GaAs/AlxGa1-xAs; GaxIn1-xAsyP1-y/AlxIn1-xAs on InP; InAs1-xSb/AlGa1-xSb on GaSb

Electron states in heterostructures

0 2 4 6 8 10

2

1.595

1.19

0.785

0.38

-0.025

k || (10 6cm -1)

E (e

V)

K||, cm-10 107

1.6

E, e

V

1.2

0.4

-0.025

0 40 80 120 160 200

2

1.539

1.078

0.617

0.156

-0.305

z (Å)

E (e

V)

z, A

E, e

V

1.54

0.16

1

0 200100-0.3

AlInAs GaInAs 80 A AlInAs

,...2,1;)(2

~)( 2||

22

nkkkm

kE neff

n

0 4 8 12 16 20

2

1.1

0.2

-0.7

-1.6

-2.5

k z (10 6cm -1)

E (

eV

)

8-band kp method(4 bands x 2 spins)

Ga0.47In0.53As

Bulk semiconductorsQuantum wells

0 40 80 120 160 200

2

1.539

1.078

0.617

0.156

-0.305

z (Å)

E (e

V)

z, A

E, e

V

1.54

0.16

1

0 200100

Optical transitions in quantum wells

0 2 4 6 8 10

2

1.595

1.19

0.785

0.38

-0.025

k || (10 6cm -1)

E (e

V)

K||, cm-10 107

1.6

E, e

V

1.2

0.4

-0.025

-0.3

AlInAs GaInAs 80 A AlInAs

0.7 0.86 1.02 1.18 1.34 1.5

15000

12000

9000

6000

3000

0

E (eV)

Ab

so

rpti

on

(1

/cm

)ab

sorp

tio

n

frequency

interband

intersubband

1

2

3

Intersubband transitions: dipole moment

dzzfz

zfz nmmn )()(*

Dipole matrix element:

zz

z

z

z

LzzkL

fzkL

f ~;sin1

~,cos1

~ 1221

Typical values ~ 10-100 ACompare with atomic transitions ~ 0.2-0.5 A

1

2

3

Intersubband transitions: selection rules

dzzfz

zfz nmmn )()(*

- Dipole matrix element:

f1 and f3 are even -> z13 = 0

- Only TM-polarization (E QW plane)

-500 -400 -300 -200 -100 0

0

0.2

0.4

0.6

0.8Intersubband transitions in asymmetric coupled QWs

Control of the optical response by engineering the shape (symmetry) of envelope functions and energies of ISBT

High optical nonlinearities: e.g. (2) ~ 106 pm/V

Short relaxation time ~ 1 ps: possibility of an ultrafast modulation

Add advantages of a semiconductor medium: electron transport and Stark effect under applied voltage, integration with other components

Saturation easily reached: /ezERabi

Large coherence can be excited 2/1~21

Rich nonlinear dynamics

measured since 1980s

Rui Yang’s talk

High voltage to align levels, high current => high heat dissipation

60 nm

520

me

V

3

2

activeregion

injector (n-doped)

injector (n-doped)

e

activeregion

QC lasersJ. Faist, F. Capasso, et al. Science 264, 553 (1994)

•Control of lifetimes: phonons, tunneling; need t32 > t2

•Cascading: high power when t_stim approaches T1

From sawtooth to staircase potential

jbjbjwjw lklk ,,,,E21 = Ephonon

From sawtooth to staircase potential

0 200 400 600 800

0

100

200

300

400

500

600

700

800

900

1000

1100

Energ

y (m

eV

)

Z (Å)

V = 0

0 200 400 600 800

0

100

200

300

400

500

600

700

800

900

1000

1100

En

erg

y (m

eV

)

Z (Å)

V = Vth

Fabrication: MBE or MOCVD

TEM / SEM image

55 nm

MINIGAP

MINIBAND3

2

1

g

ACTIVEREGION

INJECTOR

I

ACTIVEREGION

I

3

2

1

55 .1 nm

725 m

eV

0.9 nm thickwell and barrier

Rui Yang’s talk

Rui Yang’s talk

Mid-Far Infrared lasers

• IV-VI lead-salt diode lasers: 3-30 m, low-T

• Type II lasers

• Interband cascade lasers

• Intersubband (quantum) cascade lasers

What makes the QC-laser special?

• Wavelength agility– layer thicknesses determine emission wavelength

• Demonstrated applications in mid/far-IR gas sensing

• High optical power ~ 1W, room-T operation– cascading re-uses electrons

• Ultra-fast carrier dynamics– no relaxation oscillations

• Pure TM-polarization – efficient in-plane light coupling– Micro-lasers

• Small linewidth enhancement factor• Intrinsic “design potential”

HITRAN Simulation of Absorption Spectra (3.1-5.5 & 7.6-12.5 m)

NO: 5.26 m

CO: 4.66 m CH2O: 3.6 m

NH3: 10.6 m O3: 10 m

N20, CH4: 7.66 m

CO2: 4.3 m

CH4: 3.3 m

COS: 4.86 m

Frank Tittel et al.

Wide Range of Gas Sensing Applications• Urban and Industrial Emission Measurements

Industrial Plants Combustion Sources and Processes (eg. early fire detection) Automobile and Aircraft Emissions

• Rural Emission Measurements Agriculture and Animal Facilities

• Environmental Gas Monitoring Atmospheric Chemistry of Cy gases (eg global and ecosystems) Volcano Gas Emission Studies and Eruption Forecasting

• Chemical Analysis and Industrial Process Control Chemical, Pharmaceutical, Food & Semiconductor Industry Toxic Industrial Chemical Detection

• Spacecraft and Planetary Surface Monitoring Crew Health Maintenance & Advanced Human Life Support

Technology• Biomedical and Clinical Diagnostics (eg. non-invasive breath analysis)• Forensic Science and Security• Fundamental Science and Photochemistry

Life SciencesFrank Tittel et al.

Air Pollution: Houston, TX

Non-invasive Medical Diagnostics:Non-invasive Medical Diagnostics:Breath analysisBreath analysis

NO: marker of lung diseases

• Concentration in exhaled breath for a healthy adult: 7-15 ppb• For an asthma patient: 20-100 ppb

Appl. Opt. 41, 6018 (2002)

NH3: marker of kidney and liver diseases

Need fast and compact sensors

NASA Atmospheric & Mars Gas Sensor Platforms

Tunable laser sensors for earth’s stratosphere

Aircraft laser absorption spectrometers

Tunable laser planetary spectrometer

Frank Tittel et al.

Generation in the THz range

Why THz range is important

~ 100-1000 m, f ~ 0.3-3 THz

T-rays allow you to see through any dry optically opaque cover: envelope, clothing, suitcase etc, and locate non-metallic things, even read letters.

T-rays have enough specificity to distinguish “big” molecules; they can be used to detect explosives, drugs, etc.

THz spectroscopy and imaging

Three different drugs: MDMA (left), aspirin (center), and methamphetamine (right), have different images in T-rays

K. Kawase, OPN, October 2004

Q. Hu, QCL Workshop

Q. Hu, QCL Workshop

Terahertz QCLs

Highest operating temperature ~ 175 K in pulsed regime

Narrow tunability

Q. Hu (MIT), F. Capasso (Harvard), J. Faist (ETH), A. Tredicucci (Pisa)

12

Terahertz QCLs: 3 QW design

GaAs/AlGaAs

Belkin et al.

Free carriers help to reduce losses!

Metal-metal waveguide

z

xActive region

GaAs substrate

Gold

01075150m

(a)

(b)

GoldActive region

01075150m

GaAs substrate

Fig. 2. Schematic representation of (a) the semi-insulating surface-plasmon waveguide (b) the metal-metal plasmon waveguide, used in THz QCLs. The component of the magnetic field of the mode parallel to the layers of the active region (Hy) is plotted.

Heavily doped GaAs

Heterogeneous Cascades (multi- generation)

Homogeneous cascade: single stack of

~ 30 identical active regions & injectors

Stacked cascades: Interdigitated cascades:

Cooperative cascades:

Different electric field across sub-stacks

Charge transport between stages

How to design cooperation

So far:

Now:

Heterogeneous Cascades (multi- generation)

9.45 9.50 9.55Wavelength (m)

8.0 8.2Wavelength (m)

9.5 mactive region

9.5 mactive region

8.0 mactive region

Distance

Ene

rgy

Current flows in series

Design of the ultrabroadband quantum cascade laser

Act

ive

wav

egui

de c

ore

Sho

rter

wav

elen

gths

ge

nera

tion

Long

er w

avel

engt

hs

gene

ratio

n 5

6

7

8

Pea

k w

avel

engt

h ( m

)

150

175

200

225

Pea

k en

ergy

(m

eV)

250

Active region index 'i'

0 10 20 30

3

3

212

1

Ultrabroadband (6 - 8 m) spectrum

a

0.1

1

10

Pow

er (

arb.

uni

ts, l

og. s

cale

)

Wavelength (m)

5 6 7 8 9

2, 3, 4 A5 ... 13 A

activeregion

1

g3

2

4

I5

ener

gy

z 2

1

3

4

I.

5

II.

(2) ~ 105 pm/V

• Maximizing the product of dipoles d23d34d24 • Quantum interference between cascades I and II

Monolithic integration of quantum-cascade lasers Monolithic integration of quantum-cascade lasers with resonant optical nonlinearitieswith resonant optical nonlinearities

Frequency down-conversion to the THz range

• Difference frequency generation

• Stokes Raman and cascade lasing

• Parametric down-conversion

Three ways to achieve using nonlinear optics:

~ 100-1000 m, f ~ 0.3-3 THz

Current THz semiconductor lasers require cryogenic temperaturesThey are not tunable

Difference frequency generation in two-wavelength QCLs

M. Belkin, F. Capasso, A. Belyanin et al. Nature photonics 1, 288 (2007).M. Belkin, F. Xie et al., APL 96, 201101 (2008)

Difference frequency generation in two-wavelength QCLs

*)2(( qpqp EEP

1

2

3ωq

ωp

cladding

Laser1 section

Side contact layer

Laser 2 section

substrate

M. Belkin, F. Capasso, A. Belyanin et al. Nature photonics 1, 288 (2007).

M. Belkin, F. Xie et al., 2008

Results obtained by Feng Xie in Harvard in summer 2007