Search Results (90 total)

Using terahertz time-domain spectroscopy measurements, we demonstrate optical switching of the transmission of terahertz surface plasmon polaritons (SPPs) propagating on an indium antimonide surface through Bragg scattering on grooves. The switching of the in-plane transmission of SPPs through the grooves is mediated by the modification of the stop-gap width due to changes of the surface plasmon characteristics, which are induced by the optical generation of charge carriers. The use of a semiconductor surface allows us to achieve switching at extremely low excitation powers. The experimental data can be quantitatively reproduced using rigorous numerical calculations for surfaces with varying dielectric function. Such an optically active device opens further possibilities for low-frequency surface plasmon optics.

A time-resolved analysis of the amplitude and phase of THz pulses propagating through three-dimensional photonic crystals is presented. Single-cycle pulses of THz radiation allow measurements over a wide frequency range, spanning more than an octave below, at and above the bandgap of strongly dispersive photonic crystals. Transmission data provide evidence for slow group velocities at the photonic band edges and for superluminal transmission at frequencies in the gap. Our experimental results are in good agreement with finite-difference-time-domain simulations.

We present time-domain measurements of terahertz surface plasmon polaritons (SPPs) propagating on gratings structured on silicon surfaces. Using single-cycle pulses of terahertz radiation to excite SPPs in a broad frequency range, we observe that the efficient SPPs scattering on the semiconductor periodic structure introduces significant dispersion and modifies the SPPs propagation. A stop gap, or a frequency range where SPPs are Bragg reflected, is formed by the structure. This gap depends strongly on the Si doping density and type. The resonant scattering at the edge of the gap reduces the group velocity by more than a factor of 2. The measurements show a good agreement with our numerical calculations based on the reduced Rayleigh equation.

A detailed study of enhanced transmission of terahertz (THz) radiation through arrays of subwavelength apertures structured in n-type silicon is presented. The enhancement is attributed to the resonant tunneling of surface-plasmon polaritons (SPP’s) that can be excited at THz wavelengths in doped semiconductors. We investigate the dependence of the transmission as a function of aperture size and sample thickness. The transmission increases significantly as the aperture size is augmented and as the array thickness is reduced. The data confirm the tunneling character of the SPP phenomenon. Transmission efficiencies larger than unity for aperture sizes well below the cutoff wavelength are achieved.

A continuum model, based on the damped Kuramoto-Sivashinsky equation, is shown to reproduce the morphology evolution during ion sputtering quite successfully. In a very narrow range of the damping parameter α, the alignment of the structures into hexagonal domains is obtained under normal incidence of ions with striking resemblance to the experimentally observed dot patterns. The origin of this damping factor is discussed.

We present time domain measurements of surface-plasmon polaritons (SPP’s) at terahertz (THz) frequencies. SPP’s are generated by coupling THz radiation into dielectric films deposited on flat gold surfaces. In this way we are able to perform very sensitive and broadband THz measurements of thin dielectric films, demonstrating the capabilities of SPP’s for Thz thin film spectroscopy. By varying the thickness of these films we observe drastic changes in the field distribution of the SPP’s.

We present measurements of the transmission of terahertz radiation through periodic arrays of holes made in highly doped silicon wafers. The size of the holes, 70×70 μm, is significantly smaller than the radiation wavelength λ≃500 μm. Sharp resonances and enhanced transmissions are observed. This unusual transmission is attributed to the resonant tunneling of surface-plasmon polaritons that can be excited on doped semiconductors at terahertz frequencies.

The temporal evolution of the morphology of GaSb surfaces induced by ion bombardment is investigated. The erosion process with normal incident argon ions forms a regular dot pattern that shows an increase of ordering among the dots with increasing sputtering time. The pattern stabilizes to a highly uniform, hexagonally ordered dot pattern for very long times. The degree of ordering is determined by quantitative analysis of the surface roughness and the power spectral density of the surface pattern using atomic force microscopy. A comparison of the experimental results with numerical integrations of the unstabilized Kuramoto-Sivashinsky equation shows that this equation cannot reproduce essential details of the experimental pattern observed.

We experimentally demonstrate inversionless amplification of the emission from electronic transitions in a solid state system inside a cavity. A semiconductor-based terahertz (THz) emitter is exposed to a coherent, phasematched THz background field during the emission process leading to an increase in the overall emitted THz power of more than one order of magnitude. The experimental concept and results are discussed within a three-level model and an extended density matrix approach.

The coupling of Bloch oscillations to longitudinal optical phonons is investigated in a narrow-well In_0.53Ga_0.47As/In_0.52Al_0.48As superlattice. A strong increase of coherent phonon amplitudes is observed when the Bloch oscillations are subsequently tuned into resonance with different optical phonon modes. The rapid dephasing of the Bloch oscillations due to field induced tunneling from the weakly bound miniband into above-barrier continuum states leads to an additional and new excitation process of coherent phonons in superlattices at high electric fields. Polarization dependent measurements confirm their generation via Coulomb interaction excluding any contribution through impulsively stimulated Raman scattering.

We have investigated the dynamics of two-dimensional (2D) free carrier plasma oscillations in selectively doped GaAs/Al_xGa_1-xAs single quantum wells by time-domain terahertz (THz) emission spectroscopy. Considering both excitation energy and power dependence of the 2D plasmon radiation intensity, we have found two distinct excitation mechanisms of the 2D plasmon; coherent impulsive stimulated Raman process and incoherent sudden heating of the electron system by photoexcited carriers. By monitoring the phase of the emitted THz radiation, a clear crossover from one mechanism to the other is observed as the pump photon energy goes through the absorption edge of the electron system.

Ordered quantum dot patterns are generated on GaSb and InSb surfaces due to a surface instability induced by Ar⁺-ion sputtering at normal incidence. The characteristic length of the generated patterns scales with the square root of the ion energy over the energy range of 75–1800 eV. This energy dependence is compared to the solutions of the isotropic Kuramoto-Sivashinsky equation and allows the determination of the lateral width of the energy distribution deposited by the incident ions in the very-low-energy range. We show that the observed energy dependence is in agreement with the linear continuum theory under the assumption that the dominant smoothing process is due to effective ion-induced diffusion without mass transport on the surface.

We investigate coherent Bloch oscillations in GaAs/Al_xGa_1-xAs superlattices with electronic miniband widths larger than the optical phonon energy. In these superlattices the Bloch frequency can be tuned into resonance with the optical phonon. Close to resonance a direct coupling of Bloch oscillations to LO phonons is observed which gives rise to the coherent excitation of LO phonons. The density necessary for driving coherent LO phonons via Bloch oscillations is about 2 orders of magnitude smaller than the density necessary to drive coherent LO phonons in bulk GaAs. The experimental observations are confirmed by the theoretical description of this phenomenon [A.W. Ghosh et al., Phys. Rev. Lett. 85, 1084 (2000)].

Bloch oscillations excited in a biased GaAs/Al_xGa_1-xAs superlattice are investigated in a time-resolved two-color electro-optic detection scheme. The detection of resonantly excited Bloch oscillations is based on the Pockels effect probed at a wavelength in the center of the band gap. The observed birefringence is induced by the macroscopic polarization of the electronic wave packets relative to the localized holes. The off-resonant detection away from optical transitions directly monitors the spatial dynamics of the electrons in amplitude and phase. The dependence of the amplitudes of the Bloch oscillating electrons on the applied electric fields are in good agreement with the electron-hole dipole lengths calculated by a quantum-mechanical model.

The excitation of (100)-oriented KTaO₃ with 25-fs laser pulses impulsively drives phonon-pair combination states via second-order Raman scattering. Oscillations in the phonon-amplitude covariance at the sum and difference frequency of the two involved phonons are observed in a spectrally and temporally resolved pump-probe experiment. Transmission changes of the sample are dominated by contributions of wave vector conserving phonon-pair combinations from the entire Brillouin zone that have maxima in their combined density of states. For low temperatures the temperature dependence of the covariance oscillations of different phonon combinations is reproduced by a quantum-mechanical model.

Coherent zone-folded acoustic phonons are excited in GaAs/AlAs superlattices by femtosecond laser pulses via resonant impulsive stimulated Raman scattering in both forward and backward scattering directions. The relative amplitudes of three distinct modes of first and second backfolded order match well with scattering intensities calculated within an elastic continuum model. The detection of the coherent acoustic modes is based on the modulation of the interband transitions via the acoustic deformation potential and exhibits a strong enhancement at interband transitions.

In the last decade it has been shown that the low-frequency dielectric response of ferroelectrics can be investigated with great sensitivity via the impulsive excitation and phase-sensitive detection of ultrashort THz phonon-polariton wave packets. In this paper the authors review this technique and the new information obtained with it on the dielectric response and phonon properties of ferroelectrics. The technique is demonstrated with measurements on the perovskite ferroelectrics LiTaO₃ and LiNbO₃. The results are compared with those of other time-resolved studies on phonon polaritons and continuous-wave Raman and infrared-reflectivity studies on LiTaO₃, LiNbO₃ and other ferroelectric crystals. For several ferroelectrics, the polariton dispersion and polariton damping are observed to be strongly influenced by the presence of resonances at frequencies below the fundamental ferroelectric phonon frequency. The occurrence of these resonances is explained from the intrinsic anharmonicity of the ferroelectric phonon or from cross-anharmonic couplings between different normal-mode lattice vibrations.

Optical second-harmonic spectroscopy was used to probe the interface electronic structure of highly mismatched β-GaN/GaAs(001) heterostructures in the vicinity of the E₀ interband critical point of β-GaN. The resonance energy of both bulk and interface two-photon E₀ transitions from layers between 1- and 100-nm thickness are identical, indicating the absence of appreciable amounts of strain and electric fields in this materials system. This finding is in striking contrast to observations made for other materials systems, including ZnSe/GaAs and SiO₂/Si, where large shifts of several 10 meV with respect to the bulk values have been found.

The initial cooling of hot carriers and the subsequent exciton formation in GaSe are studied by time-resolved photoluminescence (PL) using femtosecond up-conversion techniques. From the time-resolved PL spectra of this layered III-VI semiconductor two different energy relaxation channels are derived. After an initial subpicosecond cooling due to Fröhlich-type interaction of carriers with longitudinal optical E^′(2²) phonons a slower regime follows, which is dominated by deformation potential interaction with the nonpolar optical A₁^′(1²) phonons. The coupling constant for nonpolar optical phonon scattering is derived. The subsequent formation of excitons is studied at different carrier densities and detection energies. A cross section for the free-exciton formation is determined based on a rate equation model.

Transient pump-probe signals of a GaAs/Al_xGa_1-xAs quantum well are investigated both theoretically and experimentally for different excitation conditions. The calculations are based on a microscopic density-matrix approach taking higher-order correlations into account. We present spectrally resolved transients which provide insights into the nature of the differential transmission change. Especially the complex phase signature of heavy-hole–light-hole quantum beats in dependence of the detection energy allows for an identification of different microscopic processes in the pump-probe signal. The theoretical results are verified experimentally by comparison with the coherent and incoherent parts of the time-resolved data. We find a dominant role of the Coulomb-induced correlations up to the χ⁽³⁾ relevant four-point level in the coherent signature in comparison to the Pauli blocking contributions.

Coherent acoustic phonons are observed in the femtosecond time-resolved reflectivity of GaAs/AlAs Fibonacci superlattices. The time-domain data reveal a complicated superposition of many folded acoustic modes induced by the quasiperiodicity of the Fibonacci sequence. We discuss the phonon spectra from the viewpoint of the polarizability modulation due to the acoustic phonons on the basis of a photoelastic model. In addition, we demonstrate that the resonance of the heavy-hole exciton transition remarkably modifies the oscillation amplitude of the coherent phonons.

THz-emission experiments are presented which demonstrate a limitation of the semiconductor Bloch equations (SBE) for the description of the Coulomb interaction in semiconductors optically excited near the band gap. In a narrow-miniband superlattice, the field dependence of the THz emission frequency shows an anticrossing behavior characteristic for excitonic transitions. This proves the dominance of excitonic contributions, contradicting predictions based upon the SBE. The data support recent theoretical work which elucidated that the inherent Hartree-Fock approximation of the SBE can lead to an incorrect description of the Coulomb interaction.

We report on the observation of the time-resolved free-induction decay (FID) in a layered III-VI semiconductor. The coherent propagation of the 1s-exciton polariton in InSe leads to severe distortions of the transmitted laser pulse shape and aperiodic modulation of the FID on a femtosecond time scale. This coherent polariton dynamic becomes operative even in optical thin samples. The characteristic dependence of the propagation beats as a function of sample thickness and excitation energy is demonstrated. The major features of the polariton dynamics are well described by coupled Maxwell and semiconductor-Bloch equations for linear pulse propagation. The scattering of polaritons by carriers and phonons is studied at different excitation densities and lattice temperatures.