Advanced Semiconductor Lasers

 Quantum cascade lasers  Single mode lasers  DFBs, VCSELs, etc. Quantum cascade laser Reminder: quantum well lasers

Conventional semiconductor laser

CB diode laser: material

VB Intersubband transitions

CB intersubband transitions

GaAs AlGaAs AlGaAs

Device applications: quantum cascade laser (QCL) and quantum well infrared photodetector (QWIP) Quantum Cascade Laser Four-level laser CB V=∞

n2 22 E  n 2m*L2

L = layer thicknesslayer thickness unipolar semiconductor laser using intersubband transitions Quantum cascade lasers: mid‐infrared light sources InGaAs/InAlAs lattice‐matched to InP

Itop 2 e 1 3 Ibott

Itop active region 2 e 1 injector

Ibott active  region Band “engineering” injector wavelength agility: InP range 5 –20 m QCL: compact, rugged light source

Grown by Molecular Beam Epitaxy

InGaAs/InAlAs lattice matched to InP Semiconductor growth: Molecular Beam Epitaxy

Prof. Manfra’s GaN and GaAs MBE machines at Purdue Device fabrication at the Birck Nanotechnology Center 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 What makes the QC‐laser special?

Fabry‐Perot Single mode DFB mode

8.0 8.2 4.96 5.00 5.04 Wavelength (m) Nonlinear light generation: Dual‐wavelength second‐harmonic

200 100 no grating

150 laser SH

Ultra‐broadband

100 50

Intensity (arb. units) (arb. Intensity Intensity (a.u.) Intensity a Intensity (a.u.) 10 50 2, 3, 4 A 4.92 4.96 5.00 5.04 7.36 7.40 7.44 7.48 5 ... 13 A Wavelength (m) 0 0 8.6 8.8 9.0 9.2 9.4 9.6 4.34.44.54.64.74.8 1 pump wavelength (m) second-harmonic (m)

0.1 Power Power (arb. units, log. scale)

56789 Wavelength (m) 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 Single‐mode and tunable QC‐DFB lasers

CO2 H2O CO2 H2O ) % (

T 0 100 4567810121418

CONO CH4 CO2 NH3 N2O Temperature (K) 0 100 200 300 4.59 4.65 5.35 5.4 8.5 8.6 9.5 9.6 9.95 10.05 16.216.22

Wavelength ( m) QCLs are ideal for sensing applications

In‐situ trace gas sensing: NO, CO, NH3, CH4, H2O (isotopes), and more complex molecules – ppm to ppb levels Chemical and biological sensing (air quality, chemical and biological weapons, breath monitoring) Remote sensing: LIDAR Non‐invasive medical diagnosis Free‐space optical telecommunications

Pranalytica’s “optical nose” http://www.pranalytica.com/core‐technologies/gas‐sensors.php 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 Reminder: lasers Lasers • Pump source • Gain medium • Optical resonator Four-level laser Population inversion Stimulated emission Gain medium

Itop 2 e 1 3 Ibott I top active  region 2  e 1 injector  I 2 bott   4 ez 1  2  32 active g  3 1  region  32  0nef Lp 2 32 injector    J  w m th g QCL research directions Design of high‐gain active region

e Understanding mid‐infrared waveguide 4 IB losses 3 2 e injector 1

active Growth of high‐purity materials injector

Heat extraction from active region Ti/Au top contact nInP, 8  1018 cm-3

nInP, 1017 cm-3

n InGaAs, 3-5  1016 cm-3

InP substrate Waveguide core: electroplated Au Active regions and injectors 30-50 stages

n InGaAs, 3-5  1016 cm-3

nInP, 1-21017 cm-3, substrate

In solder waveguide core Room‐temperature, continuous‐ wave operation MBE or MOCVD InP overgrowth Plated gold Metal electroplating

Laser core

40 450 14 400 220 K cw mode 300 K 200 K 12 220 K 240 K 280 K 350 30 260 K 240 K 10 260 K 300 280 K 300 K 250 8 20 300 K 320 K 200 6 150 Voltage (V) Voltage 4 10 320 K 100 cw output power (mW) 2 pulsed mode 50 Peak output power (mW) 0 0 0 1.0 1.5 2.0 2.5 3.0 3.5 4.0 02468 2 current density (kA/cm2) current density (kA/cm )

J. Chen, et.al., J. Vac. Sci. Tech. 25 (2007), 913. Highlights of recent results Room‐temperature high‐power cw operation (M. Razeghi et al.) Highlights of recent results Terahertz QCLs

Highest operating temperature ~ 175 K in pulsed regime

Narrow tunability

Q. Hu (MIT), F. Capasso (Harvard), J. Faist (ETH), A. Tredicucci (Pisa) Other fun stuff: Monolithic integration of QCLs with resonant optical nonlinearities

I 5 5 (2) ~ 105 pm/V 4 4 3 g 3 2 II. z 1 2 energy I. 1 active region

Difference frequency generation in QCLs 3 cladding ωq Laser1 section   Side contact layer (2) * Laser 2 section P( p  q    EpEq ωp 2

substrate 1 M. Belkin, F. Capasso, A. Belyanin et al. Nature photonics 1, 288 (2007). M. Belkin, F. Xie et al., 2008 Single mode lasers Laser modes: longitudinal and transverse

2743 DR1-3 ridge B, 77K cw QCL Longitudinal modes 4.0

3.5

3.0

2.5

2.0

1.5 400 mA Light intensity[a.u.] 300 mA 1.0 200mA 150 mA 0.5

0 1230 1240 1250 1260 1270 1280 Transverse modes

Wavenumber [cm-1] Single Mode Laser

• Single mode laser is mostly based on the index‐guided structure that supports only the fundamental transverse mode and the fundamental longitudinal mode. In order to make single mode laser we have four options: 1‐ Reducing the length of the cavity to the point where the frequency separation of the adjacent modes is larger than the laser transition line width. This is hard to handle for fabrication and results in low output power. 2‐ Vertical‐Cavity Surface Emitting laser (VCSEL) 3‐ Structures with built‐in frequency selective grating 4‐ tunable laser diodes Single Frequency Semiconductor Lasers: Distributed Bragg reflector (DBR) laser

• Frequency selective dielectric mirrors a cleaved surfaces. •Only allow a single mode to exist •Periodic corrugated structure that interfere constructively when the wavelength corresponds to twice the corrugation periodicity (Bragg wavelengths) Distributed Bragg  reflector

A q(B/2n) =  B

Active layer Corrugated (a) dielectric structure (b)

(a) Distributed Bragg reflection (DBR) laser principle. (b) Partially reflected waves at the corrugations can only constitute a reflected wave when the wavelength satisfies the Bragg condition. Reflected waves A and B interfere constructive when q(B/2n) = . © 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

q=integer, B= Bragg wavelength of the mirror output Single Frequency Semiconductor Lasers: Distributed Feedback (DFB) laser

•The corrugated layer, called the guiding layer, is now next to the active layer •In the DFB structure traveling wave are reflected partially and periodically as they propagate.  2 2n   m 1/ 2     B B q B 2nL

Ideal lasing emission Optical power  Corrugated grating Guiding layer 0.1 nm Active layer    (nm) (a) (b) B (c)

(a) Distributed feedback (DFB) laser structure. (b) Ideal lasing emission output. (c) Typical output spectrum from a DFB laser.

 2 2n   m 1/ 2     B B q B 2nL DFB (Distributed Feed‐Back) Lasers

• In DFB lasers, the optical resonator structure is due to the incorporation of Bragg grating or periodic variations of the refractive index into multilayer structure along the length of the diode.

Thermal Properties of DFB Lasers

Light output and slope efficiency decrease Wavelength shifts with temperature at high temperature •The good: Lasers can be temperature tuned for WDM systems •The bad: lasers must be temperature controlled, a problem for integration Agrawal & Dutta 1986 Vertical cavity surface emitting lasers (VCSELs) VCSEL Edge emitting vs. surface emitting laser

Ridge waveguide Laser

Vertical Cavity Surface-Emitting Laser Contact •Optical cavity axis /4n2 Dielectric mirror along the direction of /4n 1 current flow rather Active layer than perpendicular to current flow Dielectric mirror •Radiation emerges from the surface of the Substrate cavity rather than from Contact its edge •Reflectors at the Surface emission edges of the cavity are A simplified schematic illustration of a vertical cavity surface emitting laser (VCSEL). dielectric mirrors © 1999 S.O. Kasap, Optoelectronics (Prentice Hall) •20-30 layers for mirror, MQW active region Edge-emitting laser VCSEL

• Large distance between cavity modes: – single-mode laser • Circular beam shape • Low threshold and power consumption • 2D laser arrays • Wafer-scale testing • Ultrafast modulation For long wavelength laser based on InGaAsP/InP: index contrast is too low, need too many layers, the device is too resistive as a result

Current spreading, many transverse modes -> need confinement for current and for the EM field

Oxidized aperture VCSEL Other advanced optical cavities Photonic crystal lasers Microlasers: microdisk, micro‐pillar, etc.

semiconductor‐heterostructures 5 µm (n=3.3)

(McCall et al., 1992)

Phys. Rev. Lett. 98, 043906 (2007) Photonic Crystals: Opportunities

. Photonic Crystals . complex dielectric environment that controls the flow of radiation . designer vacuum for the emission and absorption of radiation

. Passive devices . dielectric mirrors for antennas . micro-resonators and waveguides

. Active devices . low-threshold nonlinear devices . microlasers and amplifiers . efficient thermal sources of light

. Integrated optics . controlled miniaturisation . pulse sculpturing Defect-Mode Photonic Crystal Microlaser

Photonic Crystal Cavity formed by a point defect

O. Painter et. al., Science (1999) Photonic Crystal Applications:PBG Laser

 The smallest defect mode laser is shown ( Axel Scherer, California Institute of PBG Defect Laser Technology).  Periodic air holes in high index material forms a 2D photonic crystal.  The center air hole is removed and forms a resonant cavity. Light is confined in the cavity.  Spontaneous emission in the band gap is prohibitted, but for the defect mode is enhanced.  This produces a microlaser with very low threshold. Light confinement in optical microresonators

n=1.47

Maxwell’s Eq. n=1 J. Wiersig, PRA 2003

MH and K. Richter, PRE 2002 QCL Microlaser

Quantum Cascade Laser (Faist, Capasso et al., 1994; Bow-tie (Gmachl et al.,1998) Sirtori et al., 1998) resonator geometry mode characteristics active (lasing) material amplification

• small amplification devices • defined direction changes upon coupling • lasing varnishes (thresholdless)