Electrically Pumped terahertz SASER device using a weakly coupled AlAs/GaAs superlattice as the gain medium

1
Electrically Pumped terahertz SASER device using
a weakly coupled AlAs/GaAs superlattice as the
gain medium
R. N. Kini, N.M. Stanton, A.J. Kent
And M. Henini
School Of Physics And Astronomy, University of
Nottingham, University Park, Nottingham, UK
INTRODUCTION We describe an electrically pumped
sound amplification by stimulated emission of
radiation (SASER) device for terahertz
frequencies. The gain medium of the device is a
weakly coupled AlAs/GaAs superlattice (SL)
contained within a multimode acoustic cavity
formed between the top (free) surface of the
structure and a SL phonon reflector. We have
studied the properties of a prototype device
using superconducting bolometers to detect the
phonons emitted. We observed an enhancement of
the phonon emission in a direction perpendicular
to the SL layers when the energy drop per period
of the gain SL, D, matched the energy of the
cavity of phonon modes. We believe these
observations provide evidence that the device is
operating as a SASER.
THE RESULTS All the measurements were carried out
at a temperature T 2K. Figure 4 shows the
current-voltage (I-V) characteristics for a 50mm
device. The device turns on at the threshold
voltage, VT 100 mV and the current increases
monotonically after that for biases upto 315 mV.
Fluctuations in the current due to electric field
domain formation can be seen for biases above
315mV. The device was energized with 1.5ms long
electrical pulses and the phonons emitted were
detected using the bolometers. Figure 5 shows the
bolometer signals, normalized to the power
dissipated in the device, as a function of D. Of
particular interest is the peak at D 2.7 meV
for q 0o. This corresponds to a phonon
frequency of 650 GHz, the same as the frequency
of the LA cavity modes. If the device was acting
as a hot phonon source, then we would have
expected to see a dip in the phonon flux at D
2.7 meV, because the phonon mirror will attenuate
the propagation of 650 GHz phonons. The peak in
the phonon emission suggests that phonon
amplification is occurring and the device is
working as a SASER. The FWHM of the peak is 0.9
meV corresponding to a phonon frequency of
220GHz. This is due to the width of stop band of
the reflector SL and the spectral broadening in
the gain SL. For q 30o, no enhancement of
phonon emission is seen. For phonons travelling
at large angles, the electron-phonon interaction
is cut off for q gt 2kF, because momentum cannot
be conserved. Hence for 650GHz phonons, the gain
medium is not efficient at q 30o. Figure 6
shows the bolometer signal normalised to the
power dissipated in a 400mm device. Again a peak
at D 2.7 meV is seen. The peak is broad due to
the larger area of the device.
THE SASER DEVICE Phonon assisted tunnelling in a
SL is indirect in momentum space, so for
interwell transitions involving emission of
phonons of energy h? lt ?, the initial states have
a higher population than the final states, see
Fig. 1. Therefore the rate of stimulated emission
can exceed the absorption rate, which gives rise
to possible phonon amplification 1. We have
observed evidence for phonon amplification in a
weakly coupled GaAs/AlAs SL 2. In this work we
have incorporated this SL into an acoustic cavity
formed between the top (free) surface of the
sample and another SL which acts as a phonon
mirror to create an electrically pumped SASER
device, the structure of which is shown in Fig.
2. The phonon mirror has 95 reflectance for
longitudinal acoustic (LA) phonons in a 90 GHz
wide band centred on 650 GHz. The length of the
cavity is 2.9 µm which leads to a mode separation
of 1 GHz for LA phonons. Device MESAs of diameter
50 µm and 400 µm were formed by etching and
GeAuNiAu contacts made to the emitter and
collector layers. The back of the substrate was
polished and superconducting Al bolometers
fabricated for phonon detection. One bolometer
was located directly opposite the SASER device
and another at an angle of 30o, see Fig. 3.
CONCLUSIONS Using bolometric detection techniques
we have studied a prototype SASER device. We find
an enhancement of phonon flux in a direction
perpendicular to the SL layers when the energy
drop per period of the SL matched the energy of
the cavity of phonon modes, indicating SASER
action.