arXiv subjects

Y. D. Jho

Publications and source records attributed to Y. D. Jho.

8 recordsLinked to original sources

arXiv2022

Nonlinear Photoelasticity to Explicate Acoustic Dephasing Dynamics

Detection and controlling of acoustic (AC) phonon phase have been strenuous tasks although such capability is crucial for further manipulating thermal properties. Here, we present a versatile formalism for tracing AC nanowaves with arbitrary strain compositions by incorporating the nonlinear photoelasticity (PE) into ultrafast acoustics where broad AC spectrum encompassing thermally important THz frequency range should be collected far beyond Brillouin frequency. The initial AC phase upon displacive carrier generation could be inherently varied depending on the bipolar AC compositions by implementing externally biased piezoelectric diodes. The importance of adopting nonlinear PE is then manifested from the transient phase shift either abrupt at the point of diffuse surface scattering or gradual during phonon-phonon or phonon-electron scattering events based on which the ratio of nonlinear to linear PE coefficient is experimentally extracted as a function of the detection probe energy, reaching 0.98 slightly below the bandgap. As the probing energy is rather set away from the bandgap, AC phase is completely invariant with any scattering events, exhibiting the conventional trend at Brillouin frequency in linear regime. Under potent influence of nonlinear PE, the AC dephasing time during the propagation are quantified as a function of AC wavepacket size and further correlated with intrinsic and extrinsic AC scattering mechanisms in electron reservoir.

physics.app-ph↗

arXiv2015

External Modulation and Switching of Acoustic Phonons: Comparative Roles of Potential Distributions

Acoustic phonons can be coherently generated by ultrafast displacive screening of potential gradients, often enhanced by the strong built-in piezoelectric fields, in wurtzite semiconductors. In such structures, transverse symmetry within the c plane hinders both the generation and detection of the transverse acoustic (TA) modes, and only longitudinal acoustic (LA) mode is generated. We show that even for c-GaN, the application of asymmetric potential distributions in the c plane can break the symmetry and selection rules, thus switching on the normally forbidden TA mode. This is in contrast to the LA mode, the strength of which varies with the symmetric potential distributions. By comparing transient differential reflectivity spectra in structures with and without asymmetric potential distributions, the role of the electrically attained anisotropy was further revealed by the digitized appearance of the TA mode, in clear contrast to the monotonic LA mode, and by modulations in the propagation velocities, optical birefringence, and geometrically varying sensitivities, the underlying mechanisms of which are modeled by electric-field-dependent perturbations of the dielectric tensors, incorporating the results of elastic modulations.

cond-mat.mtrl-sci↗

arXiv2014

Transverse Acoustic Phonon Transistor Based on Asymmetric Potential Distribution

We experimentally demonstrate a transverse acoustic (TA) phonon transistor. Phonons are coherently initiated by femtosecond photocarrier screening on potential gradients. Although translational symmetry within the isotropic plane normally prohibits optical generation of TA phonons, we show that the combined application of an external bias in the vertical and lateral directions can break the selection rules, generating the forbidden TA mode. The amplitude and on-state time of the TA mode can be modulated by the external field strength and size of the laterally biased region. The observed frequency shift with an external bias as well as the strong geometrical dependence confirm the role of the asymmetric potential distribution in electrically manipulating the crystal symmetry to control and activate the transistor.

cond-mat.mtrl-sci↗

arXiv2009

Time-Integrated Evidence for Superfluorescence from Dense Electron-Hole Magneto-Plasmas in Semiconductor Quantum Wells

Cooperative spontaneous recombination (superfluorescence) of electron-hole plasmas in semiconductors has been a challenge to observe due to ultrafast decoherence. We argue that superfluorescence can be achieved in quantum-confined semiconductor systems and present experimental evidence for superfluorescence from high-density photoexcited electron-hole plasmas in quantum wells under high magnetic fields (> 20 T). At a critical magnetic field strength and excitation fluence, we observe a clear transition in the band-edge photoluminescence from omnidirectional output to a randomly directed but highly collimated beam. Changes in the linewidth, carrier density, and magnetic field scaling of the emission spectra correlate precisely with the onset of random directionality and are consistent with cooperative recombination. We further investigate the effects of spot size, temperature, and excitation geometry on the emission properties.

cond-mat.mes-hall↗

arXiv2006

Cooperative Recombination of a Quantized High-Density Electron-Hole Plasma

We investigate photoluminescence from a high-density electron-hole plasma in semiconductor quantum wells created via intense femtosecond excitation in a strong perpendicular magnetic field, a fully-quantized and tunable system. At a critical magnetic field strength and excitation fluence, we observe a clear transition in the band-edge photoluminescence from omnidirectional output to a randomly directed but highly collimated beam. In addition, changes in the linewidth, carrier density, and magnetic field scaling of the PL spectral features correlate precisely with the onset of random directionality, indicative of cooperative recombination from a high density population of free carriers in a semiconductor environment.

cond-mat.other↗

arXiv2004

Dark-bright magneto-exciton mixing induced by Coulomb interaction in strained quantum wells

Coupled magneto-exciton states between allowed (`bright') and forbidden (`dark') transitions are found in absorption spectra of strained In$_{0.2}$Ga$_{0.8}$As/GaAs quantum wells with increasing magnetic field up to 30 T. We found large (~ 10 meV) energy splittings in the mixed states. The observed anticrossing behavior is independent of polarization, and sensitive only to the parity of the quantum confined states. Detailed experimental and theoretical investigations indicate that the excitonic Coulomb interaction rather than valence band complexity is responsible for the splittings. In addition, we determine the spin composition of the mixed states.

cond-mat.other↗

arXiv2003

Ultrafast spectroscopy of propagating coherent acoustic phonons in GaN/InGaN heterostructures

We show that large amplitude, coherent acoustic phonon wavepackets can be generated and detected in In$_x$Ga$_{1-x}$N/GaN epilayers and heterostructures in femtosecond pump-probe differential reflectivity experiments. The amplitude of the coherent phonon increases with increasing Indium fraction $x$ and unlike other coherent phonon oscillations, both \textit{amplitude} and \textit{period} are strong functions of the laser probe energy. The amplitude of the oscillation is substantially and almost instantaneously reduced when the wavepacket reaches a GaN-sapphire interface below the surface indicating that the phonon wavepackets 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 wavepackets. Our model shows that localized coherent phonon wavepackets are generated by the femtosecond pump laser in the epilayer near the surface. The wavepackets then propagate through a GaN layer changing the local index of refraction, primarily through the Franz-Keldysh effect, 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.

cond-mat.mes-hall↗

arXiv2003

Exceptionally Slow Rise in Differential Reflectivity Spectra of Excitons in GaN: Effect of Excitation-induced Dephasing

Femtosecond pump-probe (PP) differential reflectivity spectroscopy (DRS) and four-wave mixing (FWM) experiments were performed simultaneously to study the initial temporal dynamics of the exciton line-shapes in GaN epilayers. Beats between the A-B excitons were found \textit{only for positive time delay} in both PP and FWM experiments. The rise time at negative time delay for the differential reflection spectra was much slower than the FWM signal or PP differential transmission spectroscopy (DTS) at the exciton resonance. A numerical solution of a six band semiconductor Bloch equation model including nonlinearities at the Hartree-Fock level shows that this slow rise in the DRS results from excitation induced dephasing (EID), that is, the strong density dependence of the dephasing time which changes with the laser excitation energy.

cond-mat.mtrl-sci↗