High-Speed Properties of 1.55-µm-wavelength Quantum Dot Semiconductor Amplifiers and ...

236 Pages·2008·6.91 MB·English

High-Speed Properties of 1.55-µm-wavelength Quantum Dot
Semiconductor Amplifiers and Comparison with
Higher-Dimensional Structures
by
Aaron J. Zilkie
A thesis submitted in conformity with the requirements
for the degree of Doctor of Philosophy
Graduate Department of Edward S. Rogers Senior Department of
Electrical and Computer Engineering
University of Toronto
Copyright (cid:13)c 2008 by Aaron J. Zilkie
Abstract
High-Speed Properties of 1.55-µm-wavelength Quantum Dot Semiconductor Amplifiers
and Comparison with Higher-Dimensional Structures
Aaron J. Zilkie
Doctor of Philosophy
Graduate Department of Edward S. Rogers Senior Department of Electrical and
Computer Engineering
University of Toronto
2008
This thesis reports an experimental characterization of the ultrafast gain and refrac-
tive index dynamics of a novel InAs/InGaAsP/InP quantum-dot (QD) semiconductor
optical amplifier (SOA) operating near 1.55-µm wavelengths, assessing its high-speed
performance characteristics for the first time. The thesis also studies the influence of
the degree of quantum confinement on the dynamics of SOAs by comparing the zero-
dimensional (0-D) QD’s dynamics to those in 1-D InAs/InAlGaAs/InP quantum-dash
(QDash), and 2-D InGaAsP/InGaAsP/InP quantum-well (QW) SOAs, both of which
also operate near 1.55-µm wavelengths, and are made with matching or similar materi-
als and structures. The ultrafast (around 1 ps) and long-lived (up to 2 ns) amplitude
and phase dynamics of the SOAs are characterized via advanced heterodyne pump-probe
measurements with 150-femtosecond resolution. It is found that the QD SOA has an
80-picosecond amplitude, and 110-picosecond phase recovery lifetime in the gain regime,
4-6 times faster than the QDash and QW recovery lifetimes, as well as reduced ultrafast
transients, giving it the best properties for high-speed (> 100 Gb/s) all-optical signal
processing in the important telecommunications wavelength bands.
An impulse response model is developed and used to analyze the dynamics, facilitat-
ing a comparison of the gain compression factors, time-resolved linewidth enhancement
factors (α-factors), and instantaneous dynamic coefficients (two-photon absorption and
nonlinear refractive-index coefficients) amongst the three structures. The quantum-dot
ii
device is found to have the lowest effective α-factor, 2−10, compared to 8−16 in the
QW, as well as time-resolved α-factors lower than in the QW—promising for reduced-
phase-transientoperationathighbitrates. Significantdifferencesintheα-factorsoflasers
with the same structure are found, due to the differences between gain changes that are
induced optically or through the electrical bias. The relative contributions of stimulated
transitions and free-carrier absorption to the total carrier heating dynamics in SOAs of
varying dimensionality are also reported for the first time.
Examining the QD electroluminescence and linear gain spectra in combination with
the carrier dynamics also brings about conclusions on the nature of the quantum con-
finement, dot energy-level structure, and density of states—aspects of the material that
have not been previously well understood.
iii
Acknowledgements
I first would like to express my gratitude to my supervisors, Professors Stewart Aitchison
and Peter W. E. Smith, for giving me the support, and the freedom, to pursue the
research which I found most interesting and challenging. After four years of study, the
topic continues to be interesting and challenging.
I would next like to thank Claudine Allen and CIPI-S (The Canadian Institute for
Photonics Innovations Student Network) for helping to initiate the collaboration with
NRC that was the foundation for this research. This work was born out of the networking
that took place at the 2004 CIPI-S annual meeting in Sherbrooke, Quebec. Also, a big
thank-you to Philip Poole, and the IMS at NRC, for kindly donating to us at U of T the
1.55-µm QDs, the innovative material that is the product of their years of development,
so that we could have this novel material to explore and study. I hope that this work
will pave the way for continued joint research between the two institutes, and that the
full potential of 1.55-µm QDs will someday be realized!
Additional thanks goes to Joachim Meier, who offered his assistance and the mentor-
ship of his experimental mastery in the building of the heterodyne pump-probe setup,
and in improving the measurements. Thanks also to Waleed Mohammed for the regular
assistance of his experience in optics and optical theory, to Arkady Major for assistance
with getting the laser lab up and running in its early stages, and to my lab-mates Sean
Wagner, Roger D’Abreo, and Sonia Garcia-Blanco for their support over the years. I
would also like to acknowledge the time and effort put in by my thesis committee mem-
bers Professors Amr Helmy, Mo Mojahedi, and Alan Miller, who provided much valuable
feedback on the thesis.
Thanks also to my parents, and my brother, for their acceptance and endurance of
my life as a PhD student. Finally, thank-you to Michelle, for her undying support, and
for simply being the great girl that she is.
iv
Contents
Glossary xxii
1 Introduction 1
1.1 All-Optical Switching Using Semiconductor Optical Amplifiers . . . . . . 2
1.2 Nanostructured Semiconductor Optical Amplifiers as a High-Speed Solution 3
1.3 Thesis Objectives and Scope . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4 Organization of the Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2 Background: III-V Semiconductor Quantum Dots 6
2.1 Growth of Quantum Dots . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2 QD Electronic Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.3 QD Devices: Lasers and Amplifiers . . . . . . . . . . . . . . . . . . . . . 13
2.4 1.55-µm-wavelength quantum dot materials based on InP . . . . . . . . 17
2.5 Quantum Dash Materials Based on InP . . . . . . . . . . . . . . . . . . . 20
2.6 Quantum Dot Dynamics: A Review . . . . . . . . . . . . . . . . . . . . . 21
2.6.1 Fast gain recoveries in low-confinement-energy QDs –
Borri et al. 2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.6.2 Higher confinement energy QDs – Borri et al. 2001 . . . . . . . . 23
2.6.3 Longer-lived gain recoveries – van der Poel et al. 2005 . . . . . . 23
2.6.4 Effects of high-lying ES and WL states – Schneider et al. 2005,
Cesari et al. 2007, Dommers et al. 2007 . . . . . . . . . . . . . . 24
2.6.5 Quantum Dashes – van der Poel et al. 2006 . . . . . . . . . . . . 26
2.6.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3 Theory of Semiconductor Amplifier Dynamics 30
3.1 Physics of Carrier Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . 31
v
3.2 Rate Equations for the Carrier Dynamics . . . . . . . . . . . . . . . . . . 36
3.2.1 Previous Rate Equation Models . . . . . . . . . . . . . . . . . . . 37
3.2.2 Modified Dual-Lifetime CH Model . . . . . . . . . . . . . . . . . . 40
3.3 Gain Compression Factors . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.4 Impulse Response Model for Pump-Probe Measurements . . . . . . . . . 44
3.5 Phase Response and the Linewidth Enhancement Factor . . . . . . . . . 48
3.6 Example Pump-probe Responses . . . . . . . . . . . . . . . . . . . . . . 50
4 Sample Structures and Linear Characteristics 54
4.1 Semiconductor Amplifier Sample Structures . . . . . . . . . . . . . . . . 54
4.2 Linear Gain Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.2.1 Electroluminescence Spectra . . . . . . . . . . . . . . . . . . . . . 57
4.2.2 Linear Gain Coefficient Spectra . . . . . . . . . . . . . . . . . . . 58
4.2.3 Gain Saturation as a Function of Pulse Energy . . . . . . . . . . . 63
4.3 1.55-µm-wavelength Quantum Dot Electronic
Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
4.3.1 ASE Spectra Fitting . . . . . . . . . . . . . . . . . . . . . . . . . 64
4.3.2 Modelling of the Quantum Dot Electronic Structure . . . . . . . . 68
5 Nonlinear Dynamics Study 74
5.1 Heterodyne Pump-Probe Setup . . . . . . . . . . . . . . . . . . . . . . . 74
5.2 Gain and Phase Recovery Dynamics . . . . . . . . . . . . . . . . . . . . . 78
5.2.1 Long-lived Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . 78
5.2.2 Short-lived Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . 82
5.2.3 Error Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . 94
5.3 QD Dynamics Spectral Dependence . . . . . . . . . . . . . . . . . . . . . 96
5.3.1 Long-lived Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . 96
5.3.2 Short-lived Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . 99
5.3.3 Error Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . 103
5.4 Gain Compression Factors and Carrier Heating . . . . . . . . . . . . . . 103
5.4.1 The Total Pump Photon Density Sp . . . . . . . . . . . . . . . . 104
0
5.4.2 Gain Compression Factor Calculations . . . . . . . . . . . . . . . 105
5.4.3 Gain Compression Factor Results . . . . . . . . . . . . . . . . . . 107
5.4.4 Error Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . 114
vi
5.5 Summary and Device Implications . . . . . . . . . . . . . . . . . . . . . . 117
6 Linewidth Enhancement Factor Study 121
6.1 α-factors in the Literature . . . . . . . . . . . . . . . . . . . . . . . . . . 122
6.2 The Time-Resolved α-factors . . . . . . . . . . . . . . . . . . . . . . . . . 123
6.3 SOA α-factor Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
6.3.1 The All-Optical Time-Resolved α-factors . . . . . . . . . . . . . 125
6.3.2 QD Time-Resolved α-factor Spectral Dependence . . . . . . . . . 128
6.3.3 The net α-factor, α(τ) . . . . . . . . . . . . . . . . . . . . . . . . 130
6.3.4 The Effective α-factor . . . . . . . . . . . . . . . . . . . . . . . . 133
6.3.5 Error Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . 135
6.4 Laser α-factor Measurements . . . . . . . . . . . . . . . . . . . . . . . . . 136
6.4.1 The ASE Hakki-Paoli Method . . . . . . . . . . . . . . . . . . . 136
6.4.2 Laser α-factor Results . . . . . . . . . . . . . . . . . . . . . . . . 140
6.4.3 Error Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . 142
6.5 Summary and Device Implications . . . . . . . . . . . . . . . . . . . . . . 144
7 Two-Photon Absorption Study 148
7.1 Previous Work to Date . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
7.2 Calculating α and n in the Presence of Gain . . . . . . . . . . . . . . . 151
2 2
7.2.1 1/T Measurements at Transparency . . . . . . . . . . . . . . . . . 152
7.2.2 Pump-Probe Measurements . . . . . . . . . . . . . . . . . . . . . 153
7.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
7.3.1 Comparison of Pump-probe and 1/T Results . . . . . . . . . . . . 154
7.3.2 NRC QD and QW SOA α and n Results . . . . . . . . . . . . . 159
2 2
7.4 Summary and Device Implications . . . . . . . . . . . . . . . . . . . . . . 163
8 Conclusions 165
8.1 Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 165
8.2 Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
8.3 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
8.4 List of Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
A Cylindrical- and Hemispherical-Well Model Description 177
B Matlab Code 181
vii
Bibliography 191
viii
List of Tables
2.1 Summary of literature reports of heterodyne pump-probe measurements
on QD SOAs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4.1 Summary of the main characteristics of the sample structures studied in
this thesis. “D” = dimensionality; “width” = ridge width. . . . . . . . . 57
4.2 Energies of the ground state and excited state optical transitions calcu-
lated in the NRC QD SOA by fitting Equation 4.3 to the ASE spectrum
measured from the sample at 190 mA. . . . . . . . . . . . . . . . . . . . 66
4.3 Energy levels predicted in the NRC QD using the cylindrical well model
(h = 1.83 nm, r = 29.3 nm), and the hemispherical well model (h = 3.62
0
nm, r = 100 nm). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
0
5.1 Transparency currents of the QD, QDash, and QW SOAs. . . . . . . . . 87
5.2 Summary of the mean long-lived absorption and gain recovery lifetimes
in the SOAs measured in the thesis. Ranges indicate the maximum and
minimum values for the lifetimes that were found to have a bias dependence. 99
5.3 Summary of optical confinement factor Γ and effective area A in the
eff
QD and QW samples, and the parameters used in their calculation. λ
= wavelength, n = index of the QD or QW layers, n = index of the
w b
barrier layers, n = effective index of the 5 QD/QW and barrier
QW,eff
layers comprising the core of the active region, n = effective index
eff,Lum
of the waveguide mode simulated by Lumerical Mode. . . . . . . . . . . . 105
5.4 Summaryoftheparametersusedinthecalculationofthegaincompression
factors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
5.5 Summary of the relative errors in the parameters used in the calculation
of the gain compression factors. . . . . . . . . . . . . . . . . . . . . . . . 116
ix
6.1 Summary of the time-resolved α-factor quantities calculated from the
pump-probe dynamics in the NRC QD, CHTM QDash, and NRC QW
SOAs, as well as values from the literature. The ranges of values given
are the values from low bias currents (absorption) to high bias currents
(gain). The literature values are categorized into measurements made on
bulk, QW, or QD structures. . . . . . . . . . . . . . . . . . . . . . . . . . 143
7.1 Summary of the α , n , and nonlinear figure-of-merit T values found for
2 2
the NRC QD and QW SOAs, along with the parameters used in their
calculation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
x