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 ð_ß 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, m_e and m_h 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 E_g. 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 E_g 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 T_c = T_R, n = p, , where T_c is EHP temperature,  T_R 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 E_g⁰ = 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 kT_c >>  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.

1. 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).
2. I.L. Bronevoi, S.E. Kumekov, V.I. Perel. JETP Lett., 43, ¹8, 473 (1986).
3. N.N. Ageeva, I.L. Bronevoi, E.G. Dyadyushkin, B.S. Yavich. JETP Lett., 48, ¹5, 276 (1988).
4. N.N. Ageeva, I.L. Bronevoi, E.G. Dyadyushkin, V.A. Mironov, S.E. Kumekov, V.I. Perel’. Sol. St. Commun., 72, 625 (1989).
5. I.L. Bronevoi, A.N. Krivonosov, V.I. Perel’. Sol. St. Commun., 94, 363 (1995).
6. N.N. Ageeva, I.L. Bronevoi, P.B. Gornoostaev, A.N. Krivonosov et al. Book of Abstracts of the 30^th International Congress on High-Speed Imaging and Photonics ICHSIP-30, (16 - 21 September 2012 CSIR International Convention Centre Pretoria South Africa), p.2
7. N.N. Ageeva, I.L. Bronevoi, A.N. Krivonosov. Semiconductors, 35, 67 (2001).
8. I.L. Bronevoi, A.N. Krivonosov, T.A. Nalet. Sol. St. Commun. 98, 903 (1996).
9. N.N. Ageeva, I.L. Bronevoi, A.N. Krivonosov, S.E. Kumekov, S.V. Stegantsov. Semiconductors, 36, 136 (2002).
10. I.L. Bronevoi, A.N. Krivonosov. Semiconductors, 32, 479 (1998).
11. N.N. Ageeva, I.L. Bronevoi, D.N. Zabegaev, A.N. Krivonosov. Semiconductors, 44, 1121 (2010).
12. I.L. Bronevoi, A.N. Krivonosov. Semiconductors, 32, 484 (1998).
13. N.N. Ageeva, I.L. Bronevoi, A.N. Krivonosov, S.E. Kumekov, T.A. Nalet, S.V. Stegantsov. Semiconductors, 39, 650 (2005).
14. I.L. Bronevoi, A.N. Krivonosov. Semiconductors, 33, 10 (1999).
15. N.N. Ageeva, I.L. Bronevoi, D.N. Zabegaev, A.N. Krivonosov. Semiconductors, 46, 921 (2012).
16. I. L. Bronevoi, A. N. Krivonosov, V. I. Perel’. Sol. St. Commun., 94, 805 (1995).
17. N.N. Ageeva, I.L. Bronevoi, A.N. Krivonosov, T.A. Nalet, S.V. Stegantsov. Semiconductors, 41, 1398 (2007).
18. N.N. Ageeva, I.L. Bronevoi, A.N. Krivonosov. Semiconductors, 42, 1395 (2008).
19. N.N. Ageeva, I.L. Bronevoi, A.N. Krivonosov, S.V. Stegantsov. Semiconductors, 40, 785 (2006).
20. N.N. Ageeva, I.L. Bronevoi, D.N. Zabegaev, A.N. Krivonosov. Semiconductors, 44, 1285 (2010).
21. N.N. Ageeva, I.L. Bronevoi, D.N. Zabegaev, A.N. Krivonosov. Physica Status Solidi, C8, ¹4, 1211 (2011).
22. G.S. Altybaev, I.L. Bronevoi, S.E. Kumekov. Semiconductors, 38, 648 (2004).
23. 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).
24. N. N. Ageeva, I. L. Bronevoi, V. I. Mironov, S. E. Kumekov, V. I. Perel’. Sol. St. Commun., 81, 969 (1992).
25. 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