In the OSTI Collections: Ultrafast Processes

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Article Acknowledgement:
Dr. William N. Watson, Physicist
DOE Office of Scientific and Technical Information

 

Examples of ultrafast chemical and electronic processes

Further examples of electronic processes

Chemical reactions from shock waves

Ultrafast instruments

References

Research Organizations

Reports available through OSTI's SciTech Connect

Patent available through OSTI's DOepatents

Additional References

 

 

 

Because the atoms that make up a liquid or solid are so small and so close together, physical or chemical processes that involve the atoms’ interactions with their nearest neighbors involve only short-distance motion and so generally happen in a very short time.  Exactly what happens during such a process is only partially deducible from knowledge of the material’s state before and afterward.  Learning the details of such a process requires dividing its short duration into even shorter ones and looking at what’s going on in the subintervals.  Recent research reports describe how the details of numerous ultrafast processes were discovered by examining the processes during subintervals of nanoseconds, picoseconds, femtoseconds, or even smaller durations.  Other reports describe new ways to make it possible to examine such brief occurrences.  To appreciate how short these time intervals are, recall that, while light can travel through nearly 300,000 kilometers of empty space in one second—most of the way to the moon— light can only travel a bit less than 30 centimeters in just one billionth of a second (one nanosecond).  In a picosecond (a trillionth of a second), light moves just under 0.3 millimeters.  And in a femtosecond, one quadrillionth of a second, light traverses a space just under 300 nanometers across—the width of no more than a few thousand atoms side by side.  

 

Atoms are so small that the femtosecond spent by light to pass a few thousand atoms is a very short time from our usual perspective.  But the fact that light can pass thousands of atoms in a femtosecond means that a small number of atoms right next to each other can interact significantly in even one femtosecond.  Understanding the properties of matter depends on understanding the atoms and subatomic particles that it’s composed of, and part of that understanding is a knowledge of the ultrafast processes that these components undergo.  A variety of ultrafast-process investigations are described in recent research reports available through SciTech Connect and DOepatents, many of which were aimed at clarifying details of two broad types of ultrafast processes:  those involving the motion of charges in materials that can be used for electronic devices, and those involving the rearrangements of atoms (and their constituent electric charges) in chemical reactions. 

 

Examples of ultrafast chemical and electronic processes

 

Molecular desorption from a solid surface is an ultrafast chemical process whose details are affected by that surface’s features.  Desorption dynamics has many important consequences for the outcome of catalytic reactions, which involve the temporary participation of another material whose state after the reaction ends up the same as its state before the reaction.  These facts are noted in a report by researchers at SLAC National Accelerator Laboratory and several other institutions, in the paper “Strong Influence of Coadsorbate Interaction on CO Desorption Dynamics Probed by Ultrafast X-ray Spectroscopy and Ab Initio Simulations”[SciTech Connect] published in Physical Review Letters.  The spectroscopy mentioned in the title is a technique for observing different parts of a spectrum of x-ray frequencies.  The authors found that carbon monoxide molecules desorb differently from surfaces of the metal ruthenium[Wikipedia], depending on whether oxygen atoms are also present on the surface.  They exposed carbon monoxide (CO) molecules to femtosecond-long pulses of x-rays.  The x-rays, being high-frequency waves of oscillating electromagnetic fields, accelerated the electrically charged particles that made up the carbon monoxide molecules, thus transferring energy to the particles.  Depending on the molecules’ condition, which was affected by the presence or absence of oxygen on the ruthenium surface, the charged particles would absorb different amounts of energy from x-rays of different frequencies.  The different amounts indicated the different conditions of the carbon monoxide molecules as they were being desorbed from the ruthenium. 

 

In the experiment, a ruthenium crystal was first prepared, and then a gas made of either pure carbon monoxide, or a mixture of carbon monoxide and oxygen, was adsorbed onto the crystal and afterward exposed to x-rays.  The x-rays absorbed by the carbon monoxide had a narrower frequency spread and slightly higher average frequency for the pure CO than for the CO-oxygen mix.  In further experiments, CO molecules were desorbed from both crystal preparations when the systems were heated with lower-frequency electromagnetic waves in the form of visible-light laser pulses.  About 10 picoseconds after desorption, the CO molecules were exposed to a one-femtosecond x-ray pulse that resulted in still other x-ray absorption patterns, again differing by whether or not oxygen atoms accompanied CO molecules on the ruthenium surface.   These different absorption patterns indicated different arrangements of the electric charges within the desorbed COs' oxygen, which in turn indicated that different things were happening to the desorbed molecules.  The observed differences prompted computer calculations of how the presence or absence of adsorbed oxygen should affect the CO molecules’ electron distributions.  That information, combined with the results of the experiment, indicates that when oxygen is absent, some of the energy going into heated CO molecules goes into making them rotate instead of propelling them directly away from the ruthenium surface, so that many of the CO molecules stay trapped near the heated surface before finally gaining enough energy from it to desorb and escape from it.  But when the ruthenium surface is largely covered by oxygen, the oxygen atoms restrict the CO molecules’ rotation, so that more of the heat energy going into the CO molecules drives them directly away from the surface.  (See Figure 1.)  Also, when oxygen atoms are present on the surface, some of them will combine with CO molecules and form carbon dioxide, so that carbon monoxide desorption and oxidation reactions competed with each other.  However, the presence of oxygen was expected to make randomly oriented CO molecules less likely to find vacant adsorption sites or to react with oxygen atoms and form CO2. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1.  Pictures from molecular dynamics simulations showing trajectories of carbon monoxide molecules during desorption from a ruthenium surface at a temperature of 2000 kelvins.  The large blue spheres represent ruthenium atoms while the smaller green and red spheres represent carbon and oxygen atoms respectively, with the joined carbon-oxygen pairs representing molecules of carbon monoxide.  The vertical scale represents distance in Ångstrom units (tenths of a nanometer) above the nuclei of the upper layer of ruthenium atoms.  Typical desorption trajectories are expected to be quite different in the absence (a) and presence (b) of oxygen atoms adsorbed onto the surface among the carbon monoxide molecules, since the adsorbed oxygen atoms limit the possibility of energy going into the rotation of carbon monoxide molecules instead of their desorption.  {After “Strong Influence of Coadsorbate Interaction on CO Desorption Dynamics Probed by Ultrafast X-ray Spectroscopy and Ab Initio Simulations” [SciTech Connect], p. 4.}

 

Another group of researchers from SLAC, Stanford University, and Rheinisch-Westfälische Technische Hochschule Aachen University in Germany investigated an ultrafast electronic process of potential use for data storage.  Heat and electric fields applied to certain materials can switch their atomic arrangements back and forth between the regular spacing and orientation of a crystal lattice and a random amorphous arrangement.   The orderly arrangement conducts electric current easily while the amorphous one offers greater resistance to it—just the sort of binary difference that lets such materials be used as a memory device whose different states represent distinguishable 1s and 0s.  The researchers sought to learn the initial stages of how one such type of material, germanium-antimony-tellurium, reacts to electric fields and heat just before its phase actually changes.  However, conventional means of applying electric fields to materials in a circuit and measuring the induced current can only change the fields in something like a nanosecond—not fast enough to resolve the details of the materials’ initial response. 

 

The researchers used a different way to apply electric fields about a thousand times faster, by using single-oscillation terahertz-frequency light pulses, in which the electric fields rise and fall in less than a picosecond.  Since this didn’t require putting the material into a circuit, it gave the added benefit that the material’s response to the electric-field change didn’t have to be sorted out from the responses of other circuit elements.  But since no circuit was used, the response examined was not the flow of current, but the absorption of electromagnetic-wave energy by the materials’ electric charges, as was done in the carbon-monoxide desorption experiment.  The electromagnetic waves in this experiment were infrared light instead of x-rays, and supplied energy in longer pulses of about 50 femtoseconds instead of one femtosecond.  The report “Ultrafast terahertz-induced response of GeSbTe phase-change materials”[SciTech Connect], published in Applied Physics Letters, describes the observed variation of infrared light absorption with material temperature, light frequency, and time in the material’s crystalline and amorphous phases.  The electric field from the terahertz light pulses generally made the material’s electric charges more mobile, meaning that the materials were more electrically conductive, and thus more able to absorb infrared light immediately afterward, with the light absorption (and charge mobility) decreasing substantially over a period ranging from picoseconds to nanoseconds after the pulse.  The longer periods were observed in the more conductive crystalline phase.  The researchers found that the material’s electrons were heated up by the terahertz light pulse at a rate proportional to both the pulse’s intensity and the material’s electric conductivity.  They noted at the end of their report that more intense pulses “may enable extraction of the ultimate time-scales for the switching process itself” between conductive and resistive phases. 

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Further examples of electronic processes

 

In some materials, the electrons interact with the material’s atomic lattice and with each other in such a way that the combined behavior of the large set of electrons resembles the behavior of a smaller number of weakly interacting particles in empty space.  These entities, called quasiparticles[Wikipedia], may move quite differently from real electrons in some respects.  Whereas individual electrons moving at slow speed in empty space have energies almost directly proportional to their momenta squared, at least some quasiparticles moving at any speed in what are called “Dirac materials”[arXiv] have energies directly proportional to the momenta themselves instead of their squares.  A slide presentation from Los Alamos National Laboratory, “Ultrafast Probes for Dirac Materials”[SciTech Connect], addresses the question of whether quasiparticles in graphene retain this direct proportionality when they absorb a quantum of energy from light, noting that such information is important for the use of graphene in solar cells, particle detectors, and data displays. 

 

As with the aforementioned experiment with germanium-antimony-tellurium, the graphene in this experiment was exposed to one electromagnetic pulse and then examined with a different pulse, in this case to energize the quasiparticles and then find out how they behaved once they had absorbed the energy from one light quantum.  The results, according to the presentation, indicated that the energy and momentum of these quasiparticles were also directly proportional. 

 

The same slide presentation also illustrated results of experiments with two varieties of materials known as topological insulators.  The electrons in these materials only conduct electric current near the surface; a thick piece of the material acts as an electrical insulator well within its bulk.  Where it conducts near the surface, though, the charge-carrying quasiparticles have the energy-momentum direct proportionality of a Dirac material.  These experiments also involved pumping the material with energy from one ultrafast pulse and probing it with a second ultrafast pulse. 

 

Experiments summarized in an Emory University report, “Electronic Coupling Dependence of Ultrafast Interfacial Electron Transfer on Nanocrystalline Thin Films and Single Crystal”[SciTech Connect], were undertaken to enable the design and improvement of nanoparticle-based solar cells by understanding how ultrafast electron injection rates depend on the strength of the forces involving the electrons.  In these experiments, semiconducting nanoparticles were put into solutions, and femtosecond-resolving observations were made of electron transfer between the nanoparticles and materials such as dyes that were adsorbed onto them.  The experiments examined how the ultrafast electron transfer is affected by the solution pH[Wikipedia], by the composition of the solvent itself and of the semiconductor, and by which atoms in the adsorbed molecules conduct electrons into the semiconductor by direct contact with it. 

 

Quite different materials, which can retain both magnetic and electric-charge polarity, were subjected to ultrafast observation and ultrafast manipulation in experiments described in the Los Alamos slide presentation, “Ultrafast all-optical manipulation of interfacial magnetoelectric coupling”[SciTech Connect].  Some materials undergo shape changes when they are magnetized, while other materials exhibit distinct displacements of their positive and negative charges when they are stretched or squeezed.  A material that exhibits both effects—magnetostriction[Wikipedia] and piezoelectricity[Wikipedia]—would thus undergo a positive-negative charge separation when it was magnetized.  Both effects can be reversed, so that inducing charge separations in piezoelectric materials can stretch or squeeze them, while stretching or squeezing a magnetostrictive material can magnetize it.  Thus a material that is both piezoelectric and magnetostrictive could be magnetized by applying an electric field to it instead of a magnetic field:  the electric field would accelerate the material’s positive and negative charges in opposite directions, thus stretching or squeezing the material, which in turn would magnetize the material.  The electric field of an ultrashort electromagnetic pulse could thus rapidly change the material’s magnetic state.  Such ultrafast changes in a material, like the rapid crystalline-amorphous changes in the atomic arrangement of germanium-antimony-tellurium, would make the material useful for rapid data storage.  Details of these changes in magnetostrictive piezoelectrics, including the timescale of the process, were found in experiments illustrated in the slide presentation. 

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Chemical reactions from shock waves

 

Shock waves traveling through some chemical mixtures induce the mixtures to react explosively.   Two reports from Lawrence Livermore National Laboratory, prepared for a symposium on detonation phenomena, describe measurements of ultrafast processes in energetic liquids and solids made to determine their reactivity at high temperatures and pressures.  

 

In the liquid experiments, reported in “Ultrafast kinetics subsequent to shock in an unreacted, oxygen balanced mixture of nitromethane and hydrogen peroxide”[SciTech Connect], a micrometer-thick piece of aluminum was struck by a pulse of infrared light lasting about 0.35 nanoseconds, which produced a shock wave in the aluminum that acted as a piston to drive the shock wave into a small sample of nitromethane mixed with water and hydrogen peroxide.  The speeds of the shock wave and of the aluminum piston that produced it were revealed by their interactions with pairs of light pulses entering the liquid from the other side.  The two speeds were roughly proportional for piston speeds up to about 1.7 kilometers per second, as was expected for a mixture of two liquids that hadn’t chemically reacted with each other.  For larger piston speeds, though, the shockwave speeds were generally much less than proportional (Figure 4), and less correlated, suggesting that the liquids were reacting chemically in response to the shock waves. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 2.  Schematic diagram of experiments done at Lawrence Livermore National Laboratory and Sandia National Laboratories, in which shock waves were induced in energetic-material samples by a light pulse and tracked using other light pulses.  A broadband femtosecond pulse is chirped so that the various wavelengths travel different path lengths and become more separated in time, resulting in a probe pulse that lasts 300 or more picoseconds by one standard measure[Wikipedia]. One edge of a laser pulse is clipped in the beam stretcher to produce a rise-time profile of about 25 picoseconds. This chirped pulse is split by the partially-reflecting mirror at top left into two portions: a higher energy pulse to generate a shock within a target (lower left) and a low-energy pulse that propagates through a Michelson interferometer[Wikipedia] (right) to form a pulse-pair with a relative time separation of about 10-15 picoseconds (lower right). The drive pulse is focused down to a ~ 20-30 µm spot on the substrate side of an aluminum target; rapid ionization within the skin-depth of the aluminum ablator impulsively launches a shock wave that travels through the aluminum and into a sample material that is deposited onto the back side of the ablator surface. On the backside of the ablator, the probe pulses are focused to a roughly 100 micrometer spot to illuminate the entire shock break-out area, including unshocked reference regions beyond the periphery of the drive pulse spot. An objective lens is selected to image the surface of the sample onto the slit of an imaging spectrometer. A camera (CCD[Wikipedia]) attached to the spectrometer records an interference pattern created by the relative path-length differences between corresponding wavelengths in the probe pulse pair.  {After "Ultrafast Laser Diagnostics for Energetic-Material Ignition Mechanisms: Tools for Physics-Based Model Development "[SciTech Connect], p. 11.}

 

 

  

  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3.  Left, velocities at different points in a shocked sample of material; right, a more complete picture (not to scale) of how the shock is generated by heating aluminum with a “pumping” light pulse from the right and how the shocked material to the left of the aluminum is examined by light pulses from the right reflecting off the material’s front and back surfaces.  {From “Ultrafast kinetics subsequent to shock in an unreacted, oxygen balanced mixture of nitromethane and hydrogen peroxide”[SciTech Connect] , p. 5 of 10, and “Ultrafast Dynamic Response of Single Crystal PETN and Beta-HMX”[SciTech Connect], p. 5 of 11.}

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4.  Measurements of shock wave speeds compared with the speeds of the pistons that generated them. Diagonal lines (“Universal liquid Hugoniot” and “Calculated unreacted Hugoniot”) in the larger graph represent different theoretical estimates of the two speeds’ relationship.  Note that the general trend for both theoretical lines is followed by the actual data for piston speeds below 1.7 kilometers per second; higher piston speeds result in shockwave speeds that don’t correlate as much, especially for shocks generated with lower pump energies (red and blue points, 41 microjoules and 50 microjoules respectively).  Inset shows data for 50-microjoule pump energy, with the numbers indicating positions along the sample for which the speeds were measured, and the trend line being the nearest straight-line fit to the points for piston speeds under 1.7 kilometers per second.  {From “Ultrafast kinetics subsequent to shock in an unreacted, oxygen balanced mixture of nitromethane and hydrogen peroxide”[SciTech Connect], p. 6 of 10.}

 

A similar set of experiments involved, instead of liquid, single crystals of the solid explosives pentaerythritol tetranitrate (PETN)[Wikipedia] and cyclotetramethylene-tetranitramine (also known for obscure reasons as HMX)[Wikipedia], and are described in “Ultrafast Dynamic Response of Single Crystal PETN and Beta-HMX”[SciTech Connect], the “Beta” referring to a particular arrangement of the atoms in the HMX.  These experiments accounted for a characteristic of solids that liquids lack:  crystal orientation.  While the molecules of a liquid are randomly oriented, the atoms in crystalline solids have a regular, repeating arrangement.  This results in the crystal being unequally stiff along different directions, so that shock waves or other mechanical waves traveling through the crystal don’t travel equally fast in all directions as they do in liquids.  The experiments done with PETN and beta-HMX crystals thus involved sending shock waves through the crystals along different directions.  According to the researchers, while the results would seem sufficient to characterize the crystals’ elastic response to ultrafast shocks generally, they planned to check such extrapolations by repeating their experiments with shock-generating pulses that last at least twice as long (0.70 nanoseconds vs. 0.35 nanoseconds).  They also suspected from their existing data that the current mathematical model of beta-HMX may need correction, noting that the model has a limited experimental basis when it comes to shockwave data, and they planned to use more energetic pulses to produce higher-stress shocks in further experiments. 

 

Both of these reports describe how the paired laser pulses are used to examine the shocked material, but a more detailed explanation of why it works is provided in a report from Sandia National Laboratories entitled “Ultrafast Laser Diagnostics for Energetic-Material Ignition Mechanisms: Tools for Physics-Based Model Development”[SciTech Connect].  Shockwave experiments like those performed at Lawrence Livermore were also conducted at Sandia on both explosives and inert materials, and also included experiments in which the aluminum piston’s motion was examined with no such material in front of it, just to measure the response of the aluminum itself to a driving laser pulse.  The technique used is the first of three diagnostic methods that the report explains.  The other two are ultrafast methods for measuring the temperatures of micrometer-sized volumes of matter, and the amount of energy electrons have to gain to change their state of motion in a shocked material.  Since ignition mechanisms may depend on ultrafast energy transfers and complex microstructures in the material, information about temperature variations over short times and small distances are important for understanding ignition.  And different theories of how the energy of a shock wave is transformed into chemical reactions that break interatomic bonds and lead to explosions imply different shockwave effects on the energy gaps that separate possible electron motions.  Measurements of different materials’ temperatures and energy gaps by the techniques described are presented in the report.   

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Ultrafast instruments

 

In some applications of ultrafast processes, it’s useful to have the process’ pattern affect a device for detecting, recording, or otherwise reacting to the pattern, but not important to do so at the process’ actual speed in real time.  Separating the pattern from its real timeframe can even be essential when the device can’t keep up with the ultrafast process’ real pace and produce the desired result.  Two patents assigned to Lawrence Livermore National Security, LLC describe different ways to do this. 

 

“Ultrafast chirped optical waveform recorder using a time microscope”[DOepatents] provides a way to capture the pattern of either “a transient waveform or a selected number of consecutive waveforms” whose lowest and highest frequencies differ by about 10 terahertz or less.  The method used involves preparing the ultrarapidly varying waveform for capture by first running it through a device that spreads the pattern out over a longer time, the way a magnifying lens spreads the spatial pattern of a waveform over a wider area.  In both cases, the waveform is preserved while its scale is altered along certain dimensions.  Spreading the waveform over a longer time, by making an analogous pattern using lower-frequency waves, slows the pattern’s variations enough for present-day recording devices to resolve them. 

 

Some ultrarapid variations are of interest because they reflect conditions in high-radiation environments.  However, tracking the variations within those environments would damage the devices meant to track them.  This problem is addressed in the other patent, “Ultrafast transient grating radiation to optical image converter”[DOepatents].  It describes an invention that produces a lower-frequency waveform quite different from that of the ultrafast original, but one that directly corresponds to it.  In this invention, a grating is attached to a semiconducting wafer surface that can reflect low-frequency light.  When an ultrarapidly varying pulse strikes this device from the grating side, the pulse affects the electrons in those parts of the wafer not blocked by the grating, so that the changes in the electron distribution track the temporal variations in the pulse.  Meanwhile, low-frequency light waves enter the wafer from the opposite side.  If electrons in the wafer had not been affected by the ultrafast pulse, some of this light would simply be transmitted to the mirror, reflected from it, and exit the wafer to go elsewhere, while the rest of the light would be absorbed by the wafer.  The electrons’ temporal modulation by the ultrafast pulse, though, makes them into a kind of grating that spatially modulates the waveform[Wikipedia] of the low-frequency light.  While this modulation doesn’t duplicate the waveform of the original pulse, its correspondence to that waveform permits the waveform to be reconstructed from it.   Since the modulated low-frequency light can travel away from the semiconducting wafer to a remote location, this reconstruction can be made by devices far from the high-radiation environment of the original pulse. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 5.  The invention described in the patent “Ultrafast transient grating radiation to optical image converter”[DOepatents] modulates a beam of electromagnetic radiation by the device illustrated above to follow the changes in an x-ray or other beam.  This device is basically a diffraction grating that modulates the second beam more or less intensely with the increasing and decreasing intensity of the original beam.  In the patent’s “Detailed Description of the Invention” section, the above figure is described as follows:  “As shown … an x-ray to optical converter 10 of the invention is formed of a semiconductor wafer or substrate 12 having a pair of flat opposed surfaces 15, 17. An x-ray absorption grid or mask or fixed grating 14 is positioned along or adjacent to (back) surface 15 of wafer 12 (in contact with or slightly separated therefrom). An x-ray induced free carrier index grating 16 is produced in wafer 12 when an x-ray beam (flux) 18 is incident on grid 14. Part of x-ray beam 18 passes through the spaces in grid 14 and part is blocked (absorbed) by the solid parts of grid 14, thereby spatially modulating the beam 18. The spatially modulated beam 20 that passes into wafer 12 after passing through grid 14 creates a transient diffraction grating[Wikipedia]16 therein. This carrier index grating is written (produced) very quickly, e.g., in 100 fs. An optical probe beam 22 is directed at wafer 12 from the opposed (front) surface 17 to that (surface 15) adjacent to the grid 14 during the time the grating 16 is formed. The transient grating is formed for a short time. The probe beam could be on before the transient grating is formed; it could even be CW [continuous-wave]. The probe beam could also be applied just as the transient grating is formed or sometime during the existence of the transient grating. The probe beam is diffracted only when the transient grating exists. A thin (dielectric) mirror 24 on the surface 15 of the wafer 12 adjacent to grid 14 reflects optical beam 22 back on itself through the wafer (but transmits modulated x-ray beam 20) so that beam 22 passes through wafer 12 twice. Optical probe beam 22 is diffracted by grating 16. Along with the undiffracted zero order beam, only the in =+/-1 diffraction orders, 1st order diffracted beam 26 and -1st order diffracted beam 28, are shown but higher order diffracted beams are also produced. For small amplitude transient phase gratings, the higher orders are much weaker than the first order diffracted beams.” 

 

Both of these patents describe ways of detecting the pattern of ultrafast processes.  A report from Brown University describes using an ultrafast process to enable the observation of other things.  While the ultrafast process again involves electromagnetic waves—waves shorter than one picosecond duration—they aren’t used to image anything.  Rather, the electromagnetic pulses are used to produce nanometer-length waves of ultrasound in “a new type of scanning acoustic microscope” that uses the ultrasound waves to image features of nanometer-scale material structures, “including features that may be buried so as to be inaccessible to conventional lightwave or electron microscopies”.  The report, entitled “Optoacoustic Microscopy for Investigation of Material Nanostructures—Embracing the Ultrasmall, Ultrafast, and the Invisible”[SciTech Connect], describes two phases of development.  The Phase I device was a “nonfocusing” optoacoustic microscope that had nanometer vertical resolution but could only resolve horizontal features about 10,000 nanometers wide—a size that happens to be typical of living cells.  Accomplishing this in a new type of device was an advance that involved computer modeling of the underlying optics and acoustics, and devising a control system that could control the spacing between the microscope and the sample with nanometer precision.  In Phase II the inventors went further, guided by computer simulations of ultrasound-wave focusing to the nanometer scale that they developed, to produce a scanning version of the microscope with a spatial resolution around 100 nanometers.  Their work also included the invention of ways to make acoustic microlenses capable of focusing ultrasound with a broad frequency range having waves with billions of vibrations per second. 

 

The ultrasound pulses not only need to have short wavelengths to resolve nanoscale sample features, but the pulses themselves need to be ultrashort.  This is because pulses echoing back from the sample get split when they return to the microscope objective—part of the echo will enter the microscope, and part will bounce back toward the sample.  This will produce a second echo from the sample.   If the original pulse isn’t short enough, the second echo will overlap the first and will blur the sample image.  (Lengthening the distance between the sample and the objective to avoid the overlap wouldn’t help because the ultrasound wave attenuates too much to produce a clear echo over the longer distance.)  Conventional piezoelectric ultrasound generators that vibrate in response to alternating electric fields can’t be switched on and off fast enough to avoid this problem, but the inventors were able to make the vibration time short enough in their generator by giving it brief thermal stresses with ultrashort light pulses.  The ultrasound generator’s optical reflectivity also changes when it receives echoes from the sample, so ultrashort light pulses are used to detect the ultrasound echoes. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 6.  Schematic diagram of the scanning optoacoustic microscope described in “Optoacoustic Microscopy for Investigation of Material Nanostructures—Embracing the Ultrasmall, Ultrafast, and the Invisible”[SciTech Connect] (p. 46). 

 

At the time of their report, the Brown University inventors saw considerable opportunity for doubling their microscope’s resolution by making the sample-holding stage more mechanically stable and by reducing the proportion of noise in the echo-detection system.  “An acoustic microscope of this type,” they conclude, “properly engineered and manufactured as a high precision metrology tool, could be used for measurements of surface topography an also for measurements of the elastic properties of the material near to the surface of a sample, just as in a conventional acoustic microscope.  Such a tool could have multiple applications for nondestructive metrology of complex structural features of semiconductor integrated circuits during large-scale wafer-level fabrication.” 

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References

 

Wikipedia

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 Research organizations

 

Reports available through OSTI’s SciTech Connect

  • “Strong Influence of Coadsorbate Interaction on CO Desorption Dynamics Probed by Ultrafast X-ray Spectroscopy and Ab Initio Simulations” [Metadata]
  • “Ultrafast Terahertz-Induced Response of GeSbTe Phase-Change Materials” [Metadata]
  • “Ultrafast Probes for Dirac Materials” [Metadata]
  • “Electronic Coupling Dependence of Ultrafast Interfacial Electron Transfer on Nanocrystalline Thin Films and Single Crystal” [Metadata]
  • “Ultrafast all-optical manipulation of interfacial magnetoelectric coupling” [Metadata]
  • “Ultrafast kinetics subsequent to shock in an unreacted, oxygen balanced mixture of nitromethane and hydrogen peroxide” [Metadata]
  • “Ultrafast Laser Diagnostics for Energetic-Material Ignition Mechanisms: Tools for Physics-Based Model Development” [Metadata]
  • “Ultrafast Dynamic Response of Single Crystal PETN and Beta-HMX” [Metadata]
  • “Optoacoustic Microscopy for Investigation of Material Nanostructures—Embracing the Ultrasmall, Ultrafast, and the Invisible” [Metadata]

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Reports available through DOepatents

  • “Ultrafast chirped optical waveform recorder using a time microscope” [Metadata]
  • “Ultrafast transient grating radiation to optical image converter” [Metadata]

 

Additional references

  • Physical Review Letters.  “Strong Influence of Coadsorbate Interaction on CO Desorption Dynamics Probed by Ultrafast X-ray Spectroscopy and Ab Initio Simulations” appears in vol. 114, issue 15 (April 17, 2015), 156101. 
  • Applied Physics Letters.  “Ultrafast terahertz-induced response of GeSbTe phase-change materials” appears in vol. 104, issue 25 (June 23, 2014),251907. 
  • A review article entitled “Dirac materials” appears in Advances in Physics, vol. 63, issue 1 (July 23, 2014) and on arXiv.org at http://arxiv.org/abs/1405.5774. 
  • International Detonation Symposium.  “Ultrafast kinetics subsequent to shock in an unreacted, oxygen balanced mixture of nitromethane and hydrogen peroxide” and “Ultrafast Dynamic Response of Single Crystal PETN and Beta-HMX” were prepared for the 15th symposium in this series, which was held July 13-18, 2014 in San Francisco, California. 

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Last updated on Wednesday 27 July 2016