Real-time Reflectivity Measurements Research Articles

Microwave Thermal Ablation of Thyroid Nodules with the MONTREAL Technique: Preliminary Clinical Results.

Ultrasound-guided percutaneous Microwave Thermoablation (MWA) is an increasingly popular minimally invasive therapy for the treatment of benign thyroid nodules due to its remarkable heating velocity and resilience to heat sinking. We present a cohort of 26 patients (17 F, 9M, mean age 56.2yy) with 20 goiters (mean volume 34.9cc, min 6.5cc-max 84cc) and 6 autonomously functioning nodules (AFTNs, mean volume 11.6cc, min 3.5cc-max 24cc) treated with MWA, using the moving shot technique and a novel energy delivery algorithm (MONTREAL), based on intra-operative real-time reflectivity measurements: each moving shot was automatically terminated when reflectivity was observed to ramp up. 17G or 18G internally cooled 2.45 GHz applicators were used (AMICA-PROBE, HS HOSPITAL SERVICE SpA), operated at 15-30W. On average, each treatment deposited 9.9±5.5kJ during 696±325s, through 26.7±14.4 moving shots: the mean moving shot duration was 28.1±9.0s, yielding a mean ablation time per unit nodule volume of 34.3±19.1s/cc. No mild nor severe complications were recorded. Post-MWA ultrasound follow-up showed a mean volume reduction ratio of 59.1±10.2% [min 44% - max 80%] at 3 months and of 70.1±8.5% [min 53% - max 91%] at 6 months, with slightly better outcomes in the AFTNs sub-group (volume reduction of 65.3±10.2%/76.0±4.0% at 3/6 months and, in all cases, normal TSH values at 3 months upon pharmacological therapy suspension). MWA treatments of benign nodules with the MONTREAL technique appeared to be safe, fast, and effective. Further research is warranted to confirm these preliminary results.

Interfacial Silicide Formation and Stress Evolution during Sputter Deposition of Ultrathin Pd Layers on a-Si.

Synchrotron experiments combining real-time stress, X-ray diffraction, and X-ray reflectivity measurements, complemented by in situ electron diffraction and photon electron spectroscopy measurements, revealed a detailed picture of the interfacial silicide formation during deposition of ultrathin Pd layers on amorphous silicon. Initially, an amorphous Pd2Si interlayer is formed. At a critical thickness of 2.3 nm, this layer crystallizes and the resulting volume reduction leads to a tensile stress buildup. The [111] textured Pd2Si layer continues to grow up to a thickness of ≈3.7 nm and is subsequently covered by a Pd layer with [111] texture. The tensile stress relaxes already during Pd2Si growth. A comparison between the texture formation on SiOx and a-Si shows that the silicide layer serves as a template for the Pd layer, resulting in a surprisingly narrow texture of only 3° after 800 s Pd deposition. The texture formation of Pd and Pd2Si can be explained by the low lattice mismatch between Pd(111) and Pd2Si(111). The combined experimental results indicate a similar interface formation mechanism for Pd on a-Si and c-Si, whereas the resulting silicide texture depends on the Si surface. A new strain relaxation mechanism via grain boundary diffusion is proposed, taking into account the influence of the thickness-dependent crystallization on the material transport through the silicide layer. In combination with the small lattice mismatch, the grain boundary diffusion facilitates the growth of Pd clusters, explaining thus the well-defined thickness of the interfacial silicide layer, which limits the miniaturization of self-organized silicide layers for microelectronic devices.

Multi-level coding-recoding by ultrafast phase transition on Ge2Sb2Te5 thin films

Quickly switching among different states (levels) is crucial for reconfigurable metamaterials and devices. In this study, the dynamics of establishment and transformation of five amorphous or near-amorphous intermediate states with obvious optical contrasts on Ge2Sb2Te5 phase-change thin films driven by ultrashort laser pulses were investigated using real-time reflectivity measurements. The reversible coding-recoding among the five optical levels was realized by using single-shot picosecond laser pulses with designed fluences. The optical constants, crystalline states and surface morphologies before and after ultrafast multi-level coding were also compared and analyzed. These results may lay a foundation for the further design and application of dynamically reconfigurable optical/photonic devices.

Optical properties of Cr-doped Sb2Te thin films during ultrafast crystallization processes

The crystallization process of Cr-doped Sb2Te thin films induced by repeated femtosecond laser pulses was studied systematically. The threshold effects and corresponding mechanism were comprehensively analyzed by real-time reflectivity measurements, optical microscopic imaging, and Raman scattering spectra. It was found that by doping the appropriate content of Cr into Sb2Te thin films, improved optical-thermal properties could be obtained, even in ultrafast crystallization processes.

Controllable crystallization of Ge2Sb2Te5 phase-change memory thin films driven by multiple femtosecond laser pulses

In this study, controllable crystallization processes of as-deposited amorphous Ge2Sb2Te5 phase-change memory thin films driven by multiple femtosecond laser pulses were investigated in detail. The threshold effects of pulse fluence and the number of pulses were analyzed comprehensively using real-time reflectivity measurements and two-temperature model calculations. The different optical transients indicated three kinds of crystallization processes at low, medium, and high fluences. These results may provide further insights into the ultrafast phase-transition mechanics and are useful in the design of programmable multi-level logic devices based on phase change memory materials.

Adsorption calorimetry during metal vapor deposition on single crystal surfaces: Increased flux, reduced optical radiation, and real-time flux and reflectivity measurements

Thin films of metals and other materials are often grown by physical vapor deposition. To understand such processes, it is desirable to measure the adsorption energy of the deposited species as the film grows, especially when grown on single crystal substrates where the structure of the adsorbed species, evolving interface, and thin film are more homogeneous and well-defined in structure. Our group previously described in this journal an adsorption calorimeter capable of such measurements on single-crystal surfaces under the clean conditions of ultrahigh vacuum [J. T. Stuckless, N. A. Frei, and C. T. Campbell, Rev. Sci. Instrum. 69, 2427 (1998)]. Here we describe several improvements to that original design that allow for heat measurements with ~18-fold smaller standard deviation, greater absolute accuracy in energy calibration, and, most importantly, measurements of the adsorption of lower vapor-pressure materials which would have previously been impossible. These improvements are accomplished by: (1) using an electron beam evaporator instead of a Knudsen cell to generate the metal vapor at the source of the pulsed atomic beam, (2) changing the atomic beam design to decrease the relative amount of optical radiation that accompanies evaporation, (3) adding an off-axis quartz crystal microbalance for real-time measurement of the flux of the atomic beam during calorimetry experiments, and (4) adding capabilities for in situ relative diffuse optical reflectivity determinations (necessary for heat signal calibration). These improvements are not limited to adsorption calorimetry during metal deposition, but also could be applied to better study film growth of other elements and even molecular adsorbates.

Dynamics of laser-induced phase switching in GeTe films

Phase switching in GeTe thin films (grown using a modified metal organic chemical vapor deposition system) upon pulsed femtosecond and nanosecond laser irradiation has been studied. Two in situ methods, i.e., optical microscopy and real-time reflectivity measurements, have been used in order to compare the optical response before and after phase change and to follow the phase change dynamics with a time resolution close to 400 ps. The results show that cycling is possible under irradiation with both fs and ns pulses using single pulses for amorphization and multiple pulses for crystallization. The use of ns pulses favors the crystalline-to-amorphous phase transformation, with a characteristic transformation time of ∼15 ns. The presence of the liquid phase was identified and temporally resolved, featuring a well-defined transient reflectivity state, in between those of the crystalline and amorphous phases. We have also studied the role of material configuration in the phase change dynamics and the mechanisms involved in the re-crystallization process.

Enhanced thermal efficiency for amorphization in nano-structured Ge 2Sb 2Te 5–TiO x films

The dynamics of the melt-quenched amorphization in TiO x mixed Ge 2Sb 2Te 5 films were studied using real-time reflectivity measurements with a nanosecond laser pulse and various amounts of TiO x . It was observed that the laser-power required for amorphization was significantly reduced by 27% when 9.1 mol% of TiO x was incorporated with Ge 2Sb 2Te 5. The lowering of the thermal conductivity of the films, due to the increase in the number of interface between the Ge 2Sb 2Te 5 and TiO x phases, led to an increase in the thermal efficiency. Such enhanced thermal efficiency is of great technological importance in phase change storage applications.

DYNAMICS OF AMORPHIZATION INDUCED IN CRYSTALLINE Ge1Sb2Te4 FILMS BY SINGLE FEMTOSECOND PULSES

The dynamics and the conditions of amorphous transitions induced in a Ge 1 Sb 2 Te 4 system upon a single femtosecond (fs) pulse melting were studied by real-time reflectivity measurements. The system has a multilayer structure of 100 nm ZnS – SiO 2/(15–100 nm) Ge 1 Sb 2 Te 4/120 nm ZnS – SiO 2/0.6 mm polycarbonate substrate. It is shown that under optimum conditions amorphization is completed within 900 ps. The thickness of the phase change layer plays an important role in controlling the heat flow conditions in the system upon a fs pulse irradiation. The use of the fs laser pulse leads to a situation in which the pulse energy is deposited within a very short time in a thin surface layer, leading to heating or melting. The so-generated steep temperature gradient is subsequently smoothed by heat diffusion toward the substrate. The relative thermal process and effects are estimated. The calculated results are consisted with those from real-time reflectivity measurements. The mechanism of crystalline to amorphous transition triggered by single fs laser pulses is discussed.

Incubation behaviour in triazenepolymer thin films upon near-infrared femtosecond laser pulse irradiation

The effects of laser radiation induced by a sequence of ultrashort (130 fs), near-infrared (800 nm) Ti:sapphire laser pulses in ∼1 μm thick triazenepolymer films on glass substrates have been investigated by means of in-situ real-time reflectivity measurements featuring a ps-resolution streak camera and a ns-resolution photodiode set-up. The polymer films show incubation effects when each laser pulse in the sequence has a fluence below the single-pulse damage threshold. Non-damage conditions are maintained for several incubation pulses such that the reflectivity of the film shows a rapid decrease of up to 30% within 1 ns but subsequently recovers to its initial value on a ms timescale. Additional pulses lead to a permanent film damage. The critical number of laser pulses needed to generate a permanent damage of the film has been studied as a function of the laser fluence. Once damage is created, further laser pulses cause a partial removal of the film material from the glass substrate. Scanning force microscopy has been used to characterise ex-situ the irradiated surface areas. Based on these complementary measurements possible incubation mechanisms are discussed.

Laser induced damage studies in mercury cadmium telluride

We have investigated laser induced damage at 1.06 μ m laser wavelength in diamond paste polished (mirror finish) and carborundum polished Hg 0.8 Cd 0.2 Te (MCT) samples with increasing fluence as well as number of pulses. Evolution of damage morphology in two types of samples is quite different. In case of diamond paste polished samples, evolution of damage morphological features is consistent with Hg evaporation with transport of Cd/Te globules towards the periphery of the molten region. Cd/Te globules get accumulated with successive laser pulses at the periphery indicating an accumulation effect. Real time reflectivity (RTR) measurement has been done to understand melt pool dynamics. RTR measurements along with the thermal profile of the melt pool are in good agreement with thermal melting model of laser irradiated MCT samples. In case of carborundum polished samples, laser damage threshold is significantly reduced. Damage morphological features are significantly influenced by surface microstructural condition. From comparison of the morphological features in the two cases, it can be inferred that laser processing of MCT for device applications depends significantly on surface preparation conditions.

In-Situ and Ex-Situ Measurements on Silicon Thin Films Fabricated by Excimer Laser Annealing

The phase transitions occurring at the surface of amorphous silicon and a-Si/SiO2interface of the sample during XeF excimer laser annealing has been investigated by nonintrusive in-situ real-time optical reflectivity and transmissivity measurements with nanosecond time resolution. Five distinct regimes were demonstrated on the basis of various excimer laser fluences. The structural transformation scenario of silicon films based on the recrystallization mechanism is proposed to interpret the formation of the grain microstructure. The ex-situ microstructural characterizations of excimer laser-irradiated region as a function of various excimer laser energy densities were characterized by scanning electron microscopy and micro-Raman scattering measurements. A super-lateral growth length of approximately 1μm was obtained in a 90-nm-thick a-Si film by a single excimer laser pulse without any substrate heating. Micro-Raman scattering measurements as a function of various excimer laser energy densities were also carried out.

Optical evidence for reactive processes when embedding Cu nanoparticles inAl2O3 by pulsed laser deposition

The optical response of nanocomposite thin films formed by Cunanoparticles (NPs) embedded in amorphous aluminium oxide(Al2O3) prepared by pulsed laser deposition (PLD) in vacuum is studied in order to investigatethe possible existence of reactive processes on the Cu NPs during their covering withAl2O3. The study is performed as a function of the laser fluence on theAl2O3 target(0.6–4.6 J cm−2), while the laser fluence for Cu ablation is kept constant(1.8 J cm−2). The structural analysis of the films shows that they are formed by a high density of NPswith average dimensions in the 4.9–5.9 nm range. The optical response of thefilms has been followed in situ by real-time reflectivity measurements at 633 nmand after deposition by transmission measurements as a function of wavelengtharound the surface plasmon resonance (SPR). For low laser fluences on theAl2O3 target, the absorption spectrum is dominated by a well-defined SPR absorption band at 1.9 eV.As the laser fluence is increased, the intensity of the absorption band associated with the SPRdecreases and shifts to 2.1 eV. The films deposited at low fluences contain metallic Cu NPs and,as the laser fluence increases sputtering of Cu from the NPs and mixing of the species from theAl2O3 deposition with the Cu from the NPs surface takes place. The latter process leads to theformation of an Al–Cu oxide cover on the Cu NPs. The present results provide evidencefor mixing of species from the host and Cu at the surface of the NPs, and it isshown how the degree of mixing depends on the laser fluence used to ablate theAl2O3 host target.

The influence of wavelength on phase transformations induced by picosecond and femtosecond laser pulses in GeSb thin films

Cycling between the crystalline and amorphous phases of 25-nm-thick GeSb films induced by single laser pulses of duration of 100fs or 20ps is investigated in the 400–800nm wavelength range. The time evolution of the phase transformations has been studied with picosecond resolution real-time reflectivity measurements at a probe wavelength of 514.5nm and also with femtosecond and picosecond pump-probe measurements. Upon picosecond irradiation, three regimes are identified: for wavelengths below ∼550nm and above ∼750nm, the total time to transform between the crystalline and amorphous phases is of the order of 10–24ns while in the intermediate wavelength range of 600–750nm, the transformation time is only ∼650ps. Upon 100fs irradiation, the transformation times are observed to decrease with increasing wavelength with the shortest times of ∼5ns for crystallization and ∼10ns for amorphization, both occurring at 800nm. This behavior is discussed in terms of how the wavelength-dependent refractive index of the phases involved influences the initial supercooling of the molten volume and the subsequent resolidification scenario.

Optical evidence for a self-propagating molten buried layer in germanium films upon nanosecond laser irradiation

This work describes the phase transitions occurring at the film-substrate interface of amorphous germanium films upon nanosecond laser-pulse-induced melting of the surface. Films with thickness ranging from 50 to 130 nm deposited on glass substrates were studied. Real-time reflectivity measurements with subnanosecond time resolution performed both at the air-film and film-substrate interfaces were used to obtain both surface and in-depth information of the process. In the thicker films (⩾80nm), the enthalpy released upon solidification of a shallow molten surface layer induces a thin buried liquid layer that self-propagates in-depth towards the film-substrate interface. This buried liquid layer propagates with a threshold velocity of 16±1m∕s and causes, eventually, melting at the film-substrate interface. In the thinnest film (50 nm) there is no evidence of the formation of the buried layer. The presence of the self-propagating buried layer for films thicker than 80 nm at low and intermediate laser fluences is discussed in terms of the thermal gradient in the primary melt front and the heat released upon solidification.

Real-time X-ray study of roughness scaling in the initial growth of epitaxial BaTiO 3/LaNiO 3 superlattices

The evolution of surface and interface roughness during heteroepitaxial growth of strained BaTiO 3/LaNiO 3(BTO/LNO) superlattices on SrTiO 3 (0 0 1) substrates was investigated using real-time X-ray reflectivity measurements in situ with synchrotron radiation. The roughness scaling of the growth front and interface of superlattices with modulation length below (2 and 6 nm) and beyond (20 nm) the critical thickness was studied against the bilayer number. The fitted results of in situ specular X-ray reflectivity curves reveal a two-dimensional growth of BTO and LNO sublayers on the SrTiO 3 substrate for superlattices with a modulation length below the critical thickness. Moreover, a larger root-mean-square roughness of BTO/LNO interface was obtained in the superlattice with modulation length beyond the critical thickness indicating lattice relaxation in the superlattice structure. Fitted results of in situ specular X-ray reflectivity curves provide the first evidence for power-law scaling of the root-mean-square roughness of an interface and a surface in the superlattice structure. Observation of such a roughness scaling behavior indicates that strain plays an important role in the determination of microstructure in the growth of epitaxial BTO/LNO superlattices.

Femtosecond laser-induced decomposition in triazenepolymer thin films

The damage induced by ultrashort, 130 fs, near-infrared, 800 nm, Ti:sapphire laser pulses in 1 μm thick triazenepolymer films on glass substrates has been investigated. Real-time reflectivity measurements with a ps-resolution streak camera and a ns-resolution photodiode set-up have been performed to study in situ the structural transformation dynamics upon single-pulse excitation with laser fluences above the threshold of permanent damage. Scanning force microscopy has been used to probe ex situ the corresponding surface topography of the ablated spots. Modulated lateral force microscopy (M-LFM) has been applied to observe alterations of the local friction properties within and around the irradiated areas.

Phase change dynamics in a polymer thin film upon femtosecond and picosecond laser irradiation

The influence of the pulse duration on the laser-induced changes in a thin triazenepolymer film on a glass substrate has been investigated for single, near-infrared (800 nm) Ti:sapphire laser pulses with durations ranging from 130 fs up to 2.6 ps. Post-irradiation optical microscopy has been used to quantitatively determine the damage threshold fluence. The latter decreases from ∼800 mJ/cm 2 for a 2.6 ps laser pulse to ∼500 mJ/cm 2 for a pulse duration of 130 fs. In situ real-time reflectivity (RTR) measurements have been performed using a ps-resolution streak camera set-up to study the transformation dynamics upon excitation with single pulses of duration of 130 fs and fluences close to the damage threshold. Very different reflectivity transients have been observed above and below the damage threshold fluence. Above the damage threshold, an extremely complicated behaviour with oscillations of up to 100% in the transient reflectivity has been observed. Below the damage threshold, the transient reflectivity decreases by as much as 70% within 1 ns with a subsequent recovery to the initial level occurring on the ms timescale. No apparent damage could be detected by optical microscopy under these irradiation conditions. Furthermore, within the 395–410 mJ/cm 2 fluence range, the transient reflectivity increases by ∼10%. The analysis of these results indicates that the observed transformations are thermal in nature, in contrast to the known photochemical decomposition of this triazenepolymer under UV irradiation.

Amorphization induced by subnanosecond laser pulses in phase-change optical recording media.

We have investigated the dynamics of amorphization induced in phase-change optical recording media by focused laser pulses of subnanosecond duration. We initiated localized amorphism by using a focused laser beam to melt the phase-change material and completed the change by rapid cooling by means of thermal diffusion. These studies were conducted by use of real-time reflectivity measurements with a pump-and-probe technique in which both pump and probe pulses had a duration of approximately 510 ps. Our transient-reflectivity measurements indicate that the process that leads to amorphism has three distinct stages, namely, rapid melting, solidification, and slow relaxation.

Influence of pulse duration on the amorphization of GeSb thin films under ultrashort laser pulses

Laser-induced amorphization of crystalline, 25-nm-thick, Sb-rich GeSb films has been studied for pump pulse durations in the range from 100 fs up to 6 ns. The dynamics of the phase change has been investigated using real-time reflectivity measurements with picosecond time resolution performed with a streak camera. For pulses in the femtosecond regime, the time required to complete the transformation is of the order of ∼10–15 ns. When the pulse duration is increased to the 1.5–20 ps range, the transformation time decreases to values as short as 400 ps, while for nanosecond laser pulses amorphization is not achievable. This behavior is discussed in terms of the heat flow dynamics of the system and the influence of pulse duration on the initial supercooling and nucleation rate.