Detonation nanodiamonds containing silicon-vacancy (SiV) centers (SiV-DNDs) exhibit spectrally sharp optical transitions and are promising nanoscale emitters. After purification and oxidative postprocessing, SiV-DNDs are obtained with a mean particle diameter of ∼10 nm, a size scale at which single-color-center occupancy per particle may be expected. Yet, practical applications require selective enrichment from mixtures that also contain undoped nanodiamonds. Conventional separation methods lack sufficient selectivity, and resonant absorption-based optical sorting is fundamentally constrained by excited-state saturation, rendering it ineffective in the single-color-center regime. Building on our recent theoretical predictions that stimulated emission can generate a dissipative optical force beyond this limit, we demonstrate that the stimulated recoil force (SRF) provides a scalable mechanism for emission-line-selective manipulation of nanodiamonds in liquid. Using a glass capillary with counter-propagating pump beams and a manipulation beam resonant with SiV emission, we observe millimeter-scale downstream depletion and upstream enrichment of SiV-DNDs, while a spectrally distinct, off-resonant fluorescent nanodiamond population remains unchanged. The magnitude and spatial extent of the transport show that SRF overcomes Brownian diffusion and enables long-range, species-selective transport under realistic conditions. To identify the physical mechanism, we perform complementary glass-cell experiments under a well-defined focusing geometry and compare the observed enrichment with optical-force calculations based on density-matrix dynamics and Brownian-dynamics simulations. Qualitative agreement supports SRF as the dominant dissipative contribution responsible for the transport. These results demonstrate practical, emission-energy-selective optical sorting of fluorescent nanodiamonds and define design principles for extending this approach to capillaries, microfluidic systems, and other fluorescent nanomaterials.

Photon momentum is not only a means of moving matter; it is a mechanical readout of nanoscale optical response. In resonant nanomaterials, optical forces depend not only on size and refractive index, but on transition energy, oscillator strength, linewidth, dephasing, and excited-state population. Mechanical observables such as drift velocity, force balance, trapping stiffness, enrichment factor, and resonance-frequency shift can function as spectroscopic readouts. This review develops this viewpoint from resonant optical transport and optical-force spectroscopy, where absorption spectra are converted into motion, to nonlinear resonant forces, where saturation, excited-state absorption, and stimulated emission control magnitude and sign of force. We emphasize stimulated recoil force, which uses emitted-photon momentum to realize emission-line-selective enrichment of fluorescent detonation nanodiamonds. We discuss luminescence-induced optical force, in which spontaneous emission in an anisotropic photonic environment generates recoil and optical-spring effects. These developments establish optical force as a state- and emission-selective transducer for quantum nanomaterials.

We review optical responses in the nano-to-bulk crossover, where bulk translational symmetry no longer applies and the long-wavelength approximation (LWA) often fails. In this regime, the response must be treated nonlocal, and a self-consistent solution of Maxwell’s equations and the constitutive relation for excitonic polarization, with radiative boundary conditions, is required. We present a unified view based on internal-field resonance: when the internal-field profile is phase matched to quantized center-of-mass modes, radiative shifts and widths are reorganized in a mode- and size-selective way. After outlining the basics of the nonlocal formulation, we summarize theoretically predicted and experimentally observed phenomena that isolate nonlocal effects in nanostructures. Representative results include thickness-dependent level interchange, a nonmonotonic increase of linewidth with system size, ultrafast radiative decay down to 10 fs, and transitions that are forbidden in the LWA but allowed through internal-field coupling. We also note that the same nonlocal, self-consistent framework extends naturally to molecular scales, including molecule-plasmon systems and tip-enhanced spectroscopies, where structured near fields can activate nominally forbidden multipolar transitions. These connections show how a nano-to-bulk viewpoint can unify solid-state and molecular optical responses within a single formalism.

We present a simplified model for analyzing the reflection coefficients of anisotropic semiconductor thin films in terahertz magneto-optical ellipsometry. Derived from electromagnetic boundary conditions, the model is valid when the film thickness is much smaller than the wavelength. Its applicability is demonstrated through terahertz time-domain spectroscopic ellipsometry of two doped GaAs thin films under an external magnetic field. The approach allows direct extraction of sheet conductivity spectra and accurately reproduces the observed magneto-optical response, offering a non-invasive method for characterizing various thin-film materials.

Ultrafast luminescence represents a frontier in optical materials science, bridging fundamental investigations of light–matter interaction and the development of next‐generation photonic devices. Enhancing the speed of luminescence has long been a central objective, with continued relevance in both fundamental and applied contexts. In advanced photonic systems, rapid light emission enables the realization of extremely bright and temporally precise optical signals. This capability is crucial for quantum information technologies, where single‐photon sources with short radiative lifetimes are essential, as well as for classical high‐speed optical communication and sensing platforms. This review highlights recent progress in accelerating the emission process by leveraging cooperative emission effects, resonator‐enhanced coupling, and nonlocal or multipolar interactions, incorporating the authors’ original contributions and perspectives. These advances have led to the realization of novel coherent and incoherent bright light sources, high‐efficiency single‐photon emitters, radiative processes robust against thermal dephasing, and photon generation from materials typically regarded as nonluminescent due to thermal relaxation or self‐trapping. These mechanisms from both theoretical and experimental perspectives are explored, and insights into their integration for future photonic architectures are offered.

Transition-metal impurities, even at extremely low concentrations, can markedly alter proton transport in perovskite electrolytes. Here, we investigate the effects of extremely dilute transition-metal dopants (1 mol% Fe, Ni, Co, or Cu) in BaZr0.8Yb0.2O3−δ using infrared reflection spectroscopy, Raman spectroscopy, and terahertz time-domain spectroscopy. Despite the low dopant concentration, each transition metal induces distinct local structural distortions and defect configurations that reflect its intrinsic electronic-structure characteristics. Fe introduces moderate lattice disorder with localised electronic states, whereas Ni forms Alt-text is not available for this image. defect complexes that enhance hydration without significantly hindering proton incorporation. Co generates ligand holes and Co-centred defect clusters, leading to strong electronic Raman scattering, destabilisation of protonic defects, and enhanced hole conduction. Cu produces pronounced Jahn–Teller distortion, resulting in highly localised octahedral deformation. Terahertz time-domain spectroscopy reveals that these dopant-specific electronic structures directly modify proton and hole hopping barriers. These findings establish a direct connection between charge-transfer physics, defect chemistry, and transport properties in doped perovskites, offering design principles not only for minimising detrimental impurity effects but also for strategically exploiting transition-metal dopants to enhance mixed-conducting of proton and hole functionality in fuel cell electrodes.

A germanium-vacancy (GeV) centre in nanodiamond is a promising candidate for bright single-photon emitters, fluorescence labelling and quantum sensing due to its narrow zero-phonon line (ZPL) and large Debye-Waller factor. Recently, the GeV centre-containing nanodiamond (GeV-ND) was fabricated by a detonation process, which provides a scalable method capable of producing large quantities of ND particles. However, the optical properties of the GeV-ND at the single-particle level have not been well studied. In this study, we conducted spatially resolved photoluminescence (PL) mapping on sparsely distributed GeV-ND particles to examine luminescence characteristics across individual particles. The GeV-ND exhibited a narrow ZPL accompanied with a broad emission band. By comparing with well-characterized PL spectra of nitrogen-vacancy (NV) centres in detonation NDs reported in previous single-particle-level studies for magnetometry sensing applications, we attribute the broad background to luminescence from the NV centres coexisting with the GeV centres in NDs. By subtracting this background, we identified the pure GeV emission. Some GeV-NDs showed ZPL linewidths as narrow as 28 meV at room temperature, suggesting the presence of optically promising GeV centres. These findings clarify the origins of PL observed in GeV-NDs and demonstrate their potential as scalable quantum emitters.

Fluorescent nanodiamonds (F-NDs) have attracted much attention as promising materials for quantum sensing, bioimaging, and quantum information processing due to their unique optical properties. However, their widespread application is still hampered by the lack of a method to efficiently separate them from non-fluorescent nanodiamonds produced during synthesis. While optical sorting based on resonant absorption force (RAF) has been demonstrated, the RAF might be insufficient for manipulating large volumes, limiting its feasibility for industrial applications. In this work, we study two optical sorting approaches: conventional RAF and a method utilizing stimulated recoil force (SRF). We confirm that the RAF allows selective transport of silicon-vacancy-containing detonation nanodiamonds (SiV-DNDs) from germanium-vacancy-containing detonation nanodiamonds (GeV-DNDs). Compared to the RAF, the SRF-based approach significantly accelerates the sorting process and improves concentration increase, highlighting its potential for practical nanodiamond sorting applications.

Optical manipulation of nanomaterials using light resonant with material excitations holds promise for enhancing optical forces and sorting particles by unique quantum properties. Conventional resonant optical sorting mainly relies on absorption and scattering forces, making it difficult to sort nanomaterials by specific emission lines. Furthermore, emission typically induces negligible force unless the material is highly anisotropic, limiting selective manipulation via emission characteristics. To address this, we propose optical sorting of targeted emission lines by harnessing recoil forces under an inverted-occupation condition. Our theoretical evaluation of nanodiamonds (NDs) with a single-color center demonstrates the feasibility of sorting them by color-center-specific emission. These findings provide a pathway for ultrahigh concentrations of detonation NDs with single-color centers, opening avenues for advanced quantum technology and biosensing.

Helices and spirals, prevalent across various systems, play a crucial role in characterizing symmetry, describing dynamics, and imparting unique functionalities, attributed to their inherent simplicity and chiral nature. A helical excitation on a quantized vortex, an example of a one-dimensional topological defect, emerges as a Nambu-Goldstone mode following spontaneous symmetry breaking, known as a Kelvin wave. Kelvin waves play a vital role in energy dissipation within inviscid quantum fluids. However, deliberately exciting Kelvin waves has proven to be challenging. Here, we introduce a controlled method for exciting Kelvin waves on a quantized vortex in superfluid helium-4. We used a charged nanoparticle, oscillated by a time-varying electric field, to stimulate Kelvin waves on the vortex. A major breakthrough in our research is the confirmation of the helical nature of Kelvin waves through three-dimensional image reconstruction, providing visual evidence of their complex dynamics. Additionally, we determined the dispersion relation and the phase velocity of the Kelvin wave and identified the vorticity direction, enhancing our understanding of quantum fluid behavior. This work elucidates the dynamics of Kelvin waves and pioneers a novel approach for manipulating and observing quantized vortices in three dimensions, thereby opening new avenues for exploring quantum fluidic systems.

The optical force acting on an object can be significantly influenced by the resonance with the electronic transitions of the object and the incident photon flux. The optical force caused by stimulated emission acts in the opposite direction to the propagation of light and is referred to as the stimulated recoil force (SRF). Our group theoretically proposed this phenomenon in 2012, and it was experimentally verified for the first time in 2023. However, challenges remain, as generating the SRF requires prolonged exposure to high-intensity laser light, which may cause photobleaching of the target material. In this study, we consider nanodiamonds containing NV centers (NV-NDs) as the target material, as they are resistant to photobleaching even under high-intensity and long-duration laser irradiation. As the light source, we focus on optical vortices. Optical vortices are characterized by a spiral rotation of the wavefront, and it is known that irradiating a material with optical vortices causes the material to rotate in the same direction as the wavefront's rotation of the wavefront. We aim to achieve control over the rotational direction of NV-NDs by inducing stimulated emission with optical vortices to generate the SRF. Additionally, we theoretically demonstrate that this stimulated recoil force can overcome thermal fluctuations and induce rotation even in water at room temperature. These findings provide new insights into the interaction between optical vortices and nanomaterials, paving the way for new possibilities in optical manipulation.

We present two ideas to simplify the measurement and analysis of terahertz time-domain spectroscopic ellipsometry data of ultrathin films. The measurement is simplified by using a specially designed sample holder with mirrors, which can be mounted on a cryostat. It allows us to perform spectroscopic ellipsometry by simply inserting the holder into a conventional terahertz spectroscopy system for measurements in transmission geometry. The analysis of the obtained data is simplified by considering a single interface with a certain sheet conductivity σ_s (since the film thickness is significantly smaller than the wavelength of the terahertz light). We demonstrate the application of these ideas by evaluating the sheet conductivities of two perovskite rare-earth nickelate thin films in the temperature range 78–478 K. The use of this particular analytical method and the sample holder design will help to establish terahertz time-domain spectroscopic ellipsometry as a characterization technique for ultrathin films.

Nanodiamonds that contain germanium-vacancy centers (GeV-NDs) exhibit significant potential for biomedical and quantum science applications. GeV-NDs with an average particle size of 9 nm were recently fabricated through a detonation process that enables the practical-scale production of detonation NDs (DNDs). However, the optical properties of the GeV centers in the DNDs have not been studied thoroughly. In particular, the luminescence spectrum of these GeV-DNDs had an unassigned peak at 1.98 eV. Here, we investigate the optical properties of GeV-DNDs under various conditions. Although the GeV-DNDs exhibit a zero-phonon line (ZPL) with similar excitation energy dependence and photostability to their bulk counterparts, the ZPL linewidth is broader. The 1.98 eV-peak is attributed to a composite phonon sideband peak. The unique properties of the GeV centers in these small DNDs are explained by enhanced electron-phonon coupling.

We show a simplified method to analyze sheet conductivity in thin film with THz spectroscopic ellipsometry. To its demonstration, we perform THz time-domain spectroscopic ellipsometry for NdNiO 3 and SmNiO 3 thin films using the compact ellipsometry kit.

We demonstrate THz-induced martensitic transformation. We irradiated the intense THz pulses resonant for the lowest optical phonon on the surface of the partially stabilized zirconia plate and found that the clear evidence of martensitic transformation from tetragonal to monoclinic phase. We calculated the phonon dispersion in tetragonal zirconia and found the effective channel for the trigger of the transformation. Since the THz pulse excitation allows specific local shear deformation beyond thermal equilibrium, it will open new scheme of nonlinear phononics in condensed matters.

We show that protons strongly dampsome phonons in hydratedbariumzirconate. To show this, we performed terahertz spectroscopy and infraredreflection spectroscopy measurements on anhydrous and hydrous acceptor-dopedbarium zirconate sintered pellets. Last and Slater phonon modes arestrongly damped with proton doping, whereas phonons are not dampedwith trivalent dopants. This is due to picosecond motion of protons,such as hydroxyl rotation and attempt motion of proton hopping, coupledwith the phonon. This coupling strongly depends on the dopant, suggestingthat the proton motion is strongly influenced by the dopant. The resultsof Raman spectroscopy support this explanation. We consider that sucha dynamical long-range proton-lattice correlation will enablenew approaches to improve proton conduction in solid-state ionics.

Nanodiamonds (NDs) containing silicon- or germanium-vacancy centers (SiV- or GeV-NDs, respectively) have shown promising potential as fluorescent markers for bioapplications. Recently fabrications of ~10 nm-sized SiV- and GeV-NDs were demonstrated by a detonation process that enables practical-scale NDs production. In the present study, the optical properties of the SiV- and GeV-NDs, a recent addition to the family of fluorescent NDs, were spectroscopically investigated. Their luminescence bands including each zero-phonon line commonly have small Debye-Waller factors (0.47 and 0.20, respectively) and broad linewidths (32 and 59 meV, respectively) at room temperature, comparing with those of typical SiV and GeV centers. These differences in the optical properties were due to the effects of lattice distortions and surface potentials from tiny-sized diamonds. The SiV- and GeV-NDs fabricated by the detonation process are interesting materials not only in the biomedical field but also in the study of optical manipulation.

We systematically investigated the photoluminescence (PL) spectra of sintered pellets of cerium oxide, CeO2. The crystal defects were controlled by commonly used techniques in the field of solid-state ionics. We observed the transitions between the O-2p and Ce-4f energy levels in intrinsic CeO2. By doping with trivalent lanthanide ions, which results in oxygen vacancies by charge compensation, a new transition band appears. It involves electronic levels that are similar to those of Ce2O3 clusters, rather than to those of oxygen vacancies. In reduced cerium oxide CeO2-δ, where the number of Ce3+ ions is larger, electrons partially fill the energy levels of defects similar to Ce2O3 clusters, and thus an optical response derived from the Ce-4f–Ce-5d transition appears.

Yttrium-doped barium zirconate is a solid electrolyte with several properties that are advantageous for fuel cells. We measure photoluminescence and photoluminescence excitation spectra of anhydrous yttrium-doped barium zirconate to characterize the atomic arrangement near the oxygen vacancy because the local structure is known to strongly influence the protonic conduction. The photoluminescence spectra of our anhydrous sintered pellets are composed of three components: recombination at the grain boundary region, recombination at a vacancy defect with an unpaired electron, and the transition of the electronic state of the oxygen vacancy. From the peak position of the latter photoluminescence component and the corresponding photoluminescence excitation spectrum, we can obtain information on the atomic arrangement that Zr is adjacent to the oxygen vacancies. This information is important for the design of materials with high protonic conductivity.