Laser assisted particle removal (LAPR) is a technique that shows significant promise for removing both micrometer and nanometer scale particles from critical surfaces such as semiconductor wafers, high resolution photolithographic masks, high density magnetic recording media, large area high resolution optics and other critical surfaces. LAPR depends on the rapid deposition of energy provided by pulsed lasers. Several different versions of LAPR exist depending on whether the laser energy is deposited in the particle, substrate or a liquid energy transfer medium condensed under and around the particle. In this paper, the various mechanisms are reviewed and compared.

Aluminum nitride has been ablated with a KrF excimer laser (248 ran) at fluences from 1 to 60 J/cm2. Ablation depth, emission spectra and photothermal beam deflection were detected as a function of fluence. An ablation rate of 0.2 um/pulse was achieved above 30 J/cm2 in vacuum. Ablation rate decreased with decreasing fluence. Irradiated surfaces have a conductive metallic layer of reduced aluminum nitride. At low fluences, the metallic surface layer was spotty within the area of the laser beam. Below the fluence threshold for producing the metallized surface, emission spectra and photothermal beam deflection were still detected. Emission lines from Al, A1+ and Al-N were observed. Photothermal deflection data was used to calculate supersonic velocities for shock waves propagating from the sample surface. Shock waves resulted from rapid heating near the surface and expansion of ablated material from the laser spot.

Experimental measurements and calculations of stimulated Raman adiabatic passage transfer efficiencies were made on a sodium gas starting from the 3(2)S(1/2) electronic ground state, passing through the 3(2)P(1/2) and/or the 3(2)P(3/2) to the 5(2)S(1/2) state. The lasers used in the experiments had a pulse width of several picoseconds and were close to the Fourier transform limit. Although the linewidth of the laser was much smaller than the spin orbit splitting between the 3(2)P(1/2) and 3(2)P(3/2) states, Experiments and calculations reveal that both 3p states play a role in the transfer efficiency when the lasers are tuned to resonance through the 3(2)P(1/2) state, revealing evidence of quantum interference between the competing pathways.

We have applied the technique of picosecond laser spectroscopy to study the photodissociation of nitrobenzene at room temperature and 100mTorr. Three different photolysis pulses, λ2=250, 266, and 280nm, with a duration of 20–25ps, were used. The NO photofragment was detected via LIF between λ1=220 and 250nm. The profile of rotational population distribution shows a dependency on the photolysis wavelength and the delay time. The observed rotational population distributions are non-Boltzmann and bimodal for v″=0, 1, and 2.

Dual channel emission imaging of m-nitrobenzoic acid and benzoic acid was performed in order to visualize the morphology of the CN violet band emission of a TNT analogue. The CN channel was corrected for continuum emission using a simultaneously imaged background channel. Simultaneous dual channel imaging alleviated problems with shot to shot variation in the plasma morphology due to the friable substrates and showed differences between plasmas formed on the two targets.

Detection of explosives, explosive precursors, or other threat agents presents a number of technological challenges for optical sensing methods. Certainly detecting trace levels of threat agents against a complex background is chief among these challenges; however, the related issues of multiple target distances (from standoff to proximity) and sampling time scales (from passive mines to rapid rate of march convoy protection) for different applications make it unlikely that a single technique will be ideal for all sensing situations. A number of methods for spanning the range of optical sensor technologies exist which, when integrated, could produce a fused sensor system possessing a high level of sensitivity to threat agents and a moderate standoff real-time capability appropriate for portal screening of personnel or vehicles. In this work, we focus on several promising, and potentially synergistic, laser-based methods for sensing threat agents. For each method, we have briefly outlined the technique and report on the current level of capability.

Our measurements of micro-plasma following laser-induced optical breakdown of nitro compound explosive simulants, here 3-nitrobenzoic acid, show well-developed molecular spectra during the first several hundreds of nanoseconds. Analysis of recorded carbon spectra is accomplished using accurate line strengths for the diatomic molecular Swan system. Presence of hydrogen-beta allows us to infer electron density in the plasma evolution. Computational challenges include accounting for background variation and appropriate modeling of hydrogen embedded in molecular spectra. Recorded and computed spectra agree nicely for time delays on the order of 1.6 μs from optical breakdown when using a single temperature for local thermodynamic equilibrium plasma.

Nitric oxide (NO) is a major chemical byproduct of many photochemically active nitrogen-containing compounds. As a prototypical free radical with a very well characterized high-resolution spectrum, NO provides a standard spectroscopic fingerprint for indirect quantitative analysis and detection of a number of low vapor pressure nitroaromatic compounds in air through either direct photochemical decomposition of a parent molecule or from its relatively high vapor pressure chemical constituents. In this paper, we will discuss applications of picosecond laser spectroscopy for measurements and detection of NO and the nascent NO generated from photolysis of nitrobenzene. We will give a general overview of our tunable picosecond laser and detection system that we routinely use for probing and exciting the NO gamma band. This broad wavelength tuning capability of our laser allows us to set up pump-probe type experiments for detecting blue-shifted rovibronic bands and probing the relative population distribution for NO. In all cases, experiments were performed using UV laser pulses of duration less than 20 ps. Also, we studied the effect of N-2 collisions on the photoframentation spectrum of nitrobenzene in 1000 mbar of N-2 buffer gas.

The Center for Laser Studies (CLS) is a research organization within the School of Engineering at the University of Southern California. Since its founding in 1973, CLS has grown to about a dozen full-time research scientists working with about 35 graduate students on projects funded by a wide range of government and industrial agencies. Because studies of and research with lasers are frequently interdisciplinary ventures, close ties are maintained with academic departments within the University such as Electrical Engineering, Physics, Materials Science, Chemistry, Mechanical Engineering and Chemical Engineering. The need to cross traditional university department lines was one of the reasons for establishing CLS. The CLS researchers share an extensive equipment and knowledge base, which also serves as a resource facility for cooperative research with industry. This approach has led to over 200 publications in major journals and has given the Center national prominence.Six faculty members conduct a majority of their research at CLS and an approximately equal number have experiments in residence. Examples of some of the current research projects of the primary faculty are given below:1. Professor Elsa Garmire, Director, is studying nonlinear optical devices in a variety of materials with particular emphasis on using semiconductor nonlinearities in optically bistable devices and in optical phase conjugation. This group first demonstrated mid-infrared bistability in In As with a 3 mW threshold, the lowest threshold for any bistability reported at that time. Optical bistability occurs when a semiconductor etalon is illuminated near its bandgap. The nonlinear refractive index combined with reflection feedback causes the transmission or reflection to have two stable outputs at a single value of the input, depending on the device irradiation history. Typical results are shown in Figure 1. Currently under investigation is optical phase conjugation by means of degenerate four wave mixing with two wave mixing recently observed in Cr-doped GaAs.

The spatial and temporal evolution of the CN molecular emission following laser ablation of a TNT analog (3- nitrobenzoic acid) has been studied along with ablation of targets that contain neither nitro groups nor C-N bonds. At a fluence of ~104 J/cm2, behavior indicative of the ablation of native CN bonds has been observed in samples containing no native CN bonds. The recorded data show significant plasma background emissions that pose difficulties for direct spectral imaging. Spatially resolved images suggest that some of the observed phenomena are simply the result of the interaction of the plasma and the observation volume of the collection optics.

Author Institution: Arkansas Center for Laser Applications and Science and Department of Chemistry and Physics, P.O.; Box 419 State University, AR 72467; Embry Riddle Aeronautical University, 600 S. Clyde Morris Boulevard, Daytona Beach, FL 32114

Stimulated emission pumping (SEP) experiments were performed on the nitric oxide molecule in a flow cell environment using lasers with pulse widths of 17–25ps. A lambda excitation scheme, or ‘‘pump–dump” arrangement, was employed with the pump laser tuned to the T00 vibronic band origin (λpump=226.35(1)nm) of the A2Σ+(v′=0,J′)←X2Π1/2(v″=0,J″) and the dump laser scanned from 246–248nm within the A2Σ+(v′=0,J′)→X2Π1/2(v″=2,J″) transition. The rotationally resolved SEP spectra were measured by observing the total fluorescence within the A2Σ+(v′=0,J′)→X2Π1/2(v″=1,J″) transition between 235nm and 237.2nm while scanning the dump laser wavelengths. Multiple rotational states were excited due to the broad laser bandwidth. Measurements showed that the resolved rotational structure depended on the energy and bandwidth of the applied pump and dump laser pulses. Analysis of the observed fluorescence depletion signals yielded an average percent fluorescence depletion of about 19% when λpump=226.35(1)nm and λdump=247.91(1)nm. This value reflects the percent transfer of the NO population from the A2Σ+(V′=0,J′) excited electronic state to the X2Π1/2(v″=2,J″) ground electronic state. The maximum expected depletion is 50% in the limit of dump saturation. Selective excitation of NO at the bandhead provides good spectral discrimination from the background emission and noise and unambiguously confirms the identity of the emitter.

Time-resolved ultrasonic studies revealed a second, delayed ablative pressure pulse after the first primary plasma pressure pulse in a silicon wafer irradiated by a UV nanosecond laser. The intensity-dependent delay time for the second pulse indicates the existence of a corresponding intensity-dependent homogeneous vapor bubble nucleation time in the superheated molten silicon prior to its phase explosion and ablative removal, since the integral pressure correlates with the ablation rate. A transient hot ablative plasma with calculated peak temperature ∼30–90 eV and pressure ∼20–110 GPa is suggested to superheat the bulk silicon via short-wavelength recombination and Bremsstrahlung emission.