Graduate

Applied Physics/Physics Colloquium: Exciton-Polariton based Photonic Circuits

Topic: 
Exciton-Polariton based Photonic Circuits
Abstract / Description: 

Exciton-Polaritons in semiconductor microcavities are hybrid states of light and matter exhibiting a mix of electronic and photonic properties [1], including strong nonlinearity, low dephasing, ultrafast dynamics, sensitivity to electric and magnetic fields and a rich spin dynamics. These properties are ideal for the construction of a new generation of polaritonic information processing devices, such as exciton-polariton based circuits [2].

For such applications, attention is needed to overcome disorder in the system. Here, advances in the patterning of polariton potentials [3] are theoretically expected to allow for topological polaritons [4]. In analogy to photonic topological insulators [5], propagation immune to scattering with disorder is predicted.

In this presentation, I will also show preliminary work in the application of exciton-polaritons in unconventional circuit architectures such as neural networks [6].

Finally, I will consider the enhancement of nonlinear polaritonic effects at the quantum level with a viewpoint toward triggered single-photon emission [7].

Date and Time: 
Wednesday, February 25, 2015 - 2:00pm to 3:00pm
Venue: 
Spilker 232

GSEE Afternoon Tea

Topic: 
EE community event
Abstract / Description: 

Take a break this Wednesday afternoon to enjoy some beverages, snacks, and conversation!

All members of the EE community - students, faculty, and staff - are welcome.

Don't forget to bring a reusable mug if you have one. Hope to see you there!

 

- Hosted by GSEE

Date and Time: 
Wednesday, February 18, 2015 - 3:00pm to 4:00pm
Venue: 
Packard Kitchen, 2nd floor

EE380 Computer Systems Colloquium: A Family of Better Random Number Generators

Topic: 
A Family of Better Random Number Generators
Abstract / Description: 

Algorithmic random number generators are everywhere, used for all kinds of tasks, from simulation to computational creativity.

Yet most people haven't given much thought to the random number generators they use. Is the RNG you're using a good source of randomness? What does it even mean to be a good RNG?

In this talk, we will examine the desirable properties of a random number generator including performance, correctness, uniformity, and unpredictability, as well as sound mathematical grounding.

We will observe how the RNGs in widespread use lack desirable properties (most commonly failing statistical tests for randomness).

Then we will show how a simple twist on a venerable-but-flawed RNG technique can provide all the properties we desire, resulting in the PCG family of RNGs.

Date and Time: 
Wednesday, February 18, 2015 - 4:15pm to 5:15pm
Venue: 
Gates B03

Information Systems Lab Colloquium: nderstanding Visual Computations in the Primate Retina

Topic: 
nderstanding Visual Computations in the Primate Retina
Abstract / Description: 

Vision begins with neural computation in the retina, which sends a highly processed version of the visual world along multiple parallel pathways to the brain. Our research is focused on understanding visual computations in the primate retina and on using this information for the design of artificial retinas to treat blindness. I will describe the state of our understanding of visual computations in the retinal circuitry, with an emphasis on open problems for future exploration, and on achieving a synthetic understanding appropriate for diverse applications.

Date and Time: 
Thursday, February 5, 2015 - 4:15pm to 5:15pm
Venue: 
Packard 101

IT-Forum: New matrix decompositions for Gaussian communication networks

Topic: 
New matrix decompositions for Gaussian communication networks
Abstract / Description: 

A central concept in matrix analysis is the decomposition of a matrix into a product of orthogonal (or unitary) matrices and a diagonal/triangular one, e.g., unitary diagonalization of a symmetric matrix, and more generally the singular-value decomposition, and the QR decomposition. Such decompositions are of particular importance for multi-antenna point-to-point physical-layer communications, where the channel gains are represented by a (channel) matrix. Transforming the channel matrix into diagonal/triangular forms, in this case, allows to reduce the coding task to that of coding for scalar (single-antenna) channels. Thus, the modulation and coding tasks are effectively decoupled and the performance is dictated by the diagonal values. In this work we develop new joint matrix decompositions of several matrices using the same unitary matrix on one side (corresponding to a joint transmitter or receiver) to achieve desired properties for the resulting diagonals. An important special case is a transformation leading to equal diagonals for all matrices simultaneously. This, in turn, allows to construct practical schemes for various communications settings, as well as deriving new theoretic bounds for others.

Date and Time: 
Friday, February 6, 2015 - 1:00pm to 2:00pm
Venue: 
Packard 202

SystemX Seminar: Wi-Fi Everywhere: Reflections on Wi-Fi’s (R)Evolution

Topic: 
Wi-Fi Everywhere: Reflections on Wi-Fi’s (R)Evolution
Abstract / Description: 

In under 2 decades, Wi-Fi has transformed from a standards’ body curiosity to an almost essential convenience in our daily lives. With a unique perspective anchored in a successful Wi-Fi semiconductor company (Atheros), Rick will trace some of this arc in his talk. The presentation will offer an overview of Wi-Fi applications, and the attendant advance of the 802.11 standard. Particular emphasis will then be offered to the supporting technology, notably, the challenges of the past and the opportunities that lie ahead. The talk will close with a broader view of Wi-Fi’s role in the emerging communications landscape. 


Questions about this series? Please contact: 

  • Dr. Richard Dasher, SystemX Executive Director, 650-725-3621
  • or Ms. Miho Nishi, SystemX Administrative Associate, 650-725-3626​

Click to join the Stanford SystemX Seminar E-mail Announcement List

SystemX Seminars: a new forum for university-industry knowledge exchange.

Date and Time: 
Thursday, February 5, 2015 - 4:00pm to 5:00pm
Venue: 
Packard 202

EE380 Computer Systems Colloquium: Deep Speech: Scaling up end-to-end speech recognition.

Topic: 
Deep Speech: Scaling up end-to-end speech recognition.
Abstract / Description: 

Speech recognition is still an unsolved problem in AI. Humans transcribe speech substantially better than machines, particularly when the speech is noisy, accented or spoken in a natural, unaffected manner. Over the past half-century slow yet steady progress has been made in speech recognition punctuated with rare breakthroughs including the Hidden Markov Model in the 70s and, more recently, Deep Neural Networks.

In fact, the past few years have witnessed large strides in many machine learning problems including speech recognition and computer vision. This is mostly due to the resurgence of Deep Learning - a class of machine learning algorithms consisting of large neural networks with many layers. Two main drivers of progress in this field have been efficient computation at scale using GPUs and the ability to acquire or construct large labeled datasets. However, as these algorithms continue to scale up, new challenges arise. In particular capturing, annotating and efficiently accessing the data needed to train these algorithms is a resource intensive problem. Furthermore, as the dataset and model sizes continue to increase, efficiently training and evaluating these networks poses a challenge.

In this presentation I will give an overview of the current state of speech recognition technology. I will also discuss the challenges we must overcome in order to make progress and eventually approach human level performance. This presentation will include a high-level introduction to Deep Learning in addition to reviewing some of the latest applications of it. I will focus on Deep Speech, a Deep Learning based speech recognition system built at Baidu Research's Silicon Valley AI lab, which has shown great potential for rapid progress in speech recognition.

Date and Time: 
Wednesday, February 4, 2015 - 4:15pm to 5:15pm
Venue: 
Gates B03

Power Electronics Seminar

Topic: 
Power Electronics Seminar hosted by Juan Rivas
Abstract / Description: 

Today's society is seeing a rapid growth of distributed electric power sources and loads, with applications ranging from large grid-integrated photovoltaic installations and data centers to small implantable medical devices and portable electronics. These applications all require sophisticated electrical energy conversion, and the power electronics solutions that provide this must realize high efficiency, small physical size, and low cost.


In this talk I will discuss our recently developed techniques for improving the power density and efficiency of power electronics through a new hybrid switched-capacitor power conversion architecture and its associated control methods. The proposed architecture leverages the 100-1000x higher energy density of commercially available capacitors compared to inductors to achieve improved power density with maintained high efficiency. The fundamental loss mechanisms that have limited the performance of conventional switched-capacitor converters will be reviewed, as well as our proposed solution to overcome some of these limitations. A split-phase control method that achieves improved efficiency and power density will be presented, along with experimental validation using GaN-based converters ranging from tens to hundreds of volts and watts, operating at switching frequencies above 1 MHz, and power densities above 1011 W/in^3 (61.7 kW/L).

In the second part, I will present a power delivery architecture for extreme efficiency data centers. Through the use of a series-stacked structure with differential power processing converters, bulk power can flow without power conversion, and only differential power between servers is converted. This leads to extreme power efficiency, supported by the experimentally measured 99.8% record efficiency on our 4-server test-bed with real-world web server and computing loads. A quantitative comparison to a conventional 48 VDC power delivery architecture will be provided, showing a 40x reduction in power losses with our proposed method compared to state-of-the-art solutions.

Date and Time: 
Tuesday, February 3, 2015 - 4:00pm to 5:00pm
Venue: 
AllenX Auditorium

Stanford Optical Society Seminar: High-speed operation, wavelength, and mode control in vertical-cavity surface-emitting lasers

Topic: 
High-speed operation, wavelength, and mode control in vertical-cavity surface-emitting lasers
Abstract / Description: 

We address recent achievements in Vertical–Cavity Surface–Emitting Lasers for data communication. (i) Recent concepts for high speed VCSEL operation include anti–waveguiding cavity design with AlAs–rich core, further increased optical confinement factor, engineering of the density of states, thick oxide apertures and superlattice barriers aimed at prevention of the leakage of nonequilibrium carriers. Serial data transmission up to 50Gb/s is realized in laser modules without preemphasis and equalization. The expected lifetime of such VCSELs exceeds 10 years at 95oC. (ii) Electrooptically–modulated VCSELs allow optical modulation bandwidth beyond 35GHz and electrical bandwidth exceeding 60GHz. So far error-free digital data transmission at 10Gb/s is realized. With effort 100 Gb/s operation at a low current density and ultralow power consumption can become feasible. (iii) VCSEL design may allow uncooled wavelength multiplexing, for example within the narrow 840–860 nm spectral range of low modal dispersion of the standard multimode fiber. Complete temperature stability of the VCSEL is achieved due to the passive cavity concept. The gain medium is placed in the region of the bottom semiconductor distributed Bragg reflector (DBR) while the further part of the bottom DBR, the cavity region and the top DBR are made of dielectric materials. Due to the virtually no dependence of the refractive index on temperature at certain dielectric compositions, a temperature stabilized operation without cooling becomes possible. Furthermore, due to dielectric DBRs and a cavity offer a high optical confinement factor even for InP-based 1300nm - 1550nm VCSELs extending the range of VCSEL applications. (iv) Single mode VCSELs at moderate oxide diameters of the oxide aperture (5-6 µm), fully compatible to the standard technology, are feasible by the optical field engineering in the oxidized part. The leakage is engineered to suppress the high order transverse optical modes. The effect is achieved by a proper positioning of thick aperture oxide layers, inducing an optical mode suitable for the leakage. The mode engineering effect can be also used, as opposite, to create a 3D confinement of the optical modes in the microcavity allowing a long lifetime of the VCSEL modes in a broad spectral range allowing, for example, near field VCSEL. (v) Single mode operation allows to overcome effects related to significant spectral dispersion of the multimode fiber (MMF) in the 840–860 nm range. A 1000 m error–free transmission at 25Gb/s is realized in parallel MMF links using single mode VCSEL arrays in combination with commercially available array electronics and standard optical couplers assembled into parallel 12-channel transceiver and receiver boards.

This seminar is sponsored by Stanford OSA

Date and Time: 
Monday, February 2, 2015 - 2:00pm to 3:15pm
Venue: 
Y2E2 299

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