Thanh-Hung Dinh

Spectral investigation of highly ionized bismuth plasmas produced by subnanosecond Nd:YAG laser pulses

The unresolved transition arrays (UTAs) emitted from laser produced bismuth (Bi) plasma sources show potential for single-shot live cell imaging. We have measured extreme ultraviolet spectra from bismuth laser produced plasmas in the 1-7 nm region using a lambda = 1064 nm Nd: YAG laser with a pulse duration of 150 ps. Comparison of spectra obtained under different laser power densities with calculations using the Hartree-Fock with configuration interaction Cowan suite of codes and the UTA formalism, as well as consideration of previous predictions of isoelectronic trends, are employed to identify lines and a number of new features in spectra from Bi XXIII to Bi XLVII. The results show that Delta n. =. 0, n = 4-4 emission from highly charged ions merges to form intense UTAs in the 4 nm region and Delta n - 1, n - 4-5 resonance transitions UTAs dominate the 1-3 nm region of the Bi spectrum.

Soft X-ray emission from molybdenum plasmas generated by dual laser pulses

We demonstrate efficient enhancement of soft X-ray (SXR) emission from molybdenum plasmas produced using dual pulse irradiation, in which 10-ns and 150-ps pre-pulses were followed by a 150-ps main pulse. The number of photons was observed to be 5.3 × 1016 photons/sr, which corresponded to a conversion efficiency of 1.5%/sr in λ = 2.34–4.38 nm region at a pulse separation time of 1 ns with the 150-ps pre-pulse. The conversion efficiency became 1.3 times as large as that produced by a single pulse. The results indicate the advantage of dual pulse irradiation using sub-ns pre-and main pulses to produce the bright plasmas required for applications such as laboratory based SXR microscopy.

Characteristics of the soft X-ray emission from laser-produced highly charged platinum plasmas

We characterized the spectral structure of the soft X-ray emission and determined the plasma parameters in laser-produced highly charged platinum plasmas. The spectral structure observed originated from Pt21+ to Pt34+ ions, emissions from which overlapped to produce a high output flux in the carbon-window soft X-ray spectral region. Using dual laser pulse irradiation, we observed the maximum output flux, which was 20% larger than that obtained under single-laser irradiation, and the evolution of a strongly absorbed spectral structure, which was attributed to the effects of both opacity and long-scale length of the expanding pre-plasma.

Analysis of extreme ultraviolet spectra from laser produced rhenium plasmas

Extreme ultraviolet spectra of highly-charged rhenium ions were observed in the 1–7 nm region using two Nd:YAG lasers with pulse lengths of 150 ps and 10 ns, respectively, operating at a number of laser power densities. The maximum focused peak power density was 2.6 × 1014 W cm−2 for the former and 5.5 × 1012 W cm−2 for the latter. The Cowan suite of atomic structure codes and unresolved transition array (UTA) approach were used to calculate and interpret the emission properties of the different spectra obtained. The results show that n = 4-n = 4 and n = 4-n = 5 UTAs lead to two intense quasi-continuous emission bands in the 4.3–6.3 nm and 1.5–4.3 nm spectral regions. As a result of the different ion stage distributions in the plasmas induced by ps and ns laser irradiation the 1.5–4.3 nm UTA peak moves to shorter wavelength in the ps laser produced plasma spectra. For the ns spectrum, the most populated ion stage during the lifetime of this plasma that could be identified from the n = 4-n = 5 transitions was Re23+ while for the ps plasma the presence of significantly higher stages was demonstrated. For the n = 4-n = 4 4p64dN-4p54dN+1 + 4p64dN−14f transitions, the 4d-4f transitions contribute mainly in the most intense 4.7–5.5 nm region while the 4p-4d subgroup gives rise to a weaker feature in the 4.3–4.7 nm region. A number of previously unidentified spectral features produced by n = 4-n = 5 transitions in the spectra of Re XVI to Re XXXIX are identified.

Temporal behavior of unresolved transition array emission in water window soft x-ray spectral region from multiply charged ions

We have characterized the spectral structure and the temporal history of the laser-produced high-Z multi-charged ion plasmas for the efficient water window soft x-ray sources. Strong unresolved transition array emission was observed due to 4d–4f and 4f–5g transitions from Au, Pb, and Bi plasmas in the 280–700 eV photon energy region. The temporal behavior of the emission was essentially similar of that of the laser pulse with a slight delay between different transitions. These results provide feedback for accurate modeling of the atomic processes with the radiative hydrodynamic simulations.

Deep-hole drilling of amorphous silica glass by extreme ultraviolet femtosecond pulses

A free-electron laser (FEL) is a robust tool for studying the interaction of intense X-rays with matter. In this study, we investigate the damage threshold and morphology of fused silica irradiated by extreme ultraviolet femtosecond pulses of a FEL. The experimental results indicate the superiority of the FEL processing. The FEL-damage threshold of fused silica at a wavelength of 13.5 nm is 0.17 J/cm2, which is 20 times lower than that of a near infrared (NIR) femtosecond laser. The relationship between the crater depth and laser fluence reveals that the effective absorption length is αeff−1 = 58 nm. The damage threshold and the absorption length are the key values for smooth crater formation. In addition, the formation of rim structures and microcracks, which are usually the critical issues in NIR laser processing, cannot be found in the interaction region. The hole diameter is maintained below the beam size at the exit.A free-electron laser (FEL) is a robust tool for studying the interaction of intense X-rays with matter. In this study, we investigate the damage threshold and morphology of fused silica irradiated by extreme ultraviolet femtosecond pulses of a FEL. The experimental results indicate the superiority of the FEL processing. The FEL-damage threshold of fused silica at a wavelength of 13.5 nm is 0.17 J/cm2, which is 20 times lower than that of a near infrared (NIR) femtosecond laser. The relationship between the crater depth and laser fluence reveals that the effective absorption length is αeff−1 = 58 nm. The damage threshold and the absorption length are the key values for smooth crater formation. In addition, the formation of rim structures and microcracks, which are usually the critical issues in NIR laser processing, cannot be found in the interaction region. The hole diameter is maintained below the beam size at the exit.

Progress and Prospects of X-Ray Laser Research in QST

This paper reviews recent progress made in the development of coherent x-ray sources and their applications in material science and laser processing in the National Institutes for Quantum and Radiological Science and Technology (QST). The upgrade of Ti:Sapphire laser for a grazing incidence pumping (GRIP) scheme was started, to further develop a coherent x-ray source. Investigating the applications of x-ray lasers, we observed temporal evolution of metal surface ablation using an x-ray laser probe, and the damage to an EUV mirror structure by the x-ray pulse irradiation.

Development of Soft X-Ray Microscope in Water Window Using Laser-Produced Plasma Light Source

For the sake of observation of samples in solution, a contact-type soft X-ray microscope is under construction by the use of both an optical microscope for readout and a laser-produced plasma light source with a heavy metal target. Edge response is evaluated through a water layer.

Evaluation of a Flat-Field Grazing Incidence Spectrometer for Highly Charged Ion Plasma Emission in 1–10 nm

A flat-field grazing incident spectrometer was built to investigate highly charged ion (HCI) plasmas in the spectral region from 1 to 10 nm. The spectrometer consists of a flat-filed grating with 2400 lines/mm as a dispersing element and an X-ray charged-coupled device (CCD) camera as the detector. In order to produce accurate intensity calibrated spectra of the HCI plasmas, the diffraction efficiency of the grating and the sensitivity of the CCD camera were directly measured by using the reflectometer installed at the BL-11D of the Photon Factory (PF). We also studied the calibrated spectra of Gd HCI plasmas which were produced by an Nd:YAG laser.

A 10 Hz Short-Pulse CO2 Laser System for Extreme Ultraviolet Source

We produced 3–15 ns pulses at 10.6 μm wavelength to investigate efficient extreme ultraviolet and soft X-ray sources. A master oscillator power amplifier (MOPA) laser operating at a repetition rate of 10 Hz was built on transversely excited atmospheric (TEA) CO2 mediums. A scheme of a pulse-width tunable CO2 laser was proposed. The amplification by a 14-pass amplifier provided output energies of 150 and 60 mJ per pulse at durations of 15 ns and 3 ns, respectively. An initial observation of Gd plasmas produced by the CO2 laser shows that the spectral efficiency around 6.7 nm was improved by short laser pulses irradiation.