Emission of Submillimeter Electromagnetic Waves by Coherent Phonons

Abstract

We report on the first observation of the emission of electromagnetic radiation from coherent lattice vibrations in a semiconductor. Coherent optical phonons are excited by ultrashort laser pulses in singlecrystal tellurium. THz frequency radiation emitted by the Dember-field-driven phonons is detected by time-resolved THz-emission spectroscopy. The measurements are complemented by optical pump-probe experiments with a polarization-sensitive detection scheme utilizing the symmetry of the Raman tensor. 42.65.Re, 78.47.+p The investigation of eoherent phonons in solid-state materials exeited with ultrashort laser pulses is a subjeet of great eurrent interest. In reeent years, eoherent phonons have been observed in the transient optical refleeti vity and in the transmission of several semieonduetors [1][2][3]. For a eertain elass of materials including Te, only eoherent optieal phonons of Al symmetry have been deteeted in femtosecond time-resolved experiments [2], although modes of different symmetry have been identified in ew-infrared (IR) reflectivity and Raman seattering experiments. The seleetive excitation of Al modes has been explained by a displaeive exeitation mechanism, in whieh a strong interband exeitation leads to an ultrafast displacement of the atomie equilibrium position [2,4]. We present a detailed study of eoherent phonons in Te. In addition to the Al mode, phonons of E symmetry are observed in time-resolved measurements of the anisotropie refleetivity ehanges (RC), obeying selection rules given by the symmetry of the Raman tensor. Sinee all phonons in Te of symmetry different from Al are IR aetive, their eoherent excitation is expeeted to be aceompanied by the emission of eleetromagnetie radiation. In this Letter, we report on the first time-resolved observation of THz emission that originates from the maeroseopie polarization associated with coherent longitudinal optieal (LO) phonons. The radiation is detected with THz-emission speetroseopy that allows the direet measurement of the amplitude and phase of the eoherent eleetromagnetie radiation emitted after pulsed optieal exeitation [5].The IR aetivity of phonons in the single-element semieonduetor Te arises from the strong eleetronie polarizability and the laek of inversion symmetry [6]. Te erystallizes in a hexagonal lattiee (spaee group Dj or D~) eonsisting of three atoms per unit eell arranged in a helix along the c axis. The lattiee vibrations eonsist of a fully symmetrieal, only Raman-aetive Al mode (3.6 THz), two degenerate Raman-and IR-aetive E modes (E~O/LO: 2.76/3.09 THz, E~O/LO: 4.22/4.26 THz), and one only IR-aetive A 2 mode (A 2 ,To/Lo: 2.6/2.82 THz) [7]. The internal polarization is either perpendicular (E modes) or parallel (A 2 mode) to the c axis. All experiments are performed on two different Te single erystals with the c axis perpendieular (c~) and parallel (e 11) to the surfaee.To prove the excitation of eoherent phonons different from the Al mode, we perform optieal pump-probe measurements and deteet the anisotropie RC. ...

Nonlinear dynamics and band transport in a superlattice driven by a plane wave

“…The waves could be detected by the technique of time-resolved THz-emission spectroscopy, in the way similar to used in other radiating superlattice devices [46,47]. Moreover, the spectral signatures of the electron velocity unravel the potential of the acoustically pumped superlattices for an amplification of high-frequency electromagnetic signals involving mechanisms similar to the stable Bloch gain [15,48].…”

Coupled Bloch-Phonon Oscillations in Semiconductor Superlattices

“…With regard to the observation of THz electromagnetic emission from Bloch oscillations [7] and coherent phonons [26], we expect that the SL biased into the Bloch-phonon resonance is a strong emitter of electromagnetic radiation at the LO phonon frequency. Because of the longer dephasing time of the lattice mode, room temperature emission can be obtained for times exceeding the lifetime of pure electronic Bloch oscillations by an order of magnitude [27].…”

Coherent THz Phonons Driven by Light Pulses and the Sb Problem: What is the Mechanism?

“…For absorbing substances, a unique process has so far not been identified [1]. The experimental evidence indicates that Raman selection rules are strictly obeyed (see, e.g., [3,5,7,13]). However, the oscillation phase varies from material to material, and relative intensities gained from time-domain measurements and spontaneous Raman scattering (RS) do not appear to correlate [4].…”

“…Following recent advances in femtosecond laser technology, several groups demonstrated that the propagation of light pulses in solids is accompanied by intense lattice vibrations showing a high degree of spatial and temporal coherence [1][2][3][4][5][6][7][8][9]. The availability of coherent phonons at THz frequencies has led to a variety of suggestions for applications and experiments involving, in particular, time-domain spectroscopy of phonons using pumpprobe methods [1][2][3][4][5][6][7][8][9], conversion of mechanical into coherent electromagnetic energy [7], and intriguing proposals relying on photon control of the ionic motion [10,11].…”

“…The availability of coherent phonons at THz frequencies has led to a variety of suggestions for applications and experiments involving, in particular, time-domain spectroscopy of phonons using pumpprobe methods [1][2][3][4][5][6][7][8][9], conversion of mechanical into coherent electromagnetic energy [7], and intriguing proposals relying on photon control of the ionic motion [10,11]. While coherent vibrations have been produced in insulators, semiconductors, and metals using, basically, the same technique [1][2][3][4][5][6][7][8][9], the experiments reveal fundamental, but poorly understood differences between transparent and opaque materials [1,4]. This Letter presents a unifying mechanism for phonon generation that explains the differences.…”