Subpicosecond Pulse Source Research Articles

Formation of secondary electron cascades in single-crystalline plasma-deposited diamond upon exposure to femtosecond x-ray pulses

Secondary electron cascades were measured in high purity single-crystalline chemical vapor deposition (CVD) diamond, following exposure to ultrashort hard x-ray pulses (140fs full width at half maximum, 8.9keV energy) from the Sub-Picosecond Pulse Source at the Stanford Linear Accelerator Center. We report measurements of the pair creation energy and of drift mobility of carriers in two CVD diamond crystals. This was done for the first time using femtosecond x-ray excitation. Values for the average pair creation energy were found to be 12.17±0.57 and 11.81±0.59eV for the two crystals, respectively. These values are in good agreement with recent theoretical predictions. The average drift mobility of carriers, obtained by the best fit to device simulations, was μh=2750cm2∕Vs for holes and was μe=2760cm2∕Vs for electrons. These mobility values represent lower bounds for charge mobilities due to possible polarization of the samples. The results demonstrate outstanding electric properties and the enormous potential of diamond in ultrafast x-ray detectors.

Structural studies of melting on the picosecond time scale

Ultrafast structural studies of laser-induced melting have demonstrated that the solid-liquid phase transition can take place on a picosecond time scale in a variety of materials. Experimental studies using ångström wavelength X-rays from the sub-picosecond pulse source at Stanford (now retired) on non-thermal melting of semi-conductors, such as indium antimonide, employed the decay of a single Bragg-peak to measure the time component of the phase transition. These materials were found to start melting within one picosecond after the laser pulse. Recent computer simulations have described the thermal melting of ice induced by an infrared laser pulse. Here it was shown that melting can happen within a few picoseconds, somewhat slower than non-thermal melting in semi-conductors. These computer simulations are compatible with spectroscopy experiments on ice-melting, demonstrating that simulations form a very powerful complement to experiments targeting the process of phase-transitions. Here we present an overview of recent experimental and theoretical studies of melting, as well as new simulations of ice-melting where the effect of the size of the crystal on scattering is studied. Based on simulations of a near-macroscopic crystal, we predict the decay of the most intense Bragg peaks of ice following heating by laser pulse, by modeling the scattering from the melting sample in the simulations.

80.8-km BOSSNET SPECTS O-CDMA Field Trial Using Subpicosecond Pulses and a Fully Integrated, Compact AWG-Based Encoder/Decoder

We successfully demonstrate a spectral phase-encoded time-spreading (SPECTS) optical code-division multiple-access (O-CDMA) field trial on an 80.8-km link within the Boston-South Network (BOSSNET) using a fully-integrated, polarization-independent arrayed-waveguide grating (AWG)-based encoder/decoder. The subpicosecond pulse source is based on an ultrastable optical frequency comb generator employing dispersion-decreasing fiber. A novel tunable dispersion-slope compensator enables the use of subpicosecond pulses without extensive characterization of the field-deployed fiber. Bit-error rate (BER) and Ethernet traffic statistics are presented.

Technical Reports: Ultrafast X-ray Science at the Sub-Picosecond Pulse Source

The ultrafast, high brightness X-ray free electron laser (XFEL) sources of the future have the potential to revolutionize the study of time-dependent phenomena in the natural sciences. These linear accelerator (linac) sources will generate femtosecond (fs) X-ray pulses with peak flux comparable to conventional lasers, and far exceeding all other X-ray sources. The Stanford Linear Accelerator Center (SLAC) has pioneered the development of linac science and technology for decades, and since 2000 SLAC and the Stanford Synchrotron Radiation Laboratory (SSRL) have focused on the development of linac based ultrafast electron and X-ray sources.

Clocking Femtosecond X Rays

Linear-accelerator-based sources will revolutionize ultrafast x-ray science due to their unprecedented brightness and short pulse duration. However, time-resolved studies at the resolution of the x-ray pulse duration are hampered by the inability to precisely synchronize an external laser to the accelerator. At the Sub-Picosecond Pulse Source at the Stanford Linear-Accelerator Center we solved this problem by measuring the arrival time of each high energy electron bunch with electro-optic sampling. This measurement indirectly determined the arrival time of each x-ray pulse relative to an external pump laser pulse with a time resolution of better than 60 fs rms.

Soliton self-frequency shift in highly nonlinear fiber with extension by external Raman pumping.

A completely fiber-integrated, wavelength-tunable subpicosecond pulse source is demonstrated using the soliton self-frequency shift in highly nonlinear dispersion-shifted fiber from a 1.56-microm 10-GHz 400-fs signal. Solitons as short as 100 fs are obtained at tunable wavelengths as high as 1.72 microm. Raman gain from an external pump is used to extend the soliton self-frequency shift to longer wavelengths.

Optical reflectometry with micrometer resolution for the investigation of integrated optical devices

An optical time-domain reflectometry (OTDR) system using an infrared subpicosecond pulse source in conjunction with balanced heterodyne detection is discussed. Experimental results show a density of more than 100 dB and a resolution of 60 mu m in air. Taking advantage of the large tuning range of the laser system, it is possible to improve the resolution to less than 10 mu m. The applicability of the OTDR system for the diagnostics of integrated optical devices is demonstrated for a simple GaAs waveguide structure. >