Browsing by Author "Nam, C.H."
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Item A method to generate a single sub-50-as pulse by employing a 5-fs laser pulse with intensity above the saturation intensity, was proposed. The neon target medium of 0.5 mm length and 5 Torr pressure, was placed 10 mm beyond the laser focus. Harmonic radiation from neon atoms, was obtained by solving the time-dependent Schrödinger equation coupled with Maxwell equations. The results show that the positive chirp contained in the broad continuum radiation can be compensated by an x-ray filter with a negative group delay dispersion in the spectral region of interest.(American Physical Society, Single sub-50-attosecond pulse generation from chirp-compensated harmonic radiation using material dispersion) Kim, K.T.; Kim, C.M.; Baik, M.G.; Umesh, G.; Nam, C.H.2004Item Compression of harmonie pulses by using material dispersion(2004) Kim, K.T.; Kim, C.M.; Baik, M.G.; Umesh, G.; Nam, C.H.We utilize the dispersion property of an X-ray filter material for the generation of a single sub-50-as pulse from high-order harmonics. The attosecond pulse, formed by selecting the spectral range of high-order harmonic radiation, contains an intrinsic chirp corresponding to the quadratic phase variation during a half cycle of a laser pulse. We show that this chirp can be compensated by using the negative group-delay dispersion of a thin X-ray filter, compressing the attosecond pulse down to sub-50-as.Item Self-compression of attosecond high-order harmonic pulses(2007) Kim, K.T.; Kang, K.S.; Park, M.N.; Imran, T.; Umesh, G.; Nam, C.H.Self-compression of attosecond high-order harmonic pulses in the harmonic generation medium itself has been demonstrated. The attosecond pulses were generated in an argon-filled gas cell and compressed by exploiting the dispersion characteristics of argon. Since the harmonic generation medium itself was used as the compression medium, continuous chirp control was easily achieved by adjusting the gas pressure. The optimized attosecond pulse was also the most intense, and its duration of 206 as was very close to the transform-limited value of 200 as. 2007 The American Physical Society.Item Self-compression of attosecond high-order harmonic pulses(2007) Kim, K.T.; Kang, K.S.; Park, M.N.; Imran, T.; Umesh, G.; Nam, C.H.Self-compression of attosecond high-order harmonic pulses in the harmonic generation medium itself has been demonstrated. The attosecond pulses were generated in an argon-filled gas cell and compressed by exploiting the dispersion characteristics of argon. Since the harmonic generation medium itself was used as the compression medium, continuous chirp control was easily achieved by adjusting the gas pressure. The optimized attosecond pulse was also the most intense, and its duration of 206 as was very close to the transform-limited value of 200 as. © 2007 The American Physical Society.Item Single sub-50-attosecond pulse generation from chirp-compensated harmonic radiation using material dispersion(2004) Kim, K.T.; Kim, C.M.; Baik, M.-G.; Umesh, G.; Nam, C.H.A method to generate a single sub-50-as pulse by employing a 5-fs laser pulse with intensity above the saturation intensity, was proposed. The neon target medium of 0.5 mm length and 5 Torr pressure, was placed 10 mm beyond the laser focus. Harmonic radiation from neon atoms, was obtained by solving the time-dependent Schr dinger equation coupled with Maxwell equations. The results show that the positive chirp contained in the broad continuum radiation can be compensated by an x-ray filter with a negative group delay dispersion in the spectral region of interest.Item Ultrafast dynamics of autoionizing states in O2 probed by laser-field-assisted XUV photoionization(2013) Zhu, C.; Ko, D.H.; Kang, K.S.; Lee, J.; Lee, J.-H.; Umesh, G.; Krishnakumar, E.; Nam, C.H.Dynamics of the autoionizing states in O2 was investigated using the method of laser-field-assisted extreme-ultraviolet photoionization. Harmonics, ranging from 9th to 21st orders generated in Kr with 25 fs laser pulses at 820 nm, were employed to carry out photoionization of O2. Distinct autoionization features in the photoelectron spectrum, stemming from the resonant excitation of superexcited states of O2 by the 11th harmonic, were observed. The superexcited states, referred to as autoionizing states, were identified to be the ?? = 0, n = 5 Rydberg states converging to the b 4?g- state of O 2+. Infrared laser pulses were utilized as the probe to monitor time-varying characteristics of photoionization and autoionization of O2. A decay time of 21 fs was obtained for the autoionizing states of O2 by deconvoluting the pump-probe results in O2 and Ar. � 2013 American Physical Society.Item Ultrafast dynamics of autoionizing states in O2 probed by laser-field-assisted XUV photoionization(2013) Zhu, C.; Ko, D.H.; Kang, K.S.; Lee, J.; Lee, J.-H.; Umesh, G.; Krishnakumar, E.; Nam, C.H.Dynamics of the autoionizing states in O2 was investigated using the method of laser-field-assisted extreme-ultraviolet photoionization. Harmonics, ranging from 9th to 21st orders generated in Kr with 25 fs laser pulses at 820 nm, were employed to carry out photoionization of O2. Distinct autoionization features in the photoelectron spectrum, stemming from the resonant excitation of superexcited states of O2 by the 11th harmonic, were observed. The superexcited states, referred to as autoionizing states, were identified to be the ?? = 0, n = 5 Rydberg states converging to the b 4?g- state of O 2+. Infrared laser pulses were utilized as the probe to monitor time-varying characteristics of photoionization and autoionization of O2. A decay time of 21 fs was obtained for the autoionizing states of O2 by deconvoluting the pump-probe results in O2 and Ar. © 2013 American Physical Society.Item We utilize the dispersion property of an X-ray filter material for the generation of a single sub-50-as pulse from high-order harmonics. The attosecond pulse, formed by selecting the spectral range of high-order harmonic radiation, contains an intrinsic chirp corresponding to the quadratic phase variation during a half cycle of a laser pulse. We show that this chirp can be compensated by using the negative group-delay dispersion of a thin X-ray filter, compressing the attosecond pulse down to sub-50-as.(Compression of harmonie pulses by using material dispersion) Kim, K.T.; Kim, C.M.; Baik, M.G.; Umesh, G.; Nam, C.H.2004