Chapter 8 Semiconductor Saturable Absorbers

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Chapter 8

Semiconductor Saturable
Absorbers

Sofar we only considered artificial saturable absorbers, but there is of course
the possibility to use real absorbers for modelocking. A prominent candidate
for a saturable absorber is semiconductor material, which was pioneered by
Islam, Knox and Keller [1][2][3] The great advantage of using semiconductor
materials is that the wavelength range over which these absorbers operate
can be chosen by material composition and bandstructure engineering, if
semiconductor heterostructures are used (see Figure 8.1). Even though, the
basic physics of carrier dynamics in these structures is to a large extent well
understood [4], the actual development of semiconductor saturable absorbers
for mode locking is still very much ongoing.

289

290

CHAPTER 8. SEMICONDUCTOR SATURABLE ABSORBERS

Image removed due to copyright restrictions.

Please see:

Keller, U., Ultrafast Laser Physics, Institute of Quantum Electronics, Swiss Federal Institute of Technology,
ETH Hönggerberg—HPT, CH-8093 Zurich, Switzerland. Used with permission.

Figure 8.1: Energy Gap, corresponding wavelength and lattice constant for
various compound semiconductors. The dashed lines indicate indirect tran-
sitions.

30-40 Pairs

x
e
d
n
I

e
v
i
t
c
a
r
f
e
R

3.5

3.0

2.5

2.0

1.5

1.0

4

3

2

1

0

,
h
t
g
n
e
r
t
s

d
l
e
i
f

c
i
r
t
c
e
l

E

.
u
.
a

6.0

6.5

7.5

7.0
z (mm)

GaAs

AlAs

QW or Bulk Layer

Figure 8.2: Typical semiconductor saturable absorber structure. A semicon-
ductor heterostruture (here AlAs/GaAs) is grown on a GaAs-Wafer (20-40
pairs). The layer thicknesses are chosen to be quarter wave at the center
wavelength at which the laser operates. This structures acts as quarter-wave
Braggmirror. On top of the Bragg mirror a half-wave thick layer of the low
index material (here AlAs) is grown, which has a field-maximum in its center.
At the field maximum either a bulk layer of GaAlAs or a single-or multiple
Quantum Well (MQW) structure is embedded, which acts as saturable ab-
sorber for the operating wavelength of the laser.

Figure by MIT OCW.

8.1. CARRIER DYNAMICS AND SATURATION PROPERTIES

291

A typical semiconductor saturable absorber structure is shown in Figure
8.2. A semiconductor heterostruture (here AlAs/GaAs) is grown on a GaAs-
Wafer (20-40 pairs). The layer thicknesses are chosen to be quarter wave
at the center wavelength at which the laser operates. These structures act
as quarter-wave Bragg mirror. On top of the Bragg mirror, a half-wave
thick layer of the low index material (here AlAs) is grown, which has a
field-maximum in its center. At the field maximum, either a bulk layer of
a compound semiconductor or a single-or multiple Quantum Well (MQW)
structure is embedded, which acts as a saturable absorber for the operating
wavelength of the laser. The absorber mirror serves as one of the endmirrors
in the laser (see Figure 8.3).

Figure 8.3: The semiconductor saturable absorber, mounted on a heat sink,
is used as one of the cavity end mirrors. A curved mirror determines the
spot-size of the laser beam on the saturable absorber and, therefore, scales
the energy fluence on the absorber at a given intracavity energy.

8.1 Carrier Dynamics and Saturation Prop-

erties

There is a rich ultrafast carrier dynamics in these materials, which can be
favorably exploited for saturable absorber design. The carrier dynamics in
bulk semiconductors occurs on three major time scales (see Figure 8.4 [5]).
When electron-hole pairs are generated, this excitation can be considered

292

CHAPTER 8. SEMICONDUCTOR SATURABLE ABSORBERS

as an equivalent two-level system if the interaction between the carriers is
neglected, which is a very rough assumption.

E

e – e

e – LO

I

II

III

Eg

lh

hh

| k |

Figure 8.4: Carrier dynamics in a bulk semiconducotr material. Three time
scales can be distinguished. I. Coherent carrier dynamics, which at room tem-
perature may last between 10-50 fs depending on excitation density. II. Ther-
malization between the carriers due to carrier-carrier scattering and cooling
to the lattice temperature by LO-Phonon emission. III. Carrier-trapping or
recombination [5].

Figure by MIT OCW.

There is a coherent regime (I) with a duration of 10-50 fs depending on
conditions and material. Then in phase (II), carrier-carrier scattering sets
in and leads to destruction of coherence and thermalization of the electron
and hole gas at a high temperature due to the excitation of the carriers high
in the conduction or valence band. This usually happens on a 60 – 100 fs
time scale. On a 300fs – 1ps time scale, the hot carrier gas interacts with
the lattice mainly by emitting LO-phonons (37 meV in GaAs). The carrier
gas cools down to lattice temperature. After the thermalization and cooling
processes, the carriers are at the bottom of the conduction and valence band,

8.1. CARRIER DYNAMICS AND SATURATION PROPERTIES

293

respectively. The carriers vanish (III) either by getting trapped in impurity
states, which can happen on a 100 fs – 100 ps time scale, or recombine over
recombination centers or by radiation on a nanosecond time-scale. Carrier-
lifetimes in III-VI semiconductors can reach several tens of nanoseconds and
in indirect semiconductors like silicon or germanium lifetimes can be in the
millisecond range. The carrier lifetime can be engineered over a large range
of values from 100 fs – 30ns, depending on the growth conditions and purity
of the material. Special low-temperature growth that leads to the formation
or trapping and recombination centers as well as ion-bombardment can result
in very short lifetimes [9]. Figure 8.5 shows a typical pump probe response
of a semiconductor saturable absorber when excited with a 100 fs long pulse.
The typical bi-temporal behavior stems from the fast thermalization (spectral
hole-burning)[7] and carrier cooling and the slow trapping and recombination
processes.

y
t
i
v
i
t
c
e
l
f
e
R

0.5

0.4

0.3

0.2

0.1

0.0

Intraband thermalization

Carrier recombination

0.0

1.0

2.0

3.0

Time delay (ps)

Figure 8.5: Pump probe response of a semiconductor saturable absorber
mirror with a multiple-quantum well InGaAs saturable absorber grown at
low temperature [3].

Figure by MIT OCW.

With the formula for the saturation intensity of a two-level system Eq.
(2.145), we can estimate a typical value for the saturation fluence Fs (satu-
ration energy density) of a semiconductor absorber for interband transitions.
The saturation fluence FA, also related to the absorption cross-section σA, is

294

CHAPTER 8. SEMICONDUCTOR SATURABLE ABSORBERS

then given by

FA =

= IAτ A =

hf
σA

~2
2T2ZF

=

~2n0
2T2ZF 0

2

2

(cid:129)M
¯
¯
¯

¯
¯
¯

(cid:129)M
¯
¯
The value for the dipole moment for interband transitions in III-V semicon-
¯
ductors is about d = 0.5 nm with little variation for the different materials.
Together with the a dephasing time on the order of T2 = 20 fs and a linear
refractive index n0 = 3, we obtain

¯
¯
¯

(8.1)

(8.2)

(8.3)

FA =

~2n0
2T2ZF 0

(cid:129)M

2 = 35

µJ
cm2

¯
¯
Figure 8.6 shows the saturation fluence measurement and pump probe trace
¯
with 10 fs excitation pulses at 800 nm on a broadband GaAs semiconductor
saturable absorber based on a metal mirror shown in Figure 8.7 [11]. The
pump probe trace shows a 50 fs thermalization time and long time bleach-
ing of the absorption recovering on a 50 ps time scale due to trapping and
recombination.

¯
¯
¯

Image removed due to copyright restrictions.

Please see:

Jung, I. D., et al. “Semiconductor saturable absorber mirrors supporting sub-10 fs pulses.”
Applied Physics B 65 (1997): 137-150.

Figure 8.6: Saturation fluence and pump probe measurements with 10 fs
pulses on a broadband metal mirror based GaAs saturable absorber. The
dots are measured values and the solid line is the fit to a two-level saturation
characteristic [11].

8.2. HIGH FLUENCE EFFECTS

295

A typical value for the fluence at wich damage is observed on an absorber
is on the order of a few mJ/cm2. Saturating an absorber by a factor of 10
without damaging it is still possible . The damage threshold is strongly
dependent on the growth, design, fabrication and mounting (heat sinking) of
the absorber.

Image removed due to copyright restrictions.

Please see:
Fluck, R., et al. “Broadband saturable absorber for 10 fs pulse generation.” Optics Letters
21 (1996): 743-745.

Figure 8.7: GaAs saturable absorber grown an GaAs wafer and transfered
onto a metal mirror by post growth processing [10].

8.2 High Fluence Effects

To avoid Q-switched mode-locking caused by a semiconductor saturable ab-
sorber, the absorber very often is operated far above the saturation fluence
or enters this regime during Q-switched operation. Therefore it is also im-
portant to understand the nonlinear optical processes occuring at high exci-
tation levels [13]. Figure 8.8 shows differential pump probe measurements on
a semiconductor saturable absorber mirror similar to Figure 8.2 but adapted
to the 1.55 µm range for the developement of pulsed laser sources for optical

296

CHAPTER 8. SEMICONDUCTOR SATURABLE ABSORBERS

communication. The structure is a GaAs/AlAs-Bragg-mirror with an InP
half-wave layer and an embedded InGaAsP quantum well absorber with a
band edge at 1.530 µm. The mirror is matched to air with an Al203 single-
layer Ar-coating. At low fluence (5.6 µJ) the bleaching dynamics of the
QWs are dominant. At higher fluences, two-photon absorption (TPA) and
free carrier absorption (FCA) in the InP half-wave layer develop and enven-
tually dominate [13].

Figure 8.8: Differential reflectivity measurements of a semiconductor sat-
urable absorber mirror (GaAs/AlAs-Bragg-mirror and InP half-wave layer
with embedded InGaAsP quantum well absorber for the 1.55 µm range. The
mirror is matched to air with an Al203single-layer ar-coating). At low fluence
the bleaching dynamics of the QWs are dominant. At higher fluences, TPA
and FCA develop and enventually dominate [13].

Langlois, P. et al. “High fluence ultrafast dynamics of semiconductor saturable absorber mirrors.”
Applied Physics Letters 75 (1999): 3841-3483. Used with permission.

The assumption that TPA and FCA are responsible for this behaviour has
been verified experimentally. Figure 8.9 shows differential reflectivity mea-
surements under high fluence excitation at 1.56 µm for a saturable absorber
mirror structure in which absorption bleaching is negligible (solid curve). The
quantum well was placed close to a null of the field. A strong TPA peak is
5ps decay for FCA. Both of these
followed by induced FCA with a single
dynamics do not significantly depend on the wavelength of the excitation,
5ps decay is
as long as the excitation remains below the band gap. The

∼

∼