Real-time single-proton counting with transmissive perovskite nanocrystal scintillators.

High-sensitivity radiation detectors for energetic particles are essential for advanced applications in particle physics, astronomy and cancer therapy. Current particle detectors use bulk crystals, and thin-film organic scintillators have low light yields and limited radiation tolerance. Here we present transmissive thin scintillators made from CsPbBr3 nanocrystals, designed for real-time single-proton counting. These perovskite scintillators exhibit exceptional sensitivity, with a high light yield (~100,000 photons per MeV) when subjected to proton beams. This enhanced sensitivity is attributed to radiative emission from biexcitons generated through proton-induced upconversion and impact ionization. These scintillators can detect as few as seven protons per second, a sensitivity level far below the rates encountered in clinical settings. The combination of rapid response (~336 ps) and pronounced ionostability enables diverse applications, including single-proton tracing, patterned irradiation and super-resolution proton imaging. These advancements have the potential to improve proton dosimetry in proton therapy and radiography.

DC Magnetic Field Sensitivity Optimization of Spin Defects in Hexagonal Boron Nitride.

Spin defects existing in van der Waals materials attract wide attention thanks to their natural advantages for in situ quantum sensing, especially the negatively charged boron vacancy (VB-) centers in hexagonal boron nitride (h-BN). Here we systematically investigate the laser and microwave power broadening in continuous-wave optically detected magnetic resonance (ODMR) of the VB- ensemble in h-BN, by revealing the behaviors of ODMR contrast and line width as a function of the laser and microwave powers. The experimental results are well explained by employing a two-level simplified model of ODMR dynamics. Furthermore, with optimized power, the DC magnetic field sensitivity of VB- ensemble is significantly improved up to 2.87 ± 0.07 µT/Hz. Our results provide important suggestions for further applications of VB- centers in quantum information processing and ODMR-based quantum sensing.

Gate-Tunable Bound Exciton Manifolds in Monolayer MoSe2.

Two-dimensional (2D) semiconductors with point defects are predicted to host a variety of bound exciton complexes analogous to trions and biexcitons due to strong many-body effects. However, despite the common observation of defect-mediated subgap emission, the existence of such complexes remains elusive. Here, we report the observation of bound exciton (BX) complex manifolds in monolayer MoSe2 with intentionally created monoselenium vacancies (VSe) using proton beam irradiation. The emission intensity of different BX peaks is found to exhibit contrasting dependence on electrostatic doping near the onset of free electron injection. The observed trend is consistent with the model in which free excitons exist in equilibrium with excitons bound to neutral and charged VSe defects, which act as deep acceptors. These complexes are more strongly bound than trions and biexcitons, surviving up to around 180 K, and exhibit moderate valley polarization memory, indicating partial free exciton character.

Germanium-on-SOI waveguides for mid-infrared wavelengths.

We report on the development of Germanium-on-SOI waveguides for mid-infrared wavelengths. The strip waveguides have been formed in 0.85 and 2 µm thick Ge grown on SOI substrate with 220 nm thick Si overlayer. The propagation loss for various waveguide widths has been measured using the Fabry-Perot method with temperature tuning. The minimum loss of ~8 dB/cm has been achieved for 0.85 µm thick Ge core using 3.682 µm laser excitation. The transparency of these waveguides has been measured up to at least 3.82 µm.

High-gain optical waveguide amplifier based on proton beam writing of Nd:YAG crystal.

We report on an optical amplifier based on a Nd:YAG channel waveguide, which was fabricated by proton beam writing. Under the pumping of a continuous wave laser, the high-gain optical amplifications at single wavelength of 1064 nm and wavelength band of 1300 nm -1360 nm were obtained. The maximum gain was 24 dB/cm at 1064 nm and 6 dB/cm at 1319 nm, respectively. This work paves a way to apply proton beam written Nd:YAG waveguides as integrated optical amplifiers for the efficient amplification.

Ionic-resolution protoacoustic microscopy: A feasibility study.

Visualizing micro- and nano-scale biological entities requires high-resolution imaging and is conventionally achieved via optical microscopic techniques. Optical diffraction limits their resolution to ∼200 nm. This limit can be overcome by using ions with ∼1 MeV energy. Such ions penetrate through several micrometers in tissues, and their much shorter de Broglie wavelengths indicate that these ion beams can be focused to much shorter scales and hence can potentially facilitate higher resolution as compared to the optical techniques. Proton microscopy with ∼1 MeV protons has been shown to have reasonable inherent contrast between sub-cellular organelles. However, being a transmission-based modality, it is unsuitable for in vivo studies and cannot facilitate three-dimensional imaging from a single raster scan. Here, we propose proton-induced acoustic microscopy (PrAM), a technique based on pulsed proton irradiation and proton-induced acoustic signal collection. This technique is capable of label-free, super-resolution, 3D imaging with a single raster scan. Converting radiation energy into ultrasound enables PrAM with reflection mode detection, making it suitable for in vivo imaging and probing deeper than proton scanning transmission ion microscopy (STIM). Using a proton STIM image of HeLa cells, a coupled Monte Carlo+k-wave simulations-based feasibility study has been performed to demonstrate the capabilities of PrAM. We demonstrate that sub-50 nm lateral (depending upon the beam size and energy) and sub-micron axial resolution (based on acoustic detection bandwidth and proton beam pulse width) can be obtained using the proposed modality. By enabling visualization of biological phenomena at cellular and subcellular levels, this high-resolution microscopic technique enhances understanding of intricate cellular processes.

An antiferromagnetic spin phase change memory.

The electrical outputs of single-layer antiferromagnetic memory devices relying on the anisotropic magnetoresistance effect are typically rather small at room temperature. Here we report a new type of antiferromagnetic memory based on the spin phase change in a Mn-Ir binary intermetallic thin film at a composition within the phase boundary between its collinear and noncollinear phases. Via a small piezoelectric strain, the spin structure of this composition-boundary metal is reversibly interconverted, leading to a large nonvolatile room-temperature resistance modulation that is two orders of magnitude greater than the anisotropic magnetoresistance effect for a metal, mimicking the well-established phase change memory from a quantum spin degree of freedom. In addition, this antiferromagnetic spin phase change memory exhibits remarkable time and temperature stabilities, and is robust in a magnetic field high up to 60 T.

Modeling and experimental investigations of Fano resonances in free-standing LiNbO3 photonic crystal slabs.

In this paper the Fano resonance in a free-standing LiNbO(3) photonic crystal slab is demonstrated. We present a numerical analysis and experimental measurements with free space illumination where the dependence of slab thickness, radius of air holes and lattice types are investigated. The unique property of polarization dependence for LiNbO(3) photonic crystal slabs is also analyzed, and we show that the transmission spectra exhibit significant sensitivity (~25nm) to polarization. A monolithic free-standing LiNbO(3) photonic crystal slab was fabricated using ion beam enhanced etching (IBEE) technology. Measurement results of the reflection spectra agree with the numerical analysis.

Metal-assisted photonic mode for ultrasmall bending with long propagation length at visible wavelengths.

In this work, we investigate the use of metal-assisted photonic guiding in a polymer-metal waveguide as an alternative approach for high density photonic integration at visible wavelengths. We demonstrate high confinement and long propagation length in sub-wavelength dimensions down to 300nm × 200nm using leakage radiation microscopy at a wavelength of 632.8 nm. Simulations using the finite element method (FEM) show that the optimum dimension that gives good confinement and propagation length is similar to that of the predicted plasmonic mode supported in the same waveguide. Under such optimum conditions, the metal-assisted photonic mode shows a five times longer propagation length and higher transmission efficiency for all 90° bending radius down to 1 µm compared to the plasmonic mode.

Whole-cell imaging at nanometer resolutions using fast and slow focused helium ions.

Observations of the interior structure of cells and subcellular organelles are important steps in unraveling organelle functions. Microscopy using helium ions can play a major role in both surface and subcellular imaging because it can provide subnanometer resolutions at the cell surface for slow helium ions, and fast helium ions can penetrate cells without a significant loss of resolution. Slow (e.g., 10-50 keV) helium ion beams can now be focused to subnanometer dimensions (∼0.25 nm), and keV helium ion microscopy can be used to image the surfaces of cells at high resolutions. Because of the ease of neutralizing the sample charge using a flood electron beam, surface charging effects are minimal and therefore cell surfaces can be imaged without the need for a conducting metallic coating. Fast (MeV) helium ions maintain a straight path as they pass through a cell. Along the ion trajectory, the helium ion undergoes multiple electron collisions, and for each collision a small amount of energy is lost to the scattered electron. By measuring the total energy loss of each MeV helium ion as it passes through the cell, we can construct an energy-loss image that is representative of the mass distribution of the cell. This work paves the way to use ions for whole-cell investigations at nanometer resolutions through structural, elemental (via nuclear elastic backscattering), and fluorescence (via ion induced fluorescence) imaging.

Quantifying nanodiamonds biodistribution in whole cells with correlative iono-nanoscopy.

Correlative imaging and quantification of intracellular nanoparticles with the underlying ultrastructure is crucial for understanding cell-nanoparticle interactions in biological research. However, correlative nanoscale imaging of whole cells still remains a daunting challenge. Here, we report a straightforward nanoscopic approach for whole-cell correlative imaging, by simultaneous ionoluminescence and ultrastructure mapping implemented with a highly focused beam of alpha particles. We demonstrate that fluorescent nanodiamonds exhibit fast, ultrabright and stable emission upon excitation by alpha particles. Thus, by using fluorescent nanodiamonds as imaging probes, our approach enables quantification and correlative localization of single nanodiamonds within a whole cell at sub-30 nm resolution. As an application example, we show that our approach, together with Monte Carlo simulations and radiobiological experiments, can be employed to provide unique insights into the mechanisms of nanodiamond radiosensitization at the single whole-cell level. These findings may benefit clinical studies of radio-enhancement effects by nanoparticles in charged-particle cancer therapy.

Continuous wave Nd:YAG channel waveguide laser produced by focused proton beam writing.

We report on mirrorless continuous wave laser oscillation at 1064 nm from a 808 nm pumped Nd:YAG optical channel waveguide fabricated by 1 MeV focused proton beam writing. Pump power threshold has been found to be 94 mW with a laser slope efficiency of 40%. A maximum output power at 1064 nm for the waveguide laser is 63 mW at absorbed pump power of 247 mW.

An all-silicon, single-mode Bragg cladding rib waveguide.

In this paper, we demonstrate a direct method of fabricating an all-silicon, single-mode Bragg cladding rib waveguide using proton beam irradiation and subsequent electrochemical etching. The Bragg waveguide consists of porous silicon layers with a low index core of 1.4 that is bounded by eight bilayers of alternating high and low refractive index of 1.4 and 2.4. Here, the ion irradiation acts to reduce the thickness of porous silicon formed, creating an optical barrier needed for lateral confinement. Single-mode guiding with losses as low as approximately 1 dB/cm were obtained for both TE and TM polarization over a broad range of wavelengths from 1525 nm to 1625 nm. Such an approach offers a method for monolithic integration of Bragg waveguides in silicon, without the need for multiple processes of depositing alternating materials.

Three-dimensional control of optical waveguide fabrication in silicon.

In this paper, we report a direct-write technique for three-dimensional control of waveguide fabrication in silicon. Here, a focused beam of 250 keV protons is used to selectively slow down the rate of porous silicon formation during subsequent anodization, producing a silicon core surrounded by porous silicon cladding. The etch rate is found to depend on the irradiated dose, increasing the size of the core from 2.5 microm to 3.5 microm in width, and from 1.5 microm to 2.6 microm in height by increasing the dose by an order of magnitude. This ability to accurately control the waveguide profile with the ion dose at high spatial resolution provides a means of producing three-dimensional silicon waveguide tapers. Propagation losses of 6.7 dB/cm for TE and 6.8 dB/cm for TM polarization were measured in linear waveguides at the wavelength of 1550 nm.

Simple and universal model for electron-impact ionization of complex biomolecules.

We present a simple and universal approach to calculate the total ionization cross section (TICS) for electron impact ionization in DNA bases and other biomaterials in the condensed phase. Evaluating the electron impact TICS plays a vital role in ion-beam radiobiology simulation at the cellular level, as secondary electrons are the main cause of DNA damage in particle cancer therapy. Our method is based on extending the dielectric formalism. The calculated results agree well with experimental data and show a good comparison with other theoretical calculations. This method only requires information of the chemical composition and density and an estimate of the mean binding energy to produce reasonably accurate TICS of complex biomolecules. Because of its simplicity and great predictive effectiveness, this method could be helpful in situations where the experimental TICS data are absent or scarce, such as in particle cancer therapy.

Nanoscale Properties of Human Telomeres Measured with a Dual Purpose X-ray Fluorescence and Super Resolution Microscopy Gold Nanoparticle Probe.

Techniques to analyze human telomeres are imperative in studying the molecular mechanism of aging and related diseases. Two important aspects of telomeres are their length in DNA base pairs (bps) and their biophysical nanometer dimensions. However, there are currently no techniques that can simultaneously measure these quantities in individual cell nuclei. Here, we develop and evaluate a telomere "dual" gold nanoparticle-fluorescent probe simultaneously compatible with both X-ray fluorescence (XRF) and super resolution microscopy. We used silver enhancement to independently visualize the spatial locations of gold nanoparticles inside the nuclei, comparing to a standard QFISH (quantitative fluorescence in situ hybridization) probe, and showed good specificity at ∼90%. For sensitivity, we calculated telomere length based on a DNA/gold binding ratio using XRF and compared to quantitative polymerase chain reaction (qPCR) measurements. The sensitivity was low (∼10%), probably because of steric interference prohibiting the relatively large 10 nm gold nanoparticles access to DNA space. We then measured the biophysical characteristics of individual telomeres using super resolution microscopy. Telomeres that have an average length of ∼10 kbps, have diameters ranging between ∼60-300 nm. Further, we treated cells with a telomere-shortening drug and showed there was a small but significant difference in telomere diameter in drug-treated vs control cells. We discuss our results in relation to the current debate surrounding telomere compaction.

Subwavelength imaging through ion-beam-induced upconversion.

The combination of an optical microscope and a luminescent probe plays a pivotal role in biological imaging because it allows for probing subcellular structures. However, the optical resolutions are largely constrained by Abbe's diffraction limit, and the common dye probes often suffer from photobleaching. Here we present a new method for subwavelength imaging by combining lanthanide-doped upconversion nanocrystals with the ionoluminescence imaging technique. We experimentally observed that the ion beam can be used as a new form of excitation source to induce photon upconversion in lanthanide-doped nanocrystals. This approach enables luminescence imaging and simultaneous mapping of cellular structures with a spatial resolution of sub-30 nm.

A continuous-flow C. elegans sorting system with integrated optical fiber detection and laminar flow switching.

We present a high-throughput continuous-flow C. elegans sorting device that works based on integrated optical fiber detection and laminar flow switching. Two types of genetically engineered nematodes are allowed to flow into the device and their genotypes are detected based on their fluorescence, without the need for immobilization, by integrated optical fibers. A novel dynamic fluidic switch sorts the nematodes to desired outlets. By changing input pressures of the control inlets, the laminar flow path is altered to steer the nematodes to appropriate outlets. Compared to previously reported microfluidic C. elegans sorting devices, sorting in this system is conducted in a continuous flow environment without any immobilization technique or need for multilayer mechanical valves to open and close the outlets. The continuous flow sorter not only increases the throughput but also avoids any kind of invasive or possibly damaging mechanical or chemical stimulus. We have characterized both the detection and the switching accuracy of the sorting device at different flow rates, and efficiencies approaching 100% can be achieved with a high throughput of about one nematode per second. To confirm that there was no significant damage to C. elegans following sorting, we recovered the sorted worms, finding no deaths and no differences in behavior and propagation compared to control.

Sub-30 nm thick plasmonic films and structures with ultralow loss.

We report an alternative method of producing sub-30 nm thick silver films and structures with ultralow loss using gas cluster ion beam irradiation (GCIB). We have direct evidence showing that scattering from grain boundaries and voids rather than surface roughness are the main mechanisms for the increase in loss with reducing thickness. Using GCIB irradiation, we demonstrate the ability to reduce these scattering effects simultaneously through nanoscale surface smoothing, increase in grain width and lower percolation threshold. Significant improvement in electrical and optical properties by up to 4 times is obtained, before deviation from bulk silver properties starts to occur at 12 nm. We show that this is an enabling technology that can be applied post fabrication to metallic films or lithographically patterned nanostructures for enhanced plasmonic performance, especially in the ultrathin regime.

Functional multi-band THz meta-foils.

In this paper, we present the first experimental demonstration of double- and triple-band negative refraction index meta-foils in the terahertz (THz) region. Multi-band meta-foils constructed by multi-cell S-string resonators in a single structure exhibit simultaneously negative permittivity and negative permeability responses at multiple frequencies. The phenomena are confirmed by numerical simulations and Fourier transform infrared spectroscopy measurements. The flexible, freestanding multi-band meta-foils provide a promising candidate for the development of multi-frequency THz materials and devices.