Generation Of Cherenkov Radiation Research Articles

Optical fibers with a frequency-dependent Kerr nonlinearity: Theory and applications

This review provides a detailed discussion of both the mathematical treatment and the impact of a frequency-dependent Kerr nonlinearity on the propagation of short pulses in optical fibers. We revisit the theoretical framework required to deal with the frequency dependence of the nonlinear response without incurring any physical inconsistencies, such as the non-conservation of the photon number. Then, we point out the role of the zero-nonlinearity wavelength, its interplay with the zero-dispersion wavelength, and their influence on evolution of optical pulses in optical fibers, specifically by looking at soliton propagation and the ensuing generation of Cherenkov radiation. Finally, by means of a space–time analogy involving the collision of a weak control pulse and an intense soliton, we describe an all-optical switching scheme in the presence of a zero-nonlinearity wavelength within a photon-conserving framework.

Real-time revealing of Cherenkov radiation evolution in optical fibers.

The generation of Cherenkov radiation (CR) is determined by the phase-matching condition, but the experimental observation on the phase change of its transient process is still incomplete. In this paper, we use the dispersive temporal interferometer (DTI) technique to real-time reveal the buildup and evolution of CR. Experiments show that when the pump power varies, the phase-matching conditions also change, which is mainly affected by the nonlinear phase shift caused by the Kerr effect. Further simulation results propose that both pulse power and pre-chirp management have a significant impact on phase-matching. The CR wavelength can be shortened and the generation position can be moved forward by adding a suitable positive chirp or increasing the incident peak power. Our work directly reveals the evolution of CR in optical fibers and provides a method for optimizing it.

Cherenkov radiation and scattering of external dispersive waves by two-color solitons

For waveguides with two separate regions of anomalous dispersion, it is possible to create a quasistable two-color solitary wave. In this paper, we consider how those waves interact with dispersive radiation, i.e., both the generation of Cherenkov radiation and the scattering of incident dispersive waves. We derive the analytic resonance conditions and verify them through numeric experiments. We also report incident radiation driving the internal oscillations of the soliton during the scattering process in the case of an intense incident radiation. We generalize the resonance conditions for the case of an oscillating soliton and demonstrate how one can use the scattering process to probe and excite an internal mode of two-color soliton molecules.

High-efficiency threshold-less Cherenkov radiation generation by a graphene hyperbolic grating in the terahertz band

Cherenkov radiation (CR), generated when charged particles move faster than the light velocity in a medium, is an important radiation phenomenon of electron-matter interactions. Here, we demonstrate high-efficiency threshold-less in-plane CR generation in the terahertz (THz) band using a periodic graphene hyperbolic grating (GHG) structure. Our theoretical and numerical analyses proved that the phase velocity in the GHG is always lower than the electron velocity; this is owing to the graphene hyperbolic plasmon polaritons induced biaxially in the GHG. Based on the simulation results, the power of the in-plane CR in the GHG is nearly two orders of magnitude larger than that of the out-of-plane CR in the conventional graphene-dielectric hyperbolic metamaterial bulk. A full electromagnetic dipole source mode is employed to describe the fascinating in-plane hyperbolic-like spatial dispersion in the GHG. Moreover, by adjusting the size of the GHG, graphene chemical potential, and free electron energy, the CR angle and intensity can be completely controlled. This framework provides a new way to develop high-efficiency, threshold-less, and tunable CR sources in the THz range with excellent adjustability and serves as a general approach applicable to other polariton types active in a wide range of materials, such as phonon- and exciton-polaritons.

Occurrence and mechanism of visual phosphenes in external photon beam radiation therapy and how to influence them

Background and purposeTwo plausible mechanisms to explain the appearance of visual phosphenes are: direct activation of the photochemicals in the retina and the generation of Cherenkov radiation in the vitreous humour. In this clinical trial we investigated the occurrence of visual phosphenes in external photon beam radiation therapy. Material and methodsLogistic regression analysis is used to examine whether seeing light flashes and seeing steady light depended on the ambient light intensity and the dose. ResultsIn total, 465 treatments of 25 patients were analysed. The odds of seeing light flashes multiply by 0,926 as the ambient light intensity increases by 10 lux. Similarly, the odds multiply by 1,604 as the dose to the retina increases by 10 cGy. The odds of seeing steady light multiply by 1,540 as the dose to the vitreous humour increases by 10 cGy. ConclusionsWe postulate that one should reduce the dose rate, instruct patients to keep the eyes open and increase the illuminance in the treatment room to reduce the probability of experiencing visual phosphenes. We hypothesize that melanopsin is involved in the visual phosphenes and that fatigue of patients might be correlated with the observation of visual phosphenes.

Cherenkov radiation from 1550 nm pumping in tapered photonic crystal fibers.

The generation of Cherenkov radiation from soliton compression of 1550nm pulses in tapered photonic crystal fibers is analyzed numerically, with a view to generating short-wavelength-tunable output pulses in the visible range. It is shown that low-noise femtosecond light sources with spectral power densities approaching those of existing supercontinuum sources are feasible with existing fiber laser and tapering technology.

Fiber optical dose rate measurement based on the luminescence of beryllium oxide

This work presents a fiber optical dose rate measurement system based on the radioluminescence and optically stimulated luminescence of beryllium oxide. The system consists of a small, radiation sensitive probe which is coupled to a light detection unit with a long and flexible light guide. Exposing the beryllium oxide probe to ionizing radiation results in the emission of light with an intensity which is proportional to the dose rate. Additionally, optically stimulated luminescence can be used to obtain dose and dose rate information during irradiation or retrospectively. The system is capable of real time dose rate measurements in fields of high dose rates and dose rate gradients and in complex, narrow geometries. This enables the application for radiation protection measurements as well as for quality control in radiotherapy. One inherent drawback of fiber optical dosimetry systems is the generation of Cherenkov radiation and luminescence in the light guide itself when it is exposed to ionizing radiation. This so called “stem” effect leads to an additional signal which introduces a deviation in the dose rate measurement and reduces the spatial resolution of the system, hence it has to be removed. The current system uses temporal discrimination of the effect for radioluminescence measurements in pulsed radiation fields and modulated optically stimulated luminescence for continuous irradiation conditions. This work gives an overview of the major results and discusses new-found obstacles of the applied methods of stem discrimination.

Cherenkov radiation from the target with predetermined dielectric properties, produced by a 3D-printer

Most targets made of industrial materials, used for the generation of Cherenkov radiation (ChR) have a refractive index n>1.4 in millimeter wavelength region. It is often a problem to get out the radiation from such cylindrical or flat targets because the angle of incidence of ChR on the outer surface of target is greater than the angle of total internal reflection.In this work we present the solution of this problem by the usage of the targets with predetermined dielectric properties, manufactured using 3-D printer. We demonstrate the emission of ChR in millimeter wavelength region from the such flat target with the refractive index n=1.37. Suggested technique allows us to fabricate targets with turned refractive index.

TH-C-17A-08: Monte Carlo Based Design of Efficient Scintillating Fiber Dosimeters

Purpose: To accurately predict Cherenkov radiation generation in scintillating fiber dosimeters. Quantifying Cherenkov radiation provides a method for optimizing fiber dimensions, orientation, optical filters, and photodiode spectral sensitivity to achieve efficient real time imaging dosimeter designs. Methods: We develop in-house Monte Carlo simulation software to model polymer scintillation fibers' fluorescence and Cherenkov emission in megavoltage clinical beams. The model computes emissions using generation probabilities, wavelength sampling, fiber photon capture, and fiber transport efficiency and incorporates the fiber's index of refraction, optical attenuation in the Cherenkov and visible spectrum and fiber dimensions. Detector component selection based on parameters such as silicon photomultiplier efficiency and optical coupling filters separates Cherenkov radiation from the dose-proportional scintillating emissions. The computation uses spectral and geometrical separation of Cherenkov radiation, however other filtering techniques can expand the model. Results: We compute Cherenkov generation per electron and fiber capture and transmission of those photons toward the detector with incident electron beam angle dependence. The model accounts for beam obliquity and nonperpendicular electron fiber impingement, which increases Cherenkov emission and trapping. The rotational angle around square fibers shows trapping efficiency variation from the normally incident minimum to a maximum at 45 degrees rotation. For rotation in the plane formed by the fiber axis and its surface normal, trapping efficiency increases with angle from the normal. The Cherenkov spectrum follows the theoretical curve from 300nm to 800nm, the wavelength range of interest defined by silicon photomultiplier and photodiode spectral efficiency. Conclusion: We are able to compute Cherenkov generation in realistic real time scintillating fiber dosimeter geometries. Design parameters incorporate fiber dimensions, orientations, several types of detector spectral response, optical coupling filters and light transport. We can vary these parameters to design and optimize high efficiency real time dosimeters capable of enhancing external beam patient safety and treatment accuracy. This research was supported in part by a GAANN Fellowship from the Department of Education.

Dose rate measurements with a ruby-based fiber optic radioluminescent probe

Fiber optic radioluminescence dosimetry allows real-time dose rate measurements in complex, narrow geometries and at places of high dose rates, without exposing the operator or the susceptible electronics. The keys are the spatial separation of radiation sensitive probe and electronic processing system and their optical connection by a flexible light guide. The small probes are capable of measuring fields of high dose rate gradients and the sealed probe-tip qualifies for applications in the fluid milieu and even for in-vivo-dosimetry. One problem of fiber optic dosimetry is the generation of Cherenkov radiation and fiber luminescence in the irradiated light guide, the so called stem effect. Ruby (Al2O3:Cr) has a narrow radioluminescent emission at 694 nm and is a potential luminophor for fiber optic radioluminescence dosimetry. In this work the influence of the stem effect on our ruby-based fiber optic dosimetry system is examined. The behavior of ruby probes under irradiation up to 0.5 kGy, as well as their luminescence decay characteristics and the applicability for measurements in radiotherapeutic fields are investigated.

Enhanced violet Cherenkov radiation generation in GeO2-doped photonic crystal fiber

We demonstrate the generation of highly efficient Cherenkov radiation (CR) in the fundamental mode of a GeO2-doped two zero dispersion wavelengths photonic crystal fiber (PCF). Using a high power femtosecond Yb-doped PCF laser emitting 100 fs pulses as the pump source, CR with an efficiency of >40 % and a bandwidth of 38 nm is obtained in the visible-wavelength range when the average power of pump light is 1.27 W. It is that injecting the pump light in deep anomalous dispersion regime contributes to such an efficient spectrally isolated CR. The mechanism during the forming of CR is discussed and the experimental results are in good agreement with the calculation.

Conditions for generation of X-ray cherenkov radiation during motion of charges in realistic media

The influence of the spatial distribution of electrons, atoms, and nuclei in condensed media on a medium’s susceptibility and on the conditions for the formation of short-wavelength Cherenkov radiation are considered. It is shown that taking into account the inhomogeneous (atomic and electronic) structure of material media in which fast charged particles travel leads to a change in the effective susceptibility and permittivity in the X-ray range, as compared with cases of model homogeneous media with the same average concentration of electrons. The influence of the distribution functions of electrons and nuclei in a target on the conditions for Cherenkov radiation generation and its parameters and on the threshold energy of fast charged particles required to generate such radiation is studied. It is shown that the function of the spatial distribution of electrons and nuclei in a target affects the conditions for generating laser radiation in the X- and λ-ray ranges (on the problem of X- and λ-ray lasers). The obtained results show that using the Fresnel approximation in the X-ray range is insufficiently justified and can lead to significant errors.

Highly efficient Cherenkov radiation generation in the irregular point of hollow-core photonic crystal fiber

Highly efficient Cherenkov radiation (CR) is generated by the soliton self-frequency shift (SSFS) in the irregular point of a hollow-core photonic crystal fiber (HC-PCF) in our laboratory. The impacts of pump power and wavelength on the CR are investigated, and the corresponding nonlinear processes are discussed. When the average power of the 120 fs pump pulse increases from 500 mW to 700 mW, the Raman soliton shifts from 2210 nm to 2360 nm, the output power of the CR increases by 2.3 times, the maximum output power ratio of the CR at 539 nm to that of the residual pump is calculated to be 24.32:1, the width of the output optical spectrum at the visible wavelength broadens from 35 nm to 62 nm, and the conversion efficiency η of the CR in the experiment can be above 32%.

Detection of diffraction radiation in a dielectric target simultaneously with Cherenkov radiation

The effect of the simultaneous generation of Cherenkov radiation and diffraction radiation in a dielectric target by electrons that pass near it has been experimentally studied. It has been shown that diffraction radiation appears at the upstream edge of the dielectric target, propagates inside the target, and is refracted at the down-stream edge as a beam of real photons.

Optical Cherenkov radiation in ultrafast cascaded second-harmonic generation

We show through theory and numerics that when few-cycle femtosecond solitons are generated through cascaded (phase-mismatched) second-harmonic generation, these broadband solitons can emit optical Cherenkov radiation in the form of linear dispersive waves located in the red part of the spectrum. The beating between the dispersive wave and the soliton generates trailing temporal oscillations on the compressed soliton. Insertion of a simple short-wave pass filter after the crystal can restore a clean soliton. On the other hand, bandpass filtering around the dispersive wave peak results in near-transform-limited ultrashort mid-IR pulses with pulse durations much shorter than the input near-IR pulse. The Cherenkov radiation for the crystal considered ($\ensuremath{\beta}$-barium borate) is found for pump wavelengths in the range $\ensuremath{\lambda}=0.95--1.45 \mathrm{\ensuremath{\mu}}\mathrm{m}$, and is located in the regime $\ensuremath{\lambda}=1.5--3.5 \mathrm{\ensuremath{\mu}}\mathrm{m}$. For shorter pump wavelengths, the phase-matching point is located in the absorption region of the crystal, effectively absorbing the generated dispersive wave. By calculating the phase-matching curves for typically used frequency conversion crystals, we point out that the mid-IR absorption in the crystal in many cases automatically will filter away the dispersive wave. Finally, an investigation of recent experimental results uncovers a four-wave-mixing phenomenon related to Cherenkov radiation that is an additional generation mechanism of long-wavelength radiation that can occur during soliton compression. We discuss the conditions that lead to this alternative dynamics rather than generation of Cherenkov radiation.

Soliton fission and supercontinuum generation in silicon waveguides

We show through numerical simulations that silicon waveguides can be used to create a supercontinuum extending over 400 nm by launching femtosecond pulses as higher-order solitons. The physical process behind continuum generation is related to soliton fission, self-phase modulation, and generation of Cherenkov radiation. In contrast with optical fibers, stimulated Raman scattering plays little role. As low-energy (approximately 1 pJ) pulses and short waveguides (<1 cm) are sufficient for continuum generation, the proposed scheme should prove useful for practical applications.

Coherent phonon–polaritons and subluminal Cherenkov radiation

We use an optical method based on sub-picosecond pulses moving in a nonlinear medium to generate relativistic dipoles that emit Cherenkov phonon–polariton radiation. Experiments on ZnTe and ZnSe reveal emission at subluminal speeds involving frequencies below those of the corresponding infrared-active phonon. The generation of Cherenkov radiation and impulsive stimulated Raman scattering by polaritons are the same physical phenomenon.