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 Al_xGa_1-xAs-GaAs-Al_xGa_1-xAs, with an antireflection coating applied on the surfaces parallel to the epitaxial layers. The discovered physical phenomena are listed in Sections I-VII. 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 VIII a laser picosecond spectro-photo-chronometric complex is described, with which the studies were carried out. The scheme and photos of the complex are presented below.  In Section IX a list of major publications is given. In the end, contact data of IRE employees who performed the study is presented.

Layout of the laser picosecond spectrophotochronometric complex:

 

General view of the complex:                                   Photo of the complex during work:

 

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.

1. 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] [*]^{, ▲,●.}

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^*,▲,●].

3. Area of light amplification in the spectrum of fundamental light absorption in photo-pumped GaAs [4^*,▲,●,5]. The threshold of appearance [3^*,▲,●] and anisotropy of the s-emission [6^♯].

4. Picosecond "build-up" [4^*,▲,●,7] and exponential relaxation of s-emission, [7,8]. Sub-gigawatt intensity of s-emission [9^♯].

5. The slowing of picosecond stimulated radiative recombination of charge carriers at increasing of the diameter of photo pumped region [8].

6. Characteristic for the stimulated emission dependence of the spectrum of s-emission on the active region diameter and on the pump picosecond pulse energy [10].

7. Oscillating dependence of the moment of the beginning of s-emission flare-up on the energy of its photon [11].

8. Bistable self-modulation of s-emission spectrum – a new modification of the effect of competition and switching of spectral modes (CSSM) [11].

9. Mutually matched self-modulation of characteristics of s-emission emerging from the end of the sample [12^♯].

10. Electron-population Bragg grating induced in an AlxGa1–xAs–GaAs–AlxGa1–xAs heterostructure by s-emission [37].

II. Activity of s-emission with respect to stimulated Raman scattering (SRS)

1. SRS of s-emission and picosecond pumping, which occurs with the participation of optical plasmons [13,14].

2. 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 [15].

III. Electron-hole plasma (EHP) threshold state caused by s-emission and the resulting effects

1. 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 E_g, EHP temperature T_c is equal to the (experiment) room temperature T_R, the electroneutrality condition for the density of nonequilibrium electrons n and holes p is satisfied, i.e., , T_c = T_R, n = p.

2. 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 E_g: .

3. The relationship between the density of the EHP and its temperature [2^*,▲,●,14,16^*,▲,●].

4. Reversible picosecond change of the density and temperature of EHP [2^*,▲,●] and of the bleaching (increased transparency) of GaAs [1^*,▲,●].

5. 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^▲,●].

6. 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 [14].

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.

1. 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^♯].

2. "LO-phonon correlation" between the spectrum of s-emission and self-modulation of the light absorption spectrum in GaAs [6^♯].