Exciton control in a room temperature bulk semiconductor with coherent strain pulses

Abstract

The coherent manipulation of excitons in bulk semiconductors via the lattice degrees of freedom is key to the development of acousto-optic and acousto-excitonic devices. Wide-bandgap transition metal oxides exhibit strongly bound excitons that are interesting for applications in the deepultraviolet, but their properties have remained elusive due to the lack of efficient generation and detection schemes in this spectral range. Here, we perform ultrafast broadband deep-ultraviolet spectroscopy on anatase TiO 2 single crystals at room temperature, and reveal a dramatic modulation of the exciton peak amplitude due to coherent acoustic phonons. This modulation is comparable to those of nanostructures where exciton-phonon coupling is enhanced by quantum confinement, and is accompanied by a giant exciton shift of 30-50 meV. We model these results by many-body perturbation theory and show that the deformation potential coupling within the nonlinear regime is the main mechanism for the generation and detection of the coherent acoustic phonons. Our findings pave the way to the design of exciton control schemes in the deep-ultraviolet with propagating strain pulses. PACS numbers:2 New perspectives in the field of excitonics have recently developed from the discovery of strongly bound excitons that persist at room temperature (RT) in several semicondcutors, including organics [1], transition metal dichalcogenides [2] and transition metal oxides [3][4][5]. Despite their different origin, excitons in these classes of materials are strongly coupled to the lattice degrees of freedom. Indeed, since excitons can be viewed as quanta of electronic excitation energy travelling in the periodic crystal lattice, their motion is influenced by the fluctuating potential field due to lattice vibrations. On the fundamental aspect, exciton-phonon coupling is an intriguing type of boson-boson interaction that results in phenomena such as exciton self-trapping, spectral-weight transfers to phonon sidebands and Stokes-shifted emissions [6]. On the practical side, identifying the specific modes (optical or acoustic) that couple strongly to the excitons paves the way to the control of the exciton properties through the tailored application of strain, pressure or photoexcitation.Experimentally, the microscopic details of the exciton in the phonon field can be addressed via absorption and photoluminescence spectroscopy, since the shape and width of the optical spectra directly reflect the scattering of the exciton by lattice vibrations [7]. However, the information offered by these methods is mediated over all the coupled phonon modes.This calls for more advanced techniques that can yield information on the exciton-phonon coupling for specific lattice modes of interest in order to allow, in return, the phonon-selective control of the exciton properties. A powerful tool relies on setting a particular phonon mode out of equilibrium and monitoring the impact of the ionic motion on the exciton spectral features [8][9][10]. This is possible by time-and e...

Electronic States Modulation by Coherent Optical Phonons in 2D Halide Perovskites

“…Coherent modulation of the optical properties of semiconductors via lattice vibration is a viable approach to manipulate the performance of optoelectronic devices such as high‐speed actuators and transducers. [ 1–3 ] A clear understanding of this phonon modulation mechanism as well as the physical parameters is essential for their development. Self‐assembled 2D halide perovskite quantum well structures with inorganic layers stabilized by organic spacer molecules are presently in the limelight for applications as light‐emitting diodes and photodetectors.…”

Strongly Coupled Coherent Phonons in Single-Layer MoS2

“…This demonstrates that CP excitation in 1L-MoS2 can be described as a Raman-like scattering process. These results explain the CP generation process in 1L-TMDs, paving the way for coherent all-optical control of excitons in layered materials in the THz frequency range.Coherent modulation of the optical properties of a material, following impulsive photo-excitation of the lattice, is fundamentally interesting and technologically relevant because it can be used for applications in sensors[1], actuators and transducers [2][3][4], that can be operated at extremely high frequencies (up to several THz[5]). In order to exploit this effect, it is necessary to understand the mechanism underlying the coherent phonon (CP) generation process and to identify the physical parameters (such as the pump pulse photon energy) that allow their efficient excitation.…”

Giant Exciton Mott Density in Anatase TiO2

“…One solid that has recently emerged as a promising platform to explore bound exciton physics is the anatase polymorph of TiO 2 [14][15][16], a material extensively used in light-energy conversion applications [17,18] and transparent conducting substrates [19]. This system is an indirect gap insulator [ Fig.…”