We report experimental evidence that an ErAs quantum-dot array in a GaAs matrix under 1550 nm pulsed excitation produces cooperative spontaneous emission-Dicke superradiance-in the terahertz frequency region at room temperature.In 1954 Dicke proposed that cooperative spontaneous emission could occur among a collection of dipoles through interaction with a common electromagnetic radiation field 1, .He named the phenomenon "superradiance," which was subsequently observed in atomic/molecular systems 2 , and in quantum dot systems 3 . To date, almost all of these systems are either extended media or require a pencil-shaped (i.e., high directivity) beam. In this letter, we present evidence of cooperative spontaneous emission from the dipoles of a quantum dot (QD) array in a volume smaller than or comparable to the wavelength of radiation.The ErAs QD array in GaAs was obtained by growing a GaAs epitaxial layer heavily doped with erbium on a semi-insulating GaAs substrate by molecular beam epitaxy (MBE) ( Fig. 1 (a)). The thickness of the epilayer was L=2 μm, and the Er doping ~8.8x10 20 cm -3 such that the erbium incorporated into the GaAs in the form of ErAs quantum dots. This was proven by transmission-electron-microscope (TEM) imagery ( Fig. 1 (b)). The most likely diameter is estimated to be~2.0 nm, and the density of quantum dots is nQD~1×10 18 /cm 3 .

We present a study of room-temperature, ultrafast photoconductivity associated with a strong, subbandgap, resonant absorption around k ¼ 1550 nm in three MBE-grown GaAs epitaxial layers heavily doped with Er at concentrations of %2.9 Â 10 18 (control sample), 4.4 Â 10 20 , and 8.8 Â 10 20 cm À3 , respectively. Transmission-electron microscopy reveals lack of nanoparticles in the control sample, but abundant in the other two samples in the 1.0-to-3.0-nm-diameter range, which is consistent with the previously known results. We measure very high photoelectron (Hall) mobility (2.57 Â 10 3 cm 2 /V-s) and terahertz power (46 lW average) in the 4.4 Â 10 20 sample, but then, an abrupt decay in these properties as well as the dark resistivity is seen as the Er doping is increased just 2 times. The Er doping has little effect on the picosecond-scale, 1550-nm photocarrier lifetime.

Two photoconductive emitters - one with a self-complementary square spiral antenna, and the other with a resonant slot antenna - were fabricated on a GaAs epilayer embedded with ErAs quantum dots. Driven with 1550 nm mode-locked lasers, ~117 μW broadband THz power was generated from the device with the spiral antenna, and ~1.2 μW from the device with resonant slot antenna. The optical-to-THz conversion is through extrinsic photoconductivity.

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