Crystal Slab Research Articles

A new brain dedicated PET scanner with 4D detector information

Abstract In this article, we present the geometrical design and preliminary results of a high sensitivity organ-specific Positron Emission Tomography (PET) system dedicated to the study of the human brain. The system, called 4D-PET, will allow accurate imaging of brain studies due to its expected high sensitivity, high 3D spatial resolution and, by including precise photon time of flight (TOF) information, a boosted signal-to-noise ratio (SNR). The 4D-PET system incorporates an innovative detector design based on crystal slabs (semi-monolithic) that enables accurate 3D photon impact positioning (including photon Depth of Interaction (DOI) measurement), while providing a precise determination of the photon arrival time to the detector. The detector includes a novel readout system that reduces the number of detector signals in a ratio of 4:1 thus, alleviating complexity and cost. The analog output signals are fed to the TOFPET2 ASIC (PETsys) for scalability purposes. The present manuscript reports the evaluation of the 4D-PET detector, achieving best values 3D resolution values of <1.6 mm (pixelated axis), 2.7±0.5 mm (monolithic axis) and 3.4±1.1 (DOI axis) mm; 359 ± 7 ps coincidence time resolution (CTR); 10.2±1.5 % energy resolution; and sensitivity of 16.2% at the center of the scanner (simulated). Moreover, a comprehensive description of the 4D-PET architecture (that includes 320 detectors), some pictures of its mechanical assembly, and simulations on the expected image quality are provided.

Radiative heat transfer in a low-symmetry Bravais crystal

Over the last few years, broken symmetry within crystals has attracted extensive attention since it can improve the control of light propagation. In particular, low-symmetry Bravais crystal can support shear polaritons which has great potential in thermal photonics. In this work, we report a twist-induced near-field thermal control system based on the low-symmetry Bravais crystal medium (\b{eta}-Ga2O3). The near-field thermal radiation (NFTR) between such crystal slabs is nearly four orders of magnitude larger than the blackbody limit, exceeding the NFTR from other traditional dielectric materials. Moreover, we show that this crystal can serve as an excellent platform for twist-induced near-field thermal control. Due to the intrinsic shear effect, the twist-induced modulation supported by low-symmetry Bravais crystal exceeds that by high-symmetry crystal. We further clarify how the shear effect affects the twist-induced thermal-radiation modulation supported by hyperbolic and elliptical polaritons and show that the shear effect significantly enhances the twist-induced thermal control induced by the elliptical polariton mode. These results open new directions for thermal-radiation control in low-symmetry materials, including geological minerals, common oxides, and organic crystals.

Physical and materials aspects of photonic crystals for microwaves and millimetre waves

Abstract Experimental and numerical results on photonic crystals are presented for the frequency range 26–60 GHz. The material used is alumina where two techniques have been applied for fabricating the photonic crystals: manual assembly of alumina rods and rapid prototyping. The observed positions of the fundamental and higher photonic band gaps are in excellent agreement with the calculated results. A new type of defect in the 3D woodpile structure, is created by inserting interstitial rods. As a new 2D structure a square parquet lattice is investigated. The concept of a negative index of refraction is adressed including a model calculation and an experimental demonstration by the transmission through a slab of a 2D photonic crystal.

A brute-force code searching for cell of non-identical displacement for CSL grain boundaries and interfaces

Atomic simulation of coincidence-site lattice (CSL) grain boundaries (GBs) and interfaces is of importance for understanding GBs in polycrystalline materials and for making functional films. A common process of high-throughput simulation for CSL GBs and interfaces is to explore rigid body translation (RBT) of one crystal respect to the other. Cell of non-identical displacement (CNID) is the minimum cell including all non-identical RBTs in the GB or interface plane and is important for effective sampling. This work proposes an algorithm to compute the CNID of any two-dimensional CSL GB or interface based on a reciprocity relation between the displacement shift complete (DSC) and CSL. Program summaryProgram title: cnidcalCPC Library link to program files:https://doi.org/10.17632/wzy67jd56d.1Code Ocean capsule:https://codeocean.com/capsule/1648641Licensing provisions: MIT licenseProgramming language: Python Nature of problemDetermination of atomic structure of crystalline interfaces and grain boundaries is a key issue for understanding the properties of polycrystalline materials, where atomic simulation has made notable contribution in this field. Simulation of crystalline interfaces often applies a bicrystal model of a coincidence-site lattice (CSL) [1, 2] interface comprising two slabs of crystals which coincides in the interface plane. To determine the most stable structure of this interface model, a main task is often to tailor the initial bicrystal model to obtain many non-identical and non-relaxed structures for subsequent relaxation to explore the energy landscape of the system to find the ‘global minimum’ at 0 K, where one of the most important operations to tailor the model is applying rigid body translation (RBT) [1] of one crystal respect to the other by a vector confined in the interface plane. Because of symmetry of the two crystals, there exist periodicity of this translation vector which is reflected by a special cell called ‘cell of non-identical displacement’ (CNID) [1] which is the minimum cell including all non-identical choices of this vector and is key for effective sampling. Although CNID has a straightforward definition that it is a lattice including both the two overlapping two-dimensional lattices, finding a method capable to determine a primitive basis of CNID in general cases can be non-trivial and there has not been a reported programme capable to do so. Solution methodCNID in nature is a displacement shift complete (DSC) lattice [2]. While there has not been effective reported programme to compute DSC, several programmes have been made to compute CSL [3, 4, 5]. Besides, Grimmer [6] has proposed a mathematical relationship between the DSC and the CSL. In this way, based on a previously reported programme computing the CSL by brute-force method [5] and the relationship between DSC and CSL proposed by Grimmer, we generated a python code to compute CNID. This code is available to compute the CNID of a CSL interface comprising any two lattices if only they coincide even including complex cases involving heterogeneous lattices with difference in both shape and size.

Laser beam steering in a variable gradient index nematic liquid crystal structure

We have fabricated a parallel slab of nematic liquid crystal with hybrid alignment that provides “longitudinal” gradient of refractive index for the extraordinary polarized beam. This in turn generates continuously changing angular separation between ordinary and extraordinary beams while propagating inside the slab, like the optical mirage effect. For the moment the output separation angle is not large (below 1°), but it can be further optimized to obtain larger separation angles and consequently more effective electrical control.

Common-path conoscopic interferometry for enhanced picosecond ultrasound detection

We report on a common-path implementation of conoscopic interferometry in picosecond pump-probe reflectometry for simple and efficient detection of picosecond ultrasounds. The interferometric configuration proposed here is greatly simplified, involving only the insertion of a birefringent crystal in a standard reflectometry setup. Our approach is demonstrated by the optical detection of coherent acoustic phonons propagating through thin metal films under two representative geometries, one a particular case where the crystal slab is part of a sample as substrate of a metal film, and the other a more general case where the crystal slab is independent of the sample as part of the detection system. We first illustrate the former with a 300 nm thin film of polycrystalline titanium, deposited by physical vapor deposition on top of a 1 mm-thick uniaxial (0001) sapphire crystal. A signal-to-noise ratio (SNR) enhancement of more than 15 dB is achieved compared to conventional reflectometry. Next, the general case is demonstrated with a 900 nm-tungsten film sputtered on a silicon wafer substrate. More echoes can be discriminated by using the reported approach compared to standard reflectometry, which confirms the improvement in SNR and suggests broad applications for the reported method.

MoS$_2$ Broadband Coherent Perfect Absorber for Terahertz Waves

We present a comprehensive analysis of terahertz waves propagation through a bulk MoS $_2$ crystal slab. It is shown that broadband coherent perfect absorption can be achieved and the coherent absorptivity can be adjusted from 1.57% to 99.97% by means of phase modulation. Moreover, the relative bandwidth for over 90% coherent absorptivity can be as wide as 179%. By increasing the doping rate in MoS $_2$ crystal, the coherent absorptivity spectrum exhibits a clear blueshift whilst the peak coherent absorptivity remains over 99.9%. Full-wave numerical simulations are further carried out to confirm the validity of our theoretical analysis.

Measurement of the topological surface state optical conductance in bulk-insulating Sn-doped Bi1.1Sb0.9Te2S single crystals

Topological surface states have been extensively observed via optics in thin films of topological insulators. However, in typical thick single crystals of these materials, bulk states are dominant and it is difficult for optics to verify the existence of topological surface states definitively. In this work, we studied the charge dynamics of the newly formulated bulk-insulating Sn-doped Bi$_{1.1}$Sb$_{0.9}$Te$_2$S crystal by using time-domain terahertz spectroscopy. This compound shows much better insulating behavior than any other bulk-insulating topological insulators reported previously. The transmission can be enhanced an amount which is 5$\%$ of the zero-field transmission by applying magnetic field to 7 T, an effect which we believe is due to the suppression of topological surface states. This suppression is essentially independent of the thicknesses of the samples, showing the two-dimensional nature of the transport. The suppression of surface states in field allows us to use the crystal slab itself as a reference sample to extract the surface conductance, mobility, charge density and scattering rate. Our measurements set the stage for the investigation of phenomena out of the semi-classical regime, such as the topological magneto-electric effect.

Comparison of double crystal (+n,-m) and (+n,+m) settings containing a fully asymmetric diffraction geometry of a bent perfect crystal with the output beam expansion

In this paper neutron diffraction properties of the dispersive double crystal (+n,-m) and (+n,+m) settings containing the bent perfect crystal Si(220) in symmetric diffraction geometry and the bent perfect crystal Si(311) in the fully asymmetric diffraction geometry with the output beam expansion are studied and compared. It has been found that the properties, namely, the width of the output double diffracted beam, of both settings strongly depends on the relative combination of curvatures of Si(311) crystal slab with respect to the curvature of the Si(220) one. Moreover, after the beam expansion the fully asymmetric diffraction geometry provides in both cases a highly collimated and highly monochromatic beam of a rather large cross-section of several square centimetres. Application possibilities, of such a beam, namely for neutron phase contrast radiography will be discussed.

Light Emission Characteristics of Nonpolar <inline-formula> <tex-math notation="LaTeX">$a$ </tex-math> </inline-formula>-Plane GaN-Based Photonic Crystal Defect Cavities

In this paper, the light emission performance from nonpolar $a$ -plane gallium nitride (GaN)-based photonic crystal (PC) slabs with line-type defect cavities has been investigated. Via the focused ion beam milling technique, thin membrane structures of the nonpolar GaN PC defect cavities were engineered, exhibiting one dominated resonant mode occurred at 419.3 nm with a high quality factor of $1.9\times 10^{3}$ in the photoluminescence emission characterizations. In the far-field region, light emission performance shows a high polarization degree of 50% with the beam direction perpendicular to the defect cavity. Compared with the PC defect cavity patterns fabricated on $c$ -axial oriented GaN materials, the $a$ -plane PC defect cavities display not only the linear light output–input power dependence, but also an invariant resonant mode of constant light emission behavior with increasing the injection carriers, therefore providing a new perspective for the future development of promising optoelectronic devices.

Half-disordered photonic crystal slabs.

Optical transmission spectra of finite-thickness slabs of two-dimensional triangular-lattice photonic crystals of air holes in a dielectric matrix with various concentrations of randomly located vacancies (absent air holes) are studied. We focus on structures in which only one half of the structure-the incidence or transmission side-is disordered. We find vacancy-induced scattering gives rise to a strong difference in the two cases; for light incident on the disordered side, high transmission within the photonic pseudogap at normal incidence is predicted, in strong contrast to the opposite case, where low transmission is predicted throughout the pseudogap, as is observed in the case of an ideal structure with no defects.

Crystallographic new light on an old complex: NaMg[Cr(oxalato)3]·9H2O and structure redetermination of the isomorphous aluminum(III) compound

The crystal structure of sodium and magnesium tris(oxalato)chromato(III) nona hydrate, NaMg[Cr(C2O4)3]·9H2O, has been determined by X-ray diffraction methods. It crystallizes in the trigonal P3c1 space group with (in the hexagonal setting) a = b = 16.9635(4), c = 12.5247(3) Å, and Z = 6 molecules per unit cell. The Cr(III) and sodium ions are in octahedral environments coordinated to three oxalate molecules and arranged in a honeycomb-like layered structure perpendicular to the crystal c-axis. There are three different octahedral [Mg(H2O)6]2+ hydrated ions that fill the honeycomb holes giving rise to electrically neutral Na[Mg(H2O)6][Cr(oxalato)3] crystal slabs. Neighboring slabs are bridged by the remaining three water molecules through H-bonds and therefore the whole crystal can be more properly described as Na[Mg(H2O)6][Cr(oxalato)3]·3H2O. Also, the structure of isomorphous NaMg[Al(C2O4)3]·9H2O compound has been re-examined and compared with the chromium(III) complex. Infrared and Raman spectra of both complexes were also recorded and briefly discussed.

Neutron diffraction studies of a high resolution double crystal (+n,-m) setting containing Si(220) and Si(311) bent perfect crystals in symmetric and fully asymmetric diffraction geometry, respectively

In this paper studies of neutron diffraction properties of the double crystal (+n,-m) setting containing Si(220) and Si(311) bent perfect crystals (BPC) in symmetric and fully asymmetric diffraction (FAD) geometry with the output beam expansion (OBE), respectively, are presented. Namely, our attention was focused on the properties of the FAD geometry of the BPC Si(311) crystal slab. It has been found that after a beam expansion this FAD geometry can provide a monochromatic beam of a rather large cross-section and of very small divergence with some possible application use.

Multiple Bragg reflection by a thick mosaic crystal.

Symmetric Bragg-case reflections from a thick, ideally imperfect, crystal slab are studied mostly by analytical means. The scattering transfer function of a thin mosaic layer is derived and brought into a form that allows for analytical approximations or easy quadrature. The Darwin-Hamilton equations are generalized, lifting the restriction of wavevectors to a two-dimensional scattering plane. A multireflection expansion shows that wavevector diffusion can be studied independently of the real-space coordinate. Combining analytical arguments and Monte Carlo simulations, multiple Bragg reflections are found to result in a minor correction of the reflected intensity, a moderate broadening of the reflected azimuth angle distribution, a considerable modification of the polar angle distribution, and a noticeable shift and distortion of rocking curves.

Mode Reduction for Efficient Modeling of Photonic Crystal Slab Structures

Rigorous numerical simulations for photonic crystal (PhC) slab structures and devices are difficult due to the complicated three-dimensional (3-D) geometry, high index-contrast, sharp edges, and possibly inhomogeneity at infinity. The approach based on expanding the field in 1-D vertical modes has great potential, but is currently limited by the relatively large number of modes needed for maintaining the accuracy of the solutions. In this paper, we show that if a single hole is first analyzed with the full set of vertical modes, the number of modes can be reduced to less than one third of the total in the main part of the computation. This leads to a speedup of more than 27 times. The method is illustrated by computing the transmission and reflection spectra for a PhC slab with a finite number of hole arrays.

Neutron diffraction studies of a double-crystal (+n,–m) setting containing a fully asymmetric diffraction geometry of a bent perfect crystal with output beam expansion

The neutron diffraction properties of a double-crystal (+n,−m) setting, which contains a bent perfect Si(311) crystal in the fully asymmetric diffraction (FAD) geometry with output beam expansion and a bent perfect Si(220) crystal in the symmetric diffraction geometry, are presented. Generally, there are two possibilities for the FAD geometry: either with output beam compression or with output beam expansion. In this case, attention has been focused on the latter. The properties of the (+n,−m) double-bent-crystal arrangement of a bent perfect crystal FAD Si(311) geometry, in combination with a bent Si(220) crystal slab in the symmetric diffraction geometry, were studied. It was found that, after beam expansion, this FAD geometry can provide a monochromatic beam of rather large cross section but very small divergence.

DIRAC-CONE PHOTONIC SURFACE STATES IN THREE-DIMENSIONAL PHOTONIC CRYSTAL SLAB

We present a multiple-scattering method in conjunction with supercell calculations to study the electromagnetic surface states in three-dimensional photonic crystal (PC) slab. Using our technique, we obtain the first prediction of Dirac-cone photonic surface state in some three-dimensional PC slabs. Such a state can be used to investigate some extremal transmission phenomena of electromagnetic waves near the Dirac point on the surface of the crystal, which is similar to the case of electron on the surface of topological insulators.

Efficient 1856 nm emission from Tm,Mg:LiNbO_3 laser

Efficient continuous-wave laser emission at 1856 nm from a Tm,Mg:LiNbO(3) crystal slab with high Tm(3+) doping concentration is reported. A maximum output power of 2.62 W is realized with a slope efficiency of 19.6% and the beam quality factor M(2) of 1.7 at room temperature. We believe that this is the first demonstration of watt-level laser operation in Tm,Mg:LiNbO(3) crystal and the output power is four orders of magnitude higher than that reported previously in Tm-doped LiNbO(3) crystal. Performance degradation due to the photorefractive effect under high intensity 1856 nm laser is not observed thanks to the co-doping of magnesium ions. Quantitative analysis about the long-term photorefractive effect is also provided. Multi-wavelength laser operation is realized by using different narrow-band output couplers. This demonstration opens up a viable pathway towards 2-μm integrated optic devices for achieving laser oscillation, electro-optic and nonlinear optical effects within just one sample simultaneously.

ONE-DIMENSIONAL PHONONIC CRYSTAL SLABS BORDERED WITH ASYMMETRIC UNIFORM LAYERS

We investigate theoretically the propagation of Lamb waves in a one-dimensional phononic crystal (PC) slabs bordered with asymmetric uniform layers based on the supercell plane wave expansion (SC-PWE) method. The validity is proved by using the finite-element (FE) method with Comsol Multiphysics 3.5a. The effect of changing the thickness of substrate and superstrate on the variation of the band gap width and frequency are studied when the total thickness of the loading layers is fixed. We also investigate the property of band gap maps by changing the total thickness or the material of the loading layers. The results show that the band structure can be tuned by changing the material or the thickness of the substrate or the superstrate. These structures may be used as filters and acoustic sensors.

Compact Terahertz Wave Collimator Design with Photonic Crystal Slabs

A compact terahertz (THz) wave collimator is proposed, which works under the frequency from 2.4 THz to 2.7 THz with a photonic crystal (PC) slab based on the self-collimation effect. The plane wave expansion (PWE) method is used to calculate the dispersion surfaces and the equal-frequency contours (EFCs) and optimize the structure. The propagation of the THz waves in the structure is simulated and the normalized transmission is calculated by using the finite-difference time-domain (FDTD) method with perfectly matched layer (PML) absorbing boundary conditions. Numerical simulations show that the designed collimator has a good collimation property and a high transmittance.