T. Heinz - Academia.edu (original) (raw)
Papers by T. Heinz
Nano Letters, Apr 4, 2016
We report efficient nonradiative energy transfer (NRET) from core−shell, semiconducting quantum d... more We report efficient nonradiative energy transfer (NRET) from core−shell, semiconducting quantum dots to adjacent two-dimensional sheets of graphene and MoS 2 of single-and few-layer thickness. We observe quenching of the photoluminescence (PL) from individual quantum dots and enhanced PL decay rates in time-resolved PL, corresponding to energy transfer rates of 1−10 ns −1. Our measurements reveal contrasting trends in the NRET rate from the quantum dot to the van der Waals material as a function of thickness. The rate increases significantly with increasing layer thickness of graphene, but decreases with increasing thickness of MoS 2 layers. A classical electromagnetic theory accounts for both the trends and absolute rates observed for the NRET. The countervailing trends arise from the competition between screening and absorption of the electric field of the quantum dot dipole inside the acceptor layers. We extend our analysis to predict the type of NRET behavior for the near-field coupling of a chromophore to a range of semiconducting and metallic thin film materials.
Bulletin of the American Physical Society, Mar 3, 2015
The Journal of Physical Chemistry, 1986
Information on the orientation and relative composition of solute molecules at the surface of a l... more Information on the orientation and relative composition of solute molecules at the surface of a liquid solution has been obtained by the technique of second harmonic generation. In an.aqueous phenol solution, the phenol molecules at the vapor/solution interface are found to be oriented with their long axes tilted 50' from the surface normal. The orientation is found to be insensitive to the concentration of the phenol solution. A comparison with surface concentration information obtained from surface tension data is made.
We present a method for performing measurements of the absolute phase of elements of the surface ... more We present a method for performing measurements of the absolute phase of elements of the surface nonlinear susceptibility tensor χs(2) responsible for second harmonic generation (SHG) at an interface between two centrosymmetric media.
Applied Science and Convergence Technology, 2019
InAs/GaAs submonolayer quantum dots (SML-QD) were investigated by temperature dependent photorefl... more InAs/GaAs submonolayer quantum dots (SML-QD) were investigated by temperature dependent photoreflectance (PR) spectroscopy. To investigate the optical properties of SML-QD, GaAs and InAs SML-QD related PR spectra were monitored at different temperatures. Two notable signals were observed in the SML-QD and GaAs regions. The PR spectra of SML-QD region were interpreted by the third-derivative functional form method. We observe the oscillatory signal above the GaAs band gap energy (E g) due to the Franz-Keldysh effect caused by an interface electric field (F). At room temperature, the PR transition of SML-QD was obtained at near ~1.3 eV with a broadening of 29.5 meV. The F was obtained from the Aspnes' numerical PR analysis. The F was changed from 14 to 12 kV/cm by decreasing the temperature from 300 to 140 K causing a thermal induced carrier distribution near the interfaces.
... motion. Suspended single-layer graphene field-effect transistors allow for electrical detecti... more ... motion. Suspended single-layer graphene field-effect transistors allow for electrical detection of the resonances while functioning as heterodyne mixers in a manner analogous to the operation of a radio receiver. Mechanical ...
ABSTRACT It is well known that at moderate temperatures the resistivity in single wall carbon nan... more ABSTRACT It is well known that at moderate temperatures the resistivity in single wall carbon nanotubes originates primarily due to electron-phonon scattering. However, the experimental results on tube resistivity seldom matched with electron-phonon scattering theory. Previous experimental work on low bias resistivity of carbon nanotubes have found super-linear temperature dependence and much higher electron scattering rates than predicted by theory. Current work aims to study electron-phonon scattering through performing careful transport measurements on single tubes with known chirality. Here the temperature dependence of electronic mean free path of known chirality tubes will be presented. The knowledge of nanotube structure enables a precise comparison with theoretical simulations that take into account tube phonons as well as substrate phonon modes.
Nature, 2019
Discovery of topological materials has sparked a worldwide interest to harness the topological pr... more Discovery of topological materials has sparked a worldwide interest to harness the topological properties of quantum matter, with important applications in dissipationless electronics and faulttolerant quantum computing. One prime example is WTe2, which is a layered semimetal that crystallizes in a unique structure where the emergence of massless Weyl fermions in this system is sensitive to atomic-scale lattice distortions. In this talk, I will discuss the topological physics of WTe2, and show that its topological invariants are highly tunable by means of interlayer shear strain, as crystallographically measured using relativistic electron diffraction [1]. We will discuss how ultrashort pulses of terahertz (THz) electromagnetic field can be used to drive the shear mode which in turn serves as an ultrafast, energy-efficient means to induce more robust, wellseparated Weyl points or to annihilate all Weyl points of opposite chirality. These results define new methods for ultrafast manipulation of the topological properties in 2D materials that can operate at THz frequencies.
Nonlinear Optics: Materials, Fundamentals and Applications, 2002
ABSTRACT
Conference on Lasers and Electro-Optics 2012, 2012
Abstract We examine the multiple exciton population dynamics in lead chalcogenide nanostructures ... more Abstract We examine the multiple exciton population dynamics in lead chalcogenide nanostructures by ultrafast supercontinuum transient absorption. Carrier multiplication is revealed in the limit of low absorbed photon number, along with biexciton dynamics.
Frontiers in Optics 2012/Laser Science XXVIII, 2012
ABSTRACT
Submitted for the MAR14 Meeting of The American Physical Society Study of the Temperature-Depende... more Submitted for the MAR14 Meeting of The American Physical Society Study of the Temperature-Dependence of Exciton Lifetime in Single-Layer WSe 2 XIAOXIAO ZHANG, YUMENG YOU, FAN ZHANG, JAMES HONE, TONY HEINZ, Columbia University-Two-dimensional layered transition metal dichalcogenides (TMD) have recently received much attention because of their distinctive optical properties, including their strong excitonic interactions and the tightly bound trion states that they support. In this paper, we report the results of time-resolved photoluminescence measurements on exfoliated monolayer samples of WSe 2. The lifetime and quantum efficiencies of different emission features, including those from neutral and charged excitons, were found to display a strong temperature dependence over the range of 10-300K. We discuss the results in terms of the structure of the conduction band near the K-point and of the possible roles of different exciton states.
We report the identification of strongly-bound biexcitonic states in monolayer crystals of WSe 2.... more We report the identification of strongly-bound biexcitonic states in monolayer crystals of WSe 2. The presence of biexcitons was identified by the emergence of a new photoluminescence feature at the high exciton density. From the spectral shift of the biexciton emission, we infer a biexciton binding energy of about 50 meV. We also present results on the thermal stability and ultrafast dynamics of the biexciton states. In comparison with the behavior in conventional quantum-well structures, the biexciton binding energy in monolayer WSe 2 is enhanced by more than an order of magnitude. A variational calculation of the biexciton state reveals that the high binding energy arises not only from strong carrier confinement in two dimensions, but also from the reduced and nonlocal dielectric screening in this atomically thin material.
Physical Review Letters, 2006
The electronic properties of single-walled carbon nanotubes (SWNTs) are altered by intertube coup... more The electronic properties of single-walled carbon nanotubes (SWNTs) are altered by intertube coupling whenever bundles are formed. These effects are examined experimentally by applying Rayleigh scattering spectroscopy to probe the optical transitions of given individual SWNTs in their isolated and bundled forms. The transition energies of SWNTs are observed to undergo redshifts of tens of meVs upon bundling with other SWNTs. These intertube coupling effects can be understood as arising from the mutual dielectric screening of SWNTs in a bundle.
Applied Physics B: Lasers and Optics, 1999
We demonstrate and analyze a new method for probing electric field strengths using optical second... more We demonstrate and analyze a new method for probing electric field strengths using optical second-harmonic generation. The technique, based on a homodyne detection scheme, employs interference between the field-induced second-harmonic radiation from the sample and strong second-harmonic radiation from a reference. The scheme provides a linear relationship between the measured secondharmonic signal strength and the amplitude of the electric field being probed, thus providing easy calibration of the amplitude of the electric field and direct information on its sign. Experimental results are presented for direct and homodyne detection of in-plane fields in silicon structures. A discussion of the expected signal-to-noise characteristics is presented and the results are compared to experimental findings. Homodyne detection of electric fields with strengths on the order of 100 V/cm can be achieved with reasonable integration times. PACS: 42.65.-k; 42.65.Ky; 42.70.Nq Optical second-harmonic generation (SHG) has been widely recognized as a surface-sensitive probe in centrosymmetric materials [1, 2]. As is well known, this sensitivity arises from the fact that the SHG process is dipole-forbidden in a centrosymmetric medium. The breaking of the inversion symmetry at a surface or interface greatly alters SHG from the sample. The same principle is operative when an electric field E0 is applied to a centrosymmetric material. The electric field E0, as a polar vector, acts to lift the inversion symmetry of the material [2]. The efficiency for SHG is thus strongly influenced and measurement of SHG provides a sensitive probe of electric fields present within the optical probing volume. The effect of an applied electric field is thus quite different from that of an applied magnetic field. The latter can alter existing SHG contributions, but, as an axial vector, does not cause the inversion symmetry of the bulk to be lifted [2]. The high sensitivity of SHG for probing of the electric fields was demonstrated early in the history of nonlinear optics. Such an electric field-induced SHG (EFISH) process was first reported by Terhune et al. [3] for a calcite sample and extended soon thereafter to semiconductors and metals by Lee et al. [4]. More recently, systematic studies of the influence of electric fields on SHG have been pursued for centrosymmetric media [5–7], as well as for certain non-centrosymmetric materials with high symmetry [8]. The EFISH process in aqueous environments has also been intensively investigated in several interesting regimes [4, 9, 10]. It has been shown to be both of fundamental interest and a useful tool for probing chemical processes at interfaces. An especially attractive feature of the EFISH process for probing electric fields lies in the possibility for measurements with extremely high time resolution. Time resolution down to the femtosecond regime can be achieved by sampling the material system with ultrafast pulses from a modelocked laser. This approach has been applied successfully to study the influence of chargecarriers on the dynamics of internal electric fields in insulators [10] and semiconductors [11, 12], and to probe microwave [13] and ultrafast transient electric fields directly in time domain [14, 15]. While the background-free character of the EFISH process in a centrosymmetric medium constitutes the principal attractive feature of the method, it also imposes certain complications and limitations. In particular, if we do indeed observe negligible SHG in the absence of the applied field E0, then we expect the induced second-harmonic (SH) polarization to vary linearly with E0. In this case, the radiated SH field will also be linear in E0. The measured quantity is, however, the SH intensity I2ω, which will then scale quadratically with the electric field E0 being probed. Such a quadratic relation implies both the need for careful calibration and, more importantly, the loss of information on the sign of the electric field. While the sign of E0 can be recovered from a measurement of the phase of the radiated SH field, it is clearly desirable to have an experimental methodology free of these complications. Such a method is the homodyne detection scheme presented in this paper. The homodyne detection scheme is a well-established method in which a weak signal of interest is combined with
Nano Letters, Apr 4, 2016
We report efficient nonradiative energy transfer (NRET) from core−shell, semiconducting quantum d... more We report efficient nonradiative energy transfer (NRET) from core−shell, semiconducting quantum dots to adjacent two-dimensional sheets of graphene and MoS 2 of single-and few-layer thickness. We observe quenching of the photoluminescence (PL) from individual quantum dots and enhanced PL decay rates in time-resolved PL, corresponding to energy transfer rates of 1−10 ns −1. Our measurements reveal contrasting trends in the NRET rate from the quantum dot to the van der Waals material as a function of thickness. The rate increases significantly with increasing layer thickness of graphene, but decreases with increasing thickness of MoS 2 layers. A classical electromagnetic theory accounts for both the trends and absolute rates observed for the NRET. The countervailing trends arise from the competition between screening and absorption of the electric field of the quantum dot dipole inside the acceptor layers. We extend our analysis to predict the type of NRET behavior for the near-field coupling of a chromophore to a range of semiconducting and metallic thin film materials.
Bulletin of the American Physical Society, Mar 3, 2015
The Journal of Physical Chemistry, 1986
Information on the orientation and relative composition of solute molecules at the surface of a l... more Information on the orientation and relative composition of solute molecules at the surface of a liquid solution has been obtained by the technique of second harmonic generation. In an.aqueous phenol solution, the phenol molecules at the vapor/solution interface are found to be oriented with their long axes tilted 50' from the surface normal. The orientation is found to be insensitive to the concentration of the phenol solution. A comparison with surface concentration information obtained from surface tension data is made.
We present a method for performing measurements of the absolute phase of elements of the surface ... more We present a method for performing measurements of the absolute phase of elements of the surface nonlinear susceptibility tensor χs(2) responsible for second harmonic generation (SHG) at an interface between two centrosymmetric media.
Applied Science and Convergence Technology, 2019
InAs/GaAs submonolayer quantum dots (SML-QD) were investigated by temperature dependent photorefl... more InAs/GaAs submonolayer quantum dots (SML-QD) were investigated by temperature dependent photoreflectance (PR) spectroscopy. To investigate the optical properties of SML-QD, GaAs and InAs SML-QD related PR spectra were monitored at different temperatures. Two notable signals were observed in the SML-QD and GaAs regions. The PR spectra of SML-QD region were interpreted by the third-derivative functional form method. We observe the oscillatory signal above the GaAs band gap energy (E g) due to the Franz-Keldysh effect caused by an interface electric field (F). At room temperature, the PR transition of SML-QD was obtained at near ~1.3 eV with a broadening of 29.5 meV. The F was obtained from the Aspnes' numerical PR analysis. The F was changed from 14 to 12 kV/cm by decreasing the temperature from 300 to 140 K causing a thermal induced carrier distribution near the interfaces.
... motion. Suspended single-layer graphene field-effect transistors allow for electrical detecti... more ... motion. Suspended single-layer graphene field-effect transistors allow for electrical detection of the resonances while functioning as heterodyne mixers in a manner analogous to the operation of a radio receiver. Mechanical ...
ABSTRACT It is well known that at moderate temperatures the resistivity in single wall carbon nan... more ABSTRACT It is well known that at moderate temperatures the resistivity in single wall carbon nanotubes originates primarily due to electron-phonon scattering. However, the experimental results on tube resistivity seldom matched with electron-phonon scattering theory. Previous experimental work on low bias resistivity of carbon nanotubes have found super-linear temperature dependence and much higher electron scattering rates than predicted by theory. Current work aims to study electron-phonon scattering through performing careful transport measurements on single tubes with known chirality. Here the temperature dependence of electronic mean free path of known chirality tubes will be presented. The knowledge of nanotube structure enables a precise comparison with theoretical simulations that take into account tube phonons as well as substrate phonon modes.
Nature, 2019
Discovery of topological materials has sparked a worldwide interest to harness the topological pr... more Discovery of topological materials has sparked a worldwide interest to harness the topological properties of quantum matter, with important applications in dissipationless electronics and faulttolerant quantum computing. One prime example is WTe2, which is a layered semimetal that crystallizes in a unique structure where the emergence of massless Weyl fermions in this system is sensitive to atomic-scale lattice distortions. In this talk, I will discuss the topological physics of WTe2, and show that its topological invariants are highly tunable by means of interlayer shear strain, as crystallographically measured using relativistic electron diffraction [1]. We will discuss how ultrashort pulses of terahertz (THz) electromagnetic field can be used to drive the shear mode which in turn serves as an ultrafast, energy-efficient means to induce more robust, wellseparated Weyl points or to annihilate all Weyl points of opposite chirality. These results define new methods for ultrafast manipulation of the topological properties in 2D materials that can operate at THz frequencies.
Nonlinear Optics: Materials, Fundamentals and Applications, 2002
ABSTRACT
Conference on Lasers and Electro-Optics 2012, 2012
Abstract We examine the multiple exciton population dynamics in lead chalcogenide nanostructures ... more Abstract We examine the multiple exciton population dynamics in lead chalcogenide nanostructures by ultrafast supercontinuum transient absorption. Carrier multiplication is revealed in the limit of low absorbed photon number, along with biexciton dynamics.
Frontiers in Optics 2012/Laser Science XXVIII, 2012
ABSTRACT
Submitted for the MAR14 Meeting of The American Physical Society Study of the Temperature-Depende... more Submitted for the MAR14 Meeting of The American Physical Society Study of the Temperature-Dependence of Exciton Lifetime in Single-Layer WSe 2 XIAOXIAO ZHANG, YUMENG YOU, FAN ZHANG, JAMES HONE, TONY HEINZ, Columbia University-Two-dimensional layered transition metal dichalcogenides (TMD) have recently received much attention because of their distinctive optical properties, including their strong excitonic interactions and the tightly bound trion states that they support. In this paper, we report the results of time-resolved photoluminescence measurements on exfoliated monolayer samples of WSe 2. The lifetime and quantum efficiencies of different emission features, including those from neutral and charged excitons, were found to display a strong temperature dependence over the range of 10-300K. We discuss the results in terms of the structure of the conduction band near the K-point and of the possible roles of different exciton states.
We report the identification of strongly-bound biexcitonic states in monolayer crystals of WSe 2.... more We report the identification of strongly-bound biexcitonic states in monolayer crystals of WSe 2. The presence of biexcitons was identified by the emergence of a new photoluminescence feature at the high exciton density. From the spectral shift of the biexciton emission, we infer a biexciton binding energy of about 50 meV. We also present results on the thermal stability and ultrafast dynamics of the biexciton states. In comparison with the behavior in conventional quantum-well structures, the biexciton binding energy in monolayer WSe 2 is enhanced by more than an order of magnitude. A variational calculation of the biexciton state reveals that the high binding energy arises not only from strong carrier confinement in two dimensions, but also from the reduced and nonlocal dielectric screening in this atomically thin material.
Physical Review Letters, 2006
The electronic properties of single-walled carbon nanotubes (SWNTs) are altered by intertube coup... more The electronic properties of single-walled carbon nanotubes (SWNTs) are altered by intertube coupling whenever bundles are formed. These effects are examined experimentally by applying Rayleigh scattering spectroscopy to probe the optical transitions of given individual SWNTs in their isolated and bundled forms. The transition energies of SWNTs are observed to undergo redshifts of tens of meVs upon bundling with other SWNTs. These intertube coupling effects can be understood as arising from the mutual dielectric screening of SWNTs in a bundle.
Applied Physics B: Lasers and Optics, 1999
We demonstrate and analyze a new method for probing electric field strengths using optical second... more We demonstrate and analyze a new method for probing electric field strengths using optical second-harmonic generation. The technique, based on a homodyne detection scheme, employs interference between the field-induced second-harmonic radiation from the sample and strong second-harmonic radiation from a reference. The scheme provides a linear relationship between the measured secondharmonic signal strength and the amplitude of the electric field being probed, thus providing easy calibration of the amplitude of the electric field and direct information on its sign. Experimental results are presented for direct and homodyne detection of in-plane fields in silicon structures. A discussion of the expected signal-to-noise characteristics is presented and the results are compared to experimental findings. Homodyne detection of electric fields with strengths on the order of 100 V/cm can be achieved with reasonable integration times. PACS: 42.65.-k; 42.65.Ky; 42.70.Nq Optical second-harmonic generation (SHG) has been widely recognized as a surface-sensitive probe in centrosymmetric materials [1, 2]. As is well known, this sensitivity arises from the fact that the SHG process is dipole-forbidden in a centrosymmetric medium. The breaking of the inversion symmetry at a surface or interface greatly alters SHG from the sample. The same principle is operative when an electric field E0 is applied to a centrosymmetric material. The electric field E0, as a polar vector, acts to lift the inversion symmetry of the material [2]. The efficiency for SHG is thus strongly influenced and measurement of SHG provides a sensitive probe of electric fields present within the optical probing volume. The effect of an applied electric field is thus quite different from that of an applied magnetic field. The latter can alter existing SHG contributions, but, as an axial vector, does not cause the inversion symmetry of the bulk to be lifted [2]. The high sensitivity of SHG for probing of the electric fields was demonstrated early in the history of nonlinear optics. Such an electric field-induced SHG (EFISH) process was first reported by Terhune et al. [3] for a calcite sample and extended soon thereafter to semiconductors and metals by Lee et al. [4]. More recently, systematic studies of the influence of electric fields on SHG have been pursued for centrosymmetric media [5–7], as well as for certain non-centrosymmetric materials with high symmetry [8]. The EFISH process in aqueous environments has also been intensively investigated in several interesting regimes [4, 9, 10]. It has been shown to be both of fundamental interest and a useful tool for probing chemical processes at interfaces. An especially attractive feature of the EFISH process for probing electric fields lies in the possibility for measurements with extremely high time resolution. Time resolution down to the femtosecond regime can be achieved by sampling the material system with ultrafast pulses from a modelocked laser. This approach has been applied successfully to study the influence of chargecarriers on the dynamics of internal electric fields in insulators [10] and semiconductors [11, 12], and to probe microwave [13] and ultrafast transient electric fields directly in time domain [14, 15]. While the background-free character of the EFISH process in a centrosymmetric medium constitutes the principal attractive feature of the method, it also imposes certain complications and limitations. In particular, if we do indeed observe negligible SHG in the absence of the applied field E0, then we expect the induced second-harmonic (SH) polarization to vary linearly with E0. In this case, the radiated SH field will also be linear in E0. The measured quantity is, however, the SH intensity I2ω, which will then scale quadratically with the electric field E0 being probed. Such a quadratic relation implies both the need for careful calibration and, more importantly, the loss of information on the sign of the electric field. While the sign of E0 can be recovered from a measurement of the phase of the radiated SH field, it is clearly desirable to have an experimental methodology free of these complications. Such a method is the homodyne detection scheme presented in this paper. The homodyne detection scheme is a well-established method in which a weak signal of interest is combined with