IPS Laser Workshop - 1 PWJ121024
Paul W. Juodawlkis Electro-Optical Materials and Devices Group
24 October 2012
50 Years of Semiconductor Lasers
This work was sponsored by the Assistant Secretary of Defense for Research and Engineering under Air Force Contract No. FA8721-05-C-0002. The opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the United States Government.
IPS Laser Workshop - 2 PWJ121024
2012 CLEO Symposium on the “50th Anniversary of the Semiconductor Laser”
• Organized by Seth Bank (UT Austin), Dan Wasserman (Illinois), and Tom Koch (U. Arizona)
• Speakers: – Marshall Nathan, IBM, “The Invention of the Semiconductor Laser” – Herbert Kroemer, UCSB, “The Double Heterostructure” – Russel Dupuis, Georgia Tech, “Materials Development for
Semiconductor Lasers” – Charles Henry, Bell Labs, “Quantum Well Lasers” – Amnon Yariv, CalTech, “Semiconductor Lasers & OEIC's” – Don Scifres, SDL Ventures, “High Power Semiconductor Lasers, – Thomas Koch, U. Arizona, “Telecom & DFB Semiconductor Lasers” – Jack Jewell, Green VCSEL, “VCSELS” – David Welch, Infinera, “Semiconductor Photonic Integrated Circuits” – Jerome Faist, ETH Zurich, “Quantum Cascade Lasers (QCLs)” – Yasuhiko Arakawa, The University of Tokyo, “Quantum Dot Lasers” – Ming Wu, UC Berkeley, “Antenna-Coupled Nanolasers and Nano-LEDs”
• 3 sessions of standing-room-only attendance (> 200 people)
IPS Laser Workshop - 3 PWJ121024
• The First Semiconductor Laser(s) - 1962 • Semiconductor Laser Advances • Other Semiconductor Laser Structures
– Vertical-cavity surface-emitting lasers (VCSELs) – High-power lasers (Tapered lasers, SCOWLs, coherent combining) – Quantum cascade lasers (QCLs)
• Recent Results – InGaN lasers emitting at green wavelengths – “Silicon” based lasers (hybrid, strained germanium) – Large-scale integration of semiconductor lasers – Thresholdless nano-scale lasers
• Summary
Outline
IPS Laser Workshop - 4 PWJ121024
R. J. Keyes and T. M. Quist, “Recombination radiation emitted by gallium arsenide diodes,” presented at the Solid-State Device Research Conf., Durham, NC, July 1962.
R. J. Keyes and T. M. Quist, “Recombination radiation emitted by gallium arsenide,” Proc. IRE, vol. 50, pp. 1822-1823, Aug.1962.
Before the Semiconductor Laser: Light Emitting Diodes (LEDs) at Lincoln Laboratory
LED Emission Spectra • “When appropriately diffused
GaAs diodes are biased in the forward direction at 77 K, nearly all of the injected carriers, upon recombination, emit a photon whose energy is slightly but significantly smaller than the optically measured band-gap of GaAs.”
• “Our efficiency lies somewhere between 0.48 and 1 and perhaps closer to the latter.”
IPS Laser Workshop - 5 PWJ121024
R. H. Rediker, R. J. Keyes, T. M. Quist, M. J. Hudson, C. R. Grant, and R. G. Burgess, “Gallium arsenide diode sends television by infrared beam,” Electronics, vol. 35, pp. 44–45, Oct. 5, 1962.
R. J. Keyes, T. M. Quist, R. H. Rediker, M. J. Hudson, C. R. Grant, and J. W. Meyer, “Modulated infrared diode spans 30 miles,” Electronics, vol. 36, pp. 38–39, Apr. 5, 1963.
Free-Space Optical Communications with Light-Emitting Diodes, 1962-1963
Map showing 30-nmi path from Mt. Wachusett, Princeton, Mass. to Lincoln Laboratory, Lexington, Mass.
Reflecting Telescope LED Transmitter
5-Foot Searchlight + PMT Receiver Keyes Quist Rediker
IPS Laser Workshop - 6 PWJ121024
Published: November 1, 1962 Copyright The New York Times
• “In a striking coincidence, two companies announced independently yesterday the same scientific achievement, which, they said, promises to open the door to communication by light waves”
• “Compounding the coincidence was the fact that a third group of researchers, at a university laboratory, had reached similar success a week or less after one company”
• General Electric Research Laboratories – Robert N. Hall and associates “used an electric
current to ‘pump’ a semiconductor, a transistor-like device, to get it to emit coherent or ‘in-step’ light waves”
• International Business Machines Corp. – Marshall I. Nathan and associates “succeeded in
operating a new laser, using a semiconductor diode, that is powered directly by an electrical current rather than an external light source”
• MIT Lincoln Laboratory – Robert J. Keyes and Theodore M. Quist – “…the Lincoln team was not claiming priority. G.E.
achieved success ‘a couple of days or a week’ before the MIT group…” said a MIT spokesman
First Semiconductor Lasers Reported in the Popular Press
IPS Laser Workshop - 7 PWJ121024
First Semiconductor Lasers Reported in the Technical Literature
Organization Reference Material System
Date Received
Date Published
General Electric
(Schenectady)
R. N. Hall, G. E. Fenner, J. D. Kingsley, T. J. Soltys, and R. O. Carlson, “Coherent light emission from GaAs junctions,” Phys. Rev. Lett., v. 9, p. 366, 1962.
GaAs 24 Sept. 1962
1 Nov. 1962
IBM
M. I. Nathan, W. P. Dumke, G. Burns, F. H. Dill, Jr., and G. Lasher, “Stimulated emission of radiation from GaAs p-n junctions,” Appl. Phys. Lett., v. 1, p. 62, 1962.
GaAs 4 Oct. 1962
1 Nov. 1962
General Electric
(Syracuse)
N. Holonyak, Jr., and S. F. Bevacqua, “Coherent (visible) light emission from Ga(As1-xPx) junctions,” Appl. Phys. Lett., v. 1, p. 82, 1962.
GaAsP 17 Oct. 1962
1 Dec. 1962
MIT Lincoln Laboratory
T. M. Quist, R. H. Rediker, R. J. Keyes, W. E. Krag, B. Lax, A. L. McWhorter, H. J. Zeigler, “Semiconductor maser of GaAs,“ Appl. Phys. Lett, v. 1, p. 91, 1962
GaAs 5 Nov. 1962
1 Dec. 1962
IPS Laser Workshop - 8 PWJ121024
Semiconductor Laser - Version 1.0
Gallium Arsenide (GaAs) Homojunction Diode Laser
• Material grown via vapor transport
• Zinc (Zn) diffused p-n junction
• Cleaved or polished facets
• Operated at cryogenic temperatures (77 K)
• Operated under pulsed conditions (I > 10 A, τ ~ few µs, Jth ~ 10 kA/cm2)
• High optical losses (αi ~ 104 cm-1)
• Electrical-to-optical conversion efficiency ~ 0.01%
• Problems: (i) Large active volume, (ii) Large losses outside gain region
MIT-LL
GaAs
IPS Laser Workshop - 9 PWJ121024
• The First Semiconductor Laser(s) - 1962 • Semiconductor Laser Advances • Other Semiconductor Laser Structures
– Vertical-cavity surface-emitting lasers (VCSELs) – High-power lasers (Tapered lasers, SCOWLs, coherent combining) – Quantum cascade lasers (QCLs)
• Recent Results – InGaN lasers emitting at green wavelengths – “Silicon” based lasers (hybrid, strained germanium) – Large-scale integration of semiconductor lasers – Thresholdless nano-scale lasers
• Summary
Outline
IPS Laser Workshop - 10 PWJ121024
Semiconductor Laser Functional Overview
Optical Amplifier
What are the properties of the optical gain medium? • Spectral distribution (center wavelength, spectral width) • Relationship between gain and injected carrier density
IPS Laser Workshop - 11 PWJ121024
Semiconductor Laser Functional Overview
Waveguide Optical Amplifier
Heterostructure provides both optical and electronic confinement
IPS Laser Workshop - 12 PWJ121024
Fabry-Perot Laser Longitudinal
Modes
Semiconductor Laser Functional Overview
• Laser = Gain + Feedback • Multiple longitudinal modes limit applications
IPS Laser Workshop - 13 PWJ121024
Distributed Bragg Reflector (DBR) Laser
Semiconductor Laser Functional Overview
Use intracavity filter to obtain single-longitudinal-mode operation
IPS Laser Workshop - 14 PWJ121024
Distributed Bragg Reflector (DBR) Laser
Semiconductor Laser Functional Overview
Electrically adjust grating index via current injection to tune center wavelength
IPS Laser Workshop - 15 PWJ121024
Direct Modulation
Semiconductor Laser Functional Overview
• Modulate injected current to generate time-varying optical signal • Bandwidth limited by photon-carrier dynamics
IPS Laser Workshop - 16 PWJ121024
External Modulation
Semiconductor Laser Functional Overview
• Monolithically integrate modulator external to cavity to obtain higher bandwidth & lower chirp
• Laser operates continuous-wave (CW)
IPS Laser Workshop - 17 PWJ121024
Integrated Mode-Locked Laser
Semiconductor Laser Functional Overview
• Monolithically integrate modulator internal to cavity to enable active mode-locking operation
• Generate periodic train of short optical pulses
IPS Laser Workshop - 18 PWJ121024
Semiconductor Energy-Band Diagrams
Direct Bandgap Indirect Bandgap
Examples: GaAs, InP, GaN, InGaAsP Examples: Si, Ge
IPS Laser Workshop - 19 PWJ121024
Semiconductor Energy-Band Diagrams
Direct Bandgap Indirect Bandgap
Electrons
Holes
Direct Transition => Photon Emission
Photon
Indirect Transition Requires Phonon => Low Probability of Photon Emission
Photon Phonon
IPS Laser Workshop - 20 PWJ121024
Bandgap Energy vs. Lattice Constant for Common III-V Semiconductors
Lattice-Matched GaxIn1-xAsyP1-y
Ga0.47In0.53As
InP
Strained GaxIn1-xAs
Advances in Semiconductor Lasers Dependent on Improved Material Growth
• Liquid-Phase Epitaxy (LPE) • Vapor- Phase Epitaxy (VPE) • Molecular Beam Epitaxy (MBE) • Organometallic VPE (OMVPE, aka MOCVD)
Lattice-Matched AlxGa1-xAs
IPS Laser Workshop - 21 PWJ121024
Evolution of Carrier and Optical Confinement
Courtesy of Jim Coleman, University of Illinois
GaAs/AlGaAs Double Heterostructure
Double-Heterostructure Advantages: • Reduce the thickness of the active
region in the growth direction • Increase the spatial overlap between
electrons and holes • Provide index-based waveguide • Reduce the optical absorption due to
transparency of wide-gap material
Double Heterostructure
Separate-Confinement Heterostructure (SCH)
Graded-Index Separate-Confinement
Heterostructure (GRIN-SCH)
Conduction-Band Energy Profiles
Single Quantum-Well Heterostructure (QW)
Multiple Quantum-Well Heterostructure
(MQW) Ref. Herbert Kroemer, “A proposed class of hetero-junction injection lasers,” Proc. IEEE, 1963
2000 Nobel Prize
IPS Laser Workshop - 22 PWJ121024
Why Heterostructure Semiconductor Lasers Work
Holes
Electrons Intrinsic QW Region
p-type n-type
MQW Energy-Band Profile
Semiconductor Laser Cross-Section
Electrons
Conduction-Band Energy Profile
Refractive-Index Profile Transverse
Optical Mode
Material refractive index is inversely proportional to bandgap energy
=> Both carriers and optical field confined in the same region
IPS Laser Workshop - 23 PWJ121024
Impact of Spatial Carrier Confinement
Joint Density of States
3 Dimensions = Bulk
2 Dimensions = Quantum Well
1 Dimension = Quantum Wire
0 Dimension = Quantum Dot
Reduced Dimension => Reduced Density of States => Reduced Threshold Current
IPS Laser Workshop - 24 PWJ121024
First CW GaInAsP/InP Laser
Jim Hsieh
Development of near-IR (1-2 µm) lasers driven by low fiber & atmospheric losses + eye safety
IPS Laser Workshop - 25 PWJ121024
First Quantum-Well (QW) Laser
Theory (1974) Demonstration (1978)
Quantum-Well Heterostructure
IPS Laser Workshop - 26 PWJ121024
First Quantum-Dot (QD) Laser
Theory (1982) Demonstration (1994)
• Predicted negligible threshold current vs. temperature dependence for QDs
• InGaAs QDs grown via MBE using Stranski-Kranstanov method
IPS Laser Workshop - 27 PWJ121024
Basic Semiconductor Laser Fabrication: Strip-Loaded Rib Waveguide
Ridge Mask Deposition &
Patterning
Ridge Etching
Ridge Mask Removal
Insulation Deposition
Initial Epitaxial Material
Via Mask Deposition
Via Mask Patterning
Via Etching
Via Mask Removal
Top-Metal Deposition &
Patterning
Backside Wafer
Thinning
Bottom- Metal
Deposition
IPS Laser Workshop - 28 PWJ121024
Semiconductor Laser Threshold
1 2
1 1ln2
th i m
i
g
L R R
Γ α α
α
= +
= +
Threshold Modal Gain Γ = Optical Confinement Factor
gth = Threshold Material Gain Coefficient
αi = Internal Loss Coefficient
L = Cavity Length
R1,R2 = Mirror Power Reflection Coefficients
Laser Threshold Condition Round-Trip Gain = Round-Trip Loss
0 ln thth
tr
ng gn
i thth
InqV
η τ
=
=> G2A2R1R2 = 1
IPS Laser Workshop - 29 PWJ121024
Power vs. Current Characteristics
( )
( )
mO i th
i m
d th
hP I Iq
h I Iq
α νηα ανη
= − +
= −
Laser Output Power
I = Injection Current Ith = Threshold Current ηi = Internal Quantum Efficiency ηd = Differential Quantum Efficiency αi = Internal Loss Coefficient
αm = Mirror Loss Coefficient hν = Photon Energy q = Electronic Charge
O md i
i m
dPqh dI
αη ην α α
= = +
Spontaneous Emission
Stimulated Emission
Differential Quantum Efficiency
IPS Laser Workshop - 30 PWJ121024
Power vs. Current: Evidence of Lasing
T. M. Quist et al., Appl. Phys. Lett, 1962
Power-Current (L-I) Characteristic for GaAs Homojunction Laser
Amplified Spontaneous
Emission (ASE)
Spontaneous Emission
Lasing Operation
• Analysis of slope changes (“kinks”) in L-I curves are one metric for identifying lasing operation
IPS Laser Workshop - 31 PWJ121024
High-Speed Modulation
1 mW 2 2.7
5
L = 125 µm
Relaxation Oscillation Frequency Short-Cavity Laser Frequency Response
A = Differential Optical Gain Constant p0 = Steady-State Photon Density τp = Photon Lifetime
Methods to Increase Modulation Bandwidth • Increase differential gain (cooling, QWs, strain) • Increase photon density => Higher optical power • Reduce photon lifetime => Shorter cavity
Note: Also need to minimize parasitics
Bandwidth ~ Relaxation Oscillation Frequency
IPS Laser Workshop - 32 PWJ121024
Longitudinal-Mode Control
Fabry-Perot Laser Distributed Feedback
(DFB) Laser Distributed Bragg
Reflector (DBR) Laser
• Cavity formed by high- and low-reflectivity-coated facets
• Multiple longitudinal modes lasing simultaneously
• Grating fabricated along entire length of device
• Single longitudinal mode operation
• Tunable grating outside of active region
• Single longitudinal mode operation
IPS Laser Workshop - 33 PWJ121024
• The First Semiconductor Laser(s) - 1962 • Semiconductor Laser Advances • Other Semiconductor Laser Structures
– Vertical-cavity surface-emitting lasers (VCSELs) – High-power lasers (Tapered lasers, SCOWLs, coherent combining) – Quantum cascade lasers (QCLs)
• Recent Results – InGaN lasers emitting at green wavelengths – “Silicon” based lasers (hybrid, strained germanium) – Large-scale integration of semiconductor lasers – Thresholdless nano-scale lasers
• Summary
Outline
IPS Laser Workshop - 34 PWJ121024
Vertical-Cavity Surface-Emitting Lasers (VCSELs)
• Vertical confinement = High-reflectivity mirrors (> 99%) • Lateral confinement = Mesa etch + oxide layers • Cavity length ~ laser wavelength => Single longitudinal mode • Surface-emitting structure enables on-wafer testing, eliminates
the need for facet cleaving, and supports two-dimensional arrays
First Demonstration by Ken Iga and Co-Workers:
H. Soda, K. Iga, C. Kitahara, and Y. Suematsu, “GaInAsP/InP Surface Emitting Injection Lasers, Jpn. J. Appl. Phys., 1979
IPS Laser Workshop - 35 PWJ121024
High-Power Semiconductor Laser Structures
Standard Rib
Waveguide
Output Facet Cross-Section Top View Attributes
Tapered Laser
Slab-Coupled Optical
Waveguide Laser
(SCOWL)
• Low power (< 100 mW) • High gain (30 dB) • Mode size: 1 x 3 µm • High loss: 5-10 cm-1
• Stable mode profile • Simple lens-coupling
• High power (> 1 W) • High gain (30 dB) • Mode size: 1 x 200 µm • High loss: 5-10 cm-1
• Unstable mode profile • Complex lens-coupling
• High power (> 1 W) • Moderate gain (15 dB) • Mode size: > 5 x 5 µm • Low loss: ~ 0.5 cm-1
• Stable mode profile • Simple lens-coupling
Slab
IPS Laser Workshop - 36 PWJ121024
Tapered Lasers/Amplifiers
• InGaAs/AlGaAs quantum-well graded-index separate-confinement heterostructure (GRINSCH)
• Wavelength ~ 970 nm
IPS Laser Workshop - 37 PWJ121024
Slab-Coupled Optical Waveguide Concept
β = (2π/λ) x Modal Index
β z-guide
z-slab β
Continuum
Continuum Discrete Modes
E. A. J. Marcatili, Bell Syst. Tech. J., 53, 645 (1974)
x n1
n 1
n2
y
z
T
D
W
n3
• Large-diameter fiber by itself supports multiple transverse modes
• Slab guide effectively filters
out higher-order transverse modes in composite structure
• Composite structure is single-
moded (only one bound transverse mode)
Fiber Waveguide
Slab Waveguide
IPS Laser Workshop - 38 PWJ121024
Slab-Coupled Optical Waveguide Laser*
Slab-Coupled Optical Waveguide Laser (SCOWL) Cross-Section
• Small index-contrast (∆n/n) + Coupled-mode filtering => Large optical mode • Small mode-overlap with QWs => High saturation power • Small mode-overlap with p-InP => Low optical loss
Key Characteristics:
Demonstrated SCOWL wavelengths: • 9xx, 1060, 1300, 1550, and 2100 nm
Jim Walpole
Joe Donnelly
IPS Laser Workshop - 39 PWJ121024
Slab-Coupled Optical Waveguide Technology
Slab-Coupled Optical Waveguide Amplifier
(SCOWA)
Demonstrated SCOW Applications Watt-Class
Semiconductor Optical
Amplifiers (SOAs)
High-Power, Low-Noise
Mode-Locked Lasers
Single-Frequency, Narrow-Linewidth
Lasers
• Small index-contrast (∆n/n) + Coupled-mode filtering => Large optical mode (5x7 µm) • Small mode-overlap with QWs => High output power (~1 W) • Small mode-overlap with p-InP => Low optical loss (~0.5 cm-1)
High-Current Waveguide
Photodiodes P. W. Juodawlkis et al., IEEE JSTQE, 2011
IPS Laser Workshop - 40 PWJ121024
Coherent Beam Combining (CBC) of High-Power Semiconductor Optical Emitters
Seed
47 SCOWA Array 12.5 mm x 5 mm
Lincoln CBC Architecture 47-Element SCOWA Array
G. M. Smith et al., IEEE Summer Topical on High-Power Semiconductor Lasers, 2012
Upcoming Boston IPC Laser Workshop Talks: • 7 November: T. Y. Fan, MIT-LL • 17 November: Gary Smith, MIT-LL
• Coherently combined 47 SCOWA elements to produce 50-W total power with > 90% combining efficiency
• Diffractive optical element (DOE) used to achieve single beam with M2 ~ 1.2 x 1.3 λ = 1060 nm
IPS Laser Workshop - 41 PWJ121024
Interband vs. Intersubband Transitions
• Absorption above E=hν
• Broad absorption features • Long lifetime (>1 ns) • Very high radiative efficency • Transition energy <->gap
• Absorption at E = hν
• Narrow absorption features • Short lifetime (<1 ps) • Very low radiative efficiency • Transition energy <->QW thickness
Courtesy of Jérôme Faist, ETH Zurich
Interband Intersubband
IPS Laser Workshop - 42 PWJ121024
Quantum Cascade Laser (QCL) Operation
Courtesy of Jérôme Faist, ETH Zurich
• Unipolar operation (electrons only) • Each injected electron produces
multiple photons • Requires precise control of many
(~500-1000) material layers
Conduction Band Energy Profile
IPS Laser Workshop - 43 PWJ121024
Quantum-Cascade Laser Milestones
Courtesy of Jérôme Faist, ETH Zurich
1971 1994 1997 2002 2012
R. F. Kazarinov, R.A. Suris, Sov. Phys. Semicond., 1971
First Theoretical Proposal: Use intersubband transitions
in semiconductor QWs
First QCL Demonstration: Tmax = 125K (pulsed)
Pmax = 10 mW λ = 4.26 µm
J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A.L. Hutchinson, A. Y. Cho, Science,1994
Single-Mode CW Operation at Room Temperature
J. Faist et al., Appl. Phys. Lett.,1997
Improved Injector DFB Structure
Terahertz QCL Demo
R. Köhler et al., Nature,2002.
• Improvements in designs, growth and technology
• High wall-plug efficiency (27%) at 300K
• High output power • Low power (< 1 W) dissipation • QCL arrays
IPS Laser Workshop - 44 PWJ121024
• The First Semiconductor Laser(s) - 1962 • Semiconductor Laser Advances • Other Semiconductor Laser Structures
– Vertical-cavity surface-emitting lasers (VCSELs) – High-power lasers (Tapered lasers, SCOWLs, coherent combining) – Quantum cascade lasers (QCLs)
• Recent Results – InGaN lasers emitting at green wavelengths – “Silicon” based lasers (hybrid, strained germanium) – Large-scale integration of semiconductor lasers – Thresholdless nano-scale lasers
• Summary
Outline
IPS Laser Workshop - 45 PWJ121024
Bandgap Energy vs. Lattice Constant for III-N Wurtzite Materials
• The most stable form of III-N materials is the wurtzite structure – Energy gap vs. “a-plane” lattice constant shows bowing effects
IPS Laser Workshop - 46 PWJ121024
Gallium Nitride Lasers: From Ultra-Violet to Green Wavelengths
Semi-Polar 2021 Plane
λ = 530 nm, P > 100 mW
Sumitomo & Sony (2012)
Shuji Nakamura
(Nichia/UCSB)
S. Nakamura et al., Jpn. J. Appl. Phys., 1996
GaN Blue Diode Lasers and LEDs
• Quantum-well materials grown on c-plane (0001) GaN exhibit large built-in electric fields due to polarization discontinuity
• Use of nonpolar or semi-polar orientations provide improved electron-hole wavefunction overlap
Crystal Structure of GaN
IPS Laser Workshop - 47 PWJ121024
Hybrid Silicon-InP Laser
p-InP n-InP
Passive Si-Waveguide
SiO2
Si Substrate
AlGaInAs MQW Active Region
“A Chip That Can Transfer Data Using Laser Light”
Published in the New York Times September 18, 2006
• Silicon waveguide + III-V active region
• Bonding between InP and Si samples performed using low-temperature oxygen plasma assisted wafer bonding
• Bonding performed prior to III-V material patterning
John Bowers UCSB
A. W. Fang et al., Opt. Exp., 2006
Power-vs-Current Characteristic
Potential approach for realizing lasers on silicon photonic platforms for optical interconnects
IPS Laser Workshop - 48 PWJ121024
Obtaining Direct-Gap Optical Transition in Germanium (Ge)
<111>k
E
ΓL
0.80
0 eV
0.66
4 eV
(a)
<111>k
E
L
(b)
<111>k
E
L
(c)
electrons
Γ Γ
bulk Ge tensile strained i-Ge tensile strained n+ Ge
<111>k
E
ΓL
0.80
0 eV
0.66
4 eV
(a)
<111>k
E
L
(b)
<111>k
E
L
(c)
electrons
Γ Γ
bulk Ge tensile strained i-Ge tensile strained n+ Ge
Courtesy of Jurgen Michel, MIT
• Germanium (Ge) is pseudo-direct gap => ∆E between L and Γ valley is only 136 meV • Steps to achieve direct-gap optical transition in Ge:
– Add tensile strain: Ge fully relaxed at 650C growth temp; 0.2-0.3% tensile strained at room temp – Add heavy n-type doping (~1019 cm-3): Fill L-valley, scatter electrons to Γ-vally thermally
• Use novel device design to efficiently inject holes into n+ Ge
injected holes
IPS Laser Workshop - 49 PWJ121024
Electrically Pumped Germanium (Ge) Laser
Courtesy of Jurgen Michel, MIT
0 10 20 30 40 50 60 70 80
1500 1550 1600
90 kA/cm2
Inte
nsity
(a.u
.)
0 10 20 30 40 50 60 70 80
1500 1550 1600 Wavelength (nm)
511 kA/cm2
Wavelength (nm)
Inte
nsity
(a.u
.)
0
0.2
0.4
0.6
0.8
1
1.2
0 100 200 300 400
Current Density (kA/cm2)
Out
put P
ower
(mW
)
Ge Laser Cross-Section Emission Spectra
L-I Characteristic
• Use delta-doping + indiffusion of phosphorous to achieve high n-doping concentration
• Use heavily doped n+ and p+ silicon contacts => results in high optical losses (100-1000 cm-1)
• Device length ~ 300 µm • Lasing achieved at room temperature under pulsed
conditions (~50 µs @ 1000 Hz)
< 1.2 nm
Jth ~ 270 kA/cm2
POUT ~ 1 mW T = 300K
IPS Laser Workshop - 50 PWJ121024
Large-Scale Integration of Semiconductor Lasers for Optical Communications
Monolithic InP Photonic Integrated Circuit (PIC) Transmitters and Receivers
100-Gb/s Transmitter Components • 10 xTunable DFB Lasers • 10 x 10-Gb/s EA Modulators • 10 x Variable Optical Attenuators • 10 x Optical Power Monitors • 1 x Wavelength Division Mux
D. F. Welch et al., IEEE JSTQE, 2007
Terabit Transmitter PIC
• 1.12 Tb/s Coherent PM-QPSK • 10 Wavelengths • > 450 Photonic Components
P. Evans et al., Opt. Exp, 2011
IPS Laser Workshop - 51 PWJ121024
Thresholdless Nano-Laser
Ag/Al Alloy
M. Khajavikhan et al., Nature, 2012
InGaAsP Gain
Region
• Nano-scale, metal-clad coaxial structure supports only 1 mode (TEM-like) within the bandwidth of the optically pumped InGaAsP gain medium
• Evidence of “thresholdless” lasing: (i) no kink in L-I curve, (ii) Lorentzian lineshape over 105 pump-current increase, (iii) behavior or linewidth vs. pump power (i.e., no subthreshold narrowing, line narrowing only due to carrier pinning => no 1/P)
IPS Laser Workshop - 52 PWJ121024
Spectral Coverage of Semiconductor Lasers
λ (µm)
f (THz)
QCLs
InGaN/GaN GaN/AlGaN
THz QCLs
InGaAsP
InGaAs/AlGaAs AlGaInP
InGaAsSb
PbS/PbSnSe/PbSnTe HgCdTe
GaAs
= Lasers and Laser Materials Developed at Lincoln Laboratory
IPS Laser Workshop - 53 PWJ121024
Courtesy of Jim Coleman, University of Illinois
Applications of Semiconductor Lasers and LEDs
more than $7B in sales!
IPS Laser Workshop - 54 PWJ121024
• NASA’s first space lasercom
• Space terminal to fly on Lunar Atmosphere and Dust Environment Explorer (LADEE) – 2013 launch
• Key demonstration objectives - 622 Mbps optical downlink - 20 Mbps optical uplink - 200–ps time-of-flight measurement
Lunar Laser Communication Demonstration (LLCD) Program
Contains 8 Semiconductor Lasers: • 1 Master Laser for Down-Link • 4 Pump Lasers for Down-Link EDFA • 2 Pump Lasers for Up-Link EDFA • 1 Laser for Uplink Self-Test
Courtesy of Bryan Robinson, MIT-LL
IPS Laser Workshop - 55 PWJ121024
• Semiconductor lasers have gone through dramatic improvements and changes over the past 50 years through the contributions of many groups around the world
• Semiconductor lasers are having an increasing impact on many areas of technology – Ultra-high capacity optical networks – Optical interconnects for data centers, supercomputers, and
processors – Medical diagnostics and treatments – Industrial cutting and welding
• MIT Lincoln Laboratory has been actively engaged in the development and application of semiconductor lasers for 50 years
Summary
IPS Laser Workshop - 56 PWJ121024
Back-Up Charts
IPS Laser Workshop - 57 PWJ121024
• What are the properties of the optical gain? – How are the carriers injected and confined? – What is the relationship between carrier density and optical gain? – What is the spectral distribution of the gain?
• How is the optical field confined within the laser? – Spatial and spectral confinement – Overlap between optical field and the gain region (Γ)
• What are the loss mechanisms?
• How is the heat removed from the laser structure?
• What performance parameters are most critical for application? – Output power – Noise – Modulation bandwidth
Key Considerations for Semiconductor Laser Operation
IPS Laser Workshop - 58 PWJ121024
A Few Words About Modes…
Transverse Optical Modes of a Cylindrical Waveguide
Longitudinal Optical Modes of a Fabry-Perot Cavity
M = 1
M = 2
M = 23
L
=2MMcnL
ν• Frequency of Mth mode:
• For λ0 = 1.55 µm, L = 10 mm, n = 3.5: => νM = 193.4 THz and M = 45161
• Solutions to 2D Maxwell’s equations w/ appropriate boundary conditions
Spatial Domain: Intensity Profile Frequency Domain: Single-Frequency Mode-Locking
IPS Laser Workshop - 59 PWJ121024
Radiative and Non-Radiative Transitions
IPS Laser Workshop - 60 PWJ121024
Interband Optical Gain
Energy Band Diagram Carrier Distributions
EFc = Quasi-Fermi energy for electrons EFv = Quasi-Fermi energy for holes
Population Inversion Condition (Gain > 0): EG < EPH < EFc - EFv
Bulk Material
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Strained Semiconductor Materials Compressive Strain Tensile Strain
Unstrained Band Diagram
Strained Band Diagram • Material strain modifies the
band-structure of semiconductor materials
• Desirable impact on gain medium: –Strain reduces hole effective mass –Reduces density of states –Reduces carrier density required to
achieve population inversion
• Thickness of strained material limited by relaxation
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Compound Semiconductor Material Growth
• Liquid-Phase Epitaxy (LPE)—Developed 1960-62 • Vapor-Phase Epitaxy (VPE)—Developed 1960-62
– Developed for Si, Ge, and III-V’s at about the same time – Halide VPE process (mostly at atmospheric pressure) – Hydride VPE process (mostly at atmospheric pressure) – Metalorganic chemical vapor deposition (MOCVD)—1968
• MOCVD is the name first used for this VPE process in the January 1968 APL paper by Manasevit in an original series of work demonstrating the growth III-V’s on sapphire—A.K.A.: o Metalorganic vapor-phase epitaxy (MOVPE) o Metal-alkyl vapor-phase epitaxy (MAVPE) o Organometallic vapor-phase epitaxy (OMVPE) o Organometallic chemical vapor deposition (OMCVD)
• Molecular-beam epitaxy (MBE)—Developed 1968
Courtesy of Russ Dupuis, Georgia Tech
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Molecular-Beam Epitaxy (MBE)
Courtesy of Jérôme Faist, ETH Zurich
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Organometallic Vapor Phase Epitaxy (OMVPE)
Ref. C. Wang, Lincoln Laboratory Journal, 1990
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OMVPE Reactors: Then and Now
Russ Dupuis
VEECO TurboDisc MOCVD System for Gallium-Nitride (GaN)
• 45 x 2”-diameter wafers • 12 x 4”-diameter wafers • 5 x 6”-diameter wafers
MOCVD at Rockwell International for GaAs/AlGaAs
1975
• Single 2”-diameter wafer Courtesy of Russ Dupuis, Georgia Tech