Download Femtosecond pulse generation using ground-state and excited-state transitions in quantum-dot SESAMs

Femtosecond pulse generation using ground-state and excited-state transitions in
quantum-dot SESAMs
C. G. Leburn, N. K. Metzger, A. A. Lagatsky, C. T. A. Brown, W. Sibbett
M. Lumb, E. Clarke, R. Murray
J. F. Allen Research Laboratories, SUPA, School of Physics and Astronomy,
University of St Andrews, St Andrews, KY16 9SS, UK
Experimental Solid State Physics,
Imperial College London, London, SW7 2AZ, UK
Abstract
Stable mode locking is demonstrated in a Cr4+:forsterite laser incorporating quantum-dot SESAMs operating either through
ground-state or excited-state transitions to initiate and maintain the generation of pulses as short as 100fs and 122fs
respectively.
Imperial College
London
OCIS codes: (140.4050) Mode-locked lasers; (140.7090) Ultrafast lasers; (230.5590) Quantum-dot devices
Introduction
Results
 The continuing growth of applications in ultrafast science has meant that there is currently much interest in the design and construction of
femtosecond lasers that are more flexible, compact, robust and applicable than those used at present. Our current project is looking at ways
of tackling these issues.
 Here we demonstrate femtosecond-pulse generation from a laser that was mode locked using either ground-state or first-excited-state
transitions in quantum-dot-based SESAM devices and this facilitated a comparison between their respective operating regimes.
0.2
0.6
 Average output powers of 45mW.
 Time-bandwidth product of 0.33
implies
near–transform–limited
pulses
0.4
0.2
1260
1280
1300
1320
 Mode locking threshold fluence on
the SESAM was estimated to be
9J/cm2
0.0
1340
-200
0
Wavelength (nm)
200
Time (fs)
Fig. 4. Measured optical spectrum and corresponding intensity autocorrelation for the mode-locked Cr4+:forsterite using the antiresonant ground-state QD-SESAM
Excited-state QD SESAM
 Average output powers of 40mW
 Time-bandwidth product of 0.33
implies near–transform–limited
pulses
Experimental setup
 We constructed a Cr4+:forsterite laser to asses the GS and X1 devices. The laser comprised of a standard 4-mirror asymmetric, Z-fold cavity.
 Gain medium was a 11.6mm long
Brewster-cut Cr4+:forsterite crystal with
a
small-signal
pump
absorption
coefficient of 1.3cm-1.
 Self-starting stable mode locked
operation was possible
1.0
=14.8nm
centre=1290nm
0.8
0.6
0.4
0.2
 Mode locking threshold fluence
on the SESAM was estimated to
be 25J/cm2
 Self-starting stable mode locked
operation was possible
 Pump source was a Nd:YVO4 laser that
could deliver up to 10W of continuous
wave near-diffraction-limited 1064nm
laser radiation.
1.0
= 122fs
=0.33
0.8
Normalised intensity
Fig. 1a). The refractive index profile of the QD-SESAM structures and the calculated field
standing-wave pattern at 1290nm and b), room temperature PL measurements on the single
layer and bilayer dots used in the GS and X1 samples.
 Pulse durations as short as 122fs
were measured (sech2 pulses
assumed)
Normalised intensity
 The dot absorber layers were positioned in three groups of
three, separated by 40nm GaAs spacer layers. The positions
of these dots are displayed in figure 1a), which shows the
nominal refractive index profile for the SESAM and the
calculated field structure.
0.6
0.4
0.2
0.0
1260
1280
1300
Wavelength (nm)
1320
0.0
-400
-200
0
200
400
Time (fs)
Fig. 5. Measured optical spectrum and corresponding intensity autocorrelation for the mode-locked Cr4+:forsterite using the antiresonant excited-state QD-SESAM
Conclusions & future work
 Using these QD-SESAMs, pulses as short as 100fs have been generated. Future work will include more in-depth studies on the effect of
state degeneracy and recovery time of the excited-state (X1) devices, although we believe the absorption recovery time of the X1 devices is
faster than the GS devices – making X1 devices suitable for high p.r.f. systems
 A simple telescope system and
focussing lens were used to provide a
1/e2 pump beam spot radius of 35m in
the gain crystal.
The University of St Andrews is a charity registered in Scotland : No SC013532
0.4
0.0
1240
 The DBR had a design operating wavelength, 0, of 1290nm,
which corresponds to the centre of the high reflectivity stop
band in the frequency domain. Nine absorbing InAs quantum
dot layers were incorporated into a 80/4 GaAs low-finesse
cavity leading to anti-resonance at 0.
 Two fused silica prisms with a tip-to-tip
separation of 325mm were inserted in
the long arm of the cavity to provide the
appropriate dispersion compensation.
0.6
 Pulse durations as short as 100fs
were measured (sech2 pulses
assumed)
= 100fs
=0.33
0.8
Normalised intensity
Normalised intensity
 The SESAM structures comprised of a 25-period AlAs/GaAs distributed Bragg reflector (DBR) with QDs embedded in a GaAs cavity
deposited as a top layer. The dots in the ground state sample were single layer dots with an areal density of 2.2 x 1010cm-2 per layer.
The dot layers in the X1 sample had bilayers consisting of a
seed layer of QDs and a second QD layer separated by 10nm.
100
 We are also looking at the potential of resonant devices for ultrashort pulse generation. Our expectation is that this will offer more degrees
of freedom for the tailoring of fully optimised SESAM elements.
80
Ouput power (mW)
 Mode-locked operation of the laser was
obtained when HR-mirror was replaced
by one of the quantum-dot SESAMs
such that the spot radius of the incident
beam was 200m.
=18.4nm
centre=1296nm
0.8
Device structure
Ground-state QD SESAM
1.0
1.0
 Here we report experimental assessments for femtosecond-pulse generation from a Cr4+:forsterite laser system where the exploitation of the
intracavity SESAM devices incorporated within the laser cavity to initiate and sustain mode locking has involved either ground-state (GS) or
first excited-state (X1) transitions.
60
References
40
20
0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Incident power (W)
Fig. 3. Cw output powers of Cr4+:forsterite laser with
0.5% output coupler & HR in place
Fig. 2. Schematic diagram for laser cavity
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e-mail: - [email protected]
Poster MB18 - 2nd February - Advanced Solid-State Photonics 2009 – Denver, Colorado, USA