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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

, Farina K¨onig

Thomas Dekorsy

, Xihe Zhang (ÜÜÜUUUÚÚÚ)

, and Yafu Chen (æææÎÎÎ)

Department of Physics, Changchun University of Science and Technology, Changchun 130022, China

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

, 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

° 2010 Chinese Optics Letters

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