Chang Hee Nam

Adaptive quantum control of DCM fluorescence in the liquid phase

The emission efficiency, (dye fluorescence)/(excitation intensity), by multi-photon excitation of 4-(dicyanomethylene)-2-methyl-6-[p-(dimethyl-amino) styryl]-4H-pyran in methanol solution was maximized using an evolutionary algorithm. Without any a priori knowledge on the molecular system, the optimization goal was automatically achieved by improving spectral phases through feedback of experimental outputs. The spectral structure of the optimized pulse was found to have a negative chirp. It is proposed that the efficiency change induced by the negative chirp is due to wavepacket localization on locally excited states.

Space-resolving flat-field extreme ultraviolet spectrograph system and its aberration analysis with wave-front aberration

The Nam aberration of a flat-field extreme ultraviolet spectrograph system, composed of a varied line-spacing concave grating and a toroidal mirror, was analyzed by calculating the wave-front aberration with respect to an astigmatic reference surface. The toroidal mirror was used to compensate for the astigmatism that was due to the grazing incidence of light at the concave grating. The spectrograph system could form a space-resolved spectrum along the sagittal direction. The spectral and spatial resolutions of the spectrograph system were estimated from the root-mean-square spot size. The actual spectral resolution of the spectrograph system was measured from extreme ultraviolet spectra obtained from plasmas produced by an iodine laser having an energy of 0.5 J in a 4-ns duration, and it was compared with the calculated value.

The effect of prepulse on X-ray laser development using a powerful subpicosecond KrF laser

A high power UV laser has been developed as a pump source for short wavelength (down to 1 nm) X-ray lasers. Various schemes are considered and theoretical analysis is discussed. Spectroscopic studies of laser-target interaction have been performed and, in particular, the effect of a prepulse on plasma generation has been investigated. Analysis of the observed spectra indicates that reduction of the prepulse energy results in a higher temperature plasma. Investigation of the interaction using thin layered targets is also presented. These data provide evidence for initially hot plasma conditions generated from target layers ≦150 Å. Discussions of proposed laser schemes at 1–5 nm are presented.

Precise and long-term stabilization of the carrier-envelope phase of femtosecond laser pulses using an enhanced direct locking technique

We demonstrate a long-term operation with reduced phase noise in the carrier-envelope-phase (CEP) stabilization process by employing a double feedback loop and an improved signal detection in the direct locking technique [Opt. Express 13, 2969 (2005)]. A homodyne balanced detection method is employed for efficiently suppressing the dc noise in the f-2f beat signal, which is converted into the CEP noise in the direct locking loop working at around zero carrier-envelope offset frequency (f(ceo)). In order to enhance the long-term stability, we have used the double feedback scheme that modulates both the oscillator pump power for a fast control and the intracavity-prism insertion depth for a slow and high-dynamic-range control. As a result, the in-loop phase jitter is reduced from 50 mrad of the previous result to 29 mrad, corresponding to 13 as in time scale, and the long-term stable operation is achieved for more than 12 hours.

Plasma-induced frequency chirp of intense femtosecond lasers and its role in shaping high-order harmonic spectral lines

This work was supported by the Ministry of Science and Technology of Korea through the Creative Research Initiative Program.

Toward shorter wavelength lasers and soft X-ray laser microscopy

The authors present two approaches to X-ray laser development at Princeton and review progress toward the wavelength region below 10 nm. In addition, they present the first results from the application of the existing soft X-ray laser at 18.2 nm to X-ray microscopy. Particular emphasis is placed on experimentation with Li-like ions, the modeling of Li-like plasmas, cavity development, and a two-laser approach to wavelengths significantly below 10 nm. >

Radiation cooling and gain calculation for C VI 182-A line in carbon--selenium plasma

A model is developed that is capable of describing the evolution of gain resulting from both rapid radiative and expansion cooling of a recombining, freely expanding plasma. It is demonstrated for the particular case of a carbon–selenium plasma that the cooling rate that leads to optimal gain can be achieved by adjusting the admixture of an efficiently radiating material (selenium) in the gain medium (carbon). Comparison is made with a recent observation of gain in an experiment done by a group from the Naval Research Laboratory and the University of Rochester, with carbon–selenium plasma for the n = 3 → 2 transition in C vi occurring at 182 A. The predicted maximum gain is ~10 cm−1 as compared with the observation of 2–3 cm−1.

Temporal characterization of chirped femtosecond laser pulses

Chirped femtosecond laser pulses are generated by adjusting the grating separation of a pulse compressor in a chirped-pulse amplification laser and characterized using a single-shot frequency-resolved optical grating technique. Laser chirp parameters in time and frequency domains, such as group delay dispersion and linear chirp coefficient, are analyzed with respect to grating detuning. It is observed that the linear chirp coefficient rapidly increases without significant pulse broadening around the zero detuning, necessitating precise chirp characterization. The pulse broadening is slightly asymmetric to the grating detuning because of residual third- and fourth-order dispersion.

Measurement of multilayer mirror reflectivity and stimulated emission in the XUV spectral region

We present measurements of multilayer mirror reflectivity and stimulated emission in the XUV spectral region. A molybdenum–silicon multilayer mirror with 12% measured reflectivity at 182 A was found to produce a 120% enhancement of the C vi 182‐A line (3→2 transition) in a strongly recombining plasma. No such enhancement of the C v 186.7‐A line was seen, demonstrating amplification of stimulated emission at 182 A.

Incorporation of a cavity-dumped oscillator in a long-wavelength injected femtosecond terawatt Ti:sapphire laser

A cavity-dumped Ti:sapphire laser was utilized as a front-end oscillator in a femtosecond terawatt Ti:sapphire laser. The increase of the oscillator energy by 30 times helped to reduce the gain narrowing and to suppress the amplified spontaneous emission in a preamplifier. In combination with the long-wavelength injection method (J. Opt. Soc. Am. B 16 (1999) 1220), an amplified spectrum broad enough to support sub-20 fs could be generated at a multiterawatt level, and 20-fs, 3-TW pulses were demonstrated.