The interaction of high power femtosecond laser pulses with matter is now becoming a powerful tool for generating short intense X-ray pulses. In femtosecond laser plasma interactions, collisional absorption becomes less significant as an absorption mechanism compared to the nanosecond regime. A number of other collisionless absorption mechanism including resonance absorption and vacuum heating accelerate hot electrons to kinetic energies much higher than the bulk plasma temperature. These hot electrons generate X-rays via bremsstrahlung, recombination radiation in the plasma and penetrate into solid behind the plasma where they emit K_α and bremsstrahlung X-rays. These hot electrons lose their energy to the solid in a few tens of femtosecond limiting the durations of the emitted K_α pulse.
     We have investigated the generation of hot electrons and K_α radiation using Ti:sapphire laser (120fs, 800 nm, 260 µJ) pulses focused with a 10X microscope objective to obtain an intensity of 2 × 10¹⁶ W/cm² on Cu, Fe and Ag wire targets. The emission energy and spectra have been characterized using three different detector systems which include a filtered PIN diode detector, pulse height spectrometer using a CdTe detector together with a multichannel analyzer and a single hit CCD camera used in pulse height analysis mode. The copper wire target had a measured hot electron temperature of 9 keV and a conversion efficiency into K_α line radiation at 8.05 keV of approximately 4 × 10^–5 using 1 kHz laser pulses. A similar conversion efficiency of about 6 × 10^–5 is obtained using the Fe K_α X-ray Source. The silver K_α source gave about 20 times lower conversion efficiency than Cu and Fe. Higher intensities are probably required to increase and optimize the silver source emission. The measured emission spot size was ~8μm and such micro K_α Sources are of interest for high precision X-ray microscopy, phase contrast imaging and also for time resolved studies of molecular and phase transition processes using X-ray diffraction. Phase contrast microscopy is one important application for biological samples where the tissue is fairly transparent to X-rays. The phase shift at the edges of object gives greatly enhanced contrast of edge features due to near field Fresnel diffraction fringes. The principle is as shown in Fig. 1. Edge enhancement is quite pronounced in real biological specimens such as the head of a mosquito shown in Fig. 2 using the laser produced Fe K_α X-ray source. We observed stronger enhancement with Fe K_α than Cu K_α source.

Figure 1. Phase Contrast Image of Mosquito recorded for an exposure of 10 minutes using Fe X-ray Source.	Figure 2. Schematic of principle of phase-contrast imaging.

Background
Atif Ali is a graduate student in the Department of Electrical and Computer Engineering at University of Alberta. He works with the Laser Plasma Group under the supervision of Dr. Robert Fedosejevs. He is currently working on Femtosecond Laser-produced K_α Sources at the University of Alberta and on Laser Wakefield Acceleration Experiment at the Advanced Laser Light Source (ALLS), INRS, Montreal. He completed his Bachelor’s degree at Government Faridia Degree College Pakpattan Sharif (affiliated with Bahauddin Zakariya University) and then a Master of Physics from the Department of Physics, University of Engineering and Technology (UET) Lahore Pakistan. During his Master’s program he worked on Laser-generated X-ray diagnostics at the Applied Physics Lab. UET Lahore.
     Atif Ali can be reached at the Department of Electrical and Computer Engineering, University of Alberta Edmonton, Alberta, Canada T6G2V4; E-mail: atif1@ece.ualberta.ca.