Forcibly driven coherent soft phonons in GeTe with intense THz-rate pump fields

0.42 MB·English

by  Muneaki Hase

Forcibly driven coherent soft phonons in GeTe with intense THz-rate pump fields
Muneaki Hase and Masahiro Kitajima
Materials Engineering Laboratory, National Institute for Materials Science,1-2-1 Sengen, Tsukuba 305-0047, Japan
Shin-ichi Nakashima
Power Electronic Research Center, National Institute of Advanced Industrial
Science and Technology, 1-1-1 Higashi, Tsukuba 305-8561, Japan
1 Kohji Mizoguchi
1 Department of Applied Physics, Osaka City University,
0 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan
2
Weproposeanexperimentaltechniquetogeneratelargeamplitudecoherentphononswithirradi-
n
ationofTHz-ratepumppulsesandtostudythedynamicsofphasetransitioninGeTeferroelectrics.
a
J When a single pump pulse irradiates thesample at various pump power densities, the frequency of
the soft phonon decreases sub-linearly and saturates at higher pump powers. By contrast, when
1
THz-ratepumppulsesequenceirradiatesthesampleatmatchedtimeintervalstoforcibly drivethe
3
oscillation, a large red-shift of the phonon frequency is observed without saturation effects. After

excitation with a four pump pulse sequence, the coherent soft phonon becomes strongly damped
]
i leadingtoanearcritical dampingcondition. Thisconditionindicatesthatthelatticeisdriventoa
c
precursor state of the phasetransition.
s
-
l PACSnumbers: 78.47.+p,77.84.-s,42.65.Re,63.20.-e,
r
t
m
. Femtosecond-attosecond laser technologies have re- entphononwasgeneratedwithoutanysaturationeffects.
t
a cently been the focus of much attention in solid state The coherent phonon with larger amplitude showed a
m
physics because of their growing applications in observ- largedecreaseinfrequencyandbecamestronglydamped,
- ing both electronic and phononic ultrafast dynamics. In showing that the lattice is driven into a strongly anhar-
d
particular, one of the possibilities for the application of monic regime near the structural phase transition.
n
o femtosecond laser pulses is controlling the amplitudes of We chose ferroelectric GeTe as a sample because of
c coherentcollectivemotionsofatomsexcitedincondensed
its considerableinterestinopticalmemoriesapplications
[ matter by using double pump pulses or multiple pump
duetoitsreversiblestructuralchange[6]: Thiscrystalun-
2 pulses.[1–4] One can enhance the amplitude of a phonon dergoes the rhombohedral-rocksalt structural change at
v modebyapplyingin−phasepulses,orsuppressthe am- the transition temperature T = 657±100 K, attributed
4 plitude by applying out−of −phase pulses, both phe- c
to a displacive phase transition.[7] In a displacive phase
2
nomena are observed in a real time domain. The most
6 transition, the motions between the two phases gener-
remarkable advantage of the multiple pulse pump tech-
3 ally involve a soft mode vibration, whose frequency is
1. niqueistheabilitytoavoidsaturationeffectsduetohigh dramatically reduced near Tc.[8] This type of the phase
densityexcitation,i.e.,screeningofthespace-chargefield
1 transitionischaracterizedbyasinglepotentialminimum,
by electron-hole plasma.[5]
0 whosepositionshiftsatT . Bycontrast,othertypeofthe
c
1
phase transition, such as a order-disorder transition, is
: One goal in controlling coherent lattice vibrations is
v characterizedbyseveralpotentialminimaamongwhicha
to cause lattice instabilities which could lead to a phase
i choiceismadeatT .[8]Theorder-disordertransitionoc-
X transition. Wenotethatmanipulationofphononscannot c
curs with collective tunneling or thermally assisted hop-
r be demonstrated with conventional frequency-domain
a spectroscopy. Untilnow,allexperimentsincoherentcon- ping modes. In the case of GeTe, the A1 mode has been
considered to be the soft mode, which was observed by
trol of phonons have been performed under low density
Raman measurements.[7] An ab initio theoretical inves-
excitation, in which the coherent phonon amplitude was
tigationpredicted that the phase transition in GeTe was
toosmalltoobservelatticeinstability.[1–4]Inthisletter,
a fluctuation-driven first-order phase transition.[9]
we propose an experimental technique to forcibly drive
multiple coherent phonons into one larger amplitude co- Because of the strongly reduced frequency of the soft
herentphononanddemonstrateitscapabilityofinducing mode near T , monitoring the phase transition dy-
c
an extremely unstable crystal phase close to the critical namics is difficult with conventional frequency-domain
point in ferroelectric materials. We used a twin Michel- spectroscopies.[10] Motivated by these difficulties, Nel-
son interferometer to produce intense femtosecond THz- son and coworkers examined time-resolved pump-probe
rate pulse trains, which were used to repetitively push measurements at various lattice temperatures in per-
latticemotionsin-phase. Thus,alargeramplitudecoher- ovskites, and the heavily damped soft phonon was ob-
2
flectivity change obtained by a single pump pulse at the
2
pump power density of 12.7 mJ/cm . The coherent os-
cillations due to the collective motion of the crystal lat-
tice appear on the slowly varyingbackgrounddue to the
photo-excitedcarriers. Thefrequencyandtheamplitude
were obtained by fitting the time domain data using a
damped harmonic oscillator with the background,
F(t)=Ae−t/τcos(ωt+φ)+B[e−t/τ1 −e−t/τ2], (1)
whereAandB aretheamplitudeofthecoherentphonon
and the carrier contributions, respectively. τ is the de-
phasing time of the coherent phonon, τ1 and τ2 are the
relaxationandrisingtimesofthe carrierbackground,re-
spectively. ω is the frequency and φ is the initial phase
of the coherent oscillation. The time period of the oscil-
2
lation at the lowest pump power density of 0.8 mJ/cm
is∼263fs(=3.80THz),whichisclosetothatofthe A1
mode observedby Raman scattering, ∼ 3.81 THz (=127
−1
cm ),[7] and that for amorphous GeTe observed by co-
herentphononspectroscopy.[12]Asthepumppowerden-
2
sityincreasesfrom0.8to12.7mJ/cm ,thetimeperiodof
the A1 mode increases, corresponding to the red-shift of
FIG. 1: The transient reflectivity change obtained at 12.7 the phonon frequency from 3.8 to 3.0 THz, as discussed
mJ/cm2. The open circles are the experimental data, the later.
solid curve is the fit to the data with Eq. (1). The inset As the power density of the single pump pulse in-
shows the amplitude of the coherent oscillation obtained by
creases, the amplitude of the A1 mode increases and
thefitting.
saturates for the highest employed fluence, as shown
in the inset of Fig. 1. Experiments were not per-
2
formed above the fluence of 13 mJ/cm because of sam-
served near T .[11] The present study approaches the
c pledamagebylaserablationthroughgenerationofdense
critical point by increasing phonon amplitude instead of electron-hole plasma.[14] A similar saturation for the
increasing lattice temperature. The sample studied in phonon properties was observed for the optical phonons
this work was a single crystal of GeTe prepared by a va-
in semimetals[15] and in semiconductors[16] under sim-
por growthmethod andcleavedin the c crystallographic ilar conditions of high-density single pump excitation.
plane. GeTe is a narrow band-gap semiconductor, and The phonon softening observed with the single pump
the generation of the coherent A1 phonon is closely re- pulsemaybeascribedtothephononself-energyeffect[17]
lated to excitation of carriers from the valence band to or the electronic softening.[16] The observation of un-
higher energybands, i. e., displacive excitationof coher-
derdamped oscillation with single pump pulse suggests
entphonons(DECPs).[12,13]Femtosecondtime-domain that the GeTe crystal stays far from the critical point
measurementswerecarriedoutatroomtemperaturewith of the phase transition. In order to drive the A1 mode
a pump-probe technique. Femtosecond laser pulses of a
closer to the critical point while avoiding saturation
Ti:sapphire laser, operating at 800 nm, were amplified and sample damage, the multiple pump pulse excitation
to a pulse energy of 500 µJ in a 1 kHz regenerative am- technique[18, 19] was applied to the A1 mode.
plifier. After compensation of the amplifier dispersion,
By adding two mirror arms to the Michelson
the amplified pulses had 120fs duration. The pump and interferometer,[3] THz-rate pulse train with four pulse
probe beams were focused on samples to a diameter of
sequence was generated at a variable repetition rate, as
about100µm. The pump powerdensity was reducedby shown in Fig. 2 (a). By moving the mirrors of arms in
2
a neutral density filter to below 13 mJ/cm to prevent the twin interferometer, the separation time ∆t (i,j =
ij
damagingthesampleorcausinglaserablation,anditwas
1, 2, 3, 4) between the pulse components of P and P
2 i j
variedfrom0.8to 12.7mJ/cm . The probe pulse energy was controlled. The time derivatives of the transient re-
2
was also reduced and fixed at 0.3 mJ/cm . The pump- flectivity changes obtained by using the multiple pump
beamwas mechanicallychoppedat315Hz for the signal
pulses are shown in Fig. 2(b). Here, the power density
detection by a lock-in amplifier. The transient reflectiv- of each pump pulse is 3.8 mJ/cm2, such that the maxi-
ity change ∆R/R was recorded by changing the optical 2
mumtotalpumppower(3.8mJ/cm x4)exceedsthatof
path length of the probe beam. the single pump excitation (12.7 mJ/cm2) without sam-
Figure 1 shows time derivatives of the transient re- ple damage. We aimed that each pump pulse force the
3
3.8
3.6
z) 3.4
H
T
( 3.2
y
c
n 3.0
e
u
q
e 2.8
r
F
2.6
single pulse
2.4 pulse train
0 4 8 12 16
2
Pump power (mJ/cm )
FIG.3: Thepumppowerdependenceof thefrequencyofthe
A1 mode obtained for the single pump pulse and the pulse
train, respectively. The dotted curve is an eye guide and the
solid line corresponds to a fitting of the data with a linear
function.
oscillation of the coherent A1 mode through the repeti-
tive excitation. In order to well drive the soft phonon to
an in-phase motion, the time intervals in the THz-rate
pulse train was set to be unequal; as shown at the top
of Fig. 2(b), the pulse delay was increased for each sub-
sequent pulse to match the increased pulse intervals for
eachdrivenphononoscillation. Thetime-domaindatain
Fig. 2(b) clearly demonstrate both an enhancement in
the amplitude of the coherent A1 mode by a repetitive
excitation and a drastic decrease in the dephasing time
ofthe coherentphonon. The data ofFig. 2(b) wasfitted
with Eq. (1), in order to obtain the dephasing time of
the coherent phonon and the frequency of the A1 mode
for various total pump powers.
Figure3showstheobtainedfrequencyoftheA1 mode
as a function of the total pump power together with the
resultsfor single pump excitation. The A1 frequencyde-
creases linearly from 3.8 to 2.5 THz as we increase the
number of pulse sequence, while the single pulse excita-
tion produce saturation at highest fuences. The dephas-
ing time decreases from 570 to 180 fs as the number of
pulse sequence is increased. The largestdamping rate of
γ ∼ 1.8 ps−1 (=1/(180 fs)/π) observed with four pump
FIG. 2: (a) The optical layout of a twin Michelson interfer-
pulses is significantly close to the frequency of ω ∼ 2.5
ometer for the generation of the pulse train. BSs are the
−1
beam splitters. Each mirror arm labeled Pi (i=1, 2, 3, 4) ps (or 2.5 THz), showing that the A1 mode is driven
is computer-controlled. (b) Repetitive excitation of the A1g to near the point of the critical damping (ω = γ).
mode using THz-rate pulse train. P1, P2, P3, P4 are first, Wenotethatthelowestfrequencyofthesoftmodeob-
second, third, fourth pump pulses, respectively. ∆t12, ∆t23, served in the present study using THz-rate pulse train,
and ∆t34 are set to be 290 fs, 320 fs, and 345fs, respectively. of 2.5 THz, is comparable to the incoherent phonon fre-
The open circles represent the experimental data. The solid
quency at a temperature of ∼ 590 K, according to the
lines represent the fitting of the time-domain data after the
Curie-Weiss law. [7] This confirms that the crystal lat-
final pumppulse using Eq. (1).
tice is really close to the transition point.
4
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