Coherent Light Source Research Articles

Macromolecular movies, storybooks written by nature.

Twelve years ago, the first free electron laser for hard X-rays (XFEL), the Linac Coherent Light Source (LCLS), came online. Due to the extreme brilliance and the ultrashort pulse duration of their X-ray radiation, XFELs are exceptionally well positioned to conduct time-resolved studies on biological macromolecules. Here, some of the pioneering experiments and recent results are summarized.

Optically Transportable Optofluidic Microlasers with Liquid Crystal Cavities Tuned by the Electric Field.

Liquid crystal microdroplets with readily adjustable optical properties have attracted considerable attention for building reconfigurable optofluidic microsystems for sensing, imaging, and light routing applications. In this quest, development of active optical microcavities serving as versatile integrated sources of coherent light and ultra-sensitive environmental sensors has played a prominent role. Here, we study transportable optofluidic microlasers reversibly tunable by an external electric field, which are based on fluorophore-doped emulsion droplets of radial nematic liquid crystals manipulated by optical tweezers in microfluidic chips with embedded liquid electrodes. Full transparency of the electrodes formed by a concentrated electrolyte solution allows for applying an electric field to the optically trapped droplets without undesired heating caused by light absorption. Taking advantage of independent, precise control over the electric and thermal stimulation of the lasing liquid crystal droplets, we characterize their spectral tuning response at various optical trapping powers and study their relaxation upon a sudden decrease in the trapping power. Finally, we demonstrate that sufficiently strong applied electric fields can induce fully reversible phase transitions in the trapped droplets even below the bulk melting temperature of the used liquid crystal. Our observations indicate viability of creating electrically tunable, optically transported microlasers that can be prepared on-demand and operated within microfluidic chips to implement integrated microphotonic or sensing systems.

Deep learning colorful ptychographic iterative engine lens-less diffraction microscopy

The ptychographic iterative engine (PIE) is an algorithm for coherent lens-less diffraction imaging. It facilitates the development of low-cost large field of view (FOV) lens-less microscopy, as it can expand the FOV extensively by moving the light source or the bio-medical pathological sample slide. In a PIE setup, the illumination light needs to have high/partially coherence. Thus, to get a colorful microscopy image, more than 3 illumination light sources with different dominant wavelengths are required in a traditional PIE setup. In this manuscript, an improved PIE based on computational deep learning color-transferring method is proposed to achieve colorful large FOV lens-less microscopy imaging. In our method, only one high/partially coherent light source is used, where the image data are three times less than those images under multiple illuminations for colorful PIE microscopy. It is believable that our colorful PIE microscopy enhanced by the deep color-transferring method would be helpful for the development of low-cost large FOV lens-less microscopes.

Experimental demonstration of superresolution of partially coherent light sources using parity sorting: erratum.

The authors include references that appeared on arXiv during the preparation of their paper [Opt. Express29, 22034 (2021)10.1364/OE.427734].

Multimode Robust Lasing in a Dye-Doped Polymer Layer Embedded in a Wedge-Shaped Cholesteric.

Cholesteric liquid crystals (CLCs) with induced defects are one of the most prominent materials to realize compact, low-threshold and tunable coherent light sources. In this context, the investigation of optical properties of induced defect modes in such CLCs is of great interest. In particular, many studies have been devoted to the spectral control of the defect modes depending on their thickness, optical properties, distribution along the CLC, etc. In this paper, we investigate the lasing possibilities of a dye-doped polymer layer embedded in a wedge-shaped CLC. We show that multimode laser generation is possible due to the observed multiple defect modes in the PBG that enlarges the application range of the system. Furthermore, our simulations based on a Berreman 4 × 4 matrix approach for a wide range of CLC thickness show both periodic and continuous generation of defect modes along particular spectral lines inside the PBG. Such a robust spectral behaviour of induced defect modes is unique, and, to our knowledge, is not observed in similar CLC-based structures.

Random lasing at localization induced in correlated colloidal system

Random lasers have the potential for cheap and coherent light sources which, in the particular case of colloidal suspensions, are completely flexible and can take on any desired shape. Here, we studied random lasing in correlated colloidal systems composed by TiO2@Silica nanoparticles suspended in ethanol solutions of Rhodamine 6G. TiO2 particles with two different silica layers (thicknesses of 40 nm and 70 nm) were prepared. The Random laser performance improves when the silica shell is thicker (70 nm), which was attributed to a stronger localization of light (higher density of localized states) induced by stronger correlation in the scatterers' (TiO2@Silica) position as a consequence of a stronger and longer-range Coulomb interaction between the scatterers. Light diffraction patterns in both TiO2@Silica suspensions showed a stronger correlation in the scatterers’ position, being stronger when the silica shell is thicker.

Improving surrogate model accuracy for the LCLS-II injector frontend using convolutional neural networks and transfer learning

Machine learning (ML) models of accelerator systems (‘surrogate models’) are able to provide fast, accurate predictions of accelerator physics phenomena. However, approaches to date typically do not include measured input diagnostics, such as the initial beam distributions, which are critical for accurately representing the beam evolution through the system. In addition, these inputs often vary over time, and models that can account for these changing conditions are needed. As beam time for measurements is often limited, simulations are in some cases needed to provide sufficient training data. These typically represent the designed machine before construction; however, the behavior of the installed components may be quite different due to changes over time or static differences that were not modeled. Therefore, surrogate models that can leverage both simulation and measured data successfully are needed. We introduce an approach based on convolutional neural networks that uses the drive laser distribution and scalar settings as inputs for a photoinjector system model (here, the linac coherent light source II, LCLS-II, injector frontend). The model is able to predict scalar beam parameters and the transverse beam distribution downstream, taking into account the impact of time-varying non-uniformities in the initial transverse laser distribution. We also introduce and evaluate a transfer learning procedure for adapting the surrogate model from the simulation domain to the measurement domain, to account for differences between the two. Applying this approach to our test case results in a model that can predict test sample outputs within a mean absolute percent error of 9%. This is a substantial improvement over the model trained only on simulations, which has an error of 261% when applied to measured data. While we focus on the LCLS-II Injector frontend, these approaches for improving ML-based online modeling of injector systems could be easily adapted to other accelerator facilities.

Absolute contrast estimation for soft X-ray photon fluctuation spectroscopy using a variational droplet model

X-ray photon fluctuation spectroscopy using a two-pulse mode at the Linac Coherent Light Source has great potential for the study of quantum fluctuations in materials as it allows for exploration of low-energy physics. However, the complexity of the data analysis and interpretation still prevent recovering real-time results during an experiment, and can even complicate post-analysis processes. This is particularly true for high-spatial resolution applications using CCDs with small pixels, which can decrease the photon mapping accuracy resulting from the large electron cloud generation at the detector. Droplet algorithms endeavor to restore accurate photon maps, but the results can be altered by their hyper-parameters. We present numerical modeling tools through extensive simulations that mimic previous x-ray photon fluctuation spectroscopy experiments. By modification of a fast droplet algorithm, our results demonstrate how to optimize the precise parameters that lift the intrinsic counting degeneracy impeding accuracy in extracting the speckle contrast. These results allow for an absolute determination of the summed contrast from multi-pulse x-ray speckle diffraction, the modus operandi by which the correlation time for spontaneous fluctuations can be measured.

Using ASE sources in remote beamforming system with Space-Division-Multiplex fiber

Weakly-coupled Space-Division-Multiplex (SDM) fiber is a promising candidate transmission medium for the remote true-time-delay (TTD) beamforming system. However, the signal-to-crosstalk beating interference (SCBI) can result in severe phase difference (or time delay) variation between TTD channels, which deteriorates the beam direction stability. Here, we develop a model to illustrate the phase variation problem when employing the coherent light source. To obtain a stable beam direction, we propose using an ASE source instead of a coherent source as the optical carrier. When utilizing the ASE source, the signal-to-noise ratio (SNR) of the remote RF signals is also thoroughly derived. It indicates that the RF signal with high SNR can be achieved via increasing the ASE source bandwidth and pre-compensating the fiber link dispersion. We carry out a proof-of-concept experiment of the remote beamforming system with the ASE source at the end. The variation of the phase difference between different channels is less than 3°. The SNR of the RF signal is over 110 dB with a 1 Hz bandwidth. These results pave the way for exploring the SDM fiber in the remote beamforming system.

Compact common-path polarisation holographic microscope for measuring spatially-resolved Jones matrix parameters of dynamic samples in microfluidics

In this study, we propose a compact four-channel angular-multiplexing polarisation holographic microscope to quantify the spatially resolved Jones matrix of a dynamic sample in a microfluidic channel. To make the imaging system compact, a common-path off-axis configuration based on a simple object and a reference beam generator unit, consisting of a Wollaston prism and a converging lens, was introduced in the holographic microscope. Using this system and the retrieval method described in this paper, the spatially resolved Jones matrix parameters, as well as the polarisation information, including the diattenuation and phase retardation, of the target sample could be simultaneously retrieved from a single-shot hologram. We experimentally demonstrated the feasibility of our method by quantitatively measuring the birefringence parameters of a quarter-wave plate and extracting the 2-D Jones matrix and polarisation parameters of potato starch particles and C. elegans worms. The simple optical structure and common-path design reduce the system requirements for light source coherence and system stability, which provides a feasible approach for the development of a portable polarisation holographic microscope for microfluidics.

Estimating the Photon-Number Distribution of Photonic Channels for Realistic Devices and Applications in Photonic Quantum Information Processing

Characterising the input-output photon-number distribution of an unknown optical quantum channel is an important task for many applications in quantum information processing. Ideally, this would require deterministic photon-number sources and photon-number-resolving detectors, but these technologies are still work-in-progress. In this work, we propose a general method to rigorously bound the input-output photon number distribution of an unknown optical channel using standard optical devices such as coherent light sources and non-photon-number-resolving detectors/homodyne detectors. To demonstrate the broad utility of our method, we consider the security analysis of practical quantum key distribution systems based on calibrated single-photon detectors and an experimental proposal to implement time-correlated single photon counting technology using homodyne detectors instead of single-photon detectors.

Polarization measurements of deep- to extreme-ultraviolet high harmonics generated in liquid flat sheets.

Laboratory-based coherent light sources enable a wide range of applications to investigate dynamical processes in matter. High-harmonic generation (HHG) from liquid samples is a recently discovered coherent source of extreme-ultraviolet (XUV) radiation potentially capable of achieving few-femtosecond to attosecond pulse durations. However, the polarization state of this light source has so far remained unknown. In this work, we characterize the degree of polarization of both low- and high-order harmonics generated from liquid samples using linearly polarized 400 nm and 800 nm drivers. We find a remarkably high degree of linear polarization of harmonics ranging all the way from the deep-ultraviolet (160 nm) across the vacuum-ultraviolet into the XUV domain (73 nm). These results establish high-harmonic generation in liquids as a promising alternative to conventional sources of XUV radiation, combining the benefits of high target densities comparable to solids with a continuous sample renewal that avoids the limitations imposed by laser-induced damage.

Performance manipulation of the squeezed coherent light source based on four-wave mixing.

We present performance manipulation of the squeezed coherent light source based on four-wave mixing (FWM) in alkaline-earth atoms. We investigate the dynamic response of the system and the spectroscopic feature of lasing generated by resonantly enhanced wave-mixing in coherently prepared system. In this method, the spectral purity and stability of the wave-mixing lasing can be manipulated at will by choosing optimal laser parameters. We also analyze the effect of Langevin noise fluctuations on the system and the relative-intensity noise spectrum of the wave-mixing lasing is well below the standard quantum limit (down to -4.7 dB). This work opens new possibilities for alternative routes to laser stabilization and provides a promising path to realize precision metrology.

Dips in high-order harmonics spectra from a subcycle-driven two-level system reflected in the negativity structure of the time-frequency Wigner function

We investigate high-order harmonics spectra radiated from a two-level model system driven by strong, ultrabroadband, half- and single-cycle pulses, which are shorter than the inverse of the transition frequency. In this driving regime, the plateau in frequency spectra typical for radiation from strongly driven systems, has noticeable modulation in amplitude due to interference between waves of same frequency and emitted at different time instants. Specifically, there are characteristic ‘dip’ structures at a set of frequencies in the radiation spectra, where the corresponding amplitudes are suppressed by several orders of magnitude. Understanding these structures is required for applications such as the generation of attosecond pulse, where the number of composing modes and their relative phases are important. Therefore, we demonstrate a systematic way to find frequencies at which the dips are formed. To further illustrate the interference mechanism, we extract the phase information with the help of time-frequency distribution functions, namely the Husimi and Wigner functions. In particular, we found that the negativity structure of the Wigner function corresponds to each dip frequency and that the information regarding the type of interference is encoded in the pattern of the negative region of the Wigner function. Since such time-frequency Wigner function can actually be measured, we envisage utilizing its negativity structure to extract the phase information between radiation components emitted at time points within a subcycle timescale. This should provide an efficient tool for understanding and designing photonic applications, including short-wavelength coherent light sources.

Electric-field-driven excitonic instability in an organometallic manganese-cyclopentadienyl wire

Excitonic insulator is a macroscopic quantum system whose ground state consists of spontaneously formed and condensed excitons. While various excitonic insulators have been explored, they are almost exclusively characterized by the condensation of dark (optically inactive) excitons. Our first-principles $GW$-BSE calculations show that a transverse electric field will drive an organometallic manganese-cyclopentadienyl wire into a unique ``bright'' excitonic insulator phase, as the combined consequence of giant Stark effect and electric-field-induced symmetry breaking. Such bright excitonic insulators can be directly identified by experiment due to their coherent radiation. Our results are not only of scientific interest in electric-field control of exciton condensation, but may also open an opportunity to create a coherent light source based on the excitonic insulator.

Slit homogenizer introduced performance gain analysis based on the Sentinel-5/UVNS spectrometer

Abstract. Spatially heterogeneous Earth radiance scenes affect the atmospheric composition measurements of high-resolution Earth observation spectrometer missions. The scene heterogeneity creates a pseudo-random deformation of the instrument spectral response function (ISRF). The ISRF is the direct link between the forward radiative transfer model, used to retrieve the atmospheric state, and the spectra measured by the instrument. Hence, distortions of the ISRF owing to radiometric inhomogeneity of the imaged Earth scene will degrade the precision of the Level-2 retrievals. Therefore, the spectral requirements of an instrument are often parameterized in the knowledge of the ISRF over non-uniform scenes in terms of shape, centroid position of the spectral channel and the full width at half maximum (FWHM). The Sentinel-5/UVNS instrument is the first push-broom spectrometer that makes use of a concept referred to as a slit homogenizer (SH) for the mitigation of spatially non-uniform scenes. This is done by employing a spectrometer slit formed by two parallel mirrors scrambling the scene in the along track direction (ALT) and hence averaging the scene contrast only in the spectral direction. The flat mirrors do not affect imaging in the across track direction (ACT) and thus preserve the spatial information in that direction. The multiple reflections inside the SH act as coherent virtual light sources and the resulting interference pattern at the SH exit plane can be described by simulations using scalar diffraction theory. By homogenizing the slit illumination, the SH strongly modifies the spectrograph pupil illumination as a function of the input scene. In this work we investigate the impact and strength of the variations of the spectrograph pupil illumination for different scene cases and quantify the impact on the ISRF stability for different types of aberration present in the spectrograph optics.

Medium-temperature furnace baking of 1.3 GHz 9-cell superconducting cavities at IHEP

Recently, heat treatment in the range of 250 °C–500 °C has been attempted with the aim of improving the quality factor (Q) of superconducting radio-frequency (SRF) cavities worldwide. Herein, medium-temperature (mid-T) furnace baking experiments were conducted at the Institute of High Energy Physics to obtain higher Q values. First, 1.3 GHz 1-cell cavities received mid-T furnace baking at different temperatures, resulting in not only the improvement of Q but also an anti-Q-slope behavior. Furthermore, mid-T furnace baking was applied to six 1.3 GHz 9-cell cavities, all of which indicated higher Q and anti-Q-slope behavior. The average Q was 3.8 × 1010 at 16 MV m−1. The maximum gradients of the 9-cell cavities were 22.7–26.5 MV m−1. Finally, the mid-T furnace baked 1.3 GHz 9-cell cavities were welded with helium vessel, with no degradation in the vertical tests. In addition, the mid-T furnace baking process was simplified in comparison with European X-ray Free Electron Laser and Linear Coherent Light Source because of the cancellation of light electro-polishing. This simplification is beneficial in the mass production of SRF cavities with high Q, which are widely adopted in free electron lasers and high-energy colliders.

Predictive searching method for fourier ptychography with weighted apodized coherent transfer function constraint

Fourier ptychographic microscopy (FPM) is a recently developed computational imaging technology with wide field-of-view (FOV) and high resolution. However, previous studies thought that the conventional FPM algorithm approximated the LED as a coherent point light source, so FPM imaging is regarded as a coherent imaging process, and the coherent transfer function(CTF)was constrained in the Fourier domain, which seemed to be problematic. At the same time, the conventional FPM data requires the acquisition of multiple data, which takes a long time and limits its real-time application. To investigate these two problems, this paper proposes the discrete cosine (DCT) coefficient Predictive searching method for FPM with Bessel function weighted apodized coherent transfer function (B-AC) constraint. The experimental results demonstrate that without adding any algorithmic complexity and computational burden, the proposed method can reduce the number of acquired images by about 60%, while retaining the accuracy of the reconstructed results.

High-throughput spatial sensitive quantitative phase microscopy using low spatial and high temporal coherent illumination

High space-bandwidth product with high spatial phase sensitivity is indispensable for a single-shot quantitative phase microscopy (QPM) system. It opens avenue for widespread applications of QPM in the field of biomedical imaging. Temporally low coherence light sources are implemented to achieve high spatial phase sensitivity in QPM at the cost of either reduced temporal resolution or smaller field of view (FOV). In addition, such light sources have low photon degeneracy. On the contrary, high temporal coherence light sources like lasers are capable of exploiting the full FOV of the QPM systems at the expense of less spatial phase sensitivity. In the present work, we demonstrated that use of narrowband partially spatially coherent light source also called pseudo-thermal light source (PTLS) in QPM overcomes the limitations of conventional light sources. The performance of PTLS is compared with conventional light sources in terms of space bandwidth product, phase sensitivity and optical imaging quality. The capabilities of PTLS are demonstrated on both amplitude (USAF resolution chart) and phase (thin optical waveguide, height ~ 8 nm) objects. The spatial phase sensitivity of QPM using PTLS is measured to be equivalent to that for white light source and supports the FOV (18 times more) equivalent to that of laser light source. The high-speed capabilities of PTLS based QPM is demonstrated by imaging live sperm cells that is limited by the camera speed and large FOV is demonstrated by imaging histopathology human placenta tissue samples. Minimal invasive, high-throughput, spatially sensitive and single-shot QPM based on PTLS will enable wider penetration of QPM in life sciences and clinical applications.

High-energy upgrade of the Linac Coherent Light Source

High-energy upgrade of the Linac Coherent Light Source