Intrinsic picosecond stimulated emission and emission-excited picosecond optoelectronic nonlinear effects in GaAs
The above title is the subject of long-term studies carried out in Kotelnikov IRE RAS. Some of them were made in collaboration with V.I. Perel and other scientists from the A.F. Ioffe FTI RAS, with R. Gadonos and his colleagues from SCLI VSU, and with S.E. Kumekov and his colleagues from K.I. Satpayev KNRTU. The experiments were performed at room temperature. We studied the processes occurring in a thin (~ 1 µm) GaAs layer, pumped by a powerful picosecond light pulse. This layer was a part of the heterostructure AlxGa1-xAs-GaAs-AlxGa1-xAs, with an antireflection coating applied on the surfaces parallel to the epitaxial layers. The discovered physical phenomena are listed in Sections I-VI. In some sections, the title that defines the phenomenon is followed by the abstract and a list of effects caused by this phenomenon. In Section VII a laser picosecond spectro-photo-chronometric complex is described, with which the studies were carried out. In Section VIII a list of major publications is given. In the end, contact data of IRE employees who performed the study is presented.
I. Intensive intrinsic picosecond stimulated emission of GaAs (hereinafter referred to as s-emission)
Real-time measurements have proved that intense stimulated (amplified spontaneous) picosecond emission arises by powerful picosecond optical pumping of the GaAs layer, with a ~ 1 ps delay from the pumping front. This is caused by almost non-inertial formation of the emission amplification during pumping and by the fact that the rate of spontaneous recombination of charge carriers is proportional to their density. As is found, the emission relaxes exponentially with a characteristic time of about 10 ps, determined by the cooling of the plasma of nonequilibrium charge carriers. This, in turn, is caused by the interrelation between the temperature and density of plasma. This interrelation arises from the fact that intense (up to 108 W / cm2) emission does not allow significant excess of the difference between the quasi-Fermi levels of electrons and holes over the band gap width. Below, physical phenomena that revealed the above and other properties of s-emission, are listed.
Reversible picosecond change of bleaching (increasing transparency) spectrum of GaAs and therefore of the density of electron-hole plasma (EHP) - a sign of appearance of the picosecond stimulated emission during picosecond pumping [1,2] [*], ▲,●.
picosecond threshold emission of GaAs; its spectrum; the energy of its spectral components as a function of energy monopulse pumping and the delay between the two pump pulses [3*,▲,●].
The slowing of picosecond stimulated radiative recombination of charge carriers at increasing of the diameter of photo pumped region .
Characteristic for the stimulated emission dependence of the spectrum of s-emission on the active region diameter and on the pump picosecond pulse energy .
Oscillating dependence of the moment of the beginning of s-emission flare-up on the energy of its photon .
Bistable self-modulation of s-emission spectrum – a new modification of the effect of competition and switching of spectral modes (CSSM) .
Mutually matched self-modulation of characteristics of s-emission emerging from the end of the sample [12♯].
II. Activity of s-emission with respect to stimulated Raman scattering (SRS)
SRS of s-emission and picosecond pumping, which occurs with the participation of optical plasmons [13,14].
SRS of spectral modes of s-emission at interband electron oscillations in the field of s-emission. Switching of these modes and synchronization of oscillations, created by SRS .
III. Electron-hole plasma (EHP) threshold state caused by s-emission and the resulting effects
universal residual bleaching of GaAs and threshold state of the EHP at the end of s- emission [3,4] *,▲,●, at which the difference of quasi-Fermi levels of electrons and holes becomes equal to the band gap width Eg, EHP temperature Tc is equal to the (experiment) room temperature TR, the electroneutrality condition for the density of nonequilibrium electrons n and holes p is satisfied, i.e., , Tc = TR, n = p.
Over-threshold state of the EHP during s-emission [4*,▲,●,5], when the intense s-emission does not allow significant excess of the difference between the quasi-Fermi levels of electrons and holes over the band gap width Eg: .
An abnormal dependence of reversible threshold picosecond bleaching of GaAs on the pump photon energy. Influence of GaAs prebleaching on reversible picosecond change in its transparency [17▲,●].
A single parameter, i.e., the EHP density, determines: (a) distribution of electrons between the valleys, (b) narrowing of the band gap width due to Coulomb interaction of charge carriers in Γ-valley, and (c) energy of the optical plasmon .
IV. Picosecond depletion of the population of energy levels by nonequilibrium electrons, created by s-emission, when the healing of deviations from the quasi-equilibrium distribution of charge carriers decelerates
The depletion is formed due to a delay in the healing of deviations from the quasi-equilibrium distribution of charge carriers. It implies healing by the interaction of charge carriers. The slowdown is due to the energy transport of charge carriers to energy levels, from which carriers are compelled to recombine under the influence of s-emission. Slowdown could also be facilitated by the fact that the electrons are connected by the Coulomb interaction, and their energy distribution should be supported by correlated, other factors are also permissible. Due to the slowing down of healing, the following effects were realized.
LO-phonon oscillations in the spectrum of fundamental light absorption in GaAs, displaying the translation over the conduction band of the electron population depletion, created by s-emission at the bottom of the zone [18♯].
"LO-phonon correlation" between the spectrum of s-emission and self-modulation of the light absorption spectrum in GaAs [6♯].
Amplification of energy transport of electrons with emission of LO-phonons, leading to modulation of the dependence of bleaching (consequently, EHP density) on the pumping photon energy . For the formation of such a modulation is essential that EHP is in overthreshold state.
The influence of the energy transport of electrons with emission of LO-phonons on the amplitude, width and long-wavelength edge of the s-emission spectrum, including appearance of modulation of the specified spectrum parameters’ dependence on the energy [19,20]. Herewith the modulation of the long-wavelength edge of the s-emission spectrum shows the modulation of the band gap width due to interaction of electrons with LO-phonons, whose density oscillates with .
The limit value of the s-emission spectrum width in high-quality crystal as a function of the pump photon energy .
V. Picosecond self-modulation of s-emission and of the fundamental absorption of the probing picosecond light pulse in the GaAs layer is a consequence of interband synchronizing electron oscillations excited by the s-emission field or by the field plus probing field.
It is found that in the GaAs layer, which is an active resonator, with sufficient s-emission intensity, there are: (a) picosecond self-modulation of the s-emission itself and the fundamental absorption of the probing picosecond light pulse; (b) the energy exchange of the spectral modes of s-emission with each other or also with probing light. Automodulation occurs on the spectrum and in time, and on the absorption spectrum the modulation is repeated with a period determined by the energy of the LO phonon. This phenomenon can be explained with the proposed idea of electron oscillations, which: 1) are excited by the s-emission field or also by probing, as perturbation theory admits; 2) occur between the energy levels of the conduction band and the levels of the valence band; 3) arise in conditions of slowing the healing of deviations from the quasi-equilibrium distribution of charge carriers; 4) are connected and synchronized by combinational transitions. The latter: (i) consist of direct optical transitions of electrons selectively combining with acts of electron-electron or electron-LO-phonon interaction and intensifying such acts; (ii) are caused by a disturbance in the quasi-equilibrium distribution of carriers by oscillations and tend to counteract this violation, as is characteristic of stimulated Raman scattering of light, to which the combination transitions predominantly belong; (iii) create an energy exchange between the spectral components of the s-emission or also the probing light, which is typical for the synchronization of coupled oscillators. The above is confirmed by the following.
Mutually-matched self-modulation of the s-emission characteristics, specified in section I, paragraph 9.
Experimental amplitude-phase-frequency response of absorption self-modulation [25♯].
Adapted analytical expression of the perturbation theory, satisfactorily describing the experimental dependence of the frequency of synchronized self-oscillations of the population depletion on the intensity of the s-emission [9♯].
Oscillations of the absorption of the probing (p) picosecond light pulse with a fixed photon energy caused by interaction between p-pulse and s-emission .
Self-synchronization of those modulations of electron energy level populations, that are generated by: (a) the picosecond probing light pulse and the spectral component of s-emission, (b) s-emission components, one generated and the other reflected from the sample end .
The switching of the spectral modes of s-emission and the synchronization of interband electron oscillations. The first and the second are due to stimulated Raman scattering of modes .
VI. Renormalization of the GaAs band gap width due to Coulomb interaction of charge carriers
"Universal" dependence of the longwave boundary of the s-emission spectrum on the energy density of s-emission due to renormalization of the band gap caused by Coulomb interaction of charge carriers (RBGC) .
Deficit of renormalization of the band gap caused by Coulomb interaction of charge carriers, in comparison with the renormalization in the quasi-stationary state . Presumably, the deficit occurs in a threshold manner and is caused by the energy transport of charge carriers to the energy levels, from which the carriers are stimulated to recombine under the influence of s-emission.
VII. Scientific equipment for experiments
Experiments are carried out on the laser picosecond-range spectral-photo-chronometrical complex with automatic system of measuring and processing physical parameters. In its original form, the complex was manufactured by V. Sirutkaitis, R. Grigonis, and others in SCLI VSU. 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" and "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 .
VIII. The list of cited articles employees IRE RAS
N.N. Ageeva, I.L. Bronevoi, A.N. Krivonosov, S.E. Kumekov, V.I. Perel. Bulletin of the Russian Academy of Sciences: Physics, 58, №7, 89 (1994).
Senior researcher, Ph.D. N.N. Ageeva
Senior researcher, Ph.D. A.N. Krivonosov
Junior researcher, D.N. Zabegaev
Principal researcher, Dr.Sci. I.L. Bronevoi
Tel.: +7 (495) 629 34 04
[*], ▲, ●, ♯ The results are marked with the above icons are presented respectively in a brief interim reviews: , , , .