P1254413669lXGPW

1
Generation Of Transverse Acoustic Phonons In A
GaAs/AlAs Superlattice by Ultra-Fast Optical
Excitation
R.N. Kini, N.M. Stanton, A.J. Kent And M. Henini
School Of Physics And Astronomy, University of
Nottingham, University Park, Nottingham, UK
BACKGROUND Recent work has demonstrated that
coherent phonons can be generated by ultrafast
optical excitation of GaAs / AlAs superlattices
(SL) Yamamoto et. al. Phys.Rev.Lett. 73 740
(1994). Folding of the acoustic phonon spectrum
due to the differing acoustic impedances of the
SL layers leads to an acoustic miniband structure
permitting the excitation of high frequency
acoustic phonons with q 0. The generation
process has been attributed to impulsively
stimulated Raman scattering, where the resonant
excitation of an e-h pair in the well is
accompanied by the creation of a zone folded
longitudinal acoustic phonon (ZFLAP). In these
experiments, transverse phonons were not
detected, consistent with the TA modes not being
Raman active in 001 oriented samples. In
previous work Kent et. al. Appl.Phys.Lett. 81
3497 (2002) we used a bolometer to detect
directly the propagating phonons following the
pump pulse. When the condition for coherent
phonon generation was satisfied, a strong
enhancement of the LA mode intensity was
observed. Using a thick GaAs substrate we were
able to use the filter effect of the substrate to
show the ZFLAP modes were leaking into
propagating monochromatic phonons. In this work
we have used bolometric detection techniques to
study the transverse acoustic (TA) phonons which
arrive at the detector following ultrafast
excitation of a GaAs/AlAs SL.
THE RESULTS Figure 2 shows the signals detected
when the excitation spot was directly opposite
the detector. The first peak after the laser
pulse at t0 is due to direct optical excitation
of the bolometer by PL from the SL and substrate.
The second and third peaks are due to the
arrival LA and TA phonons respectively. Previous
studies of this sample showed that when the
photon energy is resonant with the E1-HH1
transition, the optical signal reduces and the
ballistic component of the LA phonon signal shows
a pronounced increase. From Fig 2, it can also
be seen that the TA component of the signal also
increases. Fig 3 shows the change in the TA
peak as the excitation wavelength is varied.
There is a pronounced increase in the ballistic
component of the signal due to TA phonons, as
shown by the increasing intensity with decreasing
wavelength, and the apparent shift of the
ballistic peak. The inset Figure shows the
change in signal intensity (TA mode) as a
function of wavelength. We see the sharp
increase in signal intensity occurs at the same
wavelength as for LA modes
FIGURE 1 The experimental arrangement. FIGURE
2 The signals detected with the excitation spot
directly opposite the bolometer. The inset is an
enlargement of the signal due to LA phonons (vs
4700 ms-1). The dashed box is the region of
interest in this work.
THE EFFECT OF THE FILTER SL Figure 4 shows the
difference in signals obtained using the filter
SL samples. We find that in the sample where the
filter is not effective for the generator
frequency, the signals again show a greater
response at earlier times. Comparing these
traces with the sample where the filter is
working, we find no change in the peak position
of the TA signal. Fig. 5 shows the result of
integrating the signal over the TA rising edge
across the wavelength range. We see that there
is a sharp increase in intensity for the
unfiltered sample, and a correspondingly small
increase for the filter (probably due to the
broad spectrum of phonons generated in carrier
relaxation.
FIGURE 3 Signals obtained when the off resonance
signal is subtracted from the series of traces.
The inset shows the result of gating the signal
over the rising edge of the TA component (vs
3000 ms-1) . As was found with the LA component,
we observe a sharp increase in TA signal on
resonance. FIGURE 4 A comparison of the signals
obtained with the filter samples. The traces
have been normalised and offset for clarity. The
dashed lines are a guide for the eye.
THE SAMPLES The samples we used were grown by MBE
on semi-insulating (001) GaAs substrates. Sample
A was a 40 period GaAs / AlAs superlattice, each
period consisting of 4 monolayers AlAs and 22
monolayers GaAs. Samples B and C consisted of a
generator SL and a filter SL. In both
samples, the filter SL consisted of 40 periods of
7ML GaAs and 7ML AlAs. The generator SL of
sample B was identical to sample A, while in
Sample C it was 24ML GaAs / 8ML AlAs. The back
surface of the substrate was polished, and a
superconducting aluminium bolometer was
fabricated by thermal evaporation.
FIGURE 6
FIGURE 5
CONCLUSIONS We have studied the TA phonons,
generated by ultrafast excitation, emitted from a
SL. We find that, as with the LA mode, when the
SL is resonantly excited there is a strong
increase in TA signal intensity. By using a
filter SL, we have observed a reduction in the
component due to resonant excitation when the
generator SL frequency matches the stop band of
the filter. Due to acoustic anisotropy, TA
phonons propagating close to the 001 direction
are not pure transverse, and as such these quasi
TA modes may be Raman active.
THE EXPERIMENT The sample is mounted in an
optical access cryostat, with the temperature
held at To2.4K, at the mid-point of the
superconducting transition. The SL was excited
by 100fs, 2nJ pulses from a mode-locked
TiSapphire laser. The excitation spot was
positioned directly opposite the phonon detector
using computer controlled galvanometer mirrors.
FIGURE 5 Gating the transverse signal over the
rising edge of the ballistic peak in the
wavelength range of interest. The sharp increase
in intensity that occurs only in the unfiltered
sample is at the same excitation wavelength as
for the LA component. FIGURE 6 The calculated
dispersion curves for the SL samples, and the
transmission rate for the filter SL using the
transfer matrix method Tamura et. al. Phys.Rev.
B 38 1427 (1988).
The authors gratefully acknowledge support for
this project from EPSRC, under grant GR/R
33595/01.