• unlimited access with print and download
    $ 37 00
  • read full document, no print or download, expires after 72 hours
    $ 4 99
More info
Unlimited access including download and printing, plus availability for reading and annotating in your in your Udini library.
  • Access to this article in your Udini library for 72 hours from purchase.
  • The article will not be available for download or print.
  • Upgrade to the full version of this document at a reduced price.
  • Your trial access payment is credited when purchasing the full version.
Buy
Continue searching

Photon manipulation in silicon nanophotonic circuits

ProQuest Dissertations and Theses, 2011
Dissertation
Author: Ali Wanis Elshaari
Abstract:
CD8+ T cells are the branch of the adaptive immune system responsible for recognizing and killing tumor cells or cells infected with intracellular pathogens, such as Listeria monocytogenes (LM). However, when CD8+ T cells target our own tissues, they can cause autoimmune diseases, such as type I diabetes, rheumatoid arthritis. For CD8+ T cells to fulfill these functions, the T cell receptors (TCRs) on CD8+ T cells must recognize pathogens or antigens presented on the surface of target cells. TCR ligation triggers multiple signaling pathways that lead to the activation and proliferation of CD8+ T cells. The goal of our research is to define the TCR-proximal signaling events that regulate CD8+ T cell-mediated immunity. To accomplish this goal, we are focusing on an adaptor protein Gads, which is critical for optimal TCR-mediated calcium mobilization. We reported the first analysis of the function of Gads in peripheral naïve CD8+ T cells. To examine the function of Gads in CD8+ T cell mediated immune responses, we crossed Gads-/- mice with mice expressing an MHC class I-restricted transgenic TCR recognizing ovalbumin (OVA). The transgenic mice are called ovalbumin-specific T cell receptor-major histocompatibility complex class I restricted (OT-I) mice. We investigated the effect of Gads on the proliferation of CD8 + T cells following stimulation with peptide antigen in vivo and in vitro. We stimulated splenocytes from Gads+/+ OT-I and Gads -/- OT-I mice with the peptide agonist. The experiments revealed that Gads is required for optimal proliferation of CD8+ T cells. The regulation of Gads is most evident at the early time points of proliferation. Then we demonstrated that Gads-/- CD8+ T cells have impaired TCR-mediated exit from G0 phase of the cell cycle. In addition, Gads-/- CD8+ T cells have delayed expression of c-myc and the activation markers CD69 and CD25, upon stimulation with peptide antigen. Next, we investigated how Gads affects CD8+ T cell-mediated immunity in the context of infection with LM. We adoptively transferred naïve CD8+ T cells from Gads+/+ OT-I mice and/or Gads -/- OT-I mice into congenic wild-type hosts. Then the recipient mice were infected with recombinant LM expressing ovalbumin (rLM-OVA). The CD8 + T cells from OT-I mice recognize and respond to the ovalbumin provided by this strain of LM. By using this system, we investigated how Gads regulates the activation of antigen-specific CD8+ T cells as well as the expansion and memory phases of CD8+ T cell-mediated immune responses following infection with rLM-OVA. We also examined the recall response of CD8+ T cells after the secondary encounter with the same pathogen. Our data demonstrated that Gads regulates the expression of activation markers CD69 and CD25 of antigen-specific CD8+ T cells but Gads is not required for the onset of accumulation of antigen-specific CD8 + T cells following infection. However, Gads is critical to sustain the expansion of CD8+ T cell-mediated immune response following infection. Although the differentiation of naïve CD8+ T cells into memory cells is independent of Gads, Gads is required for an optimal recall response. Our data indicating that Gads regulates the initiation of proliferation of CD8+ T cells upon TCR ligation by peptide antigen seemed to contradict with our in vivo infection data showing that Gads is not required for the initiation of expansion of CD8+ T cell population. In order to explain the "discrepancy", we hypothesized that the homotypic interactions among CD8+ T cells compensate for Gads deficiency at the initial stage of accumulation of antigen-specific CD8+ T cells upon infection. Our data indicated that the need for Gads in cell cycle progression of CD8+ T cells when total splenocytes were stimulated could be overcome by stimulating purified CD8 + T cells. These data suggested that the homotypic interactions among CD8+ T cells facilitate the TCR signaling so as to compensate for Gads deficiency in promoting cell cycle entry and proliferation. To conclude, the role of Gads in TCR-mediated activation and proliferation of CD8+ T cells is dependent on the interactions of CD8 + T cells and their partners. Interestingly, if CD8+ T cells interact with non-CD8+ T cells, Gads regulates the kinetics of cell cycle entry; however, if CD8+ T cells interact with other CD8+ T cells, Gads is dispensable for cell cycle entry of CD8+ T cells. Overall, these studies will help us better understand how TCR-proximal signaling regulates the activation of CD8 + T cells.

TABLE OF CONTENTS

ABSTRACT

................................ ................................ ................................ ......................

iv

DEDICATION ................................ ................................ ................................ .................... v

ACKNOWLEDGMENTS

................................ ................................ ...............................

vi

TABLE OF CONTENTS

................................ ................................ ...............................

vii

LIST OF FIGURES

................................ ................................ ................................ .........

x i

THESIS ORGANIZATION

................................ ................................ ..........................

xix

1. INTRODUCTION ................................ ................................ ................................ .......... 1

1.1

Silicon Photonics Over view

................................ ................................ ...................... 1

1.2

Resonant Photonic Structures

................................ ................................ .................... 5

1.2.1

Traveling - Wave Ring Resonators

................................ ................................ . 5

1.3

Simulation Methods

................................ ................................ ................................ .. 8

1.3.1

Time - D omain Coupled Mode Equations

................................ ...................... 8

1.3.2

Finite Difference Time Domain (FDTD) Equations

................................ ... 10

2. SINGLE PHOTON - CAVITY ELECTRODYNAMI CS (QED)

.............................. 13

2.1

Introduction

................................ ................................ ................................ ............. 13

2.2

Single Photon - Cavity Interaction

................................ ................................ ............ 1 4

2.2.1

Steady State Theoretical Model

................................ ................................ .. 1 4

2.2.2

Experimental Results

................................ ................................ ................... 23

2.2.3

Dynamic Theoretical Model

................................ ................................ ........ 27

viii

2.3

Photon - P air

Generation in Silicon Wire

................................ ................................ .. 30

2.3.1 Theoretical Model

................................ ................................ ....................... 30

2.3.2 Experi mental Results

................................ ................................ ................... 35

2.4 Summary

................................ ................................ ................................ ................. 39

3 . PHOTON EIGEN ENERGY MANIPULATION

................................ ..................... 40

3 .1

Introduction

................................ ................................ ................................ ............. 42

3 .2

Full Quantum Mechanical Analysis

................................ ................................ ........ 1 8

3 .3 Experimental Results

................................ ................................ ............................... 47

3 . 4

Summary

................................ ................................ ................................ ................. 48

4 . PHOTON TRAPPING IN EIT - LIKE SYSTEMS

................................ .................... 49

4 .1

Introduction

................................ ................................ ................................ ............. 49

4 .2 Passive Device Analysis

................................ ................................ .......................... 55

4 .3

Dynamic Photon Trapping

................................ ................................ ...................... 59

4 .4

Experimental Results

................................ ................................ ............................... 61

4 .5

Quantum Mechanical Analysis

................................ ................................ .............. 64

4.5.1

Theoretical Model

................................ ................................ ...................... 64

4.5.2

Simulation Results

................................ ................................ ...................... 67

4.6

Summary

................................ ................................ ................................ ................ 68

5 . ENGINEERED TRANSITIONS IN PHOTONIC MOLECULES

......................... 69

5 .1

Introduction

................................ ................................ ................................ ............. 69

ix

5 .2

Designing System States

................................ ................................ ......................... 71

5 . 3

Red Transitions with

Blue Index Change

................................ ................................ 74

6 . NON - MAGNETIC CMOS COMPATIBLE PHOTON ISOLATOR

..................... 79

6 .1

Introduction

................................ ................................ ................................ ............. 79

6 . 2

Adiabatic Wavelength Conversion

................................ ................................ .......... 81

6 . 2.1

Frequancy Domain Sampling Technique

................................ .................... 86

6 . 2.2

Time Domain Sampli ng Technique

................................ ........................... 88

6.3

Uni - Directional Active Isolator

................................ ................................ ............... 89

6.4

Bi - Directional Active Isolator

................................ ................................ ................. 96

6.5

Summary

................................ ................................ ................................ ................. 98

7 . BROADBAND ULTRA - FAST PHOTON SWITCH

................................ ............... 99

7.1

Introduction

................................ ................................ ................................ ............. 99

7 . 2

Modulator Design

................................ ................................ ................................ .. 101

7 . 3

Electrical and Optical Modeling

................................ ................................ ............ 102

7 . 5

Performance Analys is

................................ ................................ ............................ 104

7 . 5

Summary

................................ ................................ ................................ ............... 115

8. CONCLUSION AND FUTUR E DIRECTION

................................ ....................... 116

8.1

Conclusion

................................ ................................ ................................ ............. 116

8.2

Major Contributions

................................ ................................ .............................. 116

8.3

List of My Publications

................................ ................................ ......................... 117

x

8. 4

Future Direction

................................ ................................ ................................ .... 1 20

APPENDICES

................................ ................................ ................................ ................ 1 21

Appendix I:

Ring Resonator with Carrier Injection ( C lassical CMT m odel Matl ab )

1 21

Appendix II :

Single Photon - Cavity Dynamics (Q uantum FDTD

model Fortran )

..... 1 2 4

Appendix III :

Photonic Trapping

( Classical CMT model Matlab )

............................ 1 3 2

REFERENCES

................................ ................................ ................................ ............... 1 3 9

xi

LIST OF FIGURES

1.1

SEM of SOI waveguide with mode profle adapted from [1]

................................ ... 2

1.2

SEM

omages of some Passive SOI devices (Ring resonators, photonic crystal waveguide. and splitters)

................................ ................................ ....................... 2

1.3

SEM image of a ring resonator with different fields coupling to/frim the system…

................................ ................................ ................................ ................. 6

1. 4

Transmission spectrum of aring resonator . . ................................ ............................ 7

1. 5

Injected carrier s effect on ring resonator transmission . .

................................ ....... 10

1. 6

Field evolution order in 1D - FDTD problem, adapted from [41] . .

....................... 12

1. 7

2D simulation of SOI ring resonator on resonance . .

................................ ............ 12

2 . 1

Schematic of waveguide - cavity coupled system

. .

................................ ............... 15

2 . 2

Transmission with differe nt coupling conditions. The transmission is 0%, 35%, and 55% for critically - coupled, under - coupled, and over - coupled system respectively . .

................................ ................................ ................................ ........ 18

2 . 3

Contours in the cmplex z - plane used for the inversion of the Fourier Transform of the waveguide amplitude for

cases (a) x>0 and (b) x<0 . .

................................ 21

2 . 4

SEM image of a compact Mach - zhender interferometer with integrated ring resonators. This system was used to verify single photon interactions with resonant cavities. . .

................................ ................................ ............................... 24

2 . 5

Experimental setup used to measure single ph oton interference . .

........................ 25

xii

2 . 6

Measured Transmission of a small bandwidth (long duratation) and large bandwidth photon through the ring resonator (left) and through the output port of the device (right) . .

................................ ................................ ................................ . 26

2 . 7

Transmission of a Gaussian packet through the ca vity waveguide system

.......... 28

2 . 8

FWM process in silicon waveguides

................................ ................................ .... 32

2 . 9

Schematic of waveguide - cavity coupled system

................................ .................. 34

2 . 10

Schematic of photon pair generation in SOI ring resonator

................................ . 35

2 . 11

Experimental setup for generating correlated photons in silicon chi p

................. 36

2 . 12

Correlated photons generation in silicon micro - ring resoantor

............................ 36

2 . 13

Single photon interference in MZI

................................ ................................ ....... 38

2 . 14

Interference pattern dependence on the number of photons

................................ . 39

3 . 1

Schematic of photon energy lifter. The cavity is adiabatically tuned a t time t 0 from one stationary state to another .

................................ ................................ .... 42

3 . 2

Single photon energy state changes adiabatically as a function of the cavity tuning frequency (the tuning changes frin negative ti positive going from left to rght) .

................................ ................................ ................................ ..................... 43

3 . 3

Single photon final state as

a function of the tuning time. The conversion efficiency degrades with slower tuning (the switching time changes from zero to one photon life time going from back to front) .

................................ ................... 45

xiii

3 . 4

the state inside the cavity changes with 100% ffficiency .

During the trans isiton period the photon follows the state of the cavity (new/old wavelength amplitude changes from 0/1 after the tuning)

................................ ................................ ....... 46

3 . 5

Experimental setup for single photon state manipulation through dynamic control of silicon ring resonator .

................................ ................................ ....................... 47

3 . 6

Single photon

transitions in a ring resonator. The central large peak is the initial wavelength of the photons and is tuned to one resonance. When the resonators is dynamically tuned using the free - carrier plasma dispersion effect the photons make transitions to other resonator states (the two smaller peaks) .

..................... 48

4 .1

Schematic the system and its operation principle, (Step 1) shows the acceptance state of the system. Bits are stored as shown in (Steps 2 and 3) then released in (Steps 4 and 5) . .

................................ ................................ ................................ .... 50

4 .2

(a) Numerical simula tions of storing a 20ps pulse in the proposed system with an injected carrier density of 5E18cm - 3 . ( b)

Same data is stored in the EIT device proposed in [28]

with an injected carrier density of 5E17cm - 3 . .

......................... 53

4 .3

Numerical simulations of the stored power af ter 100picoseconds in the storage unit with different carrier densities. Our proposed scheme is in blue while a comparable EIT system in red . .

................................ ................................ ............ 55

4 .4

(a) Schematic of the capture and release process of the three ring system similar to Fig.4.1. (b) Scanning

electron microscope image of the fabricated device with three ring resonators . .

................................ ................................ ........................... 56

xiv

4 .5

(A) Shows the transmission through the middle waveguide without heating (Open EIT), while (B) shows the transmission with heating of the blue storage ring (Closed EIT),

finally (C) depicts the case when all the rings are approximately in resonance through heating of the input ring and the blue storage ring . .

................................ ................................ ................................ ...................... 58

4 .6

Experimental setup.The stored pulses are generated in an OPO crystal from 830nm Ti - Sapphire laser, while S HG is used to generate 415nm storage and release signals . .

................................ ................................ ................................ ..... 60

4 .7

Different delays are measured through changing the time between the store and the release top pumping pulses . .

................................ ................................ ........... 62

4 .8

Different delay measurements and data fit, the system has an intrinsic d ecay time of ~160ps . .

................................ ................................ ................................ ............ 64

4 .9

Schematic the single photon storage unit, (Step 1) shows the acceptance state of the system. Bits are stored as shown in (Steps 2) then released in (Steps 3) . .

..... 4 7

4 .10

Storage and release process of single photon wave packet. T he input Gaussian pulse is shown at time step=600. The stored packet amplitude extends from time steps=600 - 3600. The released wave packet decays from the storage unit at time step=3600 . .

................................ ................................ ................................ ........... 49

xv

5 .1

Photonic transitions in atom - like photonic structure. Li ght initially excites one state of the resonator (t

multiple output states are excited (t>t0). Δω is the relative shift of all of the states due to the refractive index change ..

................................ ............................ 70

5 .2

Photonic transitions in molecule - like photonic structure. Photonic transitions are engineered using the resonance of the input and output cavities. Only one state is allowed after switching the transition cavity. The remaining transitions lie inside forbidden states in the energy diagram ..

................................ ............................... 71

5 .3

The building block of the photonic molecule consists of a single cavity. The fi elds are related through coupling coefficients and time evoltuion operators equations 1 - 3. Radiation and scattering losses are lumped in the

field absorption cofficient γ ..

................................ ................................ ................................ .......... 72

5 .4

Transmission of different rings in the intial state of the system. The input cavity and the transition cavity have the same resosnance condition, while the output cavity is purposly shifted toward red wavelengths hal f FSR away ..

.................... 74

5 .5

States before the switching (blue) and after switching (red) inside the transition ring. The conversion efficiency is 96% . .

................................ .............................. 75

5 .6

(a) FDTD result of red wavelengths created in the transition ring.

(b) Field distribution in the syste m after the conversion. The converted light is now coupled to the output port through the output ring, which is initially red shifted …..

................................ ................................ ................................ ............. 76

xvi

5 .7

Red wavelength conversion efficiency vs. switching time. The efficiency decreases as switching speed is slowed d ue to the enhancement of the adiabatic shift in the resonator ..

................................ ................................ ........................... 78

6 .1

Schematic of the

isolator. A red input signal is converted to a blue output signal by a adiabatic wavelength conversion unit (Red to Blue). If any of the blue signal is reflected it is filtered out so that it cannot propagate back to the input of the circuit . .

................................ ................................ ................................ .................. 81

6 .2

Ideal wavelength conversion system . .

................................ ................................ .. 84

6 .3

L ong pulse (a), and a short pulse (b) coupled to a cavity while dynamically tuning the resonance of the cavity . .

................................ ................................ ...... 85

6 .4

F requency

sampling of the input spectrum . . ................................ ......................... 86

6 .5

S chematic of FDS system . .

................................ ................................ ................... 87

6 .6

S chematic of TDS system . .

................................ ................................ .................. 88

6 .7

Schematic of the isolator . .

................................ ................................ .................... 90

6 .8

F orward propagation simulation, where no power is coupled to the input ring ….. . .

................................ ................................ ................................ ................ 91

6.9

I nput signal (first), and the output signal (second) . .

................................ ............. 92

6.10

Mirror reflectivity with wavelength . .

................................ ................................ ... 93

6.11

Backward propagation . .

................................ ................................ ........................ 94

6.12

S pectrum of forward and backward signals . .

................................ ....................... 95

6.13

Back reflected waves couple to the input when the conversion is not used . .

....... 95

xvii

6.14

S chemati c of bidirectional isolation system . .

................................ ....................... 96

6.15

The spectrum of the wavelength conversion unit with the different resonances

,

and

(blue green and red) . .

................................ ................................ ....... 97

7 .1

Cross - section of the Schottky diode waveguide modulator. The device consists of a lightly doped center region where the light is confined. A 50nm wide Schottky contact is attached to the top. A 50nm highly doped rib is at the bottom of the waveguide whe re ohmic contacts (700nm away from the center region) are attached. The waveguide is embedded in silicon dioxide. The device length is 100 µm (not to scale) . .

................................ ................................ ........................ 102

7.2

The hole carrier concentration density profile in the devices off - state. It is seen that

there is a very low carrier concentration in the center waveguide region where the light resides . .

................................ ................................ ...................... 105

7.3

The (A) vertical magnetic field intensity and (B) horizontal electric field for the TE mode of the waveguide . .

................................ ................................ ............... 105

7.4

(A) Real part of the effectiv e index (blue) and the amount of power confined in the central silicon region where the carrier concentration is modulated (green). (B) Imaginary part of the effective index (red) and the total insertion loss of a 100 µ m long device (green) . .

................................ ................................ ............... 106

7.5

The lo g of the hole density profile with a (A) 1.25V reverse bias and (B) a 1.25V forward bias.

The change in the absorption coefficient is calculated using the change in the electron and hole concentration with the following equation …… . .

................................ ................................ ................................ .... 107

xviii

7.6

10 Gbit/s gate

voltage applied to the Schottky modulator device

...................... 110

7.7

Gate current in the Schottky modulator device. .

................................ ................ 110

7.8

Optical response of the Schottky modulator to the gate voltage applied in Fig.7.7. It is seen that a modulation depth of > 4.6dB is achieved a t a bit rate of 10 Gbit/s . .

................................ ................................ ................................ ................ 111

7.9

Hole concentration over one period of an applied gate signal. It is seen that the rise time of the concentration is only 30 picoseconds and the fall time is 60 picoseconds . .

................................ ................................ ................................ ....... 111

7.10

Insertion loss and modulation depth o f the device as a function of the device length . .

................................ ................................ ................................ ................ 114

xix

THESIS ORGANIZATION

Chapter 1 : review of silicon photonics is provided with emphasis on traveling wave cavities and their simulation methods.

Chapter 2: a fully quantum mechanical study of steady st ate and dynamic single photon - cavity interaction is provided with experimental demonstration of photon interference and generation of photon pairs.

Chapter 3:

full theor etical analysis of photon eigen energy state manipulation with experimental demonstration of adiabatic shifts and discrete transitions.

Chapter 4:

proposal, analysis, and experimental demonstration of photonic memory on chip using

EIT like system.

Chapter 5:

theoretical and numerically analysis of controlling transitions in a mole cule - like photonic structure .

Chapter 6:

proposal and numerical analysis of an optical isolator using a system of active cavities to break the time reversibility.

Chapter 7: analysis of a novel broadband photon switch by integrating Schottky diodes with si licon wires.

Chapter 8:

concludes the dissertation and states major contributions along with future direction.

1

CHAPTER ONE

INTRODUCTION

1.1

Silicon Photonics Overview

Quantum - based Silicon photonic communication system s

can potentially achieve

the ultimate security from eavesdropping and greatly

reduce

the operating powers.

Light speed transmission ,

noise immunity, and low noise properties make photons indispensable for quantum communication to transfer a quantum state through a transmission line .

As a result ,

more work needs to done

to develop the building blocks for photo n manipulation in silicon nanophotonic circuits. This is also driven by the attractive and promising improvements this field has to offer in high speed communication systems and on chip optical interconnects. Silicon has been the main platform for electron ic integrated circuits due to its desirable electronic properties, high isolation of its native oxide, low cost ,

and

well developed processing schemes . It also has desirable optical properties

to work as a platform for integrated optical systems at

the tel ecommunication wavelength of 1550nm . Some of

these properties are

the low optical loss

and the high index contrast with its native oxide . This

provide s high mode confinement for optical waveguide s ,

which

enables the integration of high density systems relying on the already developed technologies in the CMO S

processes. Fig . 1 .1 shows a Scanning Electron Microscope

(SEM) i mage of silicon on insulator (SOI) waveguide and

its mode profile.

Furthermore, silico n wires

can serve as a photon source. As shown in Fig.1.1

there is a strong light confinement due to the

high refractive - index contrast.

2

Fig. 1 .1

SEM image of SOI waveguide with the mode profile

adapted from [1]

Research has been conducted extensively in this area demonstrating many passive devices

such as splitters [2] , ring resonator based filter

[3,4] , dis k

resonator based filters

[5] ,

and

slow light photonic band - gap waveguides

[6,7] ,

to name few . But in order to broaden the functionalities

of

these devices ,

active control of the optical properties of silicon is needed.

Fig . 1 .2 sho ws

some

examples

of

SOI based passive devices.

Fig. 1 .2

SEM images of some passive SOI device (i.e. ring resonators, disk resonators, photonics crystal waveguides, and splitters).

3

1.1.1

Tuning the optical properties of silicon

In order to add

dynamic control of

the

passive devices ,

we need to tune the optical properties of silicon. There

are three main effects

to manipulate the refractive index of silicon :

Thermo - optic effect

Third order non - linearity ( χ 3 )

Free carrier dispersion effect.

The first effect results in a desirable high refractive index

change

per degree which enables broadband tuning of the optical system

[8,9] . But th e dra w back is the slow switching time

thermo - optic effect based devices

suffer from . This

is

due to the limitations of how

fast heat can be transferred in and out from the active region.

Typical switching times are

in the order of microseconds [10] .

Equation ( 1 .1) relates the change in the refractive index to the change in temperature.

]

]

1 .1

The third order nonlinearity χ 3

on th e other ha s an ultra - fast response of less than 10fs [11,12] . The Centro - symmetric structure of silicon gives rise to two types of them namely:

Franz - Keldysh and Kerr effects. The first manifests itself as an electro - optic absorption effect where the band gap of the silicon (E g =1.12 e.V.) shift s

with an

applied

electric field.

The wavelengths affected by this phenomena lie near the absorption wav elength of ~1.1 µ m with no pronounced changes at the desirable telecom wavelength

of 1.3

µ m or 1.55 µ m.

4

On the other hand, Kerr effect enables different exciting phenomena such as self - phase modulation, cross - phase

modulation, continuum generation [13,14] ,

and ultrafast all - optical

switching

[ 14,15] .

The

refractive index change of silicon can be

converted to

a change of the optical transmission using a n

interferometer or a resonator structure

[16]

to build switches .

The change in the refractive index is related directly to the intensity of the propagating wave in the system.

It

i s wor th mentioning here that ultra - fast second order nonlinearities such as Pockels effect can be introduced in silicon by breaking the symm etry of the crystal [17] . This is done by introducing strain to the active part of the silicon device. Such devices are currently far from practical due to the small nonlinear coefficient s observed in addition to the complexity of fabrication.

The followings are typical values of the third order nonlinearity parameters :

n 2

the intensity dependant refractive index and β

the two photon absorption coefficient.

]

] ]

1 .2

] ]

1 .3

The last effect is the free carrier dispersion effect where the refractive index of silicon and its absorption coefficient change with the free - carrier s

density [18]

. Many active devices have been demonstrated and proposed based on this effect such as electro - optic modulators [19 - 27] ,

storage units [28,29 ] , wavelength converters

[30 - 33] , and optical isolators [34] . The main advantage of this effect is providing a direct link between the electronics and optics using simple structures to inject carriers such as pin

diodes

[21,24,27] , Schottky diodes [23] , and M OS Capacitors [35]

. The following equations describe the change in the refractive index of silicon and the absorption coefficient as a function of the carrier density.

5

1 .4

1 .5

These carriers can also be generated through

absorption of high energy photons

(i.e 420nm

light ) with life time s

on

the order of ~500ps depending on the dimension of the waveguide [36] . The life times can be further reduce d

for ultra - fast all - optical switching by integrating reverse biased pin

diodes. This generates high E

field in the active region which sweeps the

carriers

Full document contains 168 pages
Abstract: CD8+ T cells are the branch of the adaptive immune system responsible for recognizing and killing tumor cells or cells infected with intracellular pathogens, such as Listeria monocytogenes (LM). However, when CD8+ T cells target our own tissues, they can cause autoimmune diseases, such as type I diabetes, rheumatoid arthritis. For CD8+ T cells to fulfill these functions, the T cell receptors (TCRs) on CD8+ T cells must recognize pathogens or antigens presented on the surface of target cells. TCR ligation triggers multiple signaling pathways that lead to the activation and proliferation of CD8+ T cells. The goal of our research is to define the TCR-proximal signaling events that regulate CD8+ T cell-mediated immunity. To accomplish this goal, we are focusing on an adaptor protein Gads, which is critical for optimal TCR-mediated calcium mobilization. We reported the first analysis of the function of Gads in peripheral naïve CD8+ T cells. To examine the function of Gads in CD8+ T cell mediated immune responses, we crossed Gads-/- mice with mice expressing an MHC class I-restricted transgenic TCR recognizing ovalbumin (OVA). The transgenic mice are called ovalbumin-specific T cell receptor-major histocompatibility complex class I restricted (OT-I) mice. We investigated the effect of Gads on the proliferation of CD8 + T cells following stimulation with peptide antigen in vivo and in vitro. We stimulated splenocytes from Gads+/+ OT-I and Gads -/- OT-I mice with the peptide agonist. The experiments revealed that Gads is required for optimal proliferation of CD8+ T cells. The regulation of Gads is most evident at the early time points of proliferation. Then we demonstrated that Gads-/- CD8+ T cells have impaired TCR-mediated exit from G0 phase of the cell cycle. In addition, Gads-/- CD8+ T cells have delayed expression of c-myc and the activation markers CD69 and CD25, upon stimulation with peptide antigen. Next, we investigated how Gads affects CD8+ T cell-mediated immunity in the context of infection with LM. We adoptively transferred naïve CD8+ T cells from Gads+/+ OT-I mice and/or Gads -/- OT-I mice into congenic wild-type hosts. Then the recipient mice were infected with recombinant LM expressing ovalbumin (rLM-OVA). The CD8 + T cells from OT-I mice recognize and respond to the ovalbumin provided by this strain of LM. By using this system, we investigated how Gads regulates the activation of antigen-specific CD8+ T cells as well as the expansion and memory phases of CD8+ T cell-mediated immune responses following infection with rLM-OVA. We also examined the recall response of CD8+ T cells after the secondary encounter with the same pathogen. Our data demonstrated that Gads regulates the expression of activation markers CD69 and CD25 of antigen-specific CD8+ T cells but Gads is not required for the onset of accumulation of antigen-specific CD8 + T cells following infection. However, Gads is critical to sustain the expansion of CD8+ T cell-mediated immune response following infection. Although the differentiation of naïve CD8+ T cells into memory cells is independent of Gads, Gads is required for an optimal recall response. Our data indicating that Gads regulates the initiation of proliferation of CD8+ T cells upon TCR ligation by peptide antigen seemed to contradict with our in vivo infection data showing that Gads is not required for the initiation of expansion of CD8+ T cell population. In order to explain the "discrepancy", we hypothesized that the homotypic interactions among CD8+ T cells compensate for Gads deficiency at the initial stage of accumulation of antigen-specific CD8+ T cells upon infection. Our data indicated that the need for Gads in cell cycle progression of CD8+ T cells when total splenocytes were stimulated could be overcome by stimulating purified CD8 + T cells. These data suggested that the homotypic interactions among CD8+ T cells facilitate the TCR signaling so as to compensate for Gads deficiency in promoting cell cycle entry and proliferation. To conclude, the role of Gads in TCR-mediated activation and proliferation of CD8+ T cells is dependent on the interactions of CD8 + T cells and their partners. Interestingly, if CD8+ T cells interact with non-CD8+ T cells, Gads regulates the kinetics of cell cycle entry; however, if CD8+ T cells interact with other CD8+ T cells, Gads is dispensable for cell cycle entry of CD8+ T cells. Overall, these studies will help us better understand how TCR-proximal signaling regulates the activation of CD8 + T cells.