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

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

IR laser sources Difference Frequency Generation (DFG) LiNbO3(2-5µm), AgGaS2(3-9µm),GaSe(7-18µm) QC lasers 3.4-120 µm, ~5cm -1,power 10-100mW,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)

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

Quantum well lasing AlInAs InGaAs typically

Forming a minisubband e-

Injection into upper state barrier Levels relaxed by optical phonons

~ 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)

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

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

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)

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

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

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

Gerard Wysoki Gerard Wysocki

Apparatus Schematic

Broadly tunable system External cavity length Lens Laser Grating angle

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

NO and H2O scan

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

NO v 10 R3/2(20.5)

External cavity laser: Pulsed 2 80 1 40 950 1000 1050 1100 cm-1 R. Maulini, M. Beck, J. Faist and E. Gini, Appl. Phys. Lett. 84, 1659 (2004)

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

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, 3329-3334 (2004)

Pulse train

Ethylene 100 ppb in N2

External cavity Bragg grating coupler Zhang et al, Appl. Phys. Lett. 86, 111112 (2005) T= 80K pulsed

Status of QC lasers Commercial Availability Alpes Lasers (http://www.alpeslasers.ch/) Hammatsu Photonics (http://www.hpk.co.jp/Eng/products/ld.htm)* 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 1-500 mW (mostly near low end) *Just announced, not yet on this web site

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