scispace - formally typeset
Search or ask a question
Author

Tomoki Hiraoka

Bio: Tomoki Hiraoka is an academic researcher from Kyoto University. The author has contributed to research in topics: Terahertz radiation & Resonant-tunneling diode. The author has an hindex of 2, co-authored 10 publications receiving 21 citations.

Papers
More filters
Journal ArticleDOI
TL;DR: In this article, the authors show that light's orbital angular momentum can be efficiently transferred to an elementary excitation in solids with extended electronic states, where the results obey the selection rules governed by the conservation of the total angular momentum, which is numerically confirmed by the electromagnetic field analysis.
Abstract: The nature of light-matter interaction is governed by the spatial-temporal structures of a light field and material wavefunctions. The emergence of the light beam with transverse phase vortex, or equivalently orbital angular momentum (OAM) has been providing intriguing possibilities to induce unconventional optical transitions beyond the framework of the electric dipole interaction. The uniqueness stems from the OAM transfer from light to material, as demonstrated using the bound electron of a single trapped ion. However, many aspects of the vortex light-matter interaction are still unexplored especially in solids with extended electronic states. Here, we unambiguously visualized dipole-forbidden multipolar excitations in a solid-state electron system; spoof localized surface plasmon, selectively induced by the terahertz vortex beam. The results obey the selection rules governed by the conservation of the total angular momentum, which is numerically confirmed by the electromagnetic field analysis. Our results show light's OAM can be efficiently transferred to an elementary excitation in solids.

37 citations

Journal ArticleDOI
TL;DR: This study shows that multipolar modes of the surface electromagnetic excitations are selectively induced by the terahertz vortex beam, and reveals selection rules governed by the conservation of the total angular momentum, which is confirmed by numerical simulations.
Abstract: The emergence of the vortex beam with orbital angular momentum (OAM) has provided intriguing possibilities to induce optical transitions beyond the framework of the electric dipole interaction. The uniqueness stems from the OAM transfer from light to material, as demonstrated in electronic transitions in atomic systems. In this study, we report on the OAM transfer to electrons in solid-state systems, which has been elusive to date. Using metamaterials (periodically textured metallic disks), we show that multipolar modes of the surface electromagnetic excitations (so-called spoof localized surface plasmons) are selectively induced by the terahertz vortex beam. Our results reveal selection rules governed by the conservation of the total angular momentum, which is confirmed by numerical simulations. The efficient transfer of light’s OAM to elementary excitations in solid-state systems at room temperature opens up new possibilities of OAM manipulation.

18 citations

Journal ArticleDOI
23 Feb 2021
TL;DR: In this paper, the injection-locking properties of a resonant-tunneling-diode terahertz oscillator in the small-signal injection regime with a frequency-stabilized continuous THz wave were investigated.
Abstract: We studied the injection-locking properties of a resonant-tunneling-diode terahertz oscillator in the small-signal injection regime with a frequency-stabilized continuous THz wave. The linewidth of the emission spectrum dramatically decreased to less than 120 mHz (half width at half maximum) from 4.4 MHz in the free running state as a result of the injection locking. We experimentally determined the amplitude of injection voltage at the antenna caused by the injected THz wave. The locking range was proportional to the injection amplitude and consistent with Adler’s model. While increasing the injection amplitude, we observed a decrease in the noise component of the power spectrum, which manifests the free-running state, and an alternative increase in the injection-locked component. The noise component and the injection-locked component had the same power at the threshold injection amplitude as small as 5 × 10−4 of the oscillation amplitude. This threshold behavior can be qualitatively explained by Maffezzoni’s model of noise reduction in general limit-cycle oscillators.

10 citations

Journal ArticleDOI
TL;DR: In this paper , the authors show that the resonant-tunneling-diode (RTD) oscillator can be passively mode-locked by optical feedback and generate a terahertz frequency comb.
Abstract: Optical frequency combs in the terahertz frequency range are long-awaited frequency standards for spectroscopy of molecules and high-speed wireless communications. However, a terahertz frequency comb based on a low-cost, energy-efficient, and room-temperature-operating device remains unavailable especially in the frequency range of 0.1 to 3 THz. In this paper, we show that the resonant-tunneling-diode (RTD) oscillator can be passively mode-locked by optical feedback and generate a terahertz frequency comb. The standard deviation of the spacing between the comb lines, i.e., the repetition frequency, is reduced to less than 420 mHz by applying external bias modulation. A simulation model successfully reproduces the mode-locking behavior by including the nonlinear capacitance of RTD and multiple optical feedback. Since the mode-locked RTD oscillator is a simple semiconductor device that operates at room temperature and covers the frequency range of 0.1 to 2 THz (potentially up to 3 THz), it can be used as a frequency standard for future terahertz sensing and wireless communications.

3 citations

Proceedings ArticleDOI
12 Nov 2015
TL;DR: On the recent advances in THz microscopy based on 2-dimensional electro optic imaging using an intense THz pulse source, recent achievements in optimizing the spatial resolution and acquisition time will drastically accelerate the comprehension of the subwavelength light-matter interactions at THz frequencies and enable, among others, new sensing applications.
Abstract: Plasmonics is part of the fascinating field of nanophotonics, which explores how electromagnetic fields can be confined over dimensions smaller than the wavelength. Given that THz waves may be generated and detected using ultra-short laser pulses, i.e. femtosecond (fs) laser, coherent time-domain THz measurement enable direct reading of the amplitude and phase information of the electric field. Combined with this capability, exploring the electromagnetic interactions of THz waves below diffraction limit is a powerful tool to understand the plasmonic nature of light at the metal/insulator interface. Here, we summarize on the recent advances in THz microscopy based on 2-dimensionnal electro optic (EO) imaging using an intense THz pulse source. These recent achievements in optimizing the spatial resolution and acquisition time will drastically accelerate our comprehension of the subwavelength light-matter interactions at THz frequencies and enable, among others, new sensing applications. Our discussion will include demonstrations on field enhancement in metallic-based resonators and potential avenues using THz vortex beam.

1 citations


Cited by
More filters
Journal ArticleDOI
22 Oct 2021-Science
TL;DR: In this paper, the authors describe the integration of vortex devices with optical sensing, micromanipulation, and optical communications in both classical and quantum realms, and show that their miniaturization is driven by their integration with optical sensors and optical communication.
Abstract: Rapid progress in miniaturizing vortex devices is driven by their integration with optical sensing, micromanipulation, and optical communications in both classical and quantum realms. Many such eff...

65 citations

Journal ArticleDOI
29 Mar 2021
TL;DR: The field of chirality and optical orbital angular momentum (OAM) was initiated by as discussed by the authors, who showed that optical vortices can respond differently to the handedness of + and − rays.
Abstract: Optical activity is conventionally understood as a natural difference in the optical responses of chiral materials with opposite handedness. It stems from the quantised spin angular momentum ±ħ per photon, with the ± representing either left- or right-handed circular polarisations. Less well known, until recently, was the possibility that matter might also respond in a similar, discriminatory way to the handedness of twisted light, or ‘optical vortices’, whose orbital angular momentum (OAM) is quantised as ℓℏ per photon, where ℓ is the topological charge whose sign determines a wavefront twist to the left or right. Initial studies focusing on whether, in spectroscopic applications, chiral matter might respond differently to the vortex handedness of +ℓ and −ℓ beams, failed to identify any viable mechanism. However, in the last few years, theory and experiment have both supplied ample evidence that, under certain conditions, such forms of interaction do exist—and as a result, the field of chirality and optical OAM is beginning to flourish at a pace. This topical review presents a survey of this new field, working up from a description of those initial studies to the cutting-edge experiments now taking place. Analysing the fundamental mechanisms provides for a revision of previous precepts, broadening their scope in the light of recent advances in understanding, and highlighting a vibrant synergy between the fields of optical activity and twisted light.

62 citations

Posted Content
TL;DR: In this paper, the absorption and emission of light carrying orbital angular momentum (twisted-light) by quasi-two-dimensional (disc-shaped) quantum dots in the presence of a static magnetic field was theoretically investigated.
Abstract: We theoretically investigate the absorption and emission of light carrying orbital angular momentum (twisted-light) by quasi-two-dimensional (disc-shaped) quantum dots in the presence of a static magnetic field. We calculate the transition matrix element for the light-matter interaction and use it to explore different scenarios, depending on the initial and final state of the electron undergoing the optically-induced transition. We make explicit the selection rule for the conservation of the z-projection of the orbital angular momentum. For a realistic set of parameters (quantum dots size, beam waist, photon energy, etc.) the strength of the transition induced by twisted light is 10% of that induced by plane-waves. Finally, our analysis indicates that it may be possible to select precisely the electronic level one wishes to populate using the appropriate combination of light-beam parameters suggesting technological applications to the quantum control of electronic states in quantum dots.

36 citations

Journal ArticleDOI
TL;DR: In this paper , the authors describe the physics that underlies the behavior of spoof surface plasmons and how these modes are used in applications that require the manipulation of electromagnetic fields at frequencies below optical.
Abstract: Structuring metallic surfaces allows for the support of surface electromagnetic modes at frequencies for which they would not be allowed for smooth surfaces. These modes are called ``spoof surface plasmons'' because of their similarity to surface plasmons that are supported at optical frequencies for smooth surfaces. This article describes the physics that underlies the behavior of spoof surface plasmons and how these modes are used in applications that require the manipulation of electromagnetic fields at frequencies below optical.

34 citations