1. Future Impacts Of Quantum Cascade Lasers On Spectroscopy
R. F. Curl, Anatoliy Kosterev & F. K. Tittel

2. Quantum cascade laser developments
IR laser sources
QC laser principles
Broadly tunable QC lasers
Spectroscopic demonstration

3. IR laser sources
Difference Frequency Generation (DFG) LiNbO3(2-5µm), AgGaS2(3-9µm),GaSe(7-18µm)
QC lasers µm, ~5cm -1,power mW,rugged
Lead Salt Diode Lasers each diode ~100 cm-1,3-30 µm P<1mW undesirable discontinuities, require liq N2
III-V diodes <3.8 µm P~1-10 mW
Color Center Lasers tunable from 1-4 µm, require liq N2 P~1-10mW
CO and CO2 Lasers (sidebands)
Optical Parametric Oscillators (OPO)

4. Quantum cascade laser; Basic facts
Semiconductor lasers (III-V materials)
Multiple-quantum-well heterostructure
Intersubband transitions
Band-structure engineering (emission wavelength defined by the layer thickness – MBE, MOCVD etc.)
Independent of material energy bandgap
Cascading (each electron creates N laser photons)
Number of periods N determines laser power
High reliability, long lifetime
Compact
Frank Tittel, Federico Capasso, Claire Gmachl, Jerome Faist, Rui Yang

5. Quantum well lasing
AlInAs
InGaAs
typically

6. Forming a minisubband e-

7. Injection into upper state
barrier
Levels relaxed by
optical phonons

8. ~ 1.5 periods with one active region
Injection barrier
Exit barrier
4 QW active region
Injector region
MINIGAP
MINIBAND
Faist et al., IEEE J.Quant. El.,
38, 533 (2002)

9. QCL Compared to Diode Lasers
Quantum Cascade Laser
The QC laser is a unipolar device rather than bipolar. It uses only one type of carrier for laser action [ i.e. electrons] since the material is only n-type doped. This is made possible by use of intraband (rather than interband optical transition.(Capasso-1975, 1994 (capasso), 1996 (DFB)
Electrons do not recombine with the valence band holes as in diode lasers but instead undergo a quantum jump between conduction band energy levels of a suitably designed multilayer quantum well structure which can be taylored to cover a broad wavelength region by changing layers, thickness.
An electron remains in condution band [ because thereare no holes] and therefore is recycled by injection into an identical adjacent active region and so. This cascade effect allows an electron to emit as many laser photons as the number of stages: leading to high powers typical of QC lasers. (25-75 stages are typical. Even ~ 100) by Band Structure Engineering The material is processed into laser stripes shaped as ridge waveguides. The optical cavity is defined by cleaving the strip along crystal planes as in… The resulting facets provide optical feedback. The core of the WG consists of N identical repetitions of the electron injector-active region in optical confinement normal to the layers is by provided by cladding materialInP (lower index than one!) Lateral confinement by dielectric metal coating; 1-3 mm long.  Slope represents the applied fields: 50 kv/cm; quantum design of injection region, i.e., superlattice

10. Interband QC laser (ICL)
Rui Yang
IEEE J. Q. Elect., 38, 559 (2002)

11. QCL and ICL
QCL:
Wavelengths 3.5 – 160µm (limited by the conduction-band offset on the short wavelength side)
Relatively high threshold current density (typically > 1kA/cm2)
High efficiency
High power
ICL:
Wavelengths 3 – 5 µm
Type II quantum wells
Low threshold current
High efficiency
High power
SOURCE: J. Faist and C. Sirtori (2001)

12. Tuning and frequency purity
Solitary laser - just the chip: multimode ~2-3
tunes ~3 cm-1
DFB - single mode, tunes ~3 cm-1, limits T range
Wider tuning requires:
Extended gain region
AR coated facet for external grating cavity  near RT operation OR clever outcoupling scheme

13. Bound-to-continuum design
Extended Gain
~RT CW operation
Richard Maulini, Mattias Beck & Jerome Faist,
App. Phys. Lett. 84, 1659 (2004)

14. CW external cavity QCL: output power
Richard Maulini, University of Neuchatel

15. Gerard Wysoki
Gerard Wysocki

16. Apparatus Schematic

17. Broadly tunable system
External cavity length
Lens
Laser
Grating angle

18. Scanning mode tracking
UECL – amplitude of the voltage controlling PZT tuning external cavity length
UGR – amplitude of the voltage controlling PZT tuning diffraction grating angle
EC – external cavity; FP mode – Fabry-Perot resonator mode

19. NO and H2O scan

20. Wide scan NO spectrum
R3/2(20.5)
R1/2(23.5)

21. NO v 10 R3/2(20.5)

22. External cavity laser: Pulsed2 80 1 40
950
1000
1050
1100
cm-1
R. Maulini, M. Beck, J. Faist and E. Gini, Appl. Phys. Lett. 84, 1659 (2004)

23. Fabry-Perot sideband suppression
R. Maulini, M. Beck, J. Faist and E. Gini, Appl. Phys. Lett. 84, 1659 (2004)

24. Using a pulsed QC laser
Damien Weidman, A.A. Kosterev, C. Roller, R. F. Curl, M. P. Fraser, and F. K. Tittel, Appl.Opt. 43, (2004)

25. Pulse train

26. Ethylene 100 ppb in N2

27. External cavity Bragg grating coupler
Zhang et al, Appl. Phys. Lett. 86, (2005)
T= 80K
pulsed

28. Status of QC lasers
Commercial Availability
Alpes Lasers (
Hammatsu Photonics (
Anywhere from 80 to 3300 cm-1 (most regions not yet commercially available)
Tunability - a few non-commercial lasers tunable over 100 cm-1, but typically <5 cm-1
Line width - CW <1 MHz ; pulsed ~500 MHz (chirp from heating; lifetime linewidth)
Power mW (mostly near low end)
*Just announced, not yet on this web site

29. Future Impacts Of Quantum Cascade Lasers On Spectroscopy
Depend upon QC development and commercial drive
The future impacts on spectroscopy
depend upon you!