Nomarski Interferometry Research Articles

Dependence of spectral purity of Gd plasma emission around 6.7 nm on laser irradiation conditions

Gadolinium is a compelling candidate for the next-generation 6.x nanolithography light source. We present a systematically study on the spectral purity at 6.7 nm in the gadolinium laser produced plasma. Variations in extreme ultraviolet spectra are observed over a wide range of laser focal spot sizes and laser pulse energies due to the combined effect of plasma temperature and opacity. When varying laser focal spot size, ionic populations of Gd ions at different electron temperatures were estimated by the collisional-radiative model for the explanation of changes in the spectral profile. When adjusting the laser pulse energy, modifications in the electron density profile were measured through Nomarski interferometry, and the related optical depth was quantified assisted with a one-dimensional radiation transfer model. Results show that, appropriate laser energy and focal spot size are necessary to achieve the high spectral purity, and with a lens with focal length of f = 175 mm and a laser pulse energy of 100 mJ, a spectral purity of Gd plasma up to 3.7% over range of 5.5–10 nm is achieved.

The generation of a fourth-harmonic probe and its application in Nomarski interferometry at Shengguang-II.

In this work, a design for the generation of a 4ω (263-nm) probe converted from a 1ω (1053-nm) laser is presented. The design is based on a beta-barium borate and potassium dihydrogen phosphate two-step frequency-conversion process. A suitable configuration for Nomarski interferometry based on the 4ω probe is proposed, for measuring the electron density of laser-produced plasmas. The signal-to-noise ratio of the output 4ω probe to 1ω and 2ω light after frequency quadrupling and harmonic separation is 103 with a 0.5 GW/cm21ω input but decreases to ∼102 at intensities below 0.1 GW/cm2. Additional noise suppression by a factor of 104 is achieved using filters before the interferometer recording camera. The spatial resolution of the diagnostic can reach 5.2 µm for a 10% modulation transfer function. An experiment validating the probe diagnostic system is conducted at the Shengguang-II laser facility. A clear interferogram of an aluminum plasma is obtained with 0.1 GW/cm2 input, suggesting a maximal electron density of about 2.5 × 1020 cm-3 as retrieved through an inverse-Abel transform. The design proposed in this paper is appropriate for a small laser device or a large laser facility that lacks a separate diagnostic beam, and it is an inexpensive solution as it requires small-aperture 1ω input at a relatively low intensity. All the key parameters necessary to implement the design are provided in detail, making it straightforward to reproduce or transplant the system for specific uses.

A laser wakefield acceleration facility using SG-II petawatt laser system.

Laser wakefield acceleration (LWFA) using PW-class laser pulses generally requires cm-scale laser-plasma interaction Rayleigh length, which can be realized by focusing such pulses inside a long underdense plasma with a large f-number focusing optic. Here, we present a new PW-based LWFA instrument at the SG-II 5 PW laser facility, which employs f/23 focusing. The setup also adapted an online probing of the plasma density via Nomarski interferometry using a probe laser beam having 30fs pulse duration. By focusing 1-PW, 30-fs laser pulses down to a focal spot of 230 µm, the peak laser intensity reached a mild-relativistic level of 2.6 × 1018 W/cm2, a level modest for standard LWFA experiments. Despite the large aspect ratio of >25:1 (transverse to longitudinal dimensions) of the laser pulse, electron beams were observed in our experiment only when the laser pulse experienced relativistic self-focusing at high gas-pressure thresholds, corresponding to plasma densities higher than 3 × 1018cm-3.

Spatiotemporal characteristics of high-density gas jet and absolute determination of size and density of gas clusters

Properties of gas clusters such as the size and number density when expanding into the vacuum after passing through a conical nozzle are analyzed for argon at an average density of 1020/cm3. Temporally and spatially resolved size and density distribution were measured from all-optical methods of Rayleigh scattering measurement and Nomarski interferometry using a CW laser. At the gas backing pressure of 80 bar, Ar clusters as large as 100 nm were obtained, which differs significantly from the size estimated by the conventional Hagena scaling law. The two independent methods of cluster characterization presented here would be useful to precisely determine the initial conditions in a variety of intense laser-cluster interaction driven applications such as neutron generation, thermonuclear fusion, efficient x-ray emission, and energetic ion acceleration.

Magnetic reconnection driven by Gekko XII lasers with a Helmholtz capacitor-coil target

We demonstrate a novel plasma device for magnetic reconnection, driven by Gekko XII lasers irradiating a double-turn Helmholtz capacitor-coil target. Optical probing revealed an accumulated plasma plume near the magnetic reconnection outflow. The background electron density and magnetic field were measured to be approximately 1018 cm−3 and 60 T by using Nomarski interferometry and the Faraday effect, respectively. In contrast with experiments on magnetic reconnection constructed by the Biermann battery effect, which produced high beta values, our beta value was much lower than one, which greatly extends the parameter regime of laser-driven magnetic reconnection and reveals its potential in astrophysical plasma applications.

Spatial and temporal variations of electron temperatures and densities from EUV-emitting lithium plasmas

Planar slabs of pure Li were irradiated with 1.064 nm, 6 ns Nd:YAG laser pulses. Determination of plasma densities at both the earliest times of plasma formation and near the target surface was performed using Nomarski interferometry. The plasma parameters at later times were evaluated using optical emission spectroscopy. The space- and time-dependent electron densities and temperatures of the plasma were determined from their Stark broadening and the relative intensities of the spectral lines, respectively. The advantages and disadvantages of both of these techniques are evaluated and discussed.

Space-time characterization of laser plasma interactions in the warm dense matter regime

Laser plasma interaction experiments have been performed using an fs Titanium Sapphire laser. Plasmas have been generated from planar PMMA targets using single laser pulses with 3.3 mJ pulse energy, 50 fs pulse duration at 800 nm wavelength. The electron density distributions of the plasmas in different delay times have been characterized by means of Nomarski Interferometry. Experimental data were compared with hydrodynamic simulation. First results to characterize the plasma density and temperature as a function of space and time are obtained. This work aims to generate plasmas in the warm dense matter (WDM) regime at near solid-density in an ultra-fast laser target interaction process. Plasmas under these conditions can serve as targets to develop X-ray Thomson scattering as a plasma diagnostic tool, e.g., using the Vacuum ultraviolet (VUV) free-electron laser (FLASH) at Dentsches Elektronen-Synchrotron (DESY) Hamburg.

Production of ultracollimated bunches of multi-MeV electrons by 35 fs laser pulses propagating in exploding-foil plasmas

Very collimated bunches of high energy electrons have been produced by focusing super-intense femtosecond laser pulses in submillimeter under-dense plasmas. The density of the plasma, preformed with the laser exploding-foil technique, was mapped using Nomarski interferometry. The electron beam was fully characterized: up to 109 electrons per shot were accelerated, most of which in a beam of aperture below 10−3 sterad, with energies up to 40 MeV. These measurements, which are well modeled by three-dimensional numerical simulations, validate a reliable method to generate ultrashort and ultracollimated electron bunches.

Characterization of laser plasmas for interaction studies.

Long scale length plasmas were produced by uniform laser irradiation from opposite sides of metal disks coated on thin plastic stripe targets. The plasma became fully underdense before the end of the 600-ps pulse. The evolution of the plasma was carefully characterized both in electron density and temperature, and the experimental data were compared to the predictions of a one-dimensional hydrodynamics computer code. The electron temperature was obtained from time-resolved x-ray spectra by line-ratio measurements, accounting for opacity effects. Two-dimensional electron density profiles of a large plasma region were obtained by Nomarski interferometry at different times. They showed that the expansion was axially symmetric and reproducible shot by shot. The plasma was found to be substantially free from small-scale density inhomogeneities. The plasma produced with this techique is very suitable for interaction experiments. In particular, laser stimulated plasma instabilities of relevance for laser fusion can be accurately studied in these conditions.

On the mechanism of the passivity of aluminum and aluminum alloys

The mechanism of passivation and breakdown of passive layers on Al and Al-Ta alloy has been examined at pH 8.4, 11 and 12. The methods used include: a study of the hydrogen to tritium separation factor to obtain the mechanism of the cathodic partner reaction; radiotracer examination of Cl − and SO 4 2- adsorption and absorption: X-ray photoelectron spectroscopy depth analysis; both negative and some positive secondary ion mass spectrometry (SIMS); Nomarski interferometry; scanning electron microscopy; energy dispersive X-ray spectroscopy (EDS); and scanning tunneling microscopy (STM). The separation factor H/T is 7-4 depending on potential and pH. Absorption succeeds adsorption at θ Cl − > 1 and is the forerunner of breakdown. Distribution of ionic species in the film shows non-stoichiometry of ca 8%. This gives a built-in electric field which is surprisingly high, 10 6–10 7 V cm −1. Ta species at the surface are 2%–6% depending on pH but cause a reduction in corrosion rate of ca. 10 2 times. SIMS shows the OH − :O ratio to decrease with depth from the solution/oxide interface but to remain > 0.1 through the contact film to the surface. It declines with Cl − entry. The latter behaves radically differently for pure Al, and Al + Ta films. EDS gives an Al: Ta ratio of < 9% at the surface (pH 8.4). STM shows the topography of the dissolving film and indicates “boulders” of Al, around 500 Å across. Passive layers on Al for pH 8–11 involve a porous pre-layer of Al(OH) 3 and Al 2O 3 mixed crystal. The contact layer consists largely of Al 2O 3 but a critical component of it (the reason for its protectivity) is AlO(OH), in fibril form. Breakdown involves penetration of these fibrils by Cl −, which displaces the OH − therein and diffuses out again, in the form of an AlCl n s m + complex. Interpretation of the reaction order of 2 with respect to Cl − suggests m = 1 and n = 2. The Ta protects by blocking entry to the fibrils.

Comparative electron density measurements for the refractive fringe diagnostic and Nomarski interferometry

Massive carbon targets were irradiated with a pulsed ruby laser and the laser-produced plasma electron densities were simultaneously evaluated using Nomarski interferometry and the refractive fringe diagnostic. An agreement of half an order of magnitude between the two diagnostics was obtained.

Contact of rough surfaces: A review of experimental work

In recent years engineers have come to realize that the determination of the contact geometry of rough surfaces is of critical importance in the solution of many tribological and conduction problems. The theoretical study of the contact of rough surfaces is well advanced whereas the experimental work on this topic has not received the attention that was due. This paper presents a review of the experimental investigations that have been carried out so far. Their findings and experimental methods are discussed. The main topics of experimental study are the separation of the surfaces, the real area of contact, the number of contact spots, the spatial distribution of the contact spots and the distribution of their sizes, and the relation of all these to roughness and to the normal load. Experimental techniques have included total internal reflection, Nomarski interferometry, neutrography, thermal and electrical resistance and computer simulation. The consensus of experimental results is as follows: the real area of contact increases with the load and this increase is due mainly to an increase in the number of micro-contacts, their mean size remaining approximately constant; the separation of the contacting surfaces is approximately inversely proportional to the logarithm of the load; the distribution of contact spot sizes is approximately log normal. However, it appears that the density and average size of micro-contacts can vary over several orders of magnitude for different surfaces at the same load. It also appears that, contrary to established belief, the real area of contact does not vary as the load but increases as its 0.8 power.

The measurement of small wedge angles using Nomarski interferometry, and its application to small semiconductor samples

A technique is described that enables small wedge angles to be measured. The technique uses a Nomarski interferometer and is well suited to the measurement of the angle laps that are used to determine the thickness of semiconductor layers.

On some nonelectrochemical methods for investigating the surfaces of electrodes

The uses of the Nomarski interferometry and interference contrast techniques, elipsometric technique, and Mössbauer spectroscopy in the study of electrode surfaces is discussed. More recent advances in the studies made in the Electrochemistry Laboratory at the University of Pennsylvania are reported and discussed.