Diode lasers
Lecture 23
Diode lasers
Light from diodes: LEDs
Operate in forward bias (posi=ve to p-‐type); use direct gap semiconductors. The current flux will
Determine the density in the excited state.
Opera=on of the diode to fuel inversion.
Add a cavity
Bam: diode laser…..
Direct gaps only:
GaN on SiC substrates for blue LED
Ce doped YAG powder absorbs blue
And emits yellow-‐green to combine with more blue for white light.
LaPce parameter (A)
GaN based diodes were key in opening up the white light market and full color spectrum applica=ons that included green, blue and near UV.
This was accomplished without having bulk GaN high quality single crystal material.
Substrates were essen=al:
From \ To
Sapphire
6H-‐SiC
GaN
GaN (%)
14.8
3.3
AlN (%)
12.5
1.0
InN (%)
24.4
14.0
Sapphire is the most commonly used substrate in spite if the large mismatch to GaN and wider bandgap materials.
However, it is cheap and available in large area wafers.
SiC is be\er suited, but under a closely held patent (Cree) and much more expensive to
grow. Large area wafers are also hard to grow.
Both SiC and GaN can be grown conduc=ng n-‐type or p-‐type contrary to sapphire.
Color Wavelength Voltage drop Semiconductor material
Infrared
Red
Orange
Yellow
Green
[eV]
ΔV < 1.63 Gallium arsenide (GaAs)
Aluminium gallium arsenide (AlGaAs)
1.63 < ΔV < 2.03 Aluminium gallium arsenide (AlGaAs)
Gallium arsenide phosphide (GaAsP)
Aluminium gallium indium phosphide (AlGaInP)
Gallium(III) phosphide (GaP)
2.03 < ΔV < 2.10 Gallium arsenide phosphide (GaAsP)
Aluminium gallium indium phosphide (AlGaInP)
Gallium(III) phosphide (GaP)
2.10 < ΔV < 2.18 Gallium arsenide phosphide (GaAsP)
Aluminium gallium indium phosphide (AlGaInP)
Gallium(III) phosphide (GaP)
1.9[70] < ΔV < 4.0 Tradi=onal green:
Gallium(III) phosphide (GaP)
Aluminium gallium indium phosphide (AlGaInP)
Aluminium gallium phosphide (AlGaP)
Pure green:
[nm]
λ > 760
610 < λ < 760
590 < λ < 610
570 < λ < 590
500 < λ < 570
Blue
Violet
Purple
Ultraviolet
Pink
White
450 < λ < 500
400 < λ < 450
2.48 < ΔV < 3.7
Indium gallium nitride (InGaN) / Gallium(III) nitride (GaN)
Zinc selenide (ZnSe)
Indium gallium nitride (InGaN)
Silicon carbide (SiC) as substrate
Silicon (Si) as substrate—under development
2.76 < ΔV < 4.0 Indium gallium nitride (InGaN)
Mul=ple types 2.48 < ΔV < 3.7 Dual blue/red LEDs,
λ < 400
Mul=ple types ΔV ~ 3.3[76]
3.1 < ΔV < 4.4
Broad spectrum ΔV = 3.5
blue with red phosphor,
or white with purple plas=c
Diamond (235 nm)[71]
Boron nitride (215 nm)[72][73]
Aluminium nitride (AlN) (210 nm)[74]
Aluminium gallium nitride (AlGaN)
Aluminium gallium indium nitride (AlGaInN)—down to 210 nm[75]
Blue with one or two phosphor layers:
yellow with red, orange or pink phosphor added anerwards,
Blue/UV diode with yellow phosphor such as Ce:YAG
Chromacity diagram:
Pure wavelength colors line the outer rim. Pick any three for light sources and then lines combine addi=vely
Blue LED 470 nm
Ce:YAG luminescence
540 to 560 nm (Ce concentra=on
dependent)
The “right” amount of Ce:YAG converts the ideal frac=on of blue to green to result in perceived white light. (red arrow)
Three LEDs can provide any color within a triangle.
Any linear combina=on can be achieved by individual power levels.
All colors within the triangle are possible.
Cheaper systems decrease the area in the triangle.
LED victory
Sodium vapor
1965
1879
1938
1959
1996
Laser diodes
• Semiconductor as a gain medium
• Transi=on rates for semiconductors in quasi-‐equilibrium
• Current pumping
• Laser threshold current
• Steady-‐state laser photon flux
• Power output characteris=cs
• Spa=al characteris=cs
• Spectral characteris=cs
• Wavelength tunable laser diodes
Some useful characteris<cs of semiconductor lasers:
1. Capable of emiPng high powers (e.g. con=nuous wave ~ W).
2. A rela=vely direc=onal output beam (compared with LEDs) permits high coupling efficiency (~50 %) into single-‐mode fibers.
3. A rela=vely narrow spectral width of the emi\ed light allows opera=on at high bit rates (~ 10 b/s), as fiber dispersion becomes less cri=cal for such an op=cal source.
Lasers are based on the phenomenon of s"mulated emission, the counterpart to absorp=on
E E h υ
Δ
E ( k ) k h υ h υ h υ k
1) Light
2) Electron
3) Hole
4) Energy
5) Gain
6) Thermal
environment
7) Inversion
Absorp=on S=mulated emission
must be of that matches the energy gap at k from an occupied state to an
unoccupied state: Δ E ( k ) = h υ
in lower energy state
in excited state (so to speak)
is absorbed
none
dominates
in excited state
in lower energy state
is released
2
negligible
small (but triggers s=mula=on) dominates
• A laser diode (LD) is a semiconductor op=cal amplifier (SOA) that has an op=cal feedback.
• A semiconductor op=cal amplifier is a forward-‐biased heavily-‐doped p+-‐n+ junc=on fabricated from a direct-‐bandgap semiconductor material.
• The injected current is sufficiently large to provide op=cal gain.
• The op=cal feedback is usually implemented by cleaving the semiconductor material along its crystal planes.
• The sharp refrac=ve index difference between the crystal (~3.5) and the surrounding air causes the cleaved surfaces to act as reflectors.
⇒ The semiconductor crystal therefore in general can act both as a gain medium and as a Fabry-‐
Perot op=cal resonator.
• Provided that the gain coefficient is sufficiently large, the feedback converts the op=cal amplifier into an op=cal oscillator, i.e. a laser.
• The device is called a laser diode or a diode laser or a semiconductor injec=on laser.
No bias posi=ve
Forward bias nega=ve
The cleaved surfaces func=on as the mirrors of the op=cal cavity
Current injec=on
Ac=ve region
The large change in index of refrac=on
From inside the semiconductor (e.g. in
GaAs n=3.38) to the outside air (n=1) is sufficient for large reflec=on values at R
2
.
Cleaved facets
Gain medium
Laser output
R
1
R
2
Cavity length D
For GaAs to air:
R =
( )
( )
2
2
=
(
(
3 .
38
3 .
38
−
+
1
1
)
) 2
2
= 29 .
5 % @ 1 .
5 µ m
Semiconductor as a gain medium
The basic principle: crea=on of popula"on inversion : s"mulated emission becomes more prevalent than absorp"on. The popula"on inversion is usually a\ained by electric-‐current injec"on in some form of a p + -‐n + func=on diode (also possible by op"cal pumping for basic research)
!
a forward bias voltage causes carrier pairs to be injected into the func=on region, where they recombine by means of s"mulated emission .
Here we discuss the semiconductor gain and bandwidth upon electrical pumping scheme.
No bias posi=ve
Forward bias electrons
Focus here nega=ve
electrons
E
Fc
E
g
E
Fv holes
E
Fc h υ
Absorp=on not possible
E
Fv x
Electrons are flowing into the conduc=on band while
Holes are flowing into the valence band. The popula=ons density of electrons and holes at the band edges is inverted.
E
g
Enter a photon with energy just above E g
. Absorp<on is impossible because there are now electrons in the lower energy state and there are photons in the upper energy state.
E g
<
h υ
<
(
E
Fc
−
E
Fv
)
( E
Fc
− E
Fv
)
> h
υ
> E g
The photon can, however, s<mulate emission.
An electron is triggered to drop into the valence band (combining with a hole there). A photon is emiAed , doubling the photon density.
If the original photon traveled in the correct direc=on, this can be repeated: double, quadruple, eight-‐fold…. LASing! h υ
E
Fc
E
Fv
LASing starts
Popula=on inversion in a forward-‐biased heavily doped p + -‐n + junc=on
E
E g
active region
(~ m h
m)
υ
E
E g
Fc
Fv
Upon high injec=on carrier density in a heavily-‐doped p + -‐n + junc=on there exists an ac=ve region near the deple=on layer, which contains simultaneously heavily populated electrons and holes – popula=on inverted!
Opposed to the shown schema=c, the light is emi\ed out of the plane of the paper.
