G. D. Sanders

Ultrafast magneto-optics in ferromagnetic III–V semiconductors

We investigate various ultrafast optical processes in ferromagnetic (III,Mn)V semiconductors induced by femtosecond laser pulses. Two-colour timeresolved magneto-optical spectroscopy has been developed, which allows us to observe a rich array of dynamical phenomena. We isolate several distinct temporal regimes in spin dynamics, interpreting the fast (< 1p s) dynamics as spin heating through sp–d exchange interaction between photo-carriers and Mn ions while the ∼100 ps component is interpreted as a manifestation of spin– lattice relaxation. Charge carrier and phonon dynamics were also carefully studied, showing an ultrashort charge lifetime of photo-injected electrons (∼ 2p s) and propagating coherent acoustic phonon wavepackets with a strongly probe energy dependent oscillation period, amplitude and damping. (Some figures in this article are in colour only in the electronic version)

Theory of coherent phonons in carbon nanotubes and graphene nanoribbons

We survey our recent theoretical studies on the generation and detection of coherent radial breathing mode (RBM) phonons in single-walled carbon nanotubes and coherent radial breathing like mode (RBLM) phonons in graphene nanoribbons. We present a microscopic theory for the electronic states, phonon modes, optical matrix elements and electron-phonon interaction matrix elements that allows us to calculate the coherent phonon spectrum. An extended tight-binding (ETB) model has been used for the electronic structure and a valence force field (VFF) model has been used for the phonon modes. The coherent phonon amplitudes satisfy a driven oscillator equation with the driving term depending on the photoexcited carrier density. We discuss the dependence of the coherent phonon spectrum on the nanotube chirality and type, and also on the graphene nanoribbon mod number and class (armchair versus zigzag). We compare these results with a simpler effective mass theory where reasonable agreement with the main features of the coherent phonon spectrum is found. In particular, the effective mass theory helps us to understand the initial phase of the coherent phonon oscillations for a given nanotube chirality and type. We compare these results to two different experiments for nanotubes: (i) micelle suspended tubes and (ii) aligned nanotube films. In the case of graphene nanoribbons, there are no experimental observations to date. We also discuss, based on the evaluation of the electron-phonon interaction matrix elements, the initial phase of the coherent phonon amplitude and its dependence on the chirality and type. Finally, we discuss previously unpublished results for coherent phonon amplitudes in zigzag nanoribbons obtained using an effective mass theory.

Probing strained InGaN/GaN nanostructures with ultrashort acoustic phonon wave packets generated by femtosecond lasers

Large amplitude time-domain oscillations are detected in InxGa1−xN/GaN structures via femtosecond differential reflectivity spectroscopy. The oscillation amplitude increases with increasing indium fraction and abruptly disappears at a critical time that depends on GaN thickness. We show that spatially localized, coherent acoustic phonon wave packets are generated via the photoexcited carriers and propagate into the samples modulating the reflectivity. Our results show that a system with strong built-in strain can be a very effective source for ultrafast acoustic phonon wave packets which can be used as a powerful probe for nanoscale structures.

Chirality-Selective Excitation of Coherent Phonons in Carbon Nanotubes by Femtosecond Optical Pulses

Using predesigned trains of femtosecond optical pulses, we have selectively excited coherent phonons of the radial breathing mode of specific-chirality single-walled carbon nanotubes within an ensemble sample. By analyzing the initial phase of the phonon oscillations, we prove that the tube diameter initially increases in response to ultrafast photoexcitation. Furthermore, from excitation profiles, we demonstrate that an excitonic absorption peak of carbon nanotubes periodically oscillates as a function of time when the tube diameter undergoes coherent radial breathing mode oscillations.

Femtosecond pump-probe spectroscopy of propagating coherent acoustic phonons in InxGa1-xN/GaN heterostructures

We show that large amplitude coherent acoustic phonon wave packets can be generated and detected in

Theory of coherent acoustic phonons in In x Ga 1 − x N / GaN multiple quantum wells

{\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}∕\mathrm{GaN}

Resonant Coherent Phonon Spectroscopy of Single-Walled Carbon Nanotubes

epilayers and heterostructures in femtosecond pump-probe differential reflectivity experiments. The amplitude of the coherent phonon increases with increasing indium fraction

Electronic states and cyclotron resonance in n -type InMnAs

x

Ultrahigh field electron cyclotron resonance absorption in In 1 − x Mn x As films

and unlike other coherent phonon oscillations, both amplitude and period are strong functions of the laser probe energy. The amplitude of the oscillation is substantially and almost instantaneously reduced when the wave packet reaches a GaN-sapphire interface below the surface indicating that the phonon wave packets are useful for imaging below the surface. A theoretical model is proposed which fits the experiments well and helps to deduce the strength of the phonon wave packets. Our model shows that localized coherent phonon wave packets are generated by the femtosecond pump laser in the epilayer near the surface. The wave packets then propagate through a GaN layer changing the local index of refraction and, as a result, modulate the reflectivity of the probe beam. Our model correctly predicts the experimental dependence on probe wavelength as well as epilayer thickness.

Propagating coherent acoustic phonon wave packets in In x Mn 1 − x As ∕ Ga Sb

A microscopic theory for the generation and propagation of coherent LA phonons in pseudomorphically strained wurzite (0001) InGaN/GaN multi-quantum well (MQW) p-i-n diodes is presented. The generation of coherent LA phonons is driven by photoexcitation of electron-hole pairs by an ultrafast Gaussian pump laser and is treated theoretically using the density matrix formalism. We use realistic wurzite bandstructures taking valence-band mixing and strain-induced piezo- electric fields into account. In addition, the many-body Coulomb ineraction is treated in the screened time-dependent Hartree-Fock approximation. We find that under typical experimental conditions, our microscopic theory can be simplified and mapped onto a loaded string problem which can be easily solved.