Understanding thermal transport from nanoscale heat sources is important for a fundamental description of energy flow in materials, as well as for many technological applications including thermal management in nanoelectronics and optoelectronics, thermoelectric devices, nanoenhanced photovoltaics, and nanoparticle-mediated thermal therapies. Thermal transport at the nanoscale is fundamentally different from that at the macroscale and is determined by the distribution of carrier mean free paths and energy dispersion in a material, the length scales of the heat sources, and the distance over which heat is transported. Past work has shown that Fourier's law for heat conduction dramatically overpredicts the rate of heat dissipation from heat sources with dimensions smaller than the mean free path of the dominant heat-carrying phonons. In this work, we uncover a new regime of nanoscale thermal transport that dominates when the separation between nanoscale heat sources is small compared with the dominant phonon mean free paths. Surprisingly, the interaction of phonons originating from neighboring heat sources enables more efficient diffusive-like heat dissipation, even from nanoscale heat sources much smaller than the dominant phonon mean free paths. This finding suggests that thermal management in nanoscale systems including integrated circuits might not be as challenging as previously projected. Finally, we demonstrate a unique capability to extract differential conductivity as a function of phonon mean free path in materials, allowing the first (to our knowledge) experimental validation of predictions from the recently developed first-principles calculations.nanoscale thermal transport | nondiffusive transport | mean free path spectroscopy | high harmonic generation | ultrafast X-rays C ritical applications including thermoelectrics for energy harvesting, nanoparticle-mediated thermal therapy, nanoenhanced photovoltaics, and thermal management in integrated circuits require a comprehensive understanding of energy flow at the nanoscale. Recent work has shown that the rate of heat dissipation from a heat source is reduced significantly below that predicted by Fourier's law for diffusive heat transfer when the characteristic dimension of the heat source is smaller than the mean free path (MFP) of the dominant heat carriers (phonons in dielectric and semiconductor materials) (1-6). However, a complete fundamental description of nanoscale thermal transport is still elusive, and current theoretical efforts are limited by a lack of experimental validation.Diffusive heat transfer requires many collisions among heat carriers to establish a local thermal equilibrium and a continuous temperature gradient along which energy dissipates. However, when the dimension of a heat source is smaller than the phonon MFP, the diffusion equation is intrinsically invalid because phonons move ballistically without collisions. The rate of nanoscale heat dissipation is significantly lower than the diffusive prediction such that smaller heat ...

In this work, we generate and probe the shortest wavelength surface acoustic waves to date, at 45 nm, by diffracting coherent extreme ultraviolet beams from a suboptical phononic crystal. The short acoustic wavelengths correspond to penetration depths of approximately 10 nm. We also measure the acoustic dispersion in twodimensional nanostructured phononic crystals down to this wavelength for the first time, showing that it is strongly influenced by the ultrashort acoustic penetration depth, and that advanced finite-element analysis is required to model the dispersion. Finally, we use pulse sequences to control surface acoustic wave generation in one-dimensional nanostructured gratings, to preferentially enhance higher-order surface waves, while suppressing lower frequency waves. This allows us to reduce the generated surface acoustic wavelength by a factor of two for a defined nanostructure period.

Precise characterization of the mechanical properties of ultrathin films is of paramount importance for both a fundamental understanding of nanoscale materials and for continued scaling and improvement of nanotechnology. In this work, we use coherent extreme ultraviolet beams to characterize the full elastic tensor of isotropic ultrathin films down to 11 nm in thickness. We simultaneously extract the Young's modulus and Poisson's ratio of low-k a-SiC:H films with varying degrees of hardness and average network connectivity in a single measurement. Contrary to past assumptions, we find that the Poisson's ratio of such films is not constant but rather can significantly increase from 0.25 to >0.4 for a network connectivity below a critical value of ∼2.5. Physically, the strong hydrogenation required to decrease the dielectric constant k results in bond breaking, lowering the network connectivity, and Young's modulus of the material but also decreases the compressibility of the film. This new understanding of ultrathin films demonstrates that coherent EUV beams present a new nanometrology capability that can probe a wide range of novel complex materials not accessible using traditional approaches.

Nanoscale thermal transport is becoming ever more technologically important with the development of next-generation nanoelectronics, nanomediated thermal therapies, and high efficiency thermoelectric devices. However, direct experimental measurements of nondiffusive heat flow in nanoscale systems are challenging, and first-principle models of real geometries are not yet computationally feasible. In recent work, we used ultrafast pulses of short-wavelength light to uncover a previously-unobserved regime of nanoscale thermal transport that occurs when the width and separation of heat sources are comparable to the mean free paths of the dominant heat-carrying phonons in the substrate. We now systematically compare thermal transport from gratings of metallic nanolines with different periodicities, on both silicon and fused-silica substrates, to map the entire nanoscale thermal transport landscape -from closely spaced through increasingly isolated to fully isolated heat-transfer regimes. By monitoring the surface profile dynamics with subangstrom sensitivity, we directly measure thermal transport from the nanolines into the substrate. This allows us to quantify for the first time how the nanoline separation significantly impacts thermal transport into the substrate, making it possible to reach efficiencies that are within a factor of 2 of the diffusive (i.e., thin-film) limit. We also show that partially isolated nanolines perform significantly worse, because cooling occurs in a regime that is intermediate between close-packed and fully isolated heat sources. This work thus confirms the surprising prediction that closely spaced nanoscale heat sources can cool more quickly than when far apart. These results show that our predictive model is validated by experiment over a broad parameter space, which is important for benchmarking theories that go beyond the Fourier model of heat diffusion, and for informed design of nanoengineered systems.

We use short wavelength extreme ultraviolet light to independently measure the mechanical properties of disparate layers within a bilayer film for the first time, with single-monolayer sensitivity. We show that in Ni/Ta nanostructured systems, while their density ratio is not meaningfully changed from that expected in bulk, their elastic properties are significantly modified, where nickel softens while tantalum stiffens, relative to their bulk counterparts. In particular, the presence or absence of the Ta capping layer influences the mechanical properties of the Ni film. This nondestructive nanomechanical measurement technique represents the first approach to date able to distinguish the properties of composite materials well below 100 nm in thickness. This capability is critical for understanding and optimizing the strength, flexibility and reliability of materials in a host of nanostructured electronic, photovoltaic, and thermoelectric devices.

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Diatomic potentials for krypton are computed and also probed experimentally. For a probe-laser wavelength near 811 nm, several strong dipole-dipole interactions produce purely-long-range potential wells in the singly excited manifold of (s + p) potentials and in the doubly excited manifold of (p + p) and (s + d) potentials. Evidence of resonant photoassociation into bound states of these potential wells is observed in the emission of ions and ultraviolet photons from a magneto-optically trapped krypton cloud.

Ultrathin films and multilayers, with controlled thickness down to single atomic layers, are critical for advanced technologies ranging from nanoelectronics to spintronics to quantum devices. However, for thicknesses less than 10 nm, surfaces and dopants contribute significantly to the film properties, which can differ dramatically from that of bulk materials. For amorphous films being developed as low dielectric constant interfaces for nanoelectronics, the presence of surfaces or dopants can soften films and degrade their mechanical performance. Here we use coherent short-wavelength light to fully and nondestructively characterize the mechanical properties of individual films as thin as 5 nm within a bilayer. In general, we find that the mechanical properties depend both on the amount of doping and the presence of surfaces. In very thin (5-nm) silicon carbide bilayers with low hydrogen doping, surface effects induce a substantial softening-by almost an order of magnitude-compared with the same doping in thicker (46-nm) bilayers. These findings are important for informed design of ultrathin films for a host of nano-and quantum technologies, and for improving the switching speed and efficiency of next-generation electronics.can characterize films with thicknesses on the order of a 80 fraction of a micron, when combined with advanced modeling 81 [23,24]. Surface Brillouin light scattering, which uses the 82 interaction of light and acoustic phonons, has extracted the 83 full elastic tensor of films of thicknesses down to 25 nm [25]. 84 However, it has difficulty characterizing thinner films without 85 assuming one of the elastic constants. In past work, we used 86 coherent extreme ultraviolet (EUV) beams to characterize the 87 full elastic tensor of isotropic ultrathin films down to 11 nm 88 in thickness [21]. This allowed us to simultaneously extract 89 the Young modulus and Poisson's ratio of low-k amorphous 90 SiC:H films with varying degrees of stiffness and hydrogena-91 tion, in a single measurement. 92 In this work, we show how dopants and surfaces inter-93 play to determine the elastic properties of low-k (k < 4.2) 94 dielectric films that are being developed for next-generation 95 nanoelectronics. We use coherent short-wavelength light to 96 fully and nondestructively characterize the mechanical prop-97 erties of SiOC:H films and SiC:H bilayers with individual 98 layers as thin as 5 nm. This allows us to distinguish between 99 dopant-induced and surface-induced softening. For example, 100 in very thin (5-nm) silicon carbide films with low hydrogen 101 doping, surface effects induce a substantial softening-by al-102 most an order of magnitude-compared with the same doping 103 in thicker (46-nm) films. These findings are important for 104 informed design of ultrathin films for a host of nano-and 105 quantum technologies, and for improving the switching speed 106 and efficiency of next-generation electronics. 107 II. METHODS 108 To distinguish between surface-induced softening and 109 dopant-induced softening,...