Autocorrelation Measurements Research Articles

What were the spatiotemporal evolution characteristics and influencing factors of global land use carbon emission efficiency? A case study of the 136 countries

Nations worldwide must collaborate to tackle the challenge of improving carbon emission efficiency and optimizing land use effectively. This study extends the study area from provinces and cities, regional city clusters, and countries to 136 major countries in the world, and shows the differences between countries. The paper analyzed the spatiotemporal evolution characteristics and influencing factors of land use carbon emission efficiency (LCEE) in these 136 countries from 2000 to 2019 using the super-efficiency slacks-based measure data envelopment analysis (Super-SBM DEA) model, global malmquist index, spatial autocorrelation measurement tools and Spatial Durbin model. It can help to promote low-carbon and green development while simultaneously improving the global LCEE. The findings indicate that: (1) Over the study period, there is a trend toward a decline in the LCEE worldwide. (2) The primary cause of the worldwide LCEE’s declining trend during the study period is the reduction in technical efficiency. (3) The low-low cluster type accounts for more, while the high-high agglomeration type accounts for less and exhibits a sluggish growth pattern. On the whole, there is a phenomenon of non-aggregation and non-uniformity. It is necessary to increase the driving effect of high-high cluster radiation. (4) Economic development level, investment level, population size, public service level and environmental sustainability level all have a significant impact on LCEE, while industrial development level has no significant impact on LCEE. From a global perspective, the radiation driving role of countries with high LCEE should be increased. Improve the efficiency and technological level of land use emission in light of local conditions, and promote independent innovation in green and low-carbon technologies. This is conducive to the establishment of long-term low-carbon growth.

High‐β Lasing in Self‐Assembled Photonic‐Defect Microcavities with a Transition Metal Dichalcogenide Monolayer as Active Material

AbstractThe investigation and development of innovative micro‐ and nanolasers using transition metal dichalcogenide (TMDC) monolayers as active materials is attracting considerable attention due to their unique electrical, mechanical, and optical properties. In this report, the fabrication of photonic‐defect microcavities that are self‐assembled and integrated into a dielectric distributed Bragg reflector structure that fully encapsulates a monolayer of tungsten diselenide () is detailed. The encapsulation process of the monolayer with hexagonal boron nitride generates air bubbles that induce parabolic photonic defects in the microcavity. These defects lead to a tight diameter‐dependent three‐dimensional optical confinement, which is confirmed by experimental studies and numerical simulations. In addition, a significant nonlinearity in the input‐output characteristics and excitation‐power‐dependent linewidth narrowing is observed in the resonators, indicating laser operation, which is verified by photon autocorrelation measurements. The photonic‐defect cavities are all formed on a single monolayer sample, suggesting potential advantages for multi‐wavelength emission photonic applications and facilitating TMDC‐based prestructured photonic‐defect microlasers for large‐scale fabrication.

Integration of liquid crystal optical delay and mechanical stage optical delay for measurement of ultrafast autocorrelations and terahertz pulses

To improve the temporal resolution in an optical delay system that uses a conventional mechanical delay stage, we integrate an in-line liquid crystal (LC) wave retarder. Previous implementations of LC optical delay methods are limited due to the small temporal window provided. Using a conventional mechanical delay stage system in series with the LC wave retarder, the temporal window is lengthened. Additionally, the limitation on temporal resolution resulting from the minimum optical path alteration (resolution of 400 nm) of the conventionally used mechanical delay stage is reduced via the in-line wave retarder (resolution of 50 nm). Interferometric autocorrelation measurements are conducted at multiple laser emission frequencies (349, 357, 375, 394, and 405 THz) using the in-line LC and conventional mechanical delay stage systems. The in-line LC system is compared to the conventional mechanical delay stage system to determine the improvements in temporal resolution relating to maximum resolvable frequency. This work demonstrates that the integration of the in-line LC system can extend the maximum resolvable frequency from 375 to 3000 THz. The in-line LC system is also applied for measurement of terahertz pulses.

Measuring statistics-induced entanglement entropy with a Hong–Ou–Mandel interferometer

Despite its ubiquity in quantum computation and quantum information, a universally applicable definition of quantum entanglement remains elusive. The challenge is further accentuated when entanglement is associated with other key themes, e.g., quantum interference and quantum statistics. Here, we introduce two novel motifs that characterize the interplay of entanglement and quantum statistics: an ‘entanglement pointer’ and a ‘statistics-induced entanglement entropy’. The two provide a quantitative description of the statistics-induced entanglement: (i) they are finite only in the presence of quantum entanglement underlined by quantum statistics and (ii) their explicit form depends on the quantum statistics of the particles (e.g., fermions, bosons, and anyons). We have experimentally implemented these ideas by employing an electronic Hong–Ou–Mandel interferometer fed by two highly diluted electron beams in an integer quantum Hall platform. Performing measurements of auto-correlation and cross-correlation of current fluctuations of the scattered beams (following ‘collisions’), we quantify the statistics-induced entanglement by experimentally accessing the entanglement pointer and the statistics-induced entanglement entropy. Our theoretical and experimental approaches pave the way to study entanglement in various correlated platforms, e.g., those involving anyonic Abelian and non-Abelian states.

Single photon emission from in situ created nitrogen-vacancies in chemical vapor deposition grown single crystal diamond

The work reports the manipulation of nitrogen-vacancy centres in chemical vapor deposition grown single crystal diamonds by incorporation of nitrogen, boron and oxygen during the growth process. Single photon emission measurements were performed on diamond samples by using continuous wave and pulsed laser of wavelength 532 nm. The second order auto-correlation measurement 0.3<g2(0)<0.6 on nitrogen incorporated diamond sample shows the anti-bunching behavior of photons emerging from the population of one- and two-color centres. However, the correlation measurement 0.6<g2(0)<0.8 on oxygen and boron incorporated diamond samples show the anti-bunching behaviour of photons due to population of two- and three-color centres present in the samples. The result opens the possibilities of in situ development of nitrogen-vacancy centres in diamond during the growth process for next generation quantum computing devices.

A global model of intensity autocorrelation to determine laser pulse duration

We present a new global model of collinear autocorrelation based on second harmonic generation nonlinearity. The model is rigorously derived from the nonlinear coupled wave equation specific to the autocorrelation measurement configuration, without requiring a specific form of the incident pulse function. A rigorous solution of the nonlinear coupled wave equation is obtained in the time domain and expressed in a general analytical form. The global model fully accounts for the nonlinear interaction and propagation effects within nonlinear crystals, which are not captured by the classical local model. To assess the performance of the global model compared to the classic local model, we investigate the autocorrelation signals obtained from both models for different incident pulse waveforms and different full-widthes at half-maximum (FWHMs). When the incident pulse waveform is Lorentzian with an FWHM of 200 fs, the global model predicts an autocorrelation signal FWHM of 399.9 fs, while the classic local model predicts an FWHM of 331.4 fs. The difference between the two models is 68.6 fs, corresponding to an error of 17.2%. Similarly, for a sech-type incident pulse with an FWHM of 200 fs, the global model predicts an autocorrelation signal FWHM of 343.9 fs, while the local model predicts an FWHM of 308.8 fs. The difference between the two models is 35.1 fs, with an error of 10.2%. We further examine the behavior of the models for Lorentzian pulses with FWHMs of 100 fs, 200 fs and 500 fs. The differences between the global and local models are 17.1 fs, 68.6 fs and 86.0 fs, respectively, with errors approximately around 17%. These comparative analyses clearly demonstrate the superior accuracy of the global model in intensity autocorrelation modeling.

Self-referencing similariton-based spectral interferometry for ultrashort pulse complete characterization

Similariton is used to provide a reference for the characterization of ultrashort pulses by spectral interferometry. Measured pulse profiles are compared with autocorrelation measurements and cross-checked by spectrotemporal imaging technique based on similariton-induced temporal lens. The use of similariton makes the pulse measurement method self-referencing, substantially improving its application range, whilst preserving the simplicity of the principle and setup of the classic methods of spectral interferometry that require a reference pulse. Our comparative experimental study, carried out together with numerical simulations, provides the application range and limitations of the method and evidences the high precision of the similariton-based spectral interferometry.

Efficient Frequency Upconversion Photodetection in Heterojunction of InSe and Silicon Photonic Crystal Cavity

AbstractFrequency upconversion photodetection by two‐dimensional (2D) materials with nonlinear optical response is an appealing technique for applications such as extending photodetection wavelength range, autocorrelation measurement, and high‐efficiency single photon detection. However, the frequency upconversion efficiency in 2D materials is limited due to the low efficiency of nonlinear optical processes and atomic light‐matter interaction length. Here, the efficient frequency upconversion photodetection is realized by van der Waals (vdW) integration of a few‐layer InSe with a silicon photonic crystal cavity (Si‐PPC). With the large second‐order nonlinearity of InSe and enhanced light‐matter interaction by Si‐PCC, continuous wave (CW) pumped second harmonic generation (SHG) is demonstrated with pump power down to 100 µW. Further, InSe and Si absorb the SHG photons and generate photocarriers which are separated and collected by the n‐InSe/p‐Si heterojunction. A frequency upconversion photoresponsivity of 16 µA W−1 with a CW laser at 1467 nm is obtained. More efficient SHG with pulsed laser improves the responsivity to 3.9 mA W−1 with quadratic power dependence, which is record‐high among the frequency upconversion photodetectors based on 2D materials. This work opens up opportunities for a high‐efficiency 2D material nonlinear photodetector integrated on photonic integrated circuits.

Ultrafast spectral diffusion of GaN defect single photon emitters

Defect-based single photon emitters play an important role in quantum information technologies. Quantum emitters in technologically mature direct wide bandgap semiconductors, such as nitrides, are attractive for on-chip photonic integration. GaN has recently been reported to host bright and photostable defect single photon emitters in the 600–700 nm wavelength range. Spectral diffusion caused by local electric field fluctuation around the emitter limits the photon indistinguishability, which is a key requirement for quantum applications. In this work, we investigate the spectral diffusion properties of GaN defect emitters integrated with a solid immersion lens, employing both spectral domain and time domain techniques through spectroscopy and photon autocorrelation measurements at cryogenic temperature. Our results show that the GaN defect emitter at 10 K exhibits a Gaussian line shape with a linewidth of ∼1 meV while the spectral diffusion characteristic time falls within the range of a few hundred nanoseconds to a few microseconds. We study the dependency of the spectral diffusion rate and Gaussian linewidth on the excitation laser power. Our work provides insight into the ultrafast spectral diffusion in GaN defect-based single photon emitter systems and contributes toward harnessing the potential of these emitters for applications, especially for indistinguishable single photon generation.

Flow-Data-Based Global Spatial Autocorrelation Measurements for Evaluating Spatial Interactions

Spatial autocorrelation analysis is essential for understanding the distribution patterns of spatial flow data. Existing methods focus mainly on the origins and destinations of flow units and the relationships between them. These methods measure the autocorrelation of gravity or the positional and directional autocorrelations of flow units that are treated as objects. However, the intrinsic complexity of actual flow data necessitates the consideration of not only gravity, positional, and directional autocorrelations but also the autocorrelations of the variables of interest. This study proposes a global spatial autocorrelation method to measure the variables of interest of flow data. This method mainly consists of three steps. First, the proximity constraints of the origin and destination of a flow unit are defined to ensure similarity of flow units in terms of direction, distance, and position. This undertaking aims to determine the neighborhood of flow units and generate their adjacent matrices. Second, a spatial autocorrelation measurement model for flow data is constructed on the basis of the adjacent matrix generated. Artificial data sets are also employed to test the validity of the model. Finally, the proposed method is applied to the flow data analysis of population migration in central and eastern China to prove the practical application value of the model. The proposed method is universal and can be generalized to the global spatial autocorrelation analysis of any type of flow data.

Void BAO measurements on quasars from eBOSS

Abstract We present the clustering of voids based on the quasar (QSO) sample of the extended Baryon Oscillation Spectroscopic Survey Data Release 16 in configuration space. We define voids as overlapping empty circumspheres computed by Delaunay tetrahedra spanned by quartets of quasars, allowing for an estimate of the depth of underdense regions. To maximise the BAO signal-to-noise ratio, we consider only voids with radii larger than 36h−1Mpc. Our analysis shows a negative BAO peak in the cross-correlation of QSOs and voids. The joint BAO measurement of the QSO auto-correlation and the corresponding cross-correlation with voids shows an improvement in 70% of the QSO mocks with an average improvement of $\sim 5\%$. However, on the SDSS data, we find no improvement compatible with cosmic variance. For both mocks and data, adding voids does not introduce any bias. We find under the flat ΛCDM assumption, a distance joint measurement on data at the effective redshift zeff = 1.51 of DV(zeff) = 26.558 ± 0.553. A forecast of a DESI-like survey with 1000 boxes with a similar effective volume recovers the same results as for light-cone mocks with an average of 4.8% improvement in 68% of the boxes.

Accessible interferometric autocorrelator for noise-like pulses based on a Fabry-Perot cavity.

In this work, we develop experimentally a Fabry-Perot fiber optic interferometer applied to the measurement of autocorrelation of complex dynamic pulses generated by a figure-eight fiber laser. The principle is based in the superposition of multiple pulses, which requires two partially reflecting flat surfaces in parallel, resulting in a simple and compact autocorrelator design. The autocorrelation trace obtained exhibits a typical double-scaled structure for noise-like pulses (NLPs), with an ultrashort coherence spur on the order of 100 fs riding upon a broad pedestal of 120 ps. Finally, we show experimentally that the developed Fabry-Perot device is able to measure accurately the autocorrelation of NLPs, as confirmed by comparing the measurement with that of a conventional autocorrelator scheme based on a Michelson interferometer, with the additional advantages of a more compact setup and a much easier alignment procedure compared to the latter.

Machine learning assisted quantum super-resolution microscopy

One of the main characteristics of optical imaging systems is spatial resolution, which is restricted by the diffraction limit to approximately half the wavelength of the incident light. Along with the recently developed classical super-resolution techniques, which aim at breaking the diffraction limit in classical systems, there is a class of quantum super-resolution techniques which leverage the non-classical nature of the optical signals radiated by quantum emitters, the so-called antibunching super-resolution microscopy. This approach can ensure a factor of sqrt{n} improvement in the spatial resolution by measuring the n -th order autocorrelation function. The main bottleneck of the antibunching super-resolution microscopy is the time-consuming acquisition of multi-photon event histograms. We present a machine learning-assisted approach for the realization of rapid antibunching super-resolution imaging and demonstrate 12 times speed-up compared to conventional, fitting-based autocorrelation measurements. The developed framework paves the way to the practical realization of scalable quantum super-resolution imaging devices that can be compatible with various types of quantum emitters.

Optical Autocorrelation Measurement for Ultrafast Pulses at NIR Wavelengths Using GaP, GaAsP, and Si Photoconductive Detectors

In this article, we report on an optical real-time autocorrelator readout with a 5 Hz refresh rate, equipped with a transimpedance amplified photodetector based on the two-photon absorption (TPA) of semiconductor photodiodes (PDs) for ultrashort (1 &lt; ps) pulse measurement. By replacing the GaP PD of a commercial TPA detector with GaAsP and Si PD elements, we demonstrated that the spectral response based on the TPA of each photodetector followed the linear response of the corresponding semiconductor PD within accessible wavelength regions. The TPA spectral response of the GaAsP detector exhibited a peak at 1200 nm and a long wavelength limit near 1300 nm. The TPA spectral response of the Si detector exhibited a short wavelength limit near 1170 nm and a linear response up to 1300 nm. The two types of PD were compared with the characteristics of the GaP photodiode. These photoconductive detectors are efficient, compact, and robust sensors and can be used to diagnose the pulse characteristics of ultrafast fiber lasers and light sources near IR wavelengths.

Efficient generation of single photons by quantum dots embedded in bullseye cavities with backside dielectric mirrors.

Single photons are pivotal building blocks for photonic quantum technologies. Semiconductor quantum dots are promising candidates for optimal single photon sources in terms of purity, brightness and indistinguishability. Here we embed quantum dots into bullseye cavities with a backside dielectric mirror to enhance the collection efficiency up to near 90%. Experimentally, we achieve a collection efficiency of 30%. The auto-correlation measurements reveal a multiphoton probability below 0.05±0.005. A moderate Purcell factor of 3.1 is observed. Furthermore, we propose a scheme for laser integration as well as fiber coupling. Our results represent a step forward to the practical plug-and-play single photon sources.

Quantum Dot Transfer from the Organic Phase to Acrylic Monomers for the Controlled Integration of Single-Photon Sources by Photopolymerization

This paper reports on a new strategy for obtaining homogeneous dispersion of grafted quantum dots (QDs) in a photopolymer matrix and their use for the integration of single-photon sources by two-photon polymerization (TPP) with nanoscale precision. The method is based on phase transfer of QDs from organic solvents to an acrylic matrix. The detailed protocol is described, and the corresponding mechanism is investigated and revealed. The phase transfer is done by ligand exchange through the introduction of mono-2-(methacryloyloxy) ethyl succinate (MES) that replaces oleic acid (OA). Infrared (IR) measurements show the replacement of OA on the QD surface by MES after ligand exchange. This allows QDs to move from the hexane phase to the pentaerythritol triacrylate (PETA) phase. The QDs that are homogeneously dispersed in the photopolymer without any clusterization do not show any significant broadening in their photoluminescence spectra even after more than 3 years. The ability of the hybrid photopolymer to create micro- and nanostructures by two-photon polymerization is demonstrated. The homogeneity of emission from 2D and 3D microstructures is confirmed by confocal photoluminescence microscopy. The fabrication and integration of a single-photon source in a spatially controlled manner by TPP is achieved and confirmed by auto-correlation measurements.

Geographic Information System Based Economic And Financial Risk Analysis: The Case Of Europe And Central Asia

The study has two purposes. The first purpose is, using the economic and financial risk factors determined by the International Country Risk Guide (ICRG) rating agency, to reclassify the scores that countries get from these factors with the Jenks Natural Breaks (JNB) classification technique and to compare countries by creating their thematic maps according to this classification. The second purpose of our study is to create economic and financial risk maps of countries by using the Geographical Weighted Regression (GWR) method, which is one of the Geographical Information System (GIS) analysis techniques, based on the economic and financial risk factors determined by the ICRG. Before the GWR analysis for the variables included in the research, Moran Index analysis was performed as the main measurement of spatial autocorrelation. As a result of the Moran analysis, it was determined that there was a statistically significant positive autocorrelation between the countries. In other words, it has been seen that the countries examined have spatial dependence on each other in terms of economic and financial risk. According to the results of the GWR analysis, risk maps of the examined countries were created and more dynamic, more meaningful or more sensitive, more specific and visually easier to understand results were revealed. And according to these results, it has been seen that the GWR technique can also be used in the fields of economy and finance. In addition, the study brought a different interdisciplinary perspective by bringing together the fields of economy, finance and geography.

Ultrafast Graphene-Plasmonic Hybrid Metasurface Saturable Absorber with Low Saturation Fluence.

Exploring emerging materials with enhanced optical nonlinearities at low power levels with ultrafast response and small footprints is of great interest for information processing, communication, sensing, and quantum systems. Recent progress on nonlinear metamaterials and metasurfaces suggests promising solutions to overcome the limitations of nonlinear materials in nature. Here we present a design concept for highly enhanced saturable absorption effect based on subwavelength-thick (<1/5λ0) hybrid graphene-plasmonic metasurface structures in infrared wavelengths. Our theoretical and experimental results demonstrated that, by exciting nonequilibrium carriers inside nanoscale hotspots, one could not only enhance the saturable absorption in graphene, but also reduce the saturation fluence by over 3 orders of magnitude (from ∼1 mJ/cm2 to ∼100 nJ/cm2). Our pump-probe measurement results suggested an ultrashort saturable absorption recovery time (<60 fs), which is ultimately determined by the relaxation dynamics of photoexcited carriers in graphene. We also observed pulse narrowing effects in our devices based on the autocorrelation measurement results. Such design concepts can be tailored via structure engineering to operate in broader wavelength ranges up to mid- and far- infrared spectral regions. These ultrafast low-saturation fluence saturable absorber designs can enable low-threshold, compact, self-starting mode-locked lasers, laser pulse shaping, and high-speed optical information processing.

Femtosecond Autocorrelation of Localized Surface Plasmons.

Plasmon electronic dephasing lifetime is one of the most important characteristics of localized surface plasmons, which is crucial both for understanding the related photophysics and for their applications in photonic and optoelectronic devices. This lifetime is generally shorter than 100 fs and measured using the femtosecond pump-probe technique, which requires femtosecond laser amplifiers delivering pulses with a duration even as short as 10 fs. This implies a large-scale laser system with complicated pulse compression schemes, introducing high-cost and technological challenges. Meanwhile, the strong optical pulse from an amplifier induces more thermal-related effects, disturbing the precise resolution of the pure electronic dephasing lifetime. In this work, we use a simple autocorrelator design and integrate it with the sample of plasmonic nanostructures, where a femtosecond laser oscillator supplies the incident pulses for autocorrelation measurements. Thus, the measured autocorrelation trace carries the optical modulation on the incident pulses. The dephasing lifetime can be thus determined by a comparison between the theoretical fittings to the autocorrelation traces with and without the plasmonic modulation. The measured timescale for the autocorrelation modulation is an indirect determination of the plasmonic dephasing lifetime. This supplies a simple, rapid, and low-cost method for quantitative characterization of the ultrafast optical response of localized surface plasmons.

Full characterization of superradiant pulses generated from a free-electron laser oscillator

The detailed structure of superradiant pulses generated from a free-electron laser (FEL) oscillator was experimentally revealed for the first time. Owing to the phase retrieval with a combination of linear and nonlinear autocorrelation measurements, we successfully reconstructed the temporal waveform of an FEL pulse including its phase variation. The waveform clearly exhibits the features of a superradiant pulse, the main pulse followed by a train of sub-pulses with π-phase jumps, reflecting the physics of light-matter resonant interaction. From numerical simulations, the train of sub-pulses was found to originate from repeated formation and deformation of microbunches accompanied with a temporal slippage of the electrons and light field, a process quite different from coherent many-body Rabi oscillations observed in superradiance from atomic systems.