Thermoreflectance of metal transducers for optical pump-probe studies of thermal properties

Anisotropy Reversal of Thermal Conductivity in Silicon Nanowire Networks Driven by Quasi-Ballistic Phonon Transport

Nanostructured semiconductors promise functional thermal management for microelectronics and thermoelectrics through a rich design capability. However, experimental studies on anisotropic in-plane thermal conduction remain limited, despite the demand for directional heat dissipation. Here, inspired by an oriental wave pattern, a periodic network of bent wires, we investigate anisotropic in-plane thermal conduction in nanoscale silicon phononic crystals with the thermally dead volume. We observed the anisotropy reversal of the material thermal conductivity from 1.2 at 300 K to 0.8 at 4 K, with the reversal temperature of 80 K mediated by the transition from a diffusive to a quasi-ballistic regime. Our Monte Carlo simulations revealed that the backflow of the directional phonons induces the anisotropy reversal, showing that the quasi-ballistic phonon transport introduces preferential thermal conduction channels with anomalous temperature dependence. Accordingly, the anisotropy of the effective thermal conductivity varied from 2.7 to 5.0 in the range of 4-300 K, indicating an anisotropic heat manipulation capability. Our findings demonstrate that the design of nanowire networks enables the directional thermal management of electronic devices.

Thermoreflectance-based thermometry of silicon thin films with resonantly enhanced temperature sensitivity

We demonstrate a thermoreflectance-based thermometry technique with an ultimate temperature resolution of 60 µK in a 2.6 mHz bandwidth. This temperature resolution was achieved using a 532 nm-wavelength probe laser and a ∼1 µm-thick silicon transducer film with a thermoreflectance coefficient of -4.7 × 10-3 K-1 at room temperature. The thermoreflectance sensitivity reported here is over an order-of-magnitude greater than that of metal transducers, and is comparable to the sensitivity of traditional resistance thermometers. Supporting calculations reveal that the enhancement in sensitivity is due to optical interference in the thin film.

Metal transducer-assisted acoustic deformation potential characterization via coherent acoustic phonon dynamics

Acoustic deformation potential (ADP) plays a significant role in quantifying carrier-acoustic phonon interactions in semiconductors. In this work, we report a novel ultrafast spectroscopy method to extract the ADP coupling constant of a semiconductor by jointly analyzing the coherent acoustic phonon signals with and without a metal transducer. By applying this method to GaAs, the ADP coupling constant corresponding to the band gap was extracted using a pump photon energy near the band gap, which agrees well with literature values. With a larger pump photon energy, the ADP coupling constant deviates from the one for the band gap, which is attributed to contributions from the carrier dynamics in multiple energy and wavevector states.

Frequency-domain probe beam deflection method for measurement of thermal conductivity of materials on micron length scale

Time-domain thermoreflectance and frequency-domain thermoreflectance (FDTR) have been widely used for non-contact measurement of anisotropic thermal conductivity of materials with high spatial resolution. However, the requirement of a high thermoreflectance coefficient restricts the choice of metal coating and laser wavelength. The accuracy of the measurement is often limited by the high sensitivity to the radii of the laser beams. We describe an alternative frequency-domain pump-probe technique based on probe beam deflection. The beam deflection is primarily caused by thermoelastic deformation of the sample surface, with a magnitude determined by the thermal expansion coefficient of the bulk material to measure. We derive an analytical solution to the coupled elasticity and heat diffusion equations for periodic heating of a multilayer sample with anisotropic elastic constants, thermal conductivity, and thermal expansion coefficients. In most cases, a simplified model can reliably describe the frequency dependence of the beam deflection signal without knowledge of the elastic constants and thermal expansion coefficients of the material. The magnitude of the probe beam deflection signal is larger than the maximum magnitude achievable by thermoreflectance detection of surface temperatures if the thermal expansion coefficient is greater than 5 × 10^-6 K^-1. The uncertainty propagated from laser beam radii is smaller than that in FDTR when using a large beam offset. We find a nearly perfect matching of the measured signal and model prediction, and measure thermal conductivities within 6% of accepted values for materials spanning the range of polymers to gold, 0.1-300 W/(m K).

Plasma-induced surface cooling

Plasmas are an indispensable materials engineering tool due to their unique ability to deliver a flux of species and energy to a surface. This energy flux serves to heat the surface out of thermal equilibrium with bulk material, thus enabling local physicochemical processes that can be harnessed for material manipulation. However, to-date, there have been no reports on the direct measurement of the localized, transient thermal response of a material surface exposed to a plasma. Here, we use time-resolved optical thermometry in-situ to show that the energy flux from a pulsed plasma serves to both heat and transiently cool the material surface. To identify potential mechanisms for this ‘plasma cooling,’ we employ time-resolved plasma diagnostics to correlate the photon and charged particle flux with the thermal response of the material. The results indicate photon-stimulated desorption of adsorbates from the surface is the most likely mechanism responsible for this plasma cooling.

When a plasma interacts with a surface, different thermal effects may arise. Here, the authors explore plasma interactions with a surface that produce a surface cooling effect.

Photothermal nonlinearity in plasmon-assisted photocatalysis

Understanding the intricate relationship between illumination and temperature in metallic nano-particles is crucial for elucidating the role of illumination in various physical processes which rely on plasmonic enhancement but are also sensitive to temperature. Recent studies have shown that the temperature rise in optically thick ensembles of metal nanoparticles under intense illumination is dominated by the thermal conductivity of the host, rather than by the optical properties of the metal or the host. Here, we show that the temperature dependence of the thermal conductivity of the host dominates the nonlinear photothermal response of these systems. In particular, this dependence typically causes the temperature rise to become strongly sublinear, reaching even several tens of percent. We then show that this effect can explain experimental observations in several recent plasmon-assisted photocatalysis experiments. Under certain conditions, we show that thermal emission may also contribute to photothermal nonlinearity. This shows that any claim for the dominance of non-thermal electrons in plasmon-assisted photocatalysis must account first for this photothermal nonlinear mechanism.

Pressure dependent thermoreflectance spectroscopy induced by interband transitions in metallic nano-film

Utilizing high-pressure to modulate optical properties, such as thermoreflectance (dR/dT), over a wide range has received much attention. Nevertheless, how the pressure exerts on the complex dielectric constant and finally on dR/dT remains elusive. Here, we perform a thoroughly experimental and theoretical investigation on dR/dT of Al nano-film from 0 to 25 GPa. The dR/dT values exhibit a sine-like pressure-dependence, with the zero-crossing appearing at around 6 GPa. These special phenomena are well explained from electron transition viewpoints. The first-principles calculations show that the energy difference of parallel bands is enlarged from 1.45 to 2 eV, thereby increasing the threshold for electron transitions. The lifted threshold changes the optical absorption rates of Al and the density of states of the electrons involving interband transitions; finally, the resulting dR/dT exhibits such a pressure-dependent behavior. Our findings provide a deep insight on pressure-induced electronic transitions and photon-electron interactions in metals.

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Thermoreflectance (dR/dT) of Al films was measured under a pressure range of 0–25 GPa

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A semi-quantum model well describes the pressure effect on optical properties

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Pressure enlarges interband transition energy which makes dR/dT pressure-dependent

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The resonant interband transitions let dR/dT change to negative at ∼6 GPa

Transient Hydrodynamic Lattice Cooling by Picosecond Laser Irradiation of Graphite

Recent theories and experiments have suggested hydrodynamic phonon transport features in graphite at unusually high temperatures. Here, we report a picosecond pump-probe thermal reflectance measurement of heat-pulse propagation in graphite. The measurement results reveal transient lattice cooling near the adiabatic center of a 15-μm-diameter ring-shape pump beam at temperatures between 80 and 120 K. While such lattice cooling has not been reported in recent diffraction measurements of second sound in graphite, the observation here is consistent with both hydrodynamic phonon transport theory and prior heat-pulse measurements of second sound in bulk sodium fluoride.

Thermal conductivity measurements of sub-surface buried substrates by steady-state thermoreflectance

Measuring the thermal conductivity of sub-surface buried substrates is of significant practical interests. However, this remains challenging with traditional pump-probe spectroscopies due to their limited thermal penetration depths. Here, we experimentally and numerically investigate the TPD of the recently developed optical pump-probe technique steady-state thermoreflectance (SSTR) and explore its capability for measuring the thermal properties of buried substrates. The conventional definition of the TPD (i.e., the depth at which temperature drops to 1/e value of the maximum surface temperature) does not truly represent the upper limit of how far beneath the surface SSTR can probe. For estimating the uncertainty of SSTR measurements of a buried substrate a priori, sensitivity calculations provide the best means. Thus, detailed sensitivity calculations are provided to guide future measurements. Due to the steady-state nature of SSTR, it can measure the thermal conductivity of buried substrates that are traditionally challenging by transient pump-probe techniques, exemplified by measuring three control samples. We also discuss the required criteria for SSTR to isolate the thermal properties of a buried film. Our study establishes SSTR as a suitable technique for thermal characterizations of sub-surface buried substrates in typical device geometries.

Electron-Phonon Coupling Parameter of Ferromagnetic Metal Fe and Co

The electron-phonon coupling (g) parameter plays a critical role in the ultrafast transport of heat, charge, and spin in metallic materials. However, the exact determination of the g parameter is challenging because of the complicated process during the non-equilibrium state. In this study, we investigate the g parameters of ferromagnetic 3d transition metal (FM) layers, Fe and Co, using time-domain thermoreflectance. We measure a transient increase in temperature of Au in an FM/Au bilayer; the Au layer efficiently detects the strong heat flow during the non-equilibrium between electrons and phonons in FM. The g parameter of the FM is determined by analyzing the temperature dynamics using thermal circuit modeling. The determined g values are 8.8-9.4 × 1017 W m-3 K-1 for Fe and 9.6-12.2 × 1017 W m-3 K-1 for Co. Our results demonstrate that all 3d transition FMs have a similar g value, in the order of 1018 W m-3 K-1.

Nanoscale Disorder Generates Subdiffusive Heat Transport in Self-Assembled Nanocrystal Films

Investigating the impact of nanoscale heterogeneity on heat transport requires a spatiotemporal probe of temperature on the length and time scales intrinsic to heat navigating nanoscale defects. Here, we use stroboscopic optical scattering microscopy to visualize nanoscale heat transport in disordered films of gold nanocrystals. We find that heat transport appears subdiffusive at the nanoscale. Finite element simulations show that tortuosity of the heat flow underlies the subdiffusive transport, owing to a distribution of nonconductive voids. Thus, while heat travels diffusively through contiguous regions of the film, the tortuosity causes heat to navigate circuitous pathways that make the observed mean-squared expansion of an initially localized temperature distribution appear subdiffusive on length scales comparable to the voids. Our approach should be broadly applicable to uncover the impact of both designed and unintended heterogeneities in a wide range of materials and devices that can affect more commonly used spatially averaged thermal transport measurements.

Gold-induced photothermal background in on-chip surface enhanced stimulated Raman spectroscopy

Surface enhanced Raman spectroscopy (SERS) and stimulated Raman spectroscopy (SRS) are well established techniques capable of boosting the strength of Raman scattering. The combination of both techniques (surface enhanced stimulated Raman spectroscopy, or SE-SRS) has been reported using plasmonic nanoparticles. In parallel, waveguide enhanced Raman spectroscopy has been developed using nanophotonic and nanoplasmonic waveguides. Here, we explore SE-SRS in nanoplasmonic waveguides. We demonstrate that a combined photothermal and thermo-optic effect in the gold material induces a strong background signal that limits the detection limit for the analyte. The experimental results are in line with theoretical estimates. We propose several methods to reduce or counteract this background.

Electrodeposited Thin-Film Micro-Thermoelectric Coolers with Extreme Heat Flux Handling and Microsecond Time Response

Thin-film thermoelectric coolers are emerging as a viable option for the on-chip temperature management of electronic and photonic integrated circuits. In this work, we demonstrate the record heat flux handling capability of electrodeposited Bi₂Te₃ films of 720(±60) W cm^-2 at room temperature, achieved by careful control of the contact interfaces to reduce contact resistance. The characteristic parameters of a single leg thin-film devices were measured in situ, giving a Seebeck coefficient of S = -121(±6) μV K^-1, thermal conductivity of κ = 0.85(±0.08) W m^-1 K^-1, electrical conductivity of σ = 5.2(±0.32) × 10⁴ S m^-1, and electrical contact resistivity of ∼10^-11 Ω m². These thermoelectric parameters lead to a material ZT = 0.26(±0.04), which, for our device structure, allowed a net cooling of ΔT_max = 4.4(±0.12) K. A response time of τ = 20 μs was measured experimentally. This work shows that with the correct treatment of contact interfaces, electrodeposited thin-film thermoelectrics can compete with more complicated and expensive technologies such as metal organic chemical vapor deposition (MOCVD) multilayers.

Wide bandwidth frequency-domain thermoreflectance: Volumetric heat capacity, anisotropic thermal conductivity, and thickness measurements

A frequency-domain thermoreflectance (FDTR) system with a frequency range of 1 Hz to 75 MHz is presented. The wide bandwidth of pump modulation frequency enables enhanced simultaneous thermophysical property characterization of bulk and thin-film materials. The wide bandwidth FDTR system is demonstrated with simultaneous measurements of in-plane and cross-plane thermal conductivities of sapphire and muscovite mica, thickness and thermal conductivity of gold/titanium thin films, and isotropic thermal conductivity and volumetric heat capacity of lithium niobate and silicon. Thermophysical properties measured with FDTR are compared to literature values or independent measurements for verification. Finally, at low frequencies, a negligible error was achieved in the numerical integration of the heat diffusion equation with a Gauss-Legendre quadrature method solved with 400 integration points and an upper integration limit of 16w_eff, where w_eff is the effective pump-probe 1/e² radius.

Microstructures and Mechanical Properties of Al-2Fe-xCo Ternary Alloys with High Thermal Conductivity

The microstructures, mechanical properties, and thermal conductivity (TC) of Al-2Fe-xCo (x = 0~0.8) alloys in as-cast, homogeneous annealed, and cool rolled states are systematically studied. Results indicate that appropriate Co modification (x ≤ 0.5) simultaneously improves the thermal and mechanical properties of as-cast Al-2Fe alloys. The improvement of TC is attributed to ameliorating the morphology of primary Al₃Fe phases from needles to short rods and fine particles, which decreases the scattering probability of free electrons during the electronic transmission. However, further increasing the Co content (x = 0.8) decreases the TC due to the formation of a coarse plate-like Al₂FeCo phase. Besides, the thermal conductivity of annealed Al-2Fe-xCo alloys is higher than that of as-cast alloys because of the elimination of lattice defects and spheroidization of Al₃Fe phases. After cool rolling with 80 % deformation, thermal conductivity of alloys slightly increases due to the breaking down of Al₂FeCo phases. The rolled Al-2Fe-0.3Co alloy exhibits the highest thermal conductivity, which is about 225 W/(m·K), approximately 11 % higher than the as-cast Al-2Fe sample. The ultimate tensile strength (UTS) and elongation (EL) of as-cast Al-2Fe-0.5Co (UTS: 138 MPa; EL: 22.0 %) are increased by 35 % and 69 %, respectively, compared with those of unmodified alloy (UTS: 102 MPa; EL: 13.0 %).

Phonon and Thermal Properties of Quasi-Two-Dimensional FePS3 and MnPS3 Antiferromagnetic Semiconductors

We report results of investigation of the phonon and thermal properties of the exfoliated films of layered single crystals of antiferromagnetic FePS₃ and MnPS₃ semiconductors. Raman spectroscopy was conducted using three different excitation lasers with wavelengths of 325 nm (UV), 488 nm (blue), and 633 nm (red). UV-Raman spectroscopy reveals spectral features which are not detectable via visible Raman light scattering. The thermal conductivity of FePS₃ and MnPS₃ thin films was measured by two different techniques: the steady-state Raman optothermal and transient time-resolved magneto-optical Kerr effect. The Raman optothermal measurements provided the orientation-average thermal conductivity of FePS₃ to be 1.35 ± 0.32 W m^-1 K^-1 at room temperature. The transient measurements revealed that the through-plane and in-plane thermal conductivity of FePS₃ are 0.85 ± 0.15 and 2.7 ± 0.3 W m^-1 K^-1, respectively. The films of MnPS₃ have higher thermal conductivity of 1.1 ± 0.2 W m^-1 K^-1 through-plane and 6.3 ± 1.7 W m^-1 K^-1 in-plane. The data obtained by the two techniques are in agreement and reveal strong thermal anisotropy of the films and the dominance of phonon contribution to heat conduction. The obtained results are important for the interpretation of electric switching experiments with antiferromagnetic materials as well as for the proposed applications of the antiferromagnetic semiconductors in spintronic devices.

High sensitivity pump-probe measurements of magnetic, thermal, and acoustic phenomena with a spectrally tunable oscillator

We describe an optical pump/probe system for sensitive measurements of time-resolved optical measurements of material dynamics. The instrument design is optimized for time-resolved magneto-optic Kerr effect (TR-MOKE) measurements of dynamics in magnetic materials. The system also allows for time-domain thermoreflectance (TDTR) measurements of thermal transport properties and picosecond acoustic measurements of film thickness and/or elastic constants. The system has several advantages over the conventional designs for TR-MOKE and/or TDTR systems. Measurements of pump-induced changes to the probe beam intensity are shot-noise limited. The system's design allows for MOKE and/or thermoreflectance measurements of both sides of a sample. Pumping and probing the sample on opposite sides allows nanoscale flash diffusivity measurements of transport properties. The wavelengths of the pump and probe beams are straightforward to tune between 350-525 nm and 690-1050 nm. A tunable wavelength allows for optical resonances in a wide array of materials to be excited and/or probed. Finally, the setup is calibrated to allow for the real and imaginary components of Kerr signals to be separately quantified.

Nanosecond transient thermoreflectance method for characterizing anisotropic thermal conductivity

A method is presented to characterize the anisotropic thermal properties of materials based on nanosecond transient thermoreflectance (TTR). An analytical heat transfer model is derived for the TTR signal, showing that the signal is sensitive to out-of-plane and in-plane heat conductions at distinct time scales. This sensitivity feature can be exploited to simultaneously determine the out-of-plane and in-plane thermal conductivities. Examples are given for molybdenum disulphide, hexagonal boron nitride, and highly oriented pyrolytic graphite to assess the validity of this method.

Structure Function Analysis of Temperature-Dependent Thermal Properties of Nm-Thin Nb2O5

A 166-nm-thick amorphous Niobium pentoxide layer (Nb2O5) on a silicon substrate was investigated by using time domain thermoreflectance at ambient temperatures from 25 °C to 500 °C. In the time domain thermoreflectance measurements, thermal transients with a time resolution in (sub-)nanoseconds can be obtained by a pump-probe laser technique. The analysis of the thermal transient was carried out via the established analytical approach, but also by a numerical approach. The analytical approach showed a thermal diffusivity and thermal conductivity from 0.43 mm2/s to 0.74 mm2/s and from 1.0 W/mK to 2.3 W/mK, respectively to temperature. The used numerical approach was the structure function approach to map the measured heat path in terms of a RthCth-network. The structure function showed a decrease of Rth with increasing temperature according to the increasing thermal conductivity of Nb2O5. The combination of both approaches contributes to an in-depth thermal analysis of Nb2O5 film.

A steady-state thermoreflectance method to measure thermal conductivity

We demonstrate a steady-state thermoreflectance-based optical pump-probe technique to measure the thermal conductivity of materials using a continuous wave laser heat source. The technique works in principle by inducing a steady-state temperature rise in a material via long enough exposure to heating from a pump laser. A probe beam is then used to detect the resulting change in reflectance, which is proportional to the change in temperature at the sample surface. Increasing the power of the pump beam to induce larger temperature rises, Fourier's law is used to determine the thermal conductivity. We show that this technique is capable of measuring the thermal conductivity of a wide array of materials having thermal conductivities ranging from 1 to >2000 W m^-1 K^-1, in excellent agreement with literature values.

Photoinduced Domain Pattern Transformation in Ferroelectric-Dielectric Superlattices

The nanodomain pattern in ferroelectric-dielectric superlattices transforms to a uniform polarization state under above-band-gap optical excitation. X-ray scattering reveals a disappearance of domain diffuse scattering and an expansion of the lattice. The reappearance of the domain pattern occurs over a period of seconds at room temperature, suggesting a transformation mechanism in which charge carriers in long-lived trap states screen the depolarization field. A Landau-Ginzburg-Devonshire model predicts changes in lattice parameter and a critical carrier concentration for the transformation.

Optical birefringence imaging of x-ray excited lithium tantalate

X-ray absorption in lithium tantalate induces large, long-lived (∼10⁻⁵ s) optical birefringence, visualized via scanning optical polarimetry. Similar birefringence measured from glass, sapphire, and quartz was two orders of magnitude weaker; much of this reduction can be accounted for by their smaller cross section for x-ray absorption. While x-ray induced charges can perturb local refractive indices and lead to birefringence, aligned dipoles in the non-centrosymmetric unit cell of ferroelectric LiTaO₃ create electric fields that also induce birefringence via electro-optic coupling, which shows up as a dependence on crystal orientation. Time-resolved measurements from LiTaO₃ show a prompt response on a picosecond time scale, which along with the long decay time suggest novel opportunities for optical detection of x-rays.

Metal nanospheres under intense continuous-wave illumination: A unique case of nonperturbative nonlinear nanophotonics

We show that the standard perturbative (i.e., cubic) description of the thermal nonlinear response of a single metal nanosphere to intense continuous-wave (CW) illumination is sufficient only for a temperature rise of up to 100 degrees above room temperature. Beyond this regime, the slowing down of the temperature rise requires a nonperturbative description of the nonlinear response, even though the permittivity is linearly dependent on the temperature and despite the deep subwavelength effective propagation distances involved. Using experimental data, we show that, generically, the increase of the imaginary part of the metal permittivity dominates the increase of the host permittivity as well as the resonance shift due to the joint changes to the real parts of the metal and host. Thus, the main nonlinear effect is a decrease of the quality factor of the resonance. We further analyze the relative importance of the various contributions to the temperature rise and thermal nonlinearity, compare the nonlinearity of Au and Ag, demonstrate the potential effect of the nanoparticle morphology, and show that although the thermo-optical nonlinearity of the host typically plays a minor role, its thermal conductivity and its temperature dependence is important. Finally, we discuss the differences between CW and ultrafast thermal nonlinearities.

Athermal operation of multi-section slotted tunable lasers

Two distinct athermal bias current procedures based on thermal tuning are demonstrated for a low-cost, monotlithic, three section slotted single mode laser, achieving mode-hop free wavelength stability of ± 0.04 nm / 5 GHz over a temperature range of 8-47 °C. This is the first time that athermal performance has been demonstrated for a three-section slotted laser with simple fabrication, and is well within the 50 GHz grid spacing specified for DWDM systems. This performance is similar to experiments on more complex DS-DBR lasers, indicating that strong athermal performance can be achieved using our lower-cost three section devices. An analytical model and thermoreflectance measurements provide further insight into the operation of multi-section lasers and lay the foundation for an accurate predictive tool for optimising such devices for athermal operation.

Analysis of the Thermo-Reflectivity Coefficient Influence Using Photothermal Pump-Probe Techniques

Recent improvements in the modeling of photo-induced thermo-optical-mechanical effects have broadened the application of photothermal techniques to a large class of solids and fluids. During laser excitation, changes in optical reflectivity due to temperature variation may affect the photothermal signal. In this study, the influence of the reflectivity change due to heating is analyzed for two pump-probe photothermal techniques, thermal lens and thermal mirror. A linear equation for the temperature dependence of the reflectivity is derived, and the solution is tested using optical properties of semi-transparent and opaque materials. For semi-transparent materials, the influence of the reflectivity change in photothermal signals is less than 0.01%, while for opaque materials it is lower than 3%.

A nanosecond pulsed laser heating system for studying liquid and supercooled liquid films in ultrahigh vacuum

A pulsed laser heating system has been developed that enables investigations of the dynamics and kinetics of nanoscale liquid films and liquid/solid interfaces on the nanosecond time scale in ultrahigh vacuum (UHV). Details of the design, implementation, and characterization of a nanosecond pulsed laser system for transiently heating nanoscale films are described. Nanosecond pulses from a Nd:YAG laser are used to rapidly heat thin films of adsorbed water or other volatile materials on a clean, well-characterized Pt(111) crystal in UHV. Heating rates of ∼10(10) K/s for temperature increases of ∼100-200 K are obtained. Subsequent rapid cooling (∼5 × 10(9) K/s) quenches the film, permitting in-situ, post-heating analysis using a variety of surface science techniques. Lateral variations in the laser pulse energy are ∼±2.7% leading to a temperature uncertainty of ∼±4.4 K for a temperature jump of 200 K. Initial experiments with the apparatus demonstrate that crystalline ice films initially held at 90 K can be rapidly transformed into liquid water films with T > 273 K. No discernable recrystallization occurs during the rapid cooling back to cryogenic temperatures. In contrast, amorphous solid water films heated below the melting point rapidly crystallize. The nanosecond pulsed laser heating system can prepare nanoscale liquid and supercooled liquid films that persist for nanoseconds per heat pulse in an UHV environment, enabling experimental studies of a wide range of phenomena in liquids and at liquid/solid interfaces.

Temperature- and roughness- dependent permittivity of annealed/unannealed gold films

Intrinsic absorption and subsequent heat generation have long been issues for metal-based plasmonics. Recently, thermo-plasmonics, which takes the advantage of such a thermal effect, is emerging as an important branch of plasmonics. However, although significant temperature increase is involved, characterization of metal permittivity at different temperatures and corresponding thermo-derivative are lacking. Here we measure gold permittivity from 300K to 570K, which the latter is enough for gold annealing. More than one order difference in thermo-derivative is revealed between annealed and unannealed films, resulting in a large variation of plasmonic properties. In addition, an unusual increase of imaginary permittivity after annealing is found. Both these effects can be attributed to the increased surface roughness incurred by annealing. Our results are valuable for characterizing extensively used unannealed nanoparticles, or annealed nanostructures, as building blocks in future thermo-nano-plasmonic systems.

Characterization of the temperature dependence of the thermoreflectance coefficient for conductive thin films

We describe a novel approach for calibration of the thermoreflectance coefficient, ideally suited for measurements in a vacuum thermostat, and present the high temperature thermoreflectance coefficients for several metals commonly encountered in electronic devices: gold, platinum, and aluminum. The effect of passivation on these metals is also examined, and we demonstrate the signal to noise ratio of a thermoreflectance measurement can be improved with informed selection of the dielectric layer thickness. Furthermore, the thermo-optic coefficients of the metals are extracted over a wide temperature range. The results presented here can be utilized in the optimization of experimental configurations for high temperature thermoreflectance imaging.

Thermal property microscopy with frequency domain thermoreflectance

A thermal property microscopy technique based on frequency domain thermoreflectance (FDTR) is presented. In FDTR, a periodically modulated laser locally heats a sample while a second probe beam monitors the surface reflectivity, which is related to the thermal properties of the sample with an analytical model. Here, we extend FDTR into an imaging technique capable of producing micrometer-scale maps of several thermophysical properties simultaneously. Thermal phase images are recorded at multiple frequencies chosen for maximum sensitivity to thermal properties of interest according to a thermal model of the sample. The phase versus frequency curves are then fit point-by-point to obtain quantitative thermal property images of various combinations of thermal properties in multilayer samples, including the in-plane and cross-plane thermal conductivities, heat capacity, thermal interface conductance, and film thickness. An FDTR microscope based on two continuous-wave lasers is described, and a sensitivity analysis of the technique to different thermal properties is carried out. As a demonstration, we image ~3 nm of patterned titanium under 100 nm of gold on a silicon substrate, and simultaneously create maps of the thermal interface conductance and substrate thermal conductivity. Results confirm the potential of our technique for imaging and quantifying thermal properties of buried layers, indicating its utility for mapping thermal properties in integrated circuits.

Invited article: micron resolution spatially resolved measurement of heat capacity using dual-frequency time-domain thermoreflectance

A pump-probe photothermal technique - dual-frequency time-domain thermoreflectance - was developed for measuring heat capacity with a spatial resolution on the order of 10 μm. The method was validated by measuring several common materials with known heat capacity. Rapid measurement of composition-phase-property relationships was demonstrated on Ti-TiSi2 and Ni-Zr diffusion couples; experimental values of heat capacity of the intermetallic compounds in these diffusion couples were compared with literature values and CALPHAD (CALculation of PHAse Diagram) calculations. The combination of this method and diffusion multiples provides an efficient way to generate thermodynamic data for CALPHAD modeling and database construction. The limitation of this method in measuring low thermal diffusivity materials is also discussed.