676 CHINESE OPTICS LETTERS / Vol. 8, No. 7 / July 10, 2010
Ultrafast spectroscopy of semiconductor saturable
absorber mirror
Jing Zhang (ÜÜÜ ;;;)
1,2∗
, Dominik Bauer
2
, Farina K¨onig
2
,
Thomas Dekorsy
2
, Xihe Zhang (ÜÜÜUUUÚÚÚ)
1
, and Yafu Chen (æææÎÎÎ)
1
1
Department of Physics, Changchun University of Science and Technology, Changchun 130022, China
2
Department of Physics and Center of Applied Photonics, University of Konstanz, Konstanz, Germany
∗
E-mail: zhangj@cust.edu.cn
Received January 26, 2010
Ultrafast sp ectroscopy of semiconductor saturable absorber mirror (SESAM) is measured using a femtosec-
ond pump-probe experiment. This allows dynamic responses of SESAM in the cavity to be concluded by
ultrafast sp ectroscopy. Change in reflection is measured as a function of pump-probe delay for different
pump excitation fluences. Change of nonlinear reflection of SESAM is measured as a function of incident
light energy density. When the excitation fluence increases, nonlinear change in ultrafast spectroscopy of
SESAM becomes increasingly significant. When SESAM is pumped by an ultrahigh excitation fluence, the
energy density of which is approximately 1400 µJ/cm
2
, two-photon absorption can be observed visibly in
its ultrafast spectroscopy.
OCIS co des: 140.7090, 140.4050, 320.7080.
doi: 10.3788/COL20100807.0676.
The first semiconductor saturable absorber mirror
(SESAM), the breakthrough anti-resonant Fabry-Perot
saturable absorber that first demonstrated a passively
mode-locked Nd:YLF laser without Q-switching
[1]
, was
invented in 1992. Since then, SESAM has been acknowl-
edged as the most promising mode-locking device. It
has sparked the interest of various researchers across the
globe
[1,2]
.
As a reflecting mirror in the cavity, SESAM is responsi-
ble for passive mode-locking in ultrafast laser. Ultrafast
dynamics of excited carriers inside SESAMs can directly
influence the characteristics of output ultrashort pulses.
Ultrafast dynamic processes are exceedingly short. Ma-
jority of these processes are too short to be observed
using traditional measuring technologies, such as elec-
tronic techniques, because they are limited by tempo-
ral resolution. However, ultrafast lasers have advanced
the temporal resolution of measurements into sub-10-fs
regime, allowing for direct observation of ultrafast dy-
namics of excited carriers in SESAMs
[3−5]
.
Femtosecond pump-probe technology is frequently ap-
plied in ultrafast spectroscopy detection. It makes real-
time observation of ultrafast dynamics possible. This
technique has been applied in studying saturation char-
acteristics of SESAMs
[6−10]
. In our experiment, pump-
probe setup was used for ultrafast spectroscopy detection
of dynamic response of the SESAM at varying excitation
fluences. When the excitation fluence increases, non-
linear change in ultrafast spectroscopy of the SESAM
becomes increasingly significant
[11,12]
. In this letter,
we reveal the nonlinear changes and provide a detailed
explanation of the generation mechanism of nonlinear
changes in ultrafast spectroscopy.
The sample used in the experiment—low-loss
SESAM—consists of two parts (Fig. 1). The first is
the Bragg mirror, which is grown by metal oxide chemi-
cal vapor deposition (MOCVD) with reflection exceeding
99.5%. The second is the active region, including sat-
urable absorber layer with a thickness of 21.4 nm, grown
by molecular beam epitaxy at low temp erature (400
◦
C).
They are separated by GaAs barriers, placing the quan-
tum well (QW) into the antinode of the standing wave
pattern of the laser electric field. Thickness of the ab-
sorber layer is sufficiently designed to absorb the incident
wavelength.
Figure 2 illustrates the reflection spectrum of this
SESAM with a single QW. Based on this figure, it
is evident that this structure can be characterized by
extremely high reflectivity (> 90%) in the wavelength
region of 1010−1110 nm. Meanwhile, the reflectivity
bandwidth is wide (∼100 nm), supporting the generation
of picosecond or sub-picosecond ultrashort laser pulses.
Asymmetry of reflectance spectrum at approximately
1090 nm is caused by two-dimensional (2D) density of
states (DOS), which likewise results in asymmetry of
the imaginary part of QW’s dielectric function around
1090 nm.
Figure 3 illustrates the electric field distribution of
the SESAM, which is simulated by an optical software
dubbed “Scout”. Based on the distribution pattern of
standing-wave electric field, the electric field intensity at
the InGaAs QW layer can be observed clearly. In Fig. 3,
Fig. 1. Structure of the SESAM sample.
1671-7694/2010/070676-04
c
° 2010 Chinese Optics Letters
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