Interaction of GaAs with intrinsic
stimulated intensive picosecond emission
The results of the studies of the above
interaction, obtained in Kotel'nikov IRE of RAS, are presented here in brief.
Part of this work was done in cooperation with scientists V.I. Perel’ at al.
from Ioffe PTI of RAS (Moscow, Russia), VULRC (Vilnius, Lithuania), Satpaev
KNTU (Almaty, Kazakhstan). The discovery of picosecond stimulated emission is
described in item I. Investigation of the emission properties - in item II. The
results of investigation of ultrafast processes, that appear as the effect of
this emission, are given in item III. Scientific equipment used in the
investigation, as it is today, is presented in item IV. List of main
publications on this topic of present IRE RAS employees, e-mail addresses and
phone numbers are given in item V.
I. In our investigations of the
spectrum of GaAs bleaching (transparency increase) under picosecond optical
pumping, we found that density n = p and
temperature ðß 25 meV of
electron-hole plasma (EHP) vary approximately reversibly (with inertia of about
10 ps) in relation to the pumping [1,2]. To explain such changes of EHP
density, we supposed that stimulated (increased spontaneous) emission of
picosecond duration appears in GaAs during the pumping [2]. This suggestion was
confirmed in our experiments by indirect methods [3–5]. It is also indirectly
confirmed by appearance of stimulated emission no later than 12 ps after
pumping of GaAs/AlGaAs MQWS with femtosecond
impulse
[D. Hulin et.al. J. de Phys., 48,
267 (1987)].
Only
recently, another upgrade of the spectral-photo-chronometrical complex, with
which we do our studies, allowed us to directly measure the evolution of
picosecond pulse intensity. The very first measured chronograms of emission and
pumping confirmed that the emission is of picosecond duration and it appears at
the pumping front (in case of sufficient pumping intensity) [6].
II. Before the work [6],
time-integral characteristics of intrinsic emission were investigated. It was
associated with capabilities of the lab complex at that time. Investigations
were conducted on thin (1 µm) layer of GaAs. During powerful picosecond optical
pumping of GaAs, such a dense EHP was generated (n = p ~ 1018
– 1019 cm-3) [2,4,7], that it was
accompanied by significant inversion of carrier population. Light gain due to
the population inversion was experimentally determined in [4,8,9]. As was
confirmed by calculations [4], the inversion was sufficient for ultrafast
appearance of intensive stimulated emission. This emission is anisotropic and
it propagates mainly along GaAs layer [9]. The emission appears in threshold
manner; the character of an increase of individual spectral components and emission
spectrum width narrowing with increasing pump intensity were determined in
[3,10]. For example, in [11] intrinsic emission
intensity was estimated as high as W/ßm2.
Based on the study of EHP density relaxation due to stimulated recombination
(see III), one can conjecture that after the end of pumping, characteristic
time decay of emission will be equal to ~ 10 ps [12]. Long-wave boundary of
intrinsic picosecond emission spectrum universally depends on the energy
density integrated over emission spectrum [7]. Mutually matched self-modulation
of the following characteristics of emission out the sample face was revealed:
(a) emission energy as function of distance Y between the active area and the
face; (b) emission spectrum (either there was self-modulation or was not) as
function of Y; (c) energy of individual spectral components of emission as a
function of picosecond time interval between two pulses that pumped electrons
to different energy levels of conduction band [13].
Self-modulation of these characteristics
allowed us to assume the appearance of sub-terahertz amplitude modulation of
emission. Non-monotonic, modulated with period variations
of amplitude, emission spectrum width and limit width were discovered
in the case of altering photon energy of pumping GaAs with low-defect crystal
lattice, where is
LO-phonon energy, me and mh are electron
and heavy hole masses, respectively [14,15]. Note for above and below that in
our experiments GaAs was pumped by a powerful picosecond pulse at room
temperature, and pump photon energy was
not much greater than the band gap width Eg. Correspondingly,
the temperature of generated dense EHP was not much greater than approximately
room temperature of the lattice. For example, in [4] for = 1.52 eV the
EHP temperature did not exceed 470 K.
III. The appearance of intrinsic
stimulated picosecond emission turned out to lead to initiation of various
ultrafast variations of: density and energy distribution of EHP, energy
transport of charge carriers – the factors that essentially determine emission,
fundamental absorption of light and transparency. These variations are caused by
interaction of intrinsic emission, dense hot EHP, elementary collective
excitations (optical plasmons and LO-phonons) and pumping light. In our works
we obtained that appearance of the emission leads to the following.
Reversible picosecond variation of the
spectrum of bleaching (transparency increase) of GaAs, as was mentioned above,
appears for photon energy ,
where Eg is band gap width [1,2, et al.]. It showed
that density and temperature of EHP interrelatedly and approximately reversibly
(with picosecond delay) vary with increasing and decreasing pumping pulse
intensity [2].
After the end of picosecond pumping,
relaxation of EHP temperature and density and of bleaching takes place
exponentially with characteristic time ~10 ps [12]. The characteristic time
grows with an increase of active area diameter. This anomalous dependence on
diameter is the result of negative feedback between intrinsic emission
intensity and EHP heating produced by intraband absorption of emission. Picosecond
relaxation in semiconductor comes to an end by establishment of “universal”
threshold state of EHP, in which Tc = TR, n = p,
, where Tc
is EHP temperature, TR is room
temperature, n and p is density of electrons and
holes, respectively, and
is Fermi quasi-level,
respectively [2,4].
Raman scattering (RS) of pumping light
pulse appears, stimulated by intrinsic emission and going on with participation
of optical plasmons. RS leads not only to variation of intrinsic emission but
also of EHP density and temperature and of semiconductor optical transparency
[5, 7].
Distribution of electrons
between the valleys, band gap narrowing and optical plasmon energy are
determined by a single parameter – full density of photogenerated electron-hole
pairs [7]. This was explained by the above interconnection between density and
temperature of EHP.
In the area of
light amplification of fundamental absorption spectrum, “the hole is
burned” at sufficiently high emission intensity [9]. This means appearance of
depletion of inverse carrier population. The detailed equilibrium is disturbed.
To restore it, this depletion is translated up the band with period , due to interaction of
electrons with LO-phonons [9,16,17]. Modulation of electron distribution in
conduction band is produced. So, this distribution deviates from Fermi-Dirac
distribution and becomes unstable. As far as unstable distribution is situated
in the field of intrinsic intensive emission, sub-terahertz self-oscillations
of population depletion appear [11]. They reveal themselves as sub-terahertz automodulation of fundamental light absorption spectrum [18–20]. Experimentally
obtained amplitude-phase-frequency response of absorption automodulation and
analytic expression, that satisfactorily describes experimental dependence of
self-oscillation frequency on intrinsic emission intensity, are given in
[18,11]. As emission intensity increases, depletion self-oscillations acquire
the form of slightly nonregular standing wave [11,21]. It can be assumed that
self-oscillations of population depletion and the above mentioned (in item II) automodulation of emission characteristics are interconnected.
For dense EHP, the above mentioned
deviation from Fermi distribution is not inconsistent, judging from the
following. First, the time of repairing deviation of one electron from Fermi
distribution is essentially larger than the time of electron-electron
scattering and at the same time it is close to ,
where is probability of
LO-phonon emission by electron in the absence of phonons and other electrons
[22]. Second, energetic transport of electrons towards the bottom of conduction
band and autowave of population depletion must impede the screening of
electron-LO-phonon interaction.
Non-monotonic alteration of both
amplitude and width of emission spectrum, mentioned in issue II, as well as
appearance of the width limit by
variation of pump photon energy, – all that may be proposed to be connected
with auto-modulation of electron energy distribution [15]. The width limit correlates with the
fact that periodic modulation of electron distribution in conduction band does
not permit essential increase of band
inversion [4,9,15]. If one neglects that little part of EHP density, by
subtraction of which the population inversion disappears, then smoothed energy
distribution of EHP can be approximately characterized by conditions n = p,
[4]. Interconnection of
density and temperature of EHP [2,7], that manifested itself also in [5,12],
follows from these conditions.
The above mentioned reversible EHP
density variation is explained, in particular, by the interconnection of EHP
density and temperature. Note that the reversible variation took place also
when pump photon energy was
greater than the band gap width of unexcited sample Eg0
= 1.42 eV by less than 10 meV, i.e., initially cold plasma was pumped. In
the last case, EHP heating occurred predominately due to: (a) intraband
absorption of intrinsic emission and pump light [2,12,23]; (b) the fact that
electrons were born on the levels that had more energy than the energy of the
levels from which electrons forcedly recombinated [24].
The idea of a change of the screening of
electron-LO-phonon (e-LO) interaction with increasing EHP density seems not
well-established yet. Different authors obtained that: (a) e-LO interaction is
screened at n cm–3;
(b) signs of e-LO interaction were observed up to n = cm–3; (c) by EHP temperature kTc
>> e-LO-interaction
must be screened at EHP density equal to cm–3;
(d) e-LO-interaction screening cannot be formed while dense EHP distribution
remains non-Fermian. The results of our experiments [15] testified rather to
possible compatibility or complementarity of claims (b)-(d).
IV. Scientific equipment for
experiments
Experiments are carried out on
the above-mentioned laser picosecond-range spectral-photo-chronometrical
complex with automatic system of measuring and processing physical parameters.
After the last essential upgrade (April 2012), the complex is composed of the
following components.
·
Driving YAG-laser PL PDP1-300 ("SynchroTech",
Russia), which generates single pulses of wavelength = 1.064 µm, with
controlled repetition rate and duration varied in the range T = 22 - 32
ps. Pulse energy instability 2%,
duration T < 2 ps.
·
Amplifier of pulses
generated by the driving laser, total energy gain ~102.
·
Optical frequency doublers
for amplified
pulses.
·
Two optical parametric oscillators (OPO) on LiNbO3 with temperature wavelength adjustment. For
certain experiments, a third OPO with angular wavelength adjustment is
additionally installed. The first two OPO are pumped with pulses of double
frequency (wavelength = 0.532 µm), the third OPO – with pulses of = 1.064 µm.
Pulses generated by each OPO are passed through separate channels and focused
on a single pot of sample. These pulses are used for various pumping, for
probing in “pump-probe” experiments, for EHP heating by means of intraband light absorption, etc. During experiments, time delay of sample
irradiation by pulse, pulse wavelength in the range 0.35 - 2.0 µm and its
energy are adjusted independently for each channel. Pulse duration (FWHM) 10
ps.
·
Spectrograph
SpectraPro-2500i, able to operate in
dispersion adding mode by spectral measurements and in dispersion subtraction
mode by envelope (chronogram) measurements of separate spectrum components of
picosecond light pulse. The latter mode ensures that the duration of emission
component at spectrograph output is the same as at the input.
·
CCD-camera “PIXIS”, mounted at the second output slit of the first stage of
double spectrograph. Allows instantaneous measurements of integrated-over-time
spectrum of ultrashort optical emission. Measurement resolution from 0.3 nm (in
160 nm-wide range) to 0.05 nm (in the range of 30 nm width). For measurements
in dispersion adding mode, photomultiplier is mounted at the output slit of the
second stage of spectrograph.
·
Streak-camera PS-1/S1, that works together with CCD-camera “CoolSNAP”, is connected to the second output slit of double
spectrograph and allows to measure chronograms of picosecond light pulse
components, selected by spectrograph, with resolution no worse than 2 ps.
Dynamic range of such measurements is 10 to 30, depending on light wavelength
and pulse duration. Jitter (sweep start instability) is 4.5 ps, and it is automatically compensated online by data
acquisition. Streak-camera PS-1/S1 is designed and manufactured by Prokhorov
General Physics Institute of RAS.
·
System of automatic
registration and control, where: (a) physical quantities are measured and
processed online, measurement accuracy estimated, and the results are delivered
to imaging facilities; (b) light pulse delay lines, shutters of pulse
propagation channels, spectrograph SpectraPro-2500i,
two CCD-cameras ("PIXIS" Õ "CoolSNAP"), and photomultiplier are controlled. All these functions
are realized with a special interface and a powerful computer program.
·
The complex gives the
following possibilities. 1. Various kinds of sample pumping, including
combined, synchronous or with adjustable time delay (no worse than 0.3 ps
precision), by three pulses with specially adjusted photon energies and with
various light intensity and various dimensions of focus spot on the sample. 2.
Instantaneous measurement of time-integrated spectrum of ultrashort emission.
·
The latter is particularly
necessary when investigated feature preserves its spectral position on duration
of emission pulse, and herewith experiment conditions require multiple spectrum
measurements. 3. Measurements of variations of optical absorption, transparency
and reflection during and after sample pumping. Measurements are carried out by
pump-probe method in two variants.
·
In the first variant, variations of
probing pulse energy and its time-integrated spectrum, caused by sample
pumping, are measured. In the second variant, chronogram of the whole probing
pulse or of some of its spectral components is measured. 4. Measurements of
chronograms of separate spectral components of intrinsic emission of sample.
These chronograms also allow us to reconstruct time evolution of spectrum of
ultra-short intrinsic emission.
·
Eventually, the complex
provides a rare combination of unique technical possibilities for: ultrafast
creation of powerful stimulated emission in GaAs with various parameters,
simultaneous excitation of ultrafast processes of interaction of the emission
with semiconductor, diversified optical investigation of these processes. And
all that practically without heating the crystal lattice.
·
Note that before designing
the streak-camera PS-1/S1 in Prokhorov GPI
RAS, together with the scientists of this institute
we had to lead joint study of accuracy of picosecond light pulse measurements
by streak-cameras. Non-trivial methods and results of this study are published
in [25].
V.
The list of principal publications on the described works of IRE RAS researchers.
- I.L. Bronevoi, R.A. Gadonas, V.V.
Krasauskas, T.M. Lifshits, A.S. Piskarskas, M.A. Sinitsyn, B.S. Yavich.
JETP Lett., 42, ¹8, 395 (1985).
- I.L. Bronevoi, S.E. Kumekov, V.I.
Perel. JETP Lett., 43, ¹8, 473 (1986).
- N.N. Ageeva, I.L. Bronevoi, E.G.
Dyadyushkin, B.S. Yavich. JETP Lett., 48, ¹5, 276 (1988).
- N.N. Ageeva, I.L. Bronevoi, E.G.
Dyadyushkin, V.A. Mironov, S.E. Kumekov, V.I. Perel’. Sol. St. Commun., 72,
625 (1989).
- I.L. Bronevoi, A.N. Krivonosov,
V.I. Perel’. Sol. St. Commun., 94, 363 (1995).
- N.N. Ageeva, I.L. Bronevoi, P.B.
Gornoostaev, A.N. Krivonosov et al. Book of
Abstracts of the 30th International Congress on High-Speed
Imaging and Photonics ICHSIP-30, (16 - 21 September 2012
CSIR International Convention
Centre Pretoria
South Africa), p.2
- N.N. Ageeva, I.L. Bronevoi, A.N.
Krivonosov. Semiconductors, 35, 67 (2001).
- I.L. Bronevoi, A.N. Krivonosov,
T.A. Nalet. Sol. St. Commun. 98, 903 (1996).
- N.N. Ageeva, I.L. Bronevoi, A.N.
Krivonosov, S.E. Kumekov, S.V. Stegantsov. Semiconductors, 36, 136
(2002).
- I.L. Bronevoi, A.N.
Krivonosov. Semiconductors, 32, 479 (1998).
- N.N. Ageeva, I.L. Bronevoi, D.N.
Zabegaev, A.N. Krivonosov.
Semiconductors, 44, 1121 (2010).
- I.L. Bronevoi, A.N.
Krivonosov. Semiconductors, 32, 484 (1998).
- N.N. Ageeva, I.L. Bronevoi, A.N.
Krivonosov, S.E. Kumekov, T.A. Nalet, S.V. Stegantsov. Semiconductors, 39,
650 (2005).
- I.L. Bronevoi, A.N.
Krivonosov. Semiconductors, 33, 10 (1999).
- N.N. Ageeva, I.L. Bronevoi, D.N.
Zabegaev, A.N. Krivonosov.
Semiconductors, 46, 921 (2012).
- I. L. Bronevoi, A. N. Krivonosov,
V. I. Perel’. Sol. St. Commun., 94, 805 (1995).
- N.N. Ageeva, I.L. Bronevoi, A.N.
Krivonosov, T.A. Nalet, S.V. Stegantsov. Semiconductors, 41, 1398
(2007).
- N.N. Ageeva, I.L. Bronevoi, A.N.
Krivonosov. Semiconductors, 42, 1395 (2008).
- N.N. Ageeva, I.L. Bronevoi, A.N.
Krivonosov, S.V. Stegantsov. Semiconductors, 40, 785 (2006).
- N.N. Ageeva, I.L. Bronevoi, D.N.
Zabegaev, A.N. Krivonosov.
Semiconductors, 44, 1285 (2010).
- N.N. Ageeva, I.L. Bronevoi, D.N.
Zabegaev, A.N. Krivonosov. Physica Status Solidi, C8, ¹4, 1211
(2011).
- G.S. Altybaev, I.L. Bronevoi, S.E.
Kumekov. Semiconductors, 38, 648 (2004).
- N. N. Ageeva, V. B. Borisov, I. L.
Bronevoi, V. A. Mironov, S. E. Kumekov, V. I. Perel, B. S. Yavich. Solid State Commun., 75, 167 (1990).
- N. N. Ageeva, I. L. Bronevoi, V. I.
Mironov, S. E. Kumekov, V. I. Perel’. Sol. St. Commun., 81, 969
(1992).
- N.N. Ageeva, I.L. Bronevoi, D.N.
Zabegaev, A.N. Krivonosov, N.S. Vorob'ev, P.B. Gornostaev, V.I. Lozovoi,
M.Ya. Shelev. Instruments and Experimental Techniques, 54, 548
(2011).
Contacts
Senior researcher, Ph.D. N.N. Ageeva -
ann@cplire.ru
Senior researcher, Ph.D. A.N. Krivonosov -
kan@cplire.ru
Principal researcher, Dr.Sci. I.L. Bronevoi -
bil@cplire.ru
Tel.: +7 (495) 629 34 04