FEDERICO CAPASSO

School of Engineering and Applied Sciences

Harvard University

capasso@seas.harvard.edu

http://www.seas.harvard.edu/capasso

The Quantum Cascade Laser Revolution and its

Future

Recent reviews:

Federico Capasso ,Journal of Optical Engineering 49, 111102 (2010)

Robert Curl, F. Capasso e et al. Chemical Physics Letters 487, 1 (2010)

Spectral coverage of lasers

100 200 300 400 500 600 700 800 900 1,000 3,000 30,000

Ultraviolet Visible Near-infrared Mid-infrared terahertz

HeNe 633 nm

Ruby 694 nm

Nd:YAG 1064 nm

XeF 351 nm

XeCl 308 nm

KrF 248 nm

ArF 193 nm

Ti:sapphire 700-1000 nm

Ar-ion 364-514 nm

Diode

Dye

CO2

10.6 µm

QCLs

Er:YAG

2.94 µm

QCLs: First Lasers to provide broad wavelength

coverage for a largely underdeveloped spectral

region with major commercial opportunities

Diode lasers

~ 3 - 0.3 m

Quantum Cascade

Lasers (QCLs)

(=3-300m)

Electronics

up to ~1 THz (=300m)

First demonstration: 1994

J. Faist, F. Capasso, D.L. Sivco, C. Sirtori, A.L.

Hutchinson, A.Y. Cho,

Science 264, 553, (1994)

Molecular Fingerprint Region

Microwaves THz Mid-IR Near

-IR UV

Mid-IR: Molecular Fingerprint Region

Mid-Infrared:

Every molecule has a unique

absorption fingerprint

→ chemical sensing with high

sensitivity and selectivity

Applications

• Industrial process control and Pharma

- In line process control; Compliance testing of tablet,

capsules, powders

-Quality control of chemical processes from reagents to

products

• Homeland security & DOD: standoff detection of

explosives

and hazardous gases

• Medical: breath analysis, tissue imaging

• Environment / Energy: pollution monitoring,

• atmospheric chemistry

QCLs have become unique tools for the real time, highly sensitive selective and

non-invasive detection of chemicals as well as for high-power cw applications

ELIMINATION OF BAND-GAP SLAVERY: USES STATE OF THE ART

InP BASED and GaAs BASED EPIGROWTH PLATFORMS FOR PHOTONICS AND

ELECTRONICS

Quantum design: laser properties (wavelength, gain spectrum, lifetimes

population inversion) designed from wavefunctions, energy levels, matrix elements

Higher performance (high cw power at RT) can be obtained with a four well 2-phonon design

In0.53Ga0.47As/Al0.48In0.52As/InP GaAs/AlxGa1-xAs

QCLs: quantum design and epilayer growth

What makes the QC Laser special?

Wavelength agility: layer thicknesses determines wavelength; huge

λ design range; ultrabroadband band lasing and tuning

Unipolar nature & cascading which reuses electrons: high optical power

Intersubband transition; broadening insensitive to temperature:

temperature insensitivity of laser threshold (high T0: hundreds of K)

Ultra-fast carrier lifetime: no relaxation oscillations

Negligible spontaneous emission rate compared to (lifetime)-1

linewidth limit smaller than given by standard Schawlow-Townes

formula

Design of giant nonlinear susceptibilities in active region: coherent

phenomena at room temperature and new nonlinear optical sources

Quantum Cascade Laser: compact, cryogenic-free and bright laser source in the Mid-IR

2011: Commercialization in full swing

High performance mid-ir QCL using same high volume InP technology

platform of telecom photonics (lasers, etc.) based on MOVPE epigrowth

12

Homeland Security and DOD

- Remote detection of multiple chem/bio agents and aerosols.

- Detection of explosives.

- Target illuminators and beacons.

- Infrared countermeasures.

- Directional infrared optical wireless.

Industrial Process Control

- In-line process control of water based chemical reactions.

- Monitoring complex systems in real-time and continuously.

- In-vivo algae monitoring of yield and quality.

- Detection of unwanted biological species in drinking water.

- Pollution monitoring.

- Microfluidics monitoring.

Healthcare Diagnostics

- Of single or multiple small molecules in real-time for healthcare.

- In-line compliance testing of tablets.

- In-line compliance testing of tablet, capsules, etc, through

polymeric blister packaging.

Law Enforcement

- Providing early detection and analysis for forensic services.

Energy

- Monitoring and controlling energy conversion in the

transportation industry.

Mid-infrared Market Needs & Opportunities

High Power CW Room Temperature Operation

= 4.6 µm

A. Lyakh, C. Pflügl, et al., APL 92, 111110 (2008)

Strain compensation for large barrier heights (0.7-0.8 eV) : low carrier leakage; Diamond sub-mount.

www.pranalytica.com

QCL in conjunction with FTIR spectrometers

were used for

- spectroscopy through water

- reflection measurements

- standoff detection

Future Improvements:

• development of high power broadband QCLs

• software/data treatment to improve signal to noise

• better collection optics

• take advantage of polarized emission

from QCL to increase background discrimination

QCLs in conjunction with FTIR: a good marriage

Broadband devices and with high brightness

3-to-4 orders of magnitude larger than Globars

C. Pfleugl et al. CLEO 2010 Technical Digest

Infrared

camera

PC

controller

Electronics

Laser

head

QC Laser head: QCL, wavelength 8.3-8.9 m

100mW average power

Camera: FLIR A40, 30 frames per second

320x240 microbolometer array

Wood

Plastic

Glass

Cardboard

Metal

Illuminated objects

placed 100ft. away

from laser head

Standoff-detection

Distributed feedback laser : Single mode selection by 1st

order grating placed above active region: λ/2n) = Λ

19

Melting of Arctic Icecaps and Greenland: less energy reflected from sun

Methane and Carbon Dioxide Release from Permafrost Melting

•

Global Warming and Climate change:

driven by powerful feedbacks

ATMOSPHERIC (Troposphere & Stratosphere) TRACE GAS

MEASUREMENTS WITH QCLs

TRACE

GAS

cm-1 std dev 1s

ppb

76 m path

LoD

ppb

100 s

NH3 967 0.2 0.06

C2H4 960 1 0.5

O3 1050 1.5 0.6

CH4 1270 1 0.4

N2O 1270 0.4 0.2

H2O2 1267 3 1

SO2 1370 1 0.5

NO2 1600 0.2 0.1

HONO 1700 0.6 0.3

HNO3 1723 0.6 0.3

HCHO 1765 0.3 0.15

HCOOH 1765 0.3 0.15

NO 1900 0.6 0.3

OCS 2071 0.06 0.03

CO 2190 0.4 0.2

N2O 2240 0.2 0.1

13CO2/ 12CO2 2311 0.5 ‰ 0.1 ‰

LIGHTWEIGHT

MULTIPASS

CELL (76m)

LASER 1

CH4

1270.785

N2O

1271.078

CO

2179.772

LASER 2

ABSORPTION SPECTRUM

DUAL-LASER INSTRUMENT DESIGN

Pole-to-Pole Observations (HIPPO)

of Carbon Cycle and Greenhouse Gases

Gulf Stream V Aircraft

QCLs for CO2, CO, CH4, N2O

LATITUDE AND ALTITUDE

PROFILES OF TRACERS FOR

GLOBAL CIRCULATION MODELS

PRECISION: (MIXING RATIO)

CO2 30 ppb (340 ppm)

CO 0.2 ppb (80 ppb)

CH4 0.8 ppb (1800 ppb)

N2O 0.1 ppb (320 ppb)

PI: STEVEN WOFSY, HARVARD U.

Fre

e

tro

po

sp

her

e

Lo

wer

str

ato

sp

here

ALTITUDE PROFILES

Stratospheric

intrusion

CO CH4 N2O CO2

Ozone – measured at Beijing Olympics 2008

Ozone

High of 90 – 100 ppb

Low of < 1 ppb

US EPA 8 hour average standard

= 75 ppb

Start of Olympics

Quantum Cascade Laser Open Path

System – “QCLOPS”

Ozone, ammonia, CO2, water vapor

External cavity

QC laser

TE-cooled

detector

Telescope

75m roundtrip

C. Gmachl, Princeton; Zifa Wang; Chinese Acad. Sci. Daylight Solutions Inc.

Exhaled human breath contains ~ 400 different trace gas species, mostly at ultra

low concentration levels. Many of these gases can serve as biomarkers for the

identification and monitoring of various types of human diseases or wellness

states.

Broadband external cavity quantum cascade laser Jerome Faist Group, ETH

Broadband QCLs design by combining dissimilar active regions

Continuous wave: 2 active regions,

201cm-1 tuning (8.0m – 9.6m)

135 mW average Power

Pulsed operation: 5 active regions,

432 cm-1 tuning (7.5m – 11.4m)

1 W peak power

Grating coupled external cavity

High Performance QCL Lab-On-A-Chip

20 cm

Emission spectrum of laser array

Comparison with FTIR spectrometer

• Much higher S/N due to laser rather than

thermal source: remote trace gas detection

• Higher spectral resolution due laser linewidth

• Fast electronic tuning

29

Isopropanol, Methanol, Acetone

QCL array: points

Conventional FTIR: solid line

Applications: broad band spectroscopy and sensing

• Fast spectral tuning <<1ms

• Spectral gaps between two adjacent lasers can be

covered by current tuning

• Efficient and rugged spectral been combining demonstrated: longed

distance measurement of gases performed

Highly collimated QCL using plasmonics

SEM image of a fabricated device (=8.06 m) Measured far-field mode profile

FWHM divergence angles:

=2.7o (reduction by a factor of ~30)

||=3.7o (reduction by a factor of ~10)

Apertur

e size

w1

w2

(m2)

grating

period

(m)

groove

width w

(m)

groov

e

depth

d

(m)

radius of

the first

groove r1

(m)

2.1

1.9 7.8 0.6 1.0 6.0

Large design potential still far to be exhausted

Wide range of chemical sensing applications and increasing

importance of high power applications

High cw power efficiency of QCLs > 30 % ( Max so far 25% by Razeghi

group ) and high power ~ 10 W

Higher performance at short wavelength ( down to 3 microns) and

high performance (power and temperature in THz gap)

QCL at telecom wavelengths? High temperature (T0 = 1000), high

power QCL using Nitrides; chirp free QCLs

Mode-locked / pulsed shaped QCL and mid-ir frequency combs will

open new frontiers in molecular spectroscopy and coherent control

Increased functionality using plasmonics and metamaterials

THE FUTURE