A generalized exchange-correlation functional: the Neural-Networks approach

A Neural-Networks-based approach is proposed to construct a new type of exchange-correlation functional for density functional theory. It is applied to improve B3LYP functional by taking into account of high-order contributions to the exchange-correlation functional. The improved B3LYP functional is based on a neural network whose structure and synaptic weights are determined from 116 known experimental atomization energies, ionization potentials, proton affinities or total atomic energies which were used by Becke in his pioneer work on the hybrid functionals [J. Chem. Phys. ${\bf 98}$, 5648 (1993)]. It leads to better agreement between the first-principles calculation results and these 116 experimental data. The new B3LYP functional is further tested by applying it to calculate the ionization potentials of 24 molecules of the G2 test set. The 6-311+G(3{\it df},2{\it p}) basis set is employed in the calculation, and the resulting root-mean-square error is reduced to 2.2 kcal$\cdot$mol$^{-1}$ in comparison to 3.6 kcal$\cdot$mol$^{-1}$ of conventional B3LYP/6-311+G(3{\it df},2{\it p}) calculation.Comment: 10 pages, 1figur

Scale invariant distribution functions in quantum systems with few degrees of freedom

Scale invariance usually occurs in extended systems where correlation functions decay algebraically in space and/or time. Here we introduce a new type of scale invariance, occurring in the distribution functions of physical observables. At equilibrium these functions decay over a typical scale set by the temperature, but they can become scale invariant in a sudden quantum quench. We exemplify this effect through the analysis of linear and non-linear quantum oscillators. We find that their distribution functions generically diverge logarithmically close to the stable points of the classical dynamics. Our study opens the possibility to address integrability and its breaking in distribution functions, with immediate applications to matter-wave interferometers.Comment: 8+10 pages. Scipost Submissio

Improving the imaging ability of ultrasound-modulated optical tomography with spectral-hole burning

Ultrasound-modulated optical tomography is a hybrid imaging technique based on detection of the diffused light modulated by a focused ultrasonic wave inside a scattering medium. With the combined advantages of ultrasonic resolution and optical contrast, UOT is ideal for deep tissue imaging. Its growth in popularity and application, however, is hindered by the low efficiency in detecting the modulated diffused photons. Research activities on UOT have therefore been centered on improving its signal detection efficiency by exploring various technical solutions. A prime example is the use of spectral-hole burning (SHB) in UOT. By applying SHB crystal as a spectral filter, one modulation sideband of the diffused light can be efficiently selected while all the other frequency components are strongly suppressed. Immune to both the spatial and temporal incoherence of the signal with a high enough on/off ratio, SHB can boost the UOT imaging ability dramatically and push it towards practical applications. We compare SHB with the other technologies that have been applied to UOT, and identify the unique features that make SHB a preferable tool for UOT. We also discuss the desired improvements from the SHB side, which will help UOT pave the way from research to everyday life

Focusing light into turbid media: time-reversed ultrasonically encoded (TRUE) focusing

In turbid media such as biological tissues, light undergoes multiple scattering. Consequently, it is not possible to focus light at depths beyond one transport mean free path in such media. To break through this limit, we proposed and experimentally demonstrated a novel technique, based on ultrasonic encoding of diffused laser light and optical time reversal, which effectively focuses light into a turbid medium. In the experimental implementation of the Time-Reversed Ultrasonically Encoded (TRUE) optical focusing, a turbid medium was illuminated by a laser beam with a long coherence length. The incident light was multiply scattered inside the medium and ultrasonically encoded within the ultrasonic focal zone. The wavefront of the ultrasonically encoded light was then time reversed by a Phase Conjugate Mirror (PCM) outside the medium. The time-reversed (or phase conjugated) optical wavefront traced back the trajectories of the ultrasonically encoded diffused light, and converged to the ultrasonic focal zone. With a commercially available photorefractive crystal as the PCM, the main approaches for increasing focusing depth are to improve the efficiencies of ultrasonic encoding and time reversal. Our recent experiments showed that light can be focused into a 5-mm thick tissue-mimicking phantom (optical thickness = 50, i.e., geometric thickness = 50 mean free paths) with a dynamically adjustable focus. The TRUE optical focusing opens a door to focusing light into turbid media or manipulating light-matter interactions

Focusing light into tissue

A novel technique uses ultrasonic encoding and time reversal to break the diffusion limit and enable optical imaging, manipulation, and therapy at greater depths

Dependence of optical scattering from Intralipid in gelatin-gel based tissue-mimicking phantoms on mixing temperature and time

Intralipid is widely used as an optical scattering agent in tissue-mimicking phantoms. Accurate control when using Intralipid is critical to match the optical diffusivity of phantoms to the prescribed value. Currently, most protocols of Intralipid-based hydrogel phantom fabrication focus on factors such as Intralipid brand and concentration. In this note, for the first time to our knowledge, we explore the dependence of the optical reduced scattering coefficient (at 532 nm optical wavelength) on the temperature and the time of mixing Intralipid with gelatin-water solution. The studied samples contained 1% Intralipid and were measured with oblique-incidence reflectometry. It was found that the reduced scattering coefficient increased when the Intralipid-gelatin-water mixture began to solidify at room temperature. For phantoms that had already solidified completely, the diffusivity was shown to be significantly influenced by the temperature and the duration of the mixing course. The dependence of the measured diffusivity on the mixing conditions was confirmed by experimental observations. Moreover, the mechanism behind the dependence behavior is discussed

High-sensitivity ultrasound-modulated optical tomography with a photorefractive polymer

By detecting ultrasonically tagged diffuse light, ultrasound-modulated optical tomography images optical contrast with ultrasonic resolution deep in turbid media, such as biological tissue. However, small detection etendues and weak tagged light submerged in strong untagged background light limit the signal detection sensitivity. In this Letter, we report the use of a large-area (∼5 cm×5 cm ) photorefractive polymer film that yields more than 10 times detection etendue over previous detection schemes. Our polymer-based system enabled us to resolve absorbing objects embedded inside diffused media thicker than 80 transport mean free paths, by using moderate light power and short ultrasound pulses

Localized fluorescence excitation in opaque media by time-reversed ultrasonically encoded (TRUE) optical focusing

To focus light beyond one transport mean free path, time-reversed ultrasonically encoded (TRUE) optical focusing has previously been implemented by both analog and digital devices. By allowing wavefront recording with finer resolution and larger aperture, the analog scheme, which uses photorefractive materials as the phase-conjugate mirror, generates a more complete set of time-reversed optical modes than the digital scheme. Here, we report the direct visualization of localized fluorescence excitation inside a turbid medium by photorefractive time reversal. Further, we imaged fluorescent targets embedded in a turbid phantom whose thickness was four transport mean free paths

High-efficiency time-reversed ultrasonically encoded optical focusing using a large-area photorefractive polymer

Time-reversed ultrasonically encoded (TRUE) optical focusing focuses light beyond one transport mean free path by phase-conjugating the ultrasonically tagged light. However, in previous works, only a small portion of the tagged light was phase-conjugated by using a photorefractive Bi_(12)SiO_(20) crystal, due to its small active area (1x1 cm^2). In this work, we report high-efficiency TRUE focusing using a large-area photorefractive polymer (5x5 cm^2), which demonstrated ~40 times increase in focused energy. Further, we imaged absorbers embedded in a turbid sample of thickness of ~12 transport mean free paths