Detection Of Coherent Phonons Research Articles

TRANSDUCCIÓN OPTOMECÁNICA Y BIESTABILIDADES ÓPTICAS ENMICRORRESONADORES BASADOS EN REFLECTORES DE BRAGG

In cavity optomechanics experiments with ultrashort laser pulses, the temporal dynamics of the optical modes of the system play a fundamental role in the generation and detection of coherent phonons. In this work, we analyze the feasibility of using a second continuous emission laser to optimize the detection efficiency, by setting the fundamentalótical mode to a specific spectral position. Under certain conditions, and through this continuum excitation, it is possible to modify the recovery dynamics of the optical modes after an ultrafast pulsed excitation, to the point of stopping it on a quasiequilibrium. This phenomenon is directly related to the optical bistability of the resonator.

Spectrally Resolving the Phase and Amplitude of Coherent Phonons in the Charge Density Wave State of 1T‐TaSe2

AbstractThe excitation and detection of coherent phonons have given unique insights into the condensed matter, in particular for materials with strong electron–phonon coupling. A study of coherent phonons is reported in the layered charge density wave (CDW) compound 1T‐TaSe2 performed using transient broadband reflectivity spectroscopy, in the photon energy range 1.75–2.65 eV. Several intense and long‐lasting (>20 ps) oscillations, arising from the CDW superlattice reconstruction, are observed allowing for detailed analysis of the spectral dependence of their amplitude and phase. For energies above 2.4 eV, where transitions involve Ta d‐bands, the CDW amplitude mode at 2.19 THz is found to dominate the coherent response. At lower energies, instead, beating arises between additional frequencies, with a particularly intense mode at 2.95 THz. Interestingly, the spectral analysis reveals a π phase shift at 2.4 eV. Results are discussed considering the selective coupling of specific modes to energy bands involved in the optical transitions seen in steady‐state reflectivity. The work demonstrates how coherent phonon spectroscopy can distinguish and resolve optical states strongly coupled to the CDW order and provide additional information normally hidden in conventional steady‐state techniques.

Interlayer magnetophononic coupling in MnBi2Te4

The emergence of magnetism in quantum materials creates a platform to realize spin-based applications in spintronics, magnetic memory, and quantum information science. A key to unlocking new functionalities in these materials is the discovery of tunable coupling between spins and other microscopic degrees of freedom. We present evidence for interlayer magnetophononic coupling in the layered magnetic topological insulator MnBi2Te4. Employing magneto-Raman spectroscopy, we observe anomalies in phonon scattering intensities across magnetic field-driven phase transitions, despite the absence of discernible static structural changes. This behavior is a consequence of a magnetophononic wave-mixing process that allows for the excitation of zone-boundary phonons that are otherwise ‘forbidden’ by momentum conservation. Our microscopic model based on density functional theory calculations reveals that this phenomenon can be attributed to phonons modulating the interlayer exchange coupling. Moreover, signatures of magnetophononic coupling are also observed in the time domain through the ultrafast excitation and detection of coherent phonons across magnetic transitions. In light of the intimate connection between magnetism and topology in MnBi2Te4, the magnetophononic coupling represents an important step towards coherent on-demand manipulation of magnetic topological phases.

Spectral responses of phonon optical generation and detection in superlattices

We present a thorough theoretical and experimental analysis of the generation and detection of coherent phonons in superlattices by optical femtosecond pulses. We focus on the spectral responses of both phenomena in pump-probe experiments. The samples consist of two GaAs/AlAs superlattices separated by an intermediate layer through which phonons can propagate. The spatial decoupling of the generation and detection stages in one and the other supperlattice enables the individualization of each phenomenon. We clearly verify experimentally the selection rules for the generation and detection processes, which translate into well-defined maxima at specific frequencies in the corresponding spectral responses. A thickness gradient in one of the superlattices adds a degree of freedom that permits the variation of the spectral detuning between the maxima in the generation and in the detection spectral responses. The zero-detuning case is investigated. In the analysis of both spectral responses, the full size of the sample is found to play a significant role. Thin ($\ensuremath{\sim}$3 $\ensuremath{\mu}$m) and thick ($\ensuremath{\sim}$350 $\ensuremath{\mu}$m) samples are compared. Backreflection of light at sample outer surfaces and inner interfaces induces important alterations to the spectral responses as well.

Coherent radial-breathing-like phonons in graphene nanoribbons

We have developed a microscopic theory for the generation and detection of coherent phonons in armchair and zigzag graphene nanoribbons using an extended tight-binding model for the electronic states and a valence force field model for the phonons. The coherent phonon amplitudes satisfy a driven oscillator equation with the driving term depending on photoexcited carrier density. We examine the coherent phonon radial-breathing-like mode amplitudes as a function of excitation energies and nanoribbon types. For photoexcitation near the optical absorption edge the coherent phonon driving term for the radial-breathing-like mode is much larger for zigzag nanoribbons where transitions between localized edge states provide the dominant contribution to the coherent phonon driving term. Using an effective mass theory, we explain how the armchair nanoribbon width changes in response to laser excitation.

Resonant coherent phonon spectroscopy of single-walled carbon nanotubes

Using femtosecond pump-probe spectroscopy with pulse shaping techniques, one can generate and detect coherent phonons in chirality-specific semiconducting single-walled carbon nanotubes. The signals are resonantly enhanced when the pump photon energy coincides with an interband exciton resonance, and analysis of such data provides a wealth of information on the chirality-dependence of light absorption, phonon generation, and phonon-induced band structure modulations. To explain our experimental results, we have developed a microscopic theory for the generation and detection of coherent phonons in single-walled carbon nanotubes using a tight-binding model for the electronic states and a valence force field model for the phonons. We find that the coherent phonon amplitudes satisfy a driven oscillator equation with the driving term depending on photoexcited carrier density. We compared our theoretical results with experimental results on mod 2 nanotubes and found that our model provides satisfactory overall trends in the relative strengths of the coherent phonon signal both within and between different mod 2 families. We also find that the coherent phonon intensities are considerably weaker in mod 1 nanotubes in comparison with mod~2 nanotubes, which is also in excellent agreement with experiment.

Coherent acoustic phonons in a thin gold film probed by femtosecond surface plasmon resonance

AbstractWe report on coherent phonon detection in a thin gold film by a new femtosecond pump‐probe surface plasmon resonance (SPR) technique. Acoustic coherent phonons are generated impulsively in the gold film on a prism, and they are detected in the time domain through the reflectivity modulation for probe pulses that satisfy the SPR condition. We observed not only the fundamental vibration of a longitudinal acoustic phonon but also the overtones up to the fourth. Copyright © 2008 John Wiley & Sons, Ltd.

Coherent phonons in semiconductor superlattice under DC electrical bias

Resonant pumping of superlattices SLs by femtosecond laser pulses results in the generation of coherent phonons with frequency centred approximately on s/dSL, where s is the speed of longitudinal sound and dSL is the SL period. The phonons can be detected by measuring the changes in reflectance of time-delayed probe pulses. To date, measurements have been made on nominally undoped SLs, and phonon frequencies are typically in the range 100 GHz - 1 THz. The motivation for these studies is that such hypersound could be used for acoustic probing of nanostructures. Here we describe the generation and detection of coherent phonons in a doped and electrically-biased SL. The studied SL consisted of 50×(6 nm GaAs and 4 nm AlAs), uniformly doped with Silicon to density 2×1022 m−3. Pump-probe measurements were made at λ∼770 nm on an optical MESA at T=12K. At zero bias, we observed a similar phonon spectrum as previously observed in comparable but undoped SLs, with a mode at ̃ 450GHz. Under applied bias this mode increased in amplitude, and the decay time was also increased. We discuss these observations in terms of the effects of the bias on the coherent phonon generation and detection process and also the possibility of coherent phonon amplification occurring in the structure.

Coherent Phonon Detection in Ge with a Femtosecond Laser

A highly sensitive femtosecond photodeflection spectroscopy technique wasfirst developed experimentally. Using femtosecond laser pulses we havedetected ultrafast coherent phonon emission in a Ge plate. It isfound that the ultrafast femtosecond laser pulses generate coherent phononsin the plate via the electronic mechanism. Supersonic expansion ofphoto-excited electron-hole plasma is observed.

Observation of coherent folded acoustic phonons propagating in a GaAs/AlAs superlattice by two-color pump-probe spectroscopy

The coherent folded longitudinal acoustic (FLA) phonons in a GaAs/AlAs superlattice have been studied using a two-color pump-probe technique and a designed sample in which the superlattice layer is sandwiched by ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ cladding layers. The two-color pump-probe technique makes it possible to separate regions of generation and detection of coherent phonons, which provides a powerful tool to examine the spatial dynamics of coherent phonons. We have found two types of coherent FLA phonons: the first-order FLA phonons with finite wave vectors $(q\ensuremath{\ne}0)$ and the zone-center mode of the first-order FLA phonons $(q=0).$ When ballistic acoustic phonons are generated near the surface region of the ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ cladding layer and propagate through the superlattice layer, the first-order FLA phonons with $q\ensuremath{\ne}0$ are observed in the superlattice layer. On the other hand, when the FLA phonons are generated within the superlattice layer, the zone-center FLA phonon extending from the superlattice layer is detected near the surface region of the ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ cladding layer.

Coherent Phonons in Semiconductors and Semiconductor Heterostructures

The impulsive excitation and phase-sensitive detection of coherent phonons enable the study of dynamical properties of the lattice. In pump-probe experiments with femtosecond time resolution the amplitude and phase of the coherent lattice motion can be detected with high sensitivity. Especially in semiconductors and semiconductor heterostructures, where a coherent phonon mode and free carriers are exited at the same time, important information about the carrier-phonon system far away from equilibrium is obtained. In bulk GaAs coherent plasmon-phonon modes can be traced via the associated macroscopic field. In GaAs-AlGaAs based heterostructures coupled phonon-intersubband plasmons, coupled Bloch-phonon oscillations, and coherent acoustic phonons are observed.

Coherent phonon detection from ultrafast surface vibrations.

Using an optical technique ultrafast vibrations of the surface of thin opaque films in the 0.1-THz range have been directly observed in the time domain. We propose a new type of phonon detection by monitoring the surface velocity, and demonstrate an exceptional sensitivity to atomic-scale inhomogeneities by observing an anomalous reflection from a buried film of monolayer order in thickness.