School of Physics and Astronomy, University of Nottingham, Nottingham, NG7 2RD, UK. Electrically pumped terahertz SASER device using a weakly coupled AlAs/GaAs superlattice as the gain medium R. N. Kini, N.M. Stanton, A.J. Kent and M. Henini Phonon Spectroscopy INTRODUCTION We describe an electrically pumped sound amplification by stimulated emission of radiation (SASER) device for terahertz frequencies. The gain medium of the device is a weakly coupled AlAs/GaAs superlattice (SL) contained within a multimode acoustic cavity formed between the top (free) surface of the structure and a SL phonon reflector. We have studied the properties of a prototype device using superconducting bolometers to detect the phonons emitted. We observed an enhancement of the phonon emission in a direction perpendicular to the SL layers when the energy drop per period of the gain SL, ,  matched the energy of the cavity of phonon modes. We believe these observations provide evidence that the device is operating as a SASER. THE RESULTS All the measurements were carried out at a temperature T ≈ 2K. Figure 4 shows the current-voltage (I-V) characteristics for the device. The device turns on at the threshold voltage, V T ≈ 100 mV and the current increases monotonically after that for biases up to 315 mV. Fluctuations in the current due to electric field domain formation can be seen for biases above 315mV. The device was energised with 1.5  s long electrical pulses and the phonons emitted were detected using bolometers, one directly opposite the device and another at an angle of 30 o. Figure 5 shows the bolometer signals, normalized to the power dissipated in the device, as a function of  Of particular interest is the peak at  ≈ 2.7 meV for  = 0 o. A large background signal due to spontaneous emission is also seen. The peak corresponds to a phonon frequency of 650 GHz, the same as the frequency of the LA cavity modes. If the device was acting as a hot phonon source, then we would have expected to see a dip in the phonon flux at  = 2.7 meV (see Fig. 6), because the phonon mirror will attenuate the propagation of 650 GHz phonons. The peak in the phonon emission for the SASER suggests that phonon amplification is occurring. The FWHM of the peak is 0.9 meV, corresponding to a bandwidth of 220GHz. This is due to the width of stop band of the reflector SL and the spectral broadening in the gain SL. For  = 30 o, no enhancement of phonon emission is seen. Possible reasons for this are 1) the 2k F cutoff of the electron phonon interaction due to the requirement for in-plane momentum conservation and; 2) reduced phonon feedback because of the lower reflectance of the phonon mirror for  > 0 o. CONCLUSIONS Using bolometric detection techniques we have studied a prototype SASER device. We find an enhancement of phonon flux in a direction perpendicular to the SL layers when the energy drop per period of the SL matched the energy of the cavity of phonon modes, indicating SASER action. FIGURE 1: The potential profile for two neighboring wells and the energy states under applied electric field. The process of stimulated emission of phonons in the wells is also shown. FIGURE 2: A schematic representation of the SASER structure. FIGURE 3: The experimental setup and the phonon amplification process in the acoustical cavity FIGURE 4: The current-voltage characteristics for the 50  m device. FIGURE 5: The bolometer signal, at  = 0 o and  =30 o, normalized to the power dissipated in the device plotted as a function of  FIGURE 6: To demonstrate the selective filtering action of the reflector SL, we removed the top gain SL and fabricated a metal heater to generate nonequilibrium phonons. The phonons after passing through the reflector SL were detected on the other side of the sample using an Al bolometer. A broad dip in the phonon intensity, centred at 2.7 meV, corresponding to 650 GHz is seen. REFERENCES [1] B A Glavin, V A Kochelap, T L Linnik, K W Kim and M A Stroscio, Phys. Rev B (2002) [2] R N Kini, A J Kent, N M Stanton and M Henini, Phys. Rev. Lett (submitted) GaAs substrate (0.45 mm) GaAs buffer layer 0.5  m GaAs spacer 0.5  m n + (2 x m -3 ) layer 20 nm undoped GaAs spacer Uniformly Doped (2 x m -3 ) Reflector SL (21 ML GaAs/14 ML AlAs) x 50 (7 ML GaAs/ 7 ML AlAs) x 15 Gain SL 20 nm undoped GaAs spacer 2.9  m 0.26mm 0.4mm For interwell transitions involving emission phonons of energy h < , the initial states have a higher population than the final states. Hence the current flowing through the SL gives rise to population inversion. Phonon assisted tunnelling between two adjacent wells FIGURE 1FIGURE 3FIGURE 2  = 0 o  = 30 o VTVT FIGURE 4 FIGURE 5FIGURE 6 ~ ~ THE SASER DEVICE Phonon assisted tunnelling in a SL is indirect in momentum space, so for interwell transitions involving emission of phonons of energy hν Δ, the initial states have a higher population than the final states, see Fig. 1. Therefore the rate of stimulated emission can exceed the absorption rate, which gives rise to possible phonon amplification [1]. We have observed evidence for phonon amplification in a weakly coupled GaAs/AlAs SL [2]. In this work we have incorporated this SL into an acoustic cavity formed between the top (free) surface of the sample and another SL which acts as a phonon mirror to create an electrically pumped SASER device, the structure of which is shown in Fig 2. The phonon mirror has 95% reflectance for longitudinal acoustic (LA) phonons in a 90 GHz wide band centred on 650 GHz. The length of the cavity is 2.9 μm which leads to a mode separation of 1 GHz for LA phonons. Device mesas of diameter 50μm were formed by etching and GeAuNiAu contacts made to the emitter and collector layers. The back of the substrate was polished and superconducting Al bolometers were fabricated for phonon detection. <~<~