is insufficient. The nanoantenna is thus the enabling component of these thin devices. However nanoantenna integration and fabrication vary considerably across these platforms as do the considerations taken into account during design. Here we discuss the motivation for these devices and show examples for the three material systems. Characterization results are included for the midwave infrared detector.
We show simulation results of the integration of a nanoantenna in close proximity to the active material of a photodetector. The nanoantenna allows a much thinner active layer to be used for the same amount of incident light absorption. This is accomplished through the nanoantenna coupling incoming radiation to surface plasmon modes bound to the metal surface. These modes are tightly bound and only require a thin layer of active material to allow complete absorption. Moreover, the nanoantenna impedance matches the incoming radiation to the surface waves without the need for an antireflection coating. While the nanoantenna concept may be applied to any active photodetector material, we chose to integrate the nanoantenna with an InAsSb photodiode. The addition of the nanoantenna to the photodiode requires changes to the geometry of the stack beyond the simple addition of the nanoantenna and thinning the active layer. We will show simulations of the electric fields in the nanoantenna and the active region and optimized designs to maximize absorption in the active layer as opposed to absorption in the metal of the nanoantenna. We will review the fabrication processes.
We demonstrate the effects of integrating a nanoantenna to a midwave infrared (MWIR) focal plane array (FPA). We
model an antenna-coupled photodetector with a nanoantenna fabricated in close proximity to the active material of a
photodetector. This proximity allows us to take advantage of the concentrated plasmonic fields of the nanoantenna. The
role of the nanoantenna is to convert free-space plane waves into surface plasmons bound to a patterned metal surface.
These plasmonic fields are concentrated in a small volume near the metal surface. Field concentration allows for a
thinner layer of absorbing material to be used in the photodetector design and promises improvements in cutoff
wavelength and dark current (higher operating temperature). While the nanoantenna concept may be applied to any
active photodetector material, we chose to integrate the nanoantenna with an InAsSb photodiode. The geometry of the
nanoantenna-coupled detector is optimized to give maximal carrier generation in the active region of the photodiode, and
fabrication processes must be altered to accommodate the nanoantenna structure. The intensity profiles and the carrier
generation rates in the photodetector active layers are determined by finite element method simulations, and iteration
between optical nanoantenna simulation and detector modeling is used to optimize the device structure.
We review the physics of photon-phonon coupling in guided wave systems, and discuss new opportunities for
information transduction aorded by nanoscale connement of light and phonons within a novel class of optome-
chanical waveguide systems. We present a fundamental analysis of optical forces generated through nanoscale
light-matter interactions, and use these insights to develop new approaches for broadband signal processing via
optomechanics. Recent experimental results will also be discussed.
Recent advance in controlling optical forces using nanostructures suggests that nanoscale optical waveguides are capable
of generating coherent acoustic phonons efficiently through a combination of radiation pressure and electrostriction. We
discuss the critical roles of group velocity in such processes. This photon-phonon coupling would allow an acoustic
intermediary to perform on-chip optical delay with a capacity 105 greater than photonic delay lines of the same size.
We present design, fabrication, and characterization results of a highly absorptive surface in the thermal
infrared that draws on concepts from the frequency selective surface and metamaterials communities. At
normal incidence this optically thin surface has an absorption of over 99%. Furthermore, it has a broad
angular range (over 90% absorption at 60 degrees from normal). The simple structure is composed of a
reflective metal layer, a roughly quarter-wave layer of lossy dielectric, and a top metal layer that is patterned
with an array of subwavelength apertures. The design of the aperture allows spectral and angular control of
the absorption/emission band. We will present simulation and measured results. Change in waveband and
polarization could easily be changed from pixel to pixel in a focal plane array.
Straightforward extension of canonical microwave metamaterial structures to optical and IR frequency dimensions is
complicated by both the size scale of the resulting structures, requiring cutting edge lithography to achieve the requisite
line-widths, as well as limitations on assembly/construction into final geometry. We present a scalable fabrication
approach capable of generating metamaterial structures such as split ring resonators and split wire pairs on a micron/sub-micron
size scale on concave surfaces with a radius of curvature ~ SRR diameter. This talk outlines the fabrication
method and modeling/theory based interpretation of the implications of curved metamaterial resonators.
In this paper, we will outline general mathematical techniques applied to the solution of the inverse problem for
partially coherent lithographic imaging. The forward imaging problem is reviewed and its solution is discussed
within the framework of 2D sampling and matrix coherence theory. The intensity distribution on the wafer is
shown to be a bilinear functional in the sampled mask transmission values, and represents a continuous sparse set
of variables for optimization. We review various iterative techniques to optimize the sampled mask transmission,
called a tau-map. From the optimal tau-map, a procedure is required to construct a pixelated mask representation
with restricted transmission values. This mask representation is not unique since the problem is ill-posed, and
leads to multiple mask solutions for a single optimal tau-map. Various procedures based on spectral techniques
and principle component analysis to quantize the mask are reviewed.
Coupled eigenmode (CEM) theory for TM polarized illumination is presented and applied to the 3D modeling of a linespace
reticle. In this approach, the electric and magnetic field inside a line-space reticle is described in terms of an
orthogonal set of eigenmodes of Maxwell's equations. The diffraction of light by the reticle can then be expressed as a
coherent sum of diffraction orders produced by each eigenmode independently. Fresnel transmission, overlap of
eigenmodes with diffraction orders and propagation through the mask are shown to be the interactions that determine the
complex amplitude of the diffraction orders produced by each mode. We further shown that only a small number of
eigenmodes are needed to accurately calculate image contrast under TM polarized illumination.
Coupled eigenmode (CEM) theory is presented and applied to the 3D modeling of a line-space reticle. In this approach,
the electric field inside a line-space reticle is described in terms of an orthogonal set of eigenmodes of Maxwell's
equations. The diffraction of light by the reticle can then be expressed as a coherent sum of diffraction orders produced
by each eigenmode independently. Fresnel transmission, overlap of eigenmodes with diffraction orders and propagation
through the mask are shown to be the interactions that determine the complex amplitude of the diffraction orders
produced by each mode.
CEM is then applied to the cases of a binary mask and an att-PSM under dipole illumination. It is shown that the
behavior of contrast with pitch and mask bias is primarily affected by the propagation loss of the eigenmodes, which
increases for smaller trench widths. In the case of the binary mask, this attenuation causes one eigenmode to become
dominant and the resultant image approaches the perfect imaging of a single eigenmode. In the case of att-PSM, this
attenuation causes a detuning of the transmission and phase, and thus, the image contrast is degraded.
As integrated circuit interconnect dimensions continue to shrink and signaling frequencies increase, interconnect performance degrades. The performance degradation is due to several factors such as power consumption, cross-talk, and signal attenuation. On-chip optical interconnects are a potential solution to these scaling issues because they offer the promise of providing higher bandwidth. In this paper, progress on the major on-chip optical building blocks will be reviewed. It will be shown that significant advances have been made in the design and fabrication of waveguides, detectors, and couplers. However, major challenges in high speed electrical to optical conversion and signaling remain.