Cooperative recombination of electron-hole pairs in semiconductor quantum wells under quantizing magnetic fields

We present results of detailed investigations of light emission from semiconductor multiple quantum wells at low temperatures and high magnetic fields excited by intense femtosecond laser pulses. The intensity and linewidth as well as the directional and statistical properties of photoemission strongly depended on the magnetic field strength and pump laser fluence. We also investigated the effects of spot size, temperature, excitation geometry, and excitation pulse width on the emission properties. The results suggest that the initially incoherent photoexcited electron-hole pairs spontaneously form a macroscopic coherent state upon relaxation into the low-lying magnetoexcitonic states, followed by the emission of a superfluorescent burst of radiation. We have developed a theoretical model for superfluorescent emission from semiconductor quantum wells, which successfully explained the observed characteristics.

Figure 1

(Color online) Emission spectra at various magnetic fields for a fixed pump fluence of $0.62\text{mJ}/{\text{cm}}^2$ (a) for emission through sample above the excitation spot along the laser-propagation direction and (b) for in-plane emission in an edge collection geometry. The inset of (b) shows an experimental schematic showing excitation pulses from the bottom and the collection fibers from the top. The gray dotted line denotes the position of the 0–0 Landau level of the E2-HH2 state.

Figure 2

(Color online) Emission strength (black squares) and linewidth (red circles) of the narrow peak from 0–0 and 1–1 LLs versus $B$ for different pump fluence of (a) $0.24\text{mJ}/{\text{cm}}^2$ and (b) $0.62\text{mJ}/{\text{cm}}^2$. The blue and green lines represent the superlinear and linear scaling with $B$, respectively.

Figure 3

(Color online) Narrow line emission strength as a function of magnetic field from 10 to 30 T for (a) the 1–1 LL at various pump fluences and (b) 1–1, 2–2, and 3–3 LLs at a fixed pump fluence of $0.43\text{mJ}/{\text{cm}}^2$ obtained from a line-shape analysis of the emission data. In (c), we present a contour plot of the normalized emission strength versus field and energy for a fluence of $0.43\text{mJ}/{\text{cm}}^2$. The dashed gray lines in (c) show the mixing points between various LLs and higher sublevels and the vertical lines show the correlation between the oscillation peaks and the mixing energies.

Figure 4

(Color online) Emission strength as a function of fluence at a fixed field of 20 T for different spot sizes of (a) 0.5 mm diameter, (b) 3 mm, and (c) 0.1 mm diameter. [(d)–(f)] Integrated emission intensity (black squares) and linewidth (red circles) of the narrow peak from the 0–0 LL versus fluence for different pump spot sizes. The fluence ($x$ axis) is plotted on a logarithmic scale. The blue (green) lines in (a)–(d) indicate the superlinear (linear) regime.

Figure 5

(Color online) Edge emission spectra measured through two fibers placed at $\theta =\left(\text{a}\right)0°$ and (b) $90°$, where $\theta$ is the angle between the longer beam axis and the direction of edge 2 fiber, as shown in the inset of (c). In (c), the emission strength of 0–0 LL is plotted for edge 1 (black) and edge 2 (red) as a function of angle.

Figure 6

(Color online) Four representative emission spectra at 25 T from edge 1 (black) and edge 2 (red) fibers for (a) SF regime ($9.7\text{mJ}/{\text{cm}}^2$ with 0.5 mm spot size) and for (c) ASE regime (fluence of $0.02\text{mJ}/{\text{cm}}^2$ and $3\times 3{\text{mm}}^2$ spot size), excited from single laser pulse and measured simultaneously. Normalized emission strength from the zeroth LL versus shot number in the (b) SF regime and the (d) ASE regime. The dotted triangle is the emission strength ratio between edge 1 $\left(I_{Edge1}\right)$ and edge 2 $\left(I_{Edge2}\right)$.

Figure 7

(Color online) (a) Emission spectra at 110 K at various magnetic fields. (b) The threshold field $B_{\text{TH}}\left(T\right)$ versus temperature $\left(T\right)$ for the 0–0 LL (red circles) and for the 1–1 LL (black squares) emission features. The solid lines are fittings based on acoustic and optical phonon contributions from Eq. (9).

Figure 8

(Color online) (a) Emission spectra at various of magnetic fields for an elongated excitation pulse width of 30 ps. (b) 0–0 LL peak energy positions for pulse widths of 30 ps (solid square) and 150 fs (open square), compared with low-power CW absorption. (c) Linewidth of the sharp feature from 0–0 LL versus $B$ for 30 ps (solid square) and 150 fs (open square).