Femtosecond Streak Camera Research Articles

The Effect of Photoconductive Semiconductor Materials in Improving the Resolution of Femtosecond Streak Camera

In this study, through the research on the effect of photoconductive semiconductor materials in improving the resolution of femtosecond streak camera, the effective value of applying photoconductive semiconductor materials to femtosecond streak camera is discussed and analyzed, and the effectiveness of photoconductive semiconductor materials in improving the resolution of femtosecond streak camera is verified. Method: In this study, first of all, the generation and recombination of carriers in semiconductors are studied. Solving the continuity equation is applied to further explore the law of the motion of carriers. Later, photoconductive semiconductor materials are used in the preparation of switching devices and scanning circuits. After that, the prepared circuit module is tested by dark current. The main purpose is to realize the resistance of the circuit switch in the intrinsic state and the dark state withstand voltage ability of the circuit switch. Results: It is found that the dark state withstand voltage of each switch can exceed 6000V. In addition, it is found that the current value of the switch under different light is different. When in the natural light, the current value is large. When it is in the state of shading, its current value is relatively small. The efficiency of all three switches is over 50%. The efficiency of nonlinear model is obviously higher than that of linear model, which can be more than 90%. By applying the semiconductor material to the femtosecond streak camera, the scanning circuit can be optimized effectively to meet the high standard requirements of the femtosecond streak camera for the scanning circuit, which has certain advantages, and has certain value and significance for improving the resolution of the second streak camera.

Positive and Negative Symmetric Pulses with Fast Rising Edge Generated from a GaAs Photoconductive Semiconductor Switch

In this paper, the positive and negative symmetric pulses with a fast rising edge were generated by a GaAs photoconductive semiconductor switch (PCSS). When the GaAs PCSS was biased at 2.0 kV and triggered by a femtosecond laser pulse with a pulse energy of 97.5 J, the peak voltages of the positive and negative pulses were 1.313 kV and 1.329 kV, respectively, and the rise times were 174 ps and 164 ps, respectively. Moreover, the GaAs PCSS presents good stability. The experimental results show that GaAs PCSSs can meet the requirement of a femtosecond streak camera.

Effect of capacitance on positive and negative symmetric pulse with fast rising edge based on GaAsphotoconductive semiconductor switch

Femtosecond streak camera is currently the only diagnostic device with a femtosecond time resolution. Scanning circuit with bilateral symmetrical output is an important part of femtosecond streak camera. To achieve better performance of the streak camera, high requirements are placed on the output of scanning circuit. Owing to the excellent feature of litter time jitter and fast response speed, a GaAs photoconductive semiconductor switch (PCSS) has become a core device in the scanning circuit. Investigating the positive and negative symmetric pulses with fast rising edgeof GaAs PCSS is of great significance to improving the time resolution of femtosecond streak camera. In this paper, a laser with a pulse width of 60 fs was used to trigger a GaAs PCSS with an electrode gap of 3.5 mm. Under different storage capacitors and bias voltages, the positive and negative symmetric pulses withthe fastest rise time of 149 ps and the highest voltage transmission efficiency of 92.9% were obtained. The test results meet the design requirements of streak camera to realize femtosecond time resolution. Through the comparative analysis of the experimental values, it is concluded that the storage capacitor can affect the efficiency and rise time of the output electrical pulse in the same trigger laser pulse. By calculating the multiplication rate of carriers in combination with the output electrical pulse waveform, it is concluded that the GaAs PCSS operates in linear mode. According to the working characteristics of the linear mode and the energy storage characteristics of the capacitor, the analysis indicates that, when the characteristics of the trigger laser pulse are the same, the transmission efficiency and rise time of the output electric pulse voltage increase with the increase in storage capacitor, which is consistent with the experimental results. This study has a certain guiding significance for the better application of GaAs PCSS in femtosecond streak camera, which also has a certain propelling effect on improving the time resolution of femtosecond streak camera.

Implementation and modeling of a femtosecond laser-activated streak camera.

A laser-activated streak camera was built to measure the duration of femtosecond electron pulses. The streak velocity of the device is 1.89 mrad/ps, which corresponds to a sensitivity of 34.9 fs/pixels. The streak camera also measures changes in the relative time of arrival between the laser and electron pulses with a resolution of 70 fs RMS. A full circuit analysis of the structure is presented to describe the streaking field and the general behavior of the device. We have developed a general mathematical model to analyze the streaked images. The model provides an accurate method to extract the pulse duration based on the changes of the electron beam profile when the streaking field is applied.

Traveling wave deflector design for femtosecond streak camera

In this paper, a traveling wave deflection deflector (TWD) with a slow-wave property induced by a microstrip transmission line is proposed for femtosecond streak cameras. The pass width and dispersion properties were simulated. In addition, the dynamic temporal resolution of the femtosecond camera was simulated by CST software. The results showed that with the proposed TWD a femtosecond streak camera can achieve a dynamic temporal resolution of less than 600 fs. Experiments were done to test the femtosecond streak camera, and an 800 fs dynamic temporal resolution was obtained. Guidance is provided for optimizing a femtosecond streak camera to obtain higher temporal resolution.

一种新型加速结构飞秒条纹相机的设计

一种新型加速结构飞秒条纹相机的设计

Low-budget transient imaging using photonic mixer devices

Transient imaging is an exciting a new imaging modality that can be used to understand light propagation in complex environments, and to capture and analyze scene properties such as the shape of hidden objects or the reflectance properties of surfaces. Unfortunately, research in transient imaging has so far been hindered by the high cost of the required instrumentation, as well as the fragility and difficulty to operate and calibrate devices such as femtosecond lasers and streak cameras. In this paper, we explore the use of photonic mixer devices (PMD), commonly used in inexpensive time-of-flight cameras, as alternative instrumentation for transient imaging. We obtain a sequence of differently modulated images with a PMD sensor, impose a model for local light/object interaction, and use an optimization procedure to infer transient images given the measurements and model. The resulting method produces transient images at a cost several orders of magnitude below existing methods, while simultaneously simplifying and speeding up the capture process.

Single-shot terahertz-field-driven X-ray streak camera

A few-femtosecond X-ray streak camera has been realized using a pump–probe scheme that samples the transient response of matter to ionizing soft X-ray radiation in the presence of an intense synchronized terahertz field. Borrowing its concept from attosecond metrology, the femtosecond X-ray streak camera fills the gap between conventional streak cameras with typical resolutions of hundreds of femtoseconds and streaking techniques operating in the sub-femtosecond regime. Its single-shot capability permits the duration and time structure of individual X-ray pulses to be determined. For several classes of experiments in time-resolved spectroscopy, diffraction or imaging envisaged with novel accelerator- and laser-based short-pulse X-ray sources this knowledge is essential, but represents a major challenge to X-ray metrology. Here we report on the single-shot characterization of soft X-ray pulses from the free-electron laser facility FLASH. A streak camera for characterizing the ultrashort X-ray pulses produced by a free-electron laser is reported. The scheme has a single-shot capability, a resolution of a few femtoseconds and is expected to become a useful tool for X-ray metrology, including experiments involving time-resolved spectroscopy and imaging.

Application of accelerators for the research and development of scintillators

We introduce experimental systems which use accelerators to evaluate scintillation properties such as scintillation intensity, wavelength, and lifetime. A single crystal of good optical quality is often unavailable during early stages in the research and development (R&D) of new scintillator materials. Because of their beams' high excitation power and/or low penetration depth, accelerators facilitate estimation of the properties of early samples which may only be available as powders, thin films, and very small crystals. We constructed a scintillation spectrum measurement system that uses a Van de Graaff accelerator and an optical multichannel analyzer to estimate the relative scintillation intensity. In addition, we constructed a scintillation time profile measurement system that uses an electron linear accelerator and a femtosecond streak camera or a microchannel plate photomultiplier tube followed by a digital oscilloscope to determine the scintillation lifetimes. The time resolution is approximately 10 ps. The scintillation spectra or time profiles can be obtained in a significantly shorter acquisition time in comparison with that required by conventional measuring systems. The advantages of the systems described in this study can significantly promote the R&D of novel scintillator materials.

Femtosecond single electron bunch generation by rotating longitudinal bunch phase space in magnetic field

A femtosecond (fs) electron bunching was observed in a photoinjector with a magnetic compressor by rotating the bunch in longitudinal phase space. The bunch length was obtained by measuring Cherenkov radiation of the electron beam with a femtosecond streak camera technique. A single electron bunch with rms bunch length of 98 fs was observed for a 32 MeV electron beam at a charge of 0.17 nC. The relative energy spread and the normalized transverse emittance of the electron beam were 0.2% and 3.8 mm-mrad, respectively. The effect of space charge on the bunch compression was investigated experimentally for charges from 0.17 to 1.25 nC. The dependences of the relative energy spread and the normalized beam transverse emittance on the bunch charge were measured.

Experimental Verification of Velocity Bunching via Shot-by-Shot Measurement at S-Band Photoinjector and Linac

We present an experimental verification of a bunch compression method named “velocity bunching”. Velocity bunching based on the rectilinear compression uses a traveling wave accelerating tube as a compressor. The experiment was performed using an S-band photoinjector and a linac at the Nuclear Engineering Research Laboratory, University of Tokyo. The shot-by-shot bunch length was measured to be 0.5±0.1 ps (rms) on average using a femtosecond streak camera at a bunch charge of 1 nC. The experimental result is in good agreement with the PARMELA simulation result.

Measurement and Numerical Analysis of Ultrashort Electron Bunch Using Fluctuation in Incoherent Cherenkov Radiation

Measurement of ultrashort electron bunches using the fluctuation of incoherent radiation has been pursued at Nuclear Engineering Research Laboratory (NERL), University of Tokyo. The fluctuations of Cherenkov radiation have been measured for 20- and 35-MeV electron bunches and the bunch lengths were obtained from one-dimensional evaluation as 4.5- and 30-ps FWHM respectively, while the femtosecond streak camera showed 1.0- and 1.5-ps FWHM for the respective bunches. The discrepancy comes from the finite transverse emittance. The dependence of the transverse beam emittance upon the fluctuation of Cherenkov radiation is investigated by three-dimensional numerical analysis and the new formula is deduced where the fluctuation of time-integrated intensity of Cherenkov radiation is inversely proportional to the transverse beam emittance. The model for quantitative analysis of the longitudinal electron bunch length including the effects of transverse coherent modes is presented. The possibility of applying the method to laser accelerated bunches is discussed.

Diagnostics of ultrashort electron bunch by coherent transition/diffraction radiation

Measurements of longitudinal bunch length of picosecond electron bunches have been performed by the femtosecond streak camera, the interferometer and the polychromator and their results were compared with one another. As regards the radiation, transition radiation and diffraction radiation were investigated through the diagnostics. As a result, the fine agreements between them within 200 fs were obtained and the reliability of the interferometer and the polychromator for the diagnostics of less than picosecond electron bunch were verified. Furthermore, nondestructive diagnostics for subpicosecond bunches was performed utilizing coherent diffraction radiation (CDR) interferometry.

Overall comparison of subpicosecond electron beam diagnostics by the polychromator, the interferometer and the femtosecond streak camera

Measurements of longitudinal bunch length of subpicosecond and picosecond electron beams have been performed by three methods with three radiation sources at the 35 MeV S-band twin liner accelerators at Nuclear Engineering Research Laboratory, University of Tokyo. The methods we adopt are the femtosecond streak camera with a nondispersive reflective optics, the coherent transition radiation (CTR) Michelson interferometer and the 10 ch polychromator that detects the spectrum of CTR and coherent diffraction radiation (CDR). The measurements by the two CTR methods were independently done with the streak camera and their results were consistent with one another. As a result, the reliability of the polychromator for the diagnostics of less than picosecond electron bunch and the usefulness of the diagnostics for the single shot measurement were verified. Furthermore, perfect nondestructive diagnostics for subpicosecond bunches was performed utilizing CDR interferometry. Then the good agreement between CDR interferometry and the streak camera was obtained.

フェムト秒電子シングルバンチの生成・計測・利用

A new S-band laser photocathode RF gun and chicane-type magnetic pulse compressor were installed in the twin linac system of Nuclear Engineering Research Laboratory, University of Tokyo. Low emittance (6πmm. mrad) femtosecond (240-290fs at FWHM) electron single bunch of 16MeV, 2% energy spread and 350pC/bunch was obtained. Femtosecond streak camera (200fs resolution) with dispersionless reflective optics, coherent transition radiation interferometer and far-infrared polychromator were used. We conclude that the streak camera and polychromator are the best for longer than 200fs (FWHM) bunch and for shorter, respectively. Detailed performances of the RF gun including the linac-laser synchronization and the reduction of dark currents were precisely evaluated. Further, we tried a new femtosecond quantum beam based pump-and-probe analysis, which is the time-resolved X-ray diffraction to visualize atomic motions. X-ray diffraction patterns of CuKα 1.2 from several monocrystal such as GaAs, KCl, CaF 2 were obtained using 10ps X-ray pulses. The visualization is under way. Finally, we introduce the new femtosecond ultrafast quantum phenomena research facility.

Experimental verification of laser photocathode RF gun as an injector for a laser plasma accelerator

The feasibility of the laser photocathode RF gun, BNL/GUN-IV, as an injector for a laser plasma accelerator was investigated at the subpicosecond S-band twin linac system of the Nuclear Engineering Research Laboratory, University of Tokyo. Electron beam energy of 16 MeV, emittance of 6/spl pi/ mm mrad, bunch length of 240 fs (FWHM), and charge per bunch of 350 pC were confirmed at 10 Hz. As for diagnosis of the femtosecond electron bunch, the quantitative comparison of performance of the femtosecond streak camera, the coherent transition radiation (CTR) Michelson interferometer, and the far-infrared polychromator was carried out. We concluded that the streak camera is the most reliable up to 200 fs and that the polychromator is the best for the shorter electron bunch. The 3.5-ps (rms) resolved synchronization between the YLF laser driver for the gun and the electron bunch was achieved. Based on the above experiences, we have designed and installed a much better laser-electron synchronization system using the Kerr-lens mode-locked Ti:Sapphire laser with the min harmonics synchrolocker and the stable 15-MW klystron. The timing jitter is expected to be suppressed down to 320 fs (rms).

Subpicosecond electron single-beam diagnostics by a coherent transition radiation interferometer and a streak camera

Abstract Longitudinal bunch distributions of subpicosecond and picosecond electron beams have been evaluated by the coherent transition radiation (CTR) Michelson interferometer with the reconstruction procedure obtained from the interferograms. The results were compared with those obtained from the femtosecond streak camera whose time resolution is 200 fs at the full-width at half-maximum (FWHM). From the comparison, the reliability of the interferometry to evaluate the subpicosecond electron pulse, which is close to the time resolution of the femtosecond streak camera, has been discussed in detail. Further, the validity of the application of the Kramers–Kronig relations and the extrapolation of missing data in the measurements is also discussed.

Femtosecond electron beam generation and measurement for laser synchrotron radiation

One of the S-band twin linacs (18L linac) of Nuclear Engineering Research Laboratory of University of Tokyo is modified in order to produce femtosecond electron single bunch for femtosecond X-ray generation via Thomson backward scattering, namely laser synchrotron radiation. Laser photocathode RF gun and chicane-type magnetic pulse compressor are installed at the S-band linac. 10 ps (FWHM) laser pulse generates 5 MeV, 10 ps (FWHM), 1 nC electron single bunch, which is accelerated up to 20 MeV in the S-band accelerating tube and compressed to 200 fs (FWHM) by the chicane. Design study has been performed by using the code of PARMELA and the installation has been finished. For precise and reliable measurement of the compressed pulse length, the comparison of measurement between the femtosecond streak camera and coherent transition radiation interferometry was carried out. Good agreement between them for 1–10 ps (FWHM) pulses was achieved. A new Michelson interferometer for the 200 fs pulse is now under construction.

Precise measurement of a subpicosecond electron single bunch by the femtosecond streak camera

Precise measurement of a subpicosecond electron single bunch by the femtosecond streak camera is presented. The subpicosecond electron single bunch of energy 35 MeV was generated by the achromatic magnetic pulse compressor at the S-band linear accelerator of Nuclear Engineering Research Laboratory (NERL), University of Tokyo. The electric charge per bunch and beam size are 0.5 nC and the horizontal and vertical beam sizes are 3.3 and 5.5 mm (full width at half maximum; FWHM), respectively. Pulse shape of the electron single bunch is measured via Cherenkov radiation emitted in air by the femtosecond streak camera. Optical parameters of the optical measurement system were optimized based on much experiment and numerical analysis in order to achieve a subpicosecond time resolution. By using the optimized optical measurement system, the subpicosecond pulse shape, its variation for the differents rf phases in the accelerating tube, the jitter of the total system and the correlation between measured streak images and calculated longitudinal phase space distributions were precisely evaluated. This measurement system is going to be utilized in several subpicosecond analyses for radiation physics and chemistry.

Generation of high-current (1kA) subpicosecond electron single pulse

An achromatic pulse compressor and a grid pulser system were designed for generation of a high-current single pulse in a subpicosecond time domain and were installed to the S-band (2856 MHz) linear accelerator of the University of Tokyo. 35 MeV electron pulses with the pulse width of 10 ps have been compressed by the achromatic magnetic pulse compression system. Generation of subpicosecond single-electron pulse with the charge of 1 nC has been succeeded. The shortest and average pulse widths in FWHM are 670 and 890 fs, respectively. The length of the pulse has been measured by a femtosecond streak camera with a time resolution of 200 fs. No satellite of the main pulse could be observed.