Streak Camera Measurements Research Articles

Dynamical Behavior of a Stacked Blue Laser Diode Consisting of Two Active Regions with Wide InGaN Quantum Wells and Two Tunnel Junctions

In nitride‐based laser diodes, the poor conductivity of the p‐type region, higher light absorption, and the difficult processing of an ohmic p‐contact with low resistance are major challenges. Recently interband tunnel junctions (TJs) have been used to replace much of the hole transport layers by electron transport layers, and also to allow for two n‐type contacts. In this work, GaN‐based laser diode stacks with TJs are analyzed. The devices are grown with plasma‐assisted molecular beam epitaxy and include two waveguides with active regions consisting of 25 nm wide InGaN quantum well (QW), each, which are connected through a TJ. A second TJ is placed on top to eliminate the p‐type contact. In this study, the time‐dependent spectral behavior of such structures is analyzed by streak camera measurements. The two QWs have different lasing thresholds and emission wavelength. Using the temporal resolution of the streak camera, a lasing onset of the second QW a few nanoseconds after the first one is observed. Single shot measurements show unusual intensity oscillations from both QWs, however, one is stable and the other laser diode (LD) has irregular oscillations.

Approaching an optimum time resolution for synchroscan streak-camera measurements with visible synchrotron light

The optical beam diagnostics at the BESSY II light source in Berlin have been improved significantly over the last few years. In particular, the streak-camera system has been extended in precision and sensitivity to allow two-dimensional imaging in time and space for equilibrated and non-equilibrated bunch patterns. In this paper, we prove experimentally and theoretically that we have reached a sub-ps RMS total time resolution using filtered synchrotron light. Detailed simulations, including the different physical time-dispersion mechanisms, show the influence of various band-pass and edge wavelength filters on the resolution. The limits for unfiltered near-visible synchrotron radiation (white-light) and the band-pass filter to achieve optimal time resolution are derived as well, providing a basis for more advanced beam-dynamics studies in the near future.

Fluorescence quenching in aggregates of fucoxanthin–chlorophyll protein complexes: Interplay of fluorescing and dark states

Diatoms, a major group of algae, account for about a quarter of the global primary production on Earth. These photosynthetic organisms face significant challenges due to light intensity variations in their underwater habitat. To avoid photodamage, they have developed very efficient non-photochemical quenching (NPQ) mechanisms. These mechanisms originate in their light-harvesting antenna – the fucoxanthin–chlorophyll protein (FCP) complexes. Spectroscopic studies of NPQ in vivo are often hindered by strongly overlapping signals from the photosystems and their antennae. Fortunately, in vitro FCP aggregates constitute a useful model system to study fluorescence (FL) quenching in diatoms. In this work, we present streak-camera FL measurements on FCPa and FCPb complexes, isolated from a centric diatom Cyclotella meneghiniana, and their aggregates. We find that spectra of non-aggregated FCP are dominated by a single fluorescing species, but the FL spectra of FCP aggregates additionally contain contributions from a redshifted emissive state. We relate this red state to a charge transfer state between chlorophyll c and chlorophyll a molecules. The FL quenching, on the other hand, is due to an additional dark state that involves incoherent energy transfer to the fucoxanthin carotenoids. Overall, the global picture of energy transfer and quenching in FCP aggregates is very similar to that of major light-harvesting complexes in higher plants (LHCII), but microscopic details between FCPs and LHCIIs differ significantly.

Method for solving bunch head-tail overlapping in HLS-II using a new trigger scanning module of streak-camera measurement system

This study proposed a trigger scanning module for a streak camera to overcome head–tail overlapping of bunch to achieve absolute bunch length monitoring based on the unique machine properties of the Hefei Light Source-II (HLS-II). Furthermore, precise measurement of longitudinal bunch characteristic parameters and study of longitudinal beam dynamics is crucial, and this significantly contributes for observing longitudinal bunch behavior and evolution associated with current potential-well distortion and microwave instability. First, the scanning module is calibrated thoroughly to test its performance and applicability. Afterward, the module is applied to measure the dependence of bunch length on current under single-bunch operation. Moreover, potential-well distortion and bunch lengthening are observed. Results showed that the designed trigger scanning module is appropriate for the exploration of the physical mechanism of single-bunch beam behavior, which is crucial for the investigation of bunch characteristics and beam instability in storage ring light sources.

Method for Solving Bunch Head-Tail Overlapping in Hls-Ii Using New Trigger Scanning Module of the Streak Camera Measurement System

Method for Solving Bunch Head-Tail Overlapping in Hls-Ii Using New Trigger Scanning Module of the Streak Camera Measurement System

Improved temporal contrast of streak camera measurements with periodic shadowing.

Periodic shadowing, a concept used in spectroscopy for stray light reduction, has been implemented to improve the temporal contrast of streak camera imaging. The capabilities of this technique are first proven by imaging elastically scattered picosecond laser pulses and are further applied to fluorescence lifetime imaging, where more accurate descriptions of fluorescence decay curves were observed. This all-optical approach can be adapted to various streak camera imaging systems, resulting in a robust technique to minimize space-charge induced temporal dispersion in streak cameras while maintaining temporal coverage and spatial information.

Measurement of tunable terahertz drive laser pulse train generation using streak camera

A pulse-train with tunable terahertz (THz) repetition rate can be used for driving a photocathode radio-frequency (RF) electron gun to generate electron microbunch train, which is indispensable for exciting intense narrowband THz radiation. In this paper, a train of 8 laser micro-pulses is experimentally obtained and measured by a 3-stage polarization-beam-splitter-based pulse stacker and a streak camera respectively. In addition, we performed measurements of the generation of tunable THz pulse-train with the interpulse spacings of 12.0, 9.8, and 6.3 ps, corresponding to modulated repetition frequencies of 0.089, 0.103, and 0.148 THz, respectively. As a result, preliminary experiments prove the feasibility and applicability of tunable THz laser pulse train generation based on the polarization-beam-splitter-based pulse stacking system and the streak camera measurement system.

Application and Development of the Streak Camera Measurement System at HLS-II

Application and Development of the Streak Camera Measurement System at HLS-II

Analysis of the active medium parameters of molecular nitrogen ions in ambient air.

In this Letter, we report the direct streak camera measurements of the duration of a lasing pulse from molecular nitrogen ions under various focusing conditions of 10 mJ, 950 nm femtosecond pump pulse in atmospheric pressure air. The parameters of the active medium are analyzed, and a mechanism for formation of picosecond lasing pulse duration at femtosecond seed pulse duration is proposed.

Mode rolling effects in nitride laser diodes

Due to their small separation of longitudinal modes, Fabry-Pérot type laser diodes show rich mode competition effects. We present streak camera measurements of two nitride laser diodes with different cavity lengths and model them using a fully dynamic model based on the semiconductor Bloch equations, obtaining good agreement. Both theory and experiment show that the different mode spacing has a large influence on the interactions between longitudinal modes. In contrast to rate-equation type models, our approach includes the detailed density distribution as well as the derivation of the relevant parameters, e.g. broadening, from standard material quantities, thus setting a milestone on the way towards a fully predictive laser model.

Double-propagation mode in short-gap spark discharges driven by HV pulses with sub-ns rise time

The object of this study is the investigation of spark discharges ignited by unipolar positive rectangular high voltage (HV) pulses with 200 ps rise time and (15 ± 2) kV amplitude with 3 ns duration full width at half maximum in synthetic air in a 1.2 mm pin-to-pin gap (tungsten electrodes) at atmospheric pressure. The discharge development was recorded by synchronised iCCD and streak camera measurements in single-shot operation, revealing a two-stage propagation mode. The discharge started with a fast initial breakdown across the entire gap (∼10 mm ns−1) during the HV slope, followed by a much slower (∼0.1 mm ns−1) propagation originating from both electrodes towards the gap centre. The combination of high-resolution diagnostics with numerical modelling indicated that the initial breakdown phase is caused by the rapid increase of electric field strength during the steep HV slope, which leads to the simultaneous fast propagation of a positive and a negative streamer.

Random temporal laser speckles for the robust measurement of sub-microsecond photoluminescence decay.

Time-resolved photoluminescence (PL) is commonly used to track dynamics in a broad range of materials. Thus, the search for simplification of the acquisition of PL kinetics attracts continuous attention. This paper presents a new robust and straightforward approach to the measurement of PL decay, which is based on randomly fluctuating excitation intensity. The random excitation waveform is attained by using laser speckles generated on a rotating diffuser. Owing to this, the presented technique is able to utilize any coherent excitation source without the necessity to generate short pulses or to controllably modulate the light. PL decay can be computationally reconstructed from the Fourier image of the PL trace. The paper demonstrates the performance of the method, which is able to acquire sub-microsecond dynamics as the impulse response function reaches 300 ns. The reconstructed PL decays were compared to streak camera measurements to verify the method. Finally, potential limitations and applications of the technique are discussed.

Trapping and Detrapping in Colloidal Perovskite Nanoplatelets: Elucidation and Prevention of Nonradiative Processes through Chemical Treatment.

Metal-halide perovskite nanocrystals show promise as the future active material in photovoltaics, lighting, and other optoelectronic applications. The appeal of these materials is largely due to the robustness of the optoelectronic properties to structural defects. The photoluminescence quantum yield (PLQY) of most types of perovskite nanocrystals is nevertheless below unity, evidencing the existence of nonradiative charge-carrier decay channels. In this work, we experimentally elucidate the nonradiative pathways in CsPbBr3 nanoplatelets, before and after chemical treatment with PbBr2 that improves the PLQY. A combination of picosecond streak camera and nanosecond time-correlated single-photon counting measurements is used to probe the excited-state dynamics over 6 orders of magnitude in time. We find that up to 40% of the nanoplatelets from a synthesis batch are entirely nonfluorescent and cannot be turned fluorescent through chemical treatment. The other nanoplatelets show fluorescence, but charge-carrier trapping leads to losses that are prevented by chemical treatment. Interestingly, even without chemical treatment, some losses due to trapping are mitigated because trapped carriers spontaneously detrap on nanosecond-to-microsecond timescales. Our analysis shows that multiple nonradiative pathways are active in perovskite nanoplatelets, which are affected differently by chemical treatment with PbBr2. More generally, our work highlights that in-depth studies using a combination of techniques are necessary to understand nonradiative pathways in fluorescent nanocrystals. Such understanding is essential to optimize synthesis and treatment procedures.

Longitudinal mode seeding in modulated InGaN laser diodes

The modulation of InGaN laser diodes is important for applications such as laser projection in cinemas as well as in virtual and augmented reality applications. Here, a modulation frequency in the 100 MHz to 1 GHz range is necessary. On this timescale, pixel crosstalk is a major issue that affects the intensity and spectral properties. We investigate the spectral and temporal dynamics of InGaN laser diodes driven with different pulse patterns. We identify longitudinal mode seeding as a mechanism between interacting pulses which are spaced by several nanoseconds: photons, remaining in the cavity after the trailing edge of the electric pulse, “seed” consequent pulses and promoting their own longitudinal mode. This leads to a changed spectral–temporal mode pattern of the consequent pulse. The long cavity ring-down time of these photons is a consequence of relatively low losses. Further investigations were performed by streak camera measurements of differently biased laser diodes. For a bias below, but close to, the threshold, the optical gain nearly compensates internal and mirror losses. Additional simulations are performed to confirm the experimental results.

Counter-directional polariton coupler

We report on an on-chip routing device for propagating condensates of exciton-polaritons. This counter-directional coupler implements signal control by a photonic microdisk potential, which couples two lithographically defined waveguides and reverses the condensate's propagation direction. By varying the structural sizes, we utilize the conjunction of the different dimensionalities to additionally evidence the functionality of a polaritonic resonant tunnel diode. Furthermore, we investigate the ultra-fast dynamics of the device via ps-resolved streak camera measurements, which is distinctive for the polariton platform. This scalable, all-directional coupler element is a central building block for compact non-linear on-chip photonic architectures.

Is π-Stacking Prone To Accelerate Singlet-Singlet Energy Transfers?

π-Stacking is the most common structural feature that dictates the optical and electronic properties of chromophores in the solid state. Herein, a unidirectional singlet-singlet energy-transfer dyad has been designed to test the effect of π-stacking of zinc(II) porphyrin, [Zn2], as a slipped dimer acceptor using a BODIPY unit, [bod], as the donor, bridged by the linker C6H4C≡CC6H4. The rate of singlet energy transfer, kET(S1), at 298 K ( kET(S1) = 4.5 × 1010 s-1) extracted through the change in fluorescence lifetime, τF, of [bod] in the presence (27.1 ps) and the absence of [Zn2] (4.61 ns) from Streak camera measurements, and the rise time of the acceptor signal in femtosecond transient absorption spectra (22.0 ps), is faster than most literature cases where no π-stacking effect exists (i.e., monoporphyrin units). At 77 K, the τF of [bod] increases to 45.3 ps, indicating that kET(S1) decreases by 2-fold (2.2 × 1010 s-1), a value similar to most values reported in the literature, thus suggesting that the higher value at 298 K is thermally promoted at a higher temperature.

Wavelength‐Stabilized High‐Pulse‐Power Laser Diodes for Automotive LiDAR

For automotive light detection and ranging (LiDAR) systems diode lasers emitting short optical pulses with a good beam quality and a low wavelength shift over a wide operating temperature range is needed. In this paper, theoretical and experimental results of broad area laser diodes specially designed for this application are presented. Optical pulse powers of tens of watts and pulse lengths in the 10 ns‐range for wavelengths near 905 nm and environmental temperatures between 15 and 85 °C are achieved. Due to the integrated Bragg grating the wavelength shift with temperature is as low as 65 pm K−1. The contact mesa with a width of 50 μm leads to a lateral M2 value of about 9. Streak camera measurements of the temporal evolution of the lateral near field intensity reveal that the highest intensity is emitted near the mesa edges. Simulation results of power current characteristics indicate that the power saturation experimentally observed can be attributed to nonlinear effects such as two‐photon‐absorption and gain compression.

Calculation of CsI photocathode spectral response in 10-100 keV X-ray energy region

CsI photocathode is widely applied to high energy X-ray detection. And the spectral response is an important character of CsI photocathode. In this paper, the interaction process of high energy X-ray with CsI is analyzed and the spectral response of CsI photocathode is calculated in a 10-100 keV range. The influences of Compton scattering, X-ray fluorescence radiation and Auger emission on the spectral response are analyzed in accordance with the physical process of high energy X-ray interaction with CsI photocathode. These influences prove to be negligible in comparison with photo-ionization influence. Thus only the photoelectric transition is taken into account in calculation. According to the analyses of the processes of the photoelectron creation, transition and escaping, the formula for CsI spectral response is deduced as a function of secondary electron mean escape depth and photocathode thickness. The formula of secondary electron mean escape depth is then deduced as a function of X-ray energy. These formulae indicate that the mean escape depth of the secondary electrons increases markedly with the rise of X-ray energy and has a remarkable influence on the CsI spectral response. The spectral responses for different CsI thickness values are then calculated in a range of 10-100 keV. The results show that 1000 nm CsI has the best response under 20 keV, while 10000 nm CsI has a higher response over 60 keV. Then the calculation data are compared with experimental data of Hara's and Khan's hard X-ray streak camera measurements. These data agree well with each other and prove that our calculation of CsI spectral response for high energy X-ray is reliable. The spectral responses to CsI thickness for 17.5 keV and 60 keV are also calculated and shown in figures. These calculation data match experimental data of Frumkin and Monte-Carlo simulation data of Gibrekhterman. The measurement error of Frumkin's experiment and the uncertainty of the secondary electron mean escape depth are considered to be the reasons for the deviations of calculation and experimental data. The figures of spectral responses to CsI thickness also reveal the optimal thickness values of CsI for different X-ray photon energies. It is shown that 1 m is the optimal thickness for 17.5 keV X-ray detection, and 10 m is optimal for 60 keV. Finally the spectral response of CsI photocathode in a 10-100 keV range is calculated and the formulae prove to be reliable. According to these formulae and calculations, the optimal thickness of CsI photocathode can thus be given for designing and optimizing the high energy X-ray imaging detectors.

Measurement and simulation of the temperature evolution of a short pulse laser heated buried layer target

Short pulse laser heated buried layers experiments have been performed on the Orion laser facility to study the time-resolved emission characteristics of plasmas with mass densities ≥ 1 g/cc and electron temperatures > 500 eV. Our streak camera measurements focused on the K-shell emission lines of He-like and H-like aluminum from a buried aluminum layer. The data were analyzed by comparison to synthetic spectra generated with the non-local thermodynamic equilibrium (NLTE) radiation transfer code Cretin, which yielded maximum temperatures of nearly 800 eV at near solid density. The time precise history of the temperature evolution was reproduced with a 1-D radiation hydrodynamic code; however, the known effect of lateral transport of energy out of the focal spot made exacting agree with theory difficult. Thus, we have observed densities of ≥ 1 g/cc and temperatures of > 500 eV using the 1-D analysis, which supports the idea that the aluminum plasma is locally hotter than inferred from our spatially integrating measurements and that modeling requires the inclusion of 2-dimensional effects.

Ultrafast fluorescent decay induced by metal-mediated dipole–dipole interaction in two-dimensional molecular aggregates

Two-dimensional molecular aggregate (2DMA), a thin sheet of strongly interacting dipole molecules self-assembled at close distance on an ordered lattice, is a fascinating fluorescent material. It is distinctively different from the conventional (single or colloidal) dye molecules and quantum dots. In this paper, we verify that when a 2DMA is placed at a nanometric distance from a metallic substrate, the strong and coherent interaction between the dipoles inside the 2DMA dominates its fluorescent decay at a picosecond timescale. Our streak-camera lifetime measurement and interacting lattice-dipole calculation reveal that the metal-mediated dipole-dipole interaction shortens the fluorescent lifetime to about one-half and increases the energy dissipation rate by 10 times that expected from the noninteracting single-dipole picture. Our finding can enrich our understanding of nanoscale energy transfer in molecular excitonic systems and may designate a unique direction for developing fast and efficient optoelectronic devices.