The Quantum Cascade Laser: a High Power Semiconductor Laser for Mid-Infrared Sensing Applications

The quantum cascade laser: a high power semiconductor laser for mid-infrared sensing applications

Oana Malis

Collaborators: Deborah L. Sivco, Jianxin Chen, Liming Zhang, A. Michael Sergent, Loren Pfeiffer, Kenneth West, Bell Laboratories, Lucent Technologies Claire Gmachl, Dept. of Electrical Engineering and PRISM, Princeton Univ. Alexey Belyanin, Department of Physics, Texas A&M University Introduction to quantum cascade lasers

Conventional semiconductor laser

CB diode laser: material

Quantum cascade laser: unipolar semiconductor laser using intersubband transitions CB QC- laser:

layer thickness Quantum cascade lasers: mid-infrared light sources

InGaAs/InAlAs lattice-matched to InP

mid-infrared light source 3 wavelength agile: InP range 5 – 20 mm Itop high power 2 e 1 high-speed 3 Ibott

Itop active region 2 e 1 injector

Ibott active region injector QCL: compact, rugged light source

Grown by MBE

InGaAs/InAlAs lattice matched to InP What makes the QC-laser special?

Wavelength agility: layer thicknesses determine emission wavelength High optical power: cascading re-uses electrons Fabry-Perot, single mode (DFB), or multi-wavelength (dual-wavelength, ultra-broadband) Temperature tunable Ultra-fast carrier dynamics: no relaxation oscillations Active research field in semiconductor physics QCL operating modes

Fabry-Perot Single mode DFB mode

8.0 8.2 4.96 5.00 5.04 Wavelength (mm) Nonlinear light generation: Dual-wavelength

no grating second-harmonic 200 100

Ultra-broadband 150 laser SH

Intensity (arb. units) 100 50 a 10

2, 3, 4 A Intensity (a.u.) Intensity (a.u.) 4.92 4.96 5.00 5.04 7.36 7.40 7.44 7.48 5 ... 13 A Wavelength (mm) 50

1 0 0 8.6 8.8 9.0 9.2 9.4 9.6 4.3 4.4 4.5 4.6 4.7 4.8 pump wavelength ( m m) second-harmonic (mm)

0.1 Power (arb. units, log. scale)

5 6 7 8 9 Wavelength (mm) What makes the QC-laser special?

In-situ trace gas sensing:

NO, CO, NH3, CH4, H2O (isotopes), and more complex molecules – ppm to ppb levels ÞChemical and biological sensing (air quality, chemical and biological weapons, breath Physical Sciences, Inc. monitoring) Remote sensing: LIDAR

Free-space optical telecommunications

MCT-det. QC-laser Satellite Set-Top Box DC - Source 200 m DC - Voltmeter Spectrum Analyzer Ongoing QCL research

Goal: to extend the functionality and performance of mid-infrared emitters

New materials and fabrication techniques to optimize InP QCL performance

New light generation processes:

Ø Nonlinear light generation in QCLs

Ø Hole quantum cascade laser Optimization of InP-based laser properties

Design of high-gain active region IB e 4

3 Minimization of waveguide losses 2 e injector 1

using InP top and side-claddings active

injector Growth of high-purity materials

Ti/Au top contact

n InP, 8 ´ 1018 cm -3 Thermal management n InP , 1017 cm -3

n InGaAs, 3-5 ´ 1016 cm -3

Waveguide core: Active regions and injectors 30 -50 stages InP substrate electroplated Au n InGaAs, 3-5 ´ 1016 cm -3

n InP, 1-2´1017 cm -3, substrate

In solder waveguide core Advanced fabrication and processing

MBE and MOCVD overgrowth: Liming Zhang, Jianxin Chen

MOCVD MBE MOCVD

InP substrate

Improvement of cw max. temperature by 50K (with HR coating)

Metal electroplating Plated gold

Laser core Recent highlight: room-temperature, continuous- wave operation at 8 mm

220 K 200 K cw mode 240 K 280 K 30 260 K

20 300 K

450 14 10 320 K 300 K 400 12 220 K cw output power (mW) 350 240 K 10 260 K 300 0 280 K 1.0 1.5 2.0 2.5 3.0 3.5 4.0 300 K 250 8 320 K 2 current density (kA/cm ) 200 6 150 Voltage (V) 4 100

2 pulsed mode 50 Peak output power (mW) 0 0 0 2 4 6 8 current density (kA/cm2) Ongoing QCL research

Goal: to extend the functionality and performance of mid-infrared emitters

New materials and fabrication techniques to optimize InP QCL performance

New light generation processes:

Ø Nonlinear light generation in QCLs

Ø Hole quantum cascade laser Nonlinear light generation in QCLs: Outline

Nonlinear light generation in intersubband transitions Sum-frequency and second-harmonic generation in QCLs Enhancement of second-harmonic response in InP QCLs Phase-matching for second-harmonic generation Summary and discussion of second-harmonic QCLs Future projects: parametric down-conversion Introduction to nonlinear light generation in QCLs

Motivation: Extend operation of InP-based QCLs outside the limits imposed by material system (i.e. below 5 mm) Light sources with new functionality Applications: high-resolution chemical and biological sensing to quantum cryptography

Goal: develop monolithically integrated nonlinear QC lasers Nonlinear light generation using resonant intersubband transitions

3 w 2w w 2 2w w 1

(2) 2 (3) 3 P = e0 (cE + c E + c E + …)

3 (2) e z12 z23 z13 c µ Ne e0 (hw - E12 -i× 2g 12 )×(2hw - E13 -i ×2g 13 )

M.K. Gurnick and T.A De Temple, IEEE JQE 19, 791 (1983). F. Capasso, C. Sirtori, and A.Y. Cho, IEEE JQE 30, 1313 (1994). Monolithically integrated nonlinear QCL

Conditions for efficient SHG: Efficient pumping Þ monolithic integration Phase-matching

Sum frequency and 2w1 second-harmonic w1 generation w1+ w2

SL 2w2

w1 w2 w2 active region

N. Owschimikow et al., Phys. Rev. Lett. 90, 043902 (2003). First demonstration of nonlinear QCL

• two active regions (7.1 mm and 9.5 mm) and mixing superlattice section • 7.1 mm active region includes resonant IS cascades for SFG and SHG superlattice 7.1 mm active region

60 and 80 mW of laser power Þ 30 nW SFG and 15 nW SHG Optimized nonlinear QC laser active region

e 3 N E 2 (w ) æ z z z æ n - n n - n ö z z z æ n - n n - n öö P(2w) = e x ç 23 34 24 ç 3 4 + 3 2 ÷ + 34 45 35 ç 4 5 + 3 4 ÷÷ h ç ç ÷ ç ÷÷ è G42 è G43 G32 ø G53 è G54 G43 øø Two nonlinear cascades: InGaAs/ InAlAs QCL 2 – 3 – 4, and 3 – 4 – 5 5

4 c(2) = 4.7 ´ 10-5 esu = × 3 highest measured 3 value in any material

3 system 2 1 1 injector active region injector Nonlinear QC laser general characteristics

• InGaAs/InAlAs QCL grown by MBE on n-type InP • deep-etched ridge waveguide devices • 1.5 – 2.25 mm long, 4 – 15 mm wide

200 100 Fundamental 10 K Second- 10 K MCT detector harmonic InSb detector 150 D2912

100 50

Intensity (a.u.) Intensity (a.u.)

0 0 8.6 8.8 9.0 9.2 9.4 9.6 4.3 4.4 4.5 4.6 4.7 4.8 pump wavelength (mm) second-harmonic (mm) Linear and nonlinear L-I for D2912

0.1 600 0.6 2 W) 65 mW/W (nW) m

(W) ( NL

L 0.3

2 P 49 mW/W NL 400 68 mW/W2 P 0 2 .05 0 0.005 0.01 W(2w) = h W(w) 2 (PL (W)) 200 100 Linear power P

0 0 Nonlinear power

0 1 2 3 4 5 efficiency

) 10 Current (A) 2 first optimized sample

W/W design m ( 1

0.1 Power conversion D2616 D2886 D2912 D2882 Second-harmonic generation of QC laser

128p 5S1+ e-2a2 L - 2e-a 2L cos(DkL)(1- R ) h ~ [ ] 2 2 2 2 2 2 m1 m2cl2 (Dk +a 2 )(1- R1 )

Dk = k2w – 2kw = 2w (µ2 – µ1)/c = the phase mismatch

µ1,2 = effective refractive indices of modes

a2 = total losses of a given cavity mode at l2 = l1/2 L = the cavity length,

R1,2 = reflection factors of a cavity S = nonlinear overlap factor of the two interacting modes Waveguide design for second-harmonic generation in QCLs

10 nm InGaAs 1e20 cm-3 Au contact 0 InGaAs, 6.5e18 cm-3, 850 nm -3 InAlAs, 1e17 cm , 1300 nm 2

InGaAs, 1e17 cm-3, 1600 nm

m) 4 m active region 50 stages 2475 nm 6 Distance ( InGaAs, 1e17 cm-3, 1500 nm 8 InP 1-5e17 cm-3 substrate

10 0 2 4 SH refractive index profile Modal phase-matching for second-harmonic generation in QCL

flexibility in waveguide design Þ modal phase-matching no need for birefringence or quasi phase-matching

Problem: IR refractive indices are not known accurately

IR refractive indices for InGaAs, InAlAs, and InP: Indices for undoped alloys were interpolated linearly from the published values for the end compounds for each wavelength Drude formula to calculate the complex refractive indices of the doped alloys One-dimensional solution of the wave equation assuming infinitely wide ridges Þ effective refractive indices, mode profiles Mode selection for SHG phase-matching

0.7 pump TM00 0.6 n=3.3094 SH TM Phase-matching of pump 00 0.5 TM mode with SH TM SH TM01 00 02

0.4 SH TM02 mode 0.3 0.2 n=3.2127 0.1 0.0 Spatial distribution of n=3.2868 -0.1 modes determines the n=3.1861 -0.2 overlap with each other -0.3 and with the active region Magnetic field H profile (a.u.) -0.4 -0.5

0 2 4 6 8 10 12 Thickness (mm) Exact phase-matching using ridge-width dependence

3.22 fundamental 3.20

3.18 second-harmonic 3.16

3.14 phase-match

3.12 w 3.10 refractive index 3.08

3.06

3.04 2 4 6 8 10 12 14 16 18 20 22 width (mm) InAlAs-based waveguide development for phase-matching

250 10000 ) 2 200 1000 W) W/W m m 150 100

100 10

50 Nonlinear power ( 1 Nonlinear efficiency (

0.1 2912 2927 2935 2944 2957 -1 Dk = 581 837 715 711 367 cm -1 a = 84 35 71 9 4 cm Effect of phase-matching for SHG with InAlAs waveguides

80 160 160 160 D2944: h = 35 mW/W2 120 120 ) ) ) W W W m

60 m 120 (

80 80 ( L m N ( 2 L

35 mW/W P N L 40 40 P P

r r a

e 40 80 0 0 e

w 0 0.002 0.004 0.006 n i o l

2 2 - p

P (W ) L n r o a 20 40 e N n i L 300 200 200 0 0 300 0 0.5 1.0 1.5 2.0 2.5 150 150 )

Current (A) ) 2 W ) 2.4 mW/W W m m

100 100 ( ( W L

N 200 L m P N

( 200

50 50 P L

D2957: P = 240 mW P

NL a

r 0 0 e

e 0 0.02 0.04 0.06 0.08 0.10 n i w 2 2 l - o P (W ) 100 L n

p 100

O. Malis et al., Appl. Phys. Lett. 84, 2721 (2004). r N a e n i L 0 0 0 1 2 3 Current (A) Ridge-width dependence

Agreement with calculation on the position of the maximum Recent result: 2 mW second-harmonic generation

500 2.5 2.0 450 D3014 What made it possible: 400 1.5 2.0

InP top cladding regrowth 350 1.0 2 (mW)

17 mW/W (mW) NL NL (mW) P 0.5 by Jianxin Chen L 300 1.5

HR coating of back facet 250 0.0 0.00 0.04 0.08 0.12 200 P 2(W2) 1.0 L 150

Linear power P 100 0.5

50 Nonlinear power P 0 0.0 0 1 2 3 4 Current (A)

Power >1 mW interesting for spectroscopy

O. Malis et al., Electron. Lett. 40, 1586 (2004). Far-field pattern of second-harmonic mode

cryostat

Top view Side view

2” ZnSe window

0.45

0.40 0o rotation +13o rotation 0.35 -13o rotation Far-field pattern consistent 0.30

with TM02 mode 0.25

Sharp, high feature in the 0.20

far-field 0.15

0.10

SH Intensity (arb. units) 0.05

0.00

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 angle ( o ) Discussion of experimental results

Max theoretical: Max experimental: h = 2 W/W2 h = 35 mW/W2

Experimental limitations Ridge width within 0.5 mm from wet etching Higher non-resonant mid-infrared losses due to higher doping Higher resonant losses due to accidental band alignment Future work

• Continue to improve nonlinear conversion efficiency and power

• AR coating on front facet

• CW, room temperature, and single mode operation

• Lower wavelength (< 4.5 mm) in the spectral region that is difficult to reach with InGaAs/InAlAs QC lasers Summary of second-harmonic project

Developed monolithically integrated nonlinear QCLs Developed technique for phase-matching of nonlinear QC lasers Increase by over two orders of magnitude in the second- harmonic power generation and nonlinear efficiency Milliwatt second-harmonic generation promising for applications Phase-matching technique can be applied for other parametric processes Ongoing QCL research

Goal: to extend the functionality and performance of mid-infrared emitters

New materials and fabrication techniques to optimize InP QCL performance

New light generation processes:

Ø Nonlinear light generation in QCLs

Ø Hole quantum cascade laser GaAs-based hole quantum cascade lasers

Collaborators: Loren Pfeiffer, Ken West Motivation New type of quantum cascade laser New functionality of QCLs: Ø surface emitting QCLs and VCSELs Ø Device that extends the operating range of present GaAs QCLs Ø Advantages similar to GaAs electron QCLs: better temperature behavior, lower losses Ø Alternative to InP-based devices using the mature GaAs MBE technology and GaAs substrates New physics of intersubband transitions in the valence band and hole relaxation processes Background

Previous work in hole QC structures Si/SiGe intersubband absorption, electroluminescence and photocurrent Challenges: strained heterostructures Material issues Theoretical complexity

Advantages of GaAs/AlGaAs material system Strain-free material Mature material system Extensive experience from electron GaAs QCLs Unique materials opportunities in-house Intersubband absorption

conduction band p k s• multipass waveguide

HH1 LH1 aW = ln(Ts /Tp ) / Lint SO1 L Nn HH2 QW p LH2 L = int cosq valence band phr Nn e sin 2 q a L dE = s p f ò W int if 2m0e 0cn cosq

oscillator strength Mid-infrared bound-to-bound hole intersubband absorption in GaAs/AlGaAs quantum wells

Structures: MBE grown on GaAs(001) 1% thickness control, confirmed by x-ray measurements 25 Å – 45 Å GaAs quantum wells 57% AlGaAs digital alloy barriers: 8.5 Å GaAs/ 11.3 Å AlAs superlattice P-type modulation doping with Carbon from novel solid source 1-2´1012/cm2 p-type doping Mobility 8000 cm2/Vs at 5 K Mid-infrared absorption measurements

31 Å QW

25 - 45 A QWs Þ 126 - 206 meV

FWHMInGaAs = 20 meV < FWHMGaAs < FWHMSiGe = 30 meV dipole matrix element: z = 6 Å Comparison of experimental and simulation results

250 experiment 6-band k·p calculations with calculation 57% analog alloy calculation digital alloy nextnano3 package

200 digital alloy effectively increases the band offset

31 Å QW 150

0.0 HH hole transition energy (meV)

100 25 30 35 40 45

QW width (Å) -0.2

Parameters: Energy (eV) Band offset: 0.51 eV Doping: 1.6·1012/cm2 -0.4 GaAs: g =8.64, g =2.44, g =3.27 1 2 3 10 20 30 AlAs: g1=5.03, g2=0.8, g3=1.55 Position (nm) O. Malis et al., Appl. Phys. Lett.. 87, 091116 (2005). Intersubband hole electroluminescence

GaAs/AlAs/Al0.3Ga0.7As E = 57kV/cm 2-level system h Al0.3Ga0.7As injector 1.2

hh1

Energy (eV) h 1.0

20 40 distance (nm) Absorption and photocurrent measurements on luminescence structures

p-polarized p-pol/s-pol

Absorption (a.u.)

160 180 200 220 240 4 Energy (meV) zero bias Photocurrent (a.u.) expected DEhh1 = 210 meV 0

200 300 400 500 Energy (meV) Electroluminescence results

hh1 thermal 3 peaks: hh1, hh2, thermal hh2

60 expected DEhh1 = 190 meV

Absorption (a.u.) measured DEhh1 = 162 meV

120 140 160 180 200 Energy (meV) J = 1.34 kA/cm2

1.2

hh1

30 hh2 2 Intensity (a.u.) J = 1.08 kA/cm Energy (eV) 1.0

20 40 distance (nm) J = 0.54 kA/cm2

100 200 300 400 Energy (meV) Effect of active QW thickness

2 J = 1.08 kA/cm Using growth non-uniformity: 10% thickness difference (1ML) Þ Þ 15 meV energy difference

30 thicker QWs Intensity (a.u.) Broadening of the hh emission peak: FWHM hh1 20 - 45 meV

thinner QWs

200 400 Energy (meV) Electroluminescence L-I-V

3 P = Ihcoll NhQChu / e 30 -4 Power (nW) hcoll = 4×10 2 -5 20 NhQC = 2.3×10

Voltage (V) 1 assuming z = 5 Å 10

0 0.0 0.4 0.8 1.2 J (kA/cm2)

Upper-level lifetime of approx. 0.4 ps * Lifetime consistent with estimate based on m GaAs=0.266, * m AlAs=0.2915 from Luttinger parameters

O. Malis et al., Appl. Phys. Lett.. 88, 081117 (2006). Summary of hole intersubband absorption and electroluminescence

Mid-infrared bound-to-bound hole intersubband absorption range of 126 – 206 meV for 25 – 45 Å C-doped GaAs/AlAs QWs Agreement between experimental results for hh-transitions in wide wells and calculations considering the full band structure Heavy-to-light transitions and hh-transitions in narrow QWs still challenging Hole intersubband electroluminescence and photocurrent measurements Emission wavelength slightly lower in energy than expected Additional emission peaks possibly due to other hh-transitions Broadening of the hh emission peak Upper-level lifetime of approx. 0.4 ps The quantum cascade laser

Collaborators: Deborah L. Sivco, Jianxin Chen, Liming Zhang, Loren Pfeiffer, Kenneth West, A. Michael Sergent, Claire Gmachl, Alexey Belyanin

Unipolar, intersubband laser operating in the mid-infrared range Applications in trace-gas sensing and free- space communications Active field of research into new materials and new light emission processes Ø New materials and fabrication techniques to optimize InP QCL performance Ø Nonlinear light generation in QCLs Ø Hole quantum cascade laser