Droplet Laser Research Articles

Effects of the dynamics of droplet-based laser-produced plasma on angular extreme ultraviolet emission profile

The emission distribution of extreme ultraviolet (EUV) radiation from droplet targets is dependent on the dynamics of the laser-produced plasma. The EUV emission is measured on a 2% bandwidth centered at 13.5 nm (in-band). The targets of the laser are small (sub-50 μm) tin droplets, and the in-band emission distribution is measured for different laser irradiances and droplet sizes at various angular positions. Larger droplets lead to a faster decay of EUV emission at larger angles with respect to the laser axis. A decrease in laser irradiance has the opposite effect. The measurements are used together with an analytical model to estimate plume dynamics. Additionally, the model is used to estimate EUV emission distribution for a desired droplet diameter and laser irradiance.

A New Liquid Droplet Laser Desorption Source Combined with Supersonic Jet Expansion: Application to Phenol and its Water Clusters

Abstract We have developed a new laser source, for the spectroscopy of nonvolatile molecules in gas phase. It is based on a laser induced liquid bead ion desorption source (LILBID) combined with a supersonic beam. The cold molecules produced with this technique are sampled with Resonant Two Photon Ionization spectroscopy (R2PI) to measurement of the gas phase optical spectra. LILBID allows to bring nonvolatile molecule from liquid phase (out of a droplet) into gas phase, by means of multi photon ablation with IR photons exciting the vibrations of the solvent. Phenol and its different water clusters have been used as an example to demonstrate the method and to standardise the new experimental setup. The recorded R2PI spectral data of phenol monomer and its different water clusters obtained from this laser desorption technique are in very good agreement with the previously published data. This technique opens a new door for the measurement of molecules under microsolvation and potentially for zwitterion formation of thermally labile, nonvolatile molecules such as amino acids or biomolecules.

Effect of velocity on microdroplet fluorescence quantified by laser-induced fluorescence.

Microdroplets generated inside microfluidic devices have been widely used as miniaturized chemical and biological reactors allowing important reductions in experimental fluid volumes and making it possible to carry out high-throughput assays. Laser-induced fluorescence (LIF) is commonly used to detect and quantify the product, marker or cell content inside each individual droplet. In this work, we employed this technique to characterize the response of in-flow microdroplets filled with fluorescein dye at different laser powers and flow velocities. Using two parallel laser beams closely focused inside a microchannel we determined the microdroplet velocities and showed that the droplet fluorescence intensity decreases exponentially with reducing velocities because of the effects of photobleaching. In contrast, the fluorescence intensity increases linearly with laser power in the 4-10 mW range. When LIF is used for microdroplet measurements it is important to consider not just the fluorophore concentration but also the droplet velocity and laser power in the development of quantitative assays.

Biomaterial Laser: All‐Biomaterial Laser Using Vitamin and Biopolymers (Adv. Mater. 41/2013)

Vitamin pills sitting on a substrate are illustrated. A similar arrangement, on a much smaller microscopic scale, is used to build a vitamin droplet laser made of FDA-approved human-compatible biomaterials. Daily-taken vitamin B2 is used as the optical gain medium, and vitamin-doped microdroplet lasers with a low-level threshold are embedded on a super-hydrophobic poly-L-lactic acid (PLLA) substrate. This work is described by S. H. Yun and co-workers on page 5943.

Dye lasing in optically manipulated liquid aerosols

We report lasing in airborne, rhodamine B-doped glycerol-water droplets with diameters ranging between 7.7 and 11.0 μm, which were localized using optical tweezers. While being trapped near the focal point of an infrared laser, the droplets were pumped with a Q-switched green laser. Our experiments revealed nonlinear dependence of the intensity of the droplet whispering gallery modes (WGMs) on the pump laser fluence, indicating dye lasing. The average wavelength of the lasing WGMs could be tuned between 600 and 630 nm by changing the droplet size. These results may lead to new ways of probing airborne particles, exploiting the high sensitivity of stimulated emission to small perturbations in the droplet laser cavity and the gain medium.

Polarization and Droplet Size Effects in the Laser-Trapping-Induced Reconfiguration in Individual Nematic Liquid Crystal Microdroplets

We experimentally demonstrate reordering throughout the inside of an individual bipolar nematic liquid-crystalline microdroplet optically trapped by a highly focused laser beam, when the laser powers are above a definite threshold. The threshold depends on the droplet size and laser polarization. A physical interpretation of the results is presented by considering the nonlocal orientations of the nematic liquid-crystal molecules in the droplets with the dimensions on the order of the focal spot diameter or larger. On the basis of the finite size approximation, we show that the dependence of threshold power on the droplet size is calculated to be in qualitative agreement with the experimental data.

一维云粒子激光探测仪研制及探测数据分析

一维云粒子激光探测仪研制及探测数据分析

On the correlation between droplet volume and irradiation conditions in the laser forward transfer of liquids

The analysis of the morphology of droplets printed through laser-induced forward transfer (LIFT) of liquid films shows that: (i) the droplet volume is linearly related with the energy of the laser pulse that originated it, (ii) the liquid ejection process is activated by an energy density threshold F 0, and (iii) the droplet volume can be correlated with a dimensional parameter of the laser beam through an oversimple model that states that the amount of printed liquid equals the liquid contained in the cylindrical portion of an irradiated film whose base corresponds to the cross-sectional area of the beam with energy density higher than F 0. Although these issues seem to describe correctly the LIFT process, some problematic instances arise from them. Thus, the linear relation between droplet volume and laser pulse energy seems to be inconsistent with the existence of the threshold F 0. On the other hand, the compatibility between the model and the aforementioned linear relation requires to be explained. Finally, the model is based on the idea that transfer takes place in a way analogous to the LIFT of solid films, but time-resolved imaging studies have demonstrated that liquid ejection follows a dynamics which seems quite unsuited with that idea. In this work previous results are re-analyzed and new experiments are performed in an attempt to clarify these questions. It is then shown that the inconsistencies pointed out are only apparent, and that the validity of the model is limited to irradiation conditions where the beam dimensions are significantly larger than the thickness of the liquid film. Furthermore, an explanation is provided for the dependence of the success and failure of the model on those irradiation conditions in terms of the diverse liquid ejection dynamics taking place.

Spectral and polarization characteristics of a deformed droplet laser

A fluorescent microdroplet was created in an elastomer by using a glycerol, polyethylene-glycol, or polysorbate solution of rhodamine 6G. Droplets exhibited a whispering gallery mode emission at a specific spectral band (yellow-green to orange), which shifted to a longer wavelength range (redshift) as the droplet diameter increased. The TE mode emission was stronger than the TM mode emission. As the elastomer was pressed in one direction, the droplet expanded in the perpendicular directions. Consequently, the resonant emission disappeared in the pressure direction, while the emission in the perpendicular direction exhibited the redshift of the resonance peaks.

Versatile optofluidic ring resonator lasers based on microdroplets

We develop a novel nL-sized microdroplet laser based on the capillary optofluidic ring resonator (OFRR). The microdroplet is generated in a microfluidic channel using two immiscible fluids and is subsequently delivered to the capillary OFRR downstream. Despite the presence of the high refractive index (RI) carrier fluid, the lasing emission can still be achieved for the droplet formed by low RI solution. The lasing threshold of 1.54 µJ/mm(2) is achieved, >6 times lower than the state-of-the-art, thanks to the high Q-factor of the OFRR. Furthermore, the lasing emission can be conveniently coupled into an optical fiber. Finally, tuning of the lasing wavelength is achieved via highly efficient fluorescence resonance energy transfer processes by merging two different dye droplets in the microfluidic channel. Versatility combined with improved lasing characteristics makes our OFRR droplet laser an attractive platform for high performance optofluidic lasers and bio/chemical sensing with small sample volumes.

Probing the limits of liquid droplet laser desorption mass spectrometry in the analysis of oligonucleotides and nucleic acids

In the present work we demonstrate the advantages of LILBID mass spectrometry (laser-induced liquid bead ion desorption) in the analysis of nucleic acids and large oligonucleotides. For established methods like matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI), the mass analysis of oligonucleotides or of noncovalent oligonucleotide-protein complexes, in particular of very large ones, still represents a considerable challenge either due to the lack of native solutions or nonspecific adduct formation or due to a reduced salt tolerance or a high charge state of the ions. With LILBID, oligonucleotides, solvated in micro-droplets of aqueous buffer at certain pH and ion strength, are brought into the gas phase by laser ablation. We show that our method is able to detect single- and double-stranded oligonucleotides with high softness, demonstrated by the buffer dependence of the melting of a duplex. The absolute sensitivity is in the attomole range concomitant with a total analyte consumption in the femtomole region. The upper mass limit of oligonucleotides still detected with good signal-to-noise ratio with LILBID is the 1.66 MDa plasmid pUC19. With DNA ladders from short duplexes with sticky ends, we show that LILBID correctly reflects the relative thermodynamic stabilities of the ladders. Moreover, as an example for a specific DNA-protein complex we show that a NF-kappaB p50 homodimer binds sequence specifically to its match DNA. In summary we demonstrate that LILBID, although presently performed only with low mass resolution, due to these advantages, is an alternative mass spectrometric method for the analysis of oligonucleotides in general and of specific noncovalent nucleic acid-protein complexes in particular.

Effect of static magnetic field on a thermal conductivity measurement of a molten droplet using an electromagnetic levitation technique

Recently, a novel method of measuring the thermophysical properties, particularly thermal conductivity, of high-temperature molten materials using the electromagnetic levitation technique has been developed by Kobatake et al. [H. Kobatake, H. Fukuyama, I. Minato, T. Tsukada, S. Awaji, Noncontact measurement of thermal conductivity of liquid silicon in a static magnetic field, Appl. Phys. Lett. 90 (2007) 094102]; this method is based on a periodic laser-heating method, and entails the superimposing of a static magnetic field to suppress convection in an electromagnetically levitated droplet. In this work, to confirm the fact that a static magnetic field really suppresses convection in a molten silicon droplet in an electromagnetic levitator, numerical simulations of convection in the droplet and periodic laser heating in the presence of convection have been carried out. Here, the convections driven by buoyancy force, thermocapillary force due to the temperature dependence of the surface tension on the melt surface, and electromagnetic force in the droplet were considered. As a result, it was found that applying a static magnetic field of 4 T can suppress convection in a molten silicon droplet enough to measure the real thermal conductivity of molten silicon.

13.5 nm EUV generation from tin-doped droplets using a fiber laser

A comprehensive study of the spectral and Mo-Si mirror inband EUV emission from tin-doped droplet laser plasma targets irradiated with a single 1064 nm beam from an Yb:doped fiber laser is reported.With pre-pulse enhancement, in-band conversion efficiency of approximately 2.1% is measured for laser irradiance intensities near 8 x 10(10) W/cm(2). This is the first study to be reported that uses a high-power, high repetition rate fiber laser with the high repetition rate droplet targets where EUV generation from plasmas is measured.

Ion emission measurements and mirror erosion studies for extreme ultraviolet lithography

Mirror erosion by high energy ion emission from extreme UV light sources is one of the main factors contributing to EUVL collector mirror reflectivity degradation. We are measuring ion energy distributions at the mirror distance from the plasma utilizing three different ion diagnostics for the case of tin-doped microscopic droplet laser plasmas. Typical ion energy distributions measured by an electrostatic spectrometer are described. From the ion energy distributions, an estimate of mirror erosion is obtained. The effectiveness of electrostatic field mitigation is evaluated for the EUVL source requirement.

Dried-droplet laser ablation ICP-MS of HPLC fractions for the determination of selenomethionine in yeast

Offline coupling of high performance liquid chromatography (HPLC) to dried droplet laser ablation (LA) ICP-MS detection is described. The method is based on the offline spotting of HPLC fractions onto a substrate followed by LA introduction of the dried sample residues to an ICP-MS for elemental analysis. Quantitation of selenomethionine (SeMet) in yeast using species specific isotope dilution (ID) was achieved following digestion of samples in 4 M methanesulfonic acid and HPLC separation of species. Chromatographic fractions having retention times of 180–210 seconds were collected for each sample. Dried micro-droplets from each fraction were ablated from a polystyrene substrate and quantitated for SeMet. Concentrations of 3301 ± 18 and 3309 ± 24 μg g−1 (one standard deviation, n = 4) with RSDs of 0.55% and 0.73% were obtained based on measured 78Se/74Se and 82Se/74Se ratios, in good agreement with the values of 3309 ± 19 and 3305 ± 26 μg g−1 (one standard deviation, n = 4, RSDs of 0.58% and 0.79%), respectively, obtained by direct HPLC-ICP-MS analysis. The proposed method provides a satisfactory alternative technique for the quantitation of SeMet in yeast.

Studies of high-repetition-rate laser plasma EUV sources from droplet targets

The water droplet laser plasma source has been shown to have many attractive features as a continuous, almost debris-free source for extreme ultraviolet (EUV) and X-ray applications. Through a dual experimental and theoretical study, we analyze the interaction physics between the laser light and the target. The hydrodynamic laser plasma simulation code, Medusa103 is used to model the electron density distribution for comparison to electron density distributions obtained through Abel inversion of plasma interferograms. In addition, flat field EUV spectra are compared to synthetic spectra calculated with the atomic physics code RATION.

Dynamics of Mass-Limited Laser Plasma Targets as Sources for Extreme Ultraviolet Lithography

The droplet laser plasma source has many attractive features as a continuous, almost debris-free source for extreme ultraviolet (EUV) and X-ray radiation applications. In a combined experimental and theoretical study, we are analyzing the interaction physics between the laser light and microscopic spherical liquid droplet targets over a range of conditions.

EUCLIDES: European EUVL Program

In August 1998 a new European research program named extreme ultraviolet concept lithography development system (EUCLIDES) was started. The program, headed by ASM Lithography (ASML), partnered by Carl Zeiss and Oxford Instruments, is evaluating extreme ultraviolet lithography (EUVL) as a viable lithographic solution for resolutions of 70 nm and smaller. In this article a summary of program objectives and status will be given, followed by an overview of recent highlights obtained by the various program partners and subcontractors. Results of the system architecture study will be presented. These include the (preliminary) results of the comparison study between laser plasma and synchrotron sources in terms of throughput and cost-of-ownership. A novel technique will be presented which mitigates resist outgassing without the need of a window between the projection optics and the wafer. Using this idea the EUV flux on the wafer can be doubled hence increasing the throughput. An EUVL test bench, containing an EUV interferometer is under construction. The test bench contains an operational water droplet laser plasma source. Experimental results will be presented. Part of the program focuses on optical and mechanical design, substrate fabrication, metrology, and multilayer coatings. Most recent results on coatings and optical substrate manufacturing will be presented.

Solid‐state droplet laser made from an organic blend with a conjugated polymer emitter

Lasers based on organic materials have been produced with a wide range of resonator design and in a variety of geometries. A new strategy is presented for fabricating permanently near‐spherical whispering gallery mode (WGM) lasers from a blend of PPV7 and PBD (see Figure) by a melting and resolidification process. The thresholds and quality factors of these resonators are estimated and discussed.magnified image

Core-resonance capillary-fiber whispering-gallery-mode laser

We describe the observation of laser emission at wavelengths corresponding to whispering-gallery modes from a highly refractive dye-doped solvent flowing in a normally illuminated silica capillary fiber. The cylindrical whispering-gallery-mode laser described here differs from the well-known spherical droplet laser in that it has an internal refractive index discontinuity. Boundary-value computations are presented that reveal that the feedback responsible for the observed lasing modes is associated with total internal reflection at the liquidsilica interface. No laser peaks are observed when the refractive index of the solvent is less than that of the silica.