Session E46: Advances in Scanning Probe Microscopy II: High Frequencies and Optical Techniques

Towards microwave imaging of single two-level defects in dielectric materials

Two-level fluctuators (TLF) are a major source of decoherence in quantum devices and significant effort is invested towards better understanding and eliminating these types of material defects. Here we propose that a near-field scanning microwave microscope (NSMM) can be used to image individual two-level defects on the nano-scale, provided that such a microscope operates in the right regime [1]. Not only would such a ‘coherent’ NSMM be able to obtain nano-scale spatial distributions of defects and their locations within dielectric materials, it would also be able to determine the relative orientation of the TLF dipole with respect to the dielectric crystal, giving vital information about the nature of the TLF. We theoretically describe the operation and capabilities of a ‘coherent’ NSMM and show that individual defects can be imaged in dielectric materials with low enough loss tangent, such as sapphire and silicon dioxide, relevant for solid state quantum technologies. We describe the requirements for constructing such an NSMM and report on our recent progress in setting up this technology [2]. [1] S. E. de Graaf, et al., Sci. Rep., in press (2015). [2] S. E. de Graaf, et al., Rev. Sci. Instrum. 84, 023706 (2013).   

Advances in imaging and quantification of electrical properties at the nanoscale using Scanning Microwave Impedance Microscopy (sMIM)

Scanning Microwave Impedance Microscopy (sMIM) is a mode for Atomic Force Microscopy (AFM) enabling imaging of unique contrast mechanisms and measurement of local permittivity and conductivity at the 10’s of nm length scale. sMIM has been applied to a variety of systems including nanotubes, nanowires, 2D materials, photovoltaics and semiconductor devices. Early results were largely semi-quantitative. This talk will focus on techniques for extracting quantitative physical parameters such as permittivity, conductivity, doping concentrations and thin film properties from sMIM data. Particular attention will be paid to non-linear materials where sMIM has been used to acquire nano-scale capacitance-voltage curves. These curves can be used to identify the dopant type (n vs p) and doping level in doped semiconductors, both bulk samples and devices.   

Magnetic resonance force detection using a membrane resonator

Silicon nitride (Si$_3$N$_4$) membranes are commercially-available, versatile structures that have a variety of applications. Although most commonly used as the support structure for transmission electron microscopy (TEM) studies, membranes are also ultrasensitive high-frequency mechanical oscillators. The sensitivity stems from the high quality factor $Q\sim10^6$, which has led to applications in sensitive quantum optomechanical experiments. The high sensitivity also opens the door to ultrasensitive force detection applications. We report force detection of electron spin magnetic resonance at 300 K using a Si$_3$N$_4$ membrane with a force sensitivity of 4 fN/$\sqrt{\mathrm{Hz}}$, and a potential low temperature sensitivity of 25 aN/$\sqrt{\mathrm{Hz}}$. Given membranes' sensitivity, robust construction, large surface area and low cost, SiN membranes can potentially serve as the central component of a compact room-temperature ESR and NMR instrument that has superior spatial resolution to conventional NMR.   

Dipolar Decoupling in Magnetic Resonance Force Microscopy using Optimal Control Pulses

We present data showing how a modified gradient ascent pulse engineering method can be used to design nuclear magnetic resonance pulses that perform a single unitary transformation over a large range of maximum Rabi field strengths $(B_1)$, while decoupling the secular dipolar interactions between spins. We designed dipolar-decoupling $\pi$-pulses that perform well over spins feeling maximum $B_1$ fields from $131-274G$. By combining these $\pi$-pulses into a simple multiple pulse sequence, with fields produced by a silver microwire, we have increased $T2^*$ in a polystyrene sample attached to the tip of a silicon nanowire from $11\mu s$ to $\sim250ms$. This dipolar decoupling could be used to improve the spatial resolution of nano-MRI experiments and to allow spectroscopy of chemical shifts in nanoscale samples.   

Force-Detected Magnetic Resonance Imaging in Micron-Scale Liquids

We report our efforts in the development of Nuclear Magnetic Resonance Force Microscopy (NMRFM) for the study of biological materials in liquid media at the micron scale. Our probe contains microfluidic samples sealed in thin-walled (~few µm) quartz tubes, with a micro-oscillator sensor nearby in vacuum to maintain its high mechanical resonance quality factor. An initial demonstration utilizes a permalloy magnet on the oscillator tip, which provides a resonant slice of thickness ~0.5 µm and an area of diameter ~10µm; these first measurements aim to demonstrate a single-shot measurement of the longitudinal relaxation time T1 in aqueous solutions of Cu2SO4. We also aim to implement a sawtooth 2? cyclic inversion of the nuclear spins, a detection scheme that effectively eliminates common measurement artifacts. At the micron scale, both spin diffusion and physical diffusion in liquids tend to blur images in conventional magnetic resonance imaging (MRI); we aim to exploit the local nature of the NMRFM probe to obtain higher resolution dynamical images, with the ultimate goal of imaging within individual biological cells.   

Time-Resolved SQUID Sensor with a Nyquist Frequency up to 25 GHz

We demonstrate a time-resolved scanning Superconducting QUantum Interference Device (SQUID) sensor with an expected maximum sampling rate of 50 GHz. The time-resolved SQUID sampler is operated by a pump-probe pulse sequence and will be particularly useful in studying high-frequency magnetic devices and the transient behavior of magnetic materials. The high sampling rate is achieved through a Josephson-interferometry technique developed at IBM[1][2]. We tested our sampler with flux signals of order 10 m$\Phi_{0}$ (where $\Phi_{0}$ is the magnetic flux quantum), which corresponds to 25 million Bohr magnetons located 1 micron directly below the pickup loop. Operating in this regime, our sampler will have much higher sensitivity than bulk sensors like conventional SQUIDs and much larger spatial scanning range than single-spin sensors like NV centers. The SQUID sampler will thus be well-suited to characterize individual mesoscopic samples as well as bulk samples with mesoscopic features. [1]S. M. Faris, APL 36, 1005 (1980). [2]J. R. Kirtley, et al., "Advanced sensors for scanning SQUID microscopy", Superconductive Electronics Conference (ISEC 2013), invited paper F2.   

Toward single atom qubits on a surface: Pump-probe spectroscopy and electrically-driven spin resonance

We will discuss the characterization of spin dynamics by pump-probe spectroscopy and the use of gigahertz-frequency electric fields to drive spin resonance of a Fe atom on a MgO/Ag(001) surface. In the spirit of this session, the technical challenges in applying a precise voltage to the tip sample junction across a wide radio-frequency bandwidth will be described. The energy relaxation time, T1, of single spins on surfaces can be measured by spin-polarized pump-probe STM (scanning tunneling microscopy) [1]. To date, the relaxation times reported for Fe-Cu dimers on Cu2N insulating films have been of the order $\sim$100 ns [1]. A three-order-of-magnitude enhancement of lifetime, to $\sim$200 $\mu$s, was recently demonstrated for Co on a single-monolayer of MgO [2]. Here, we report on the tailoring of the T1 lifetime of single Fe atoms on single- and multi- layer MgO films grown on Ag(001). Next, we demonstrate electron spin resonance of an individual single Fe atom, driven by a gigahertz-frequency electric field applied across the tip-sample junction, and detected by a spin-polarized tunneling current. The principle parameters of the spin resonance experiment, namely the phase coherence time T2 and the Rabi rate, are characterized for Fe atoms adsorbed to the monolayer MgO film. [1] Loth et al., Science 329, 1628 (2010) [2] Rau and Baumann et al., Science 344, 988 (2014) [3] Baumann and Paul et al., Science 350, 417 (2015)   

Cantilever detection of electron spin resonance in the terahertz region

Electron spin resonance (ESR) is used in a wide range of research areas. Most commercially available spectrometers operate at the X- band ($\sim$10 GHz). However, high-frequency ESR ($>$100 GHz) has many advantages, such as the high spectral resolution, the ESR detection beyond the zero-field splitting etc. We report the cantilever detection of electron spin resonance in the terahertz region. This technique mechanically detects ESR as a change in magnetic torque that acts on the cantilever, while the conventional method, such as the cavity perturbation and the transmission method, directly measures the absorption of electromagnetic wave power. Backward wave oscillators (BWO) were used as THz-wave sources. Despite the small sample mass ($m=4\ \mathrm{\mu g}$) and low power output of the BWO ($P< 4\ \mathrm{mW}$ above 1 THz), we observed ESR absorption of Co Tutton salt, Co(NH$_4$)$_2$(SO$_4$)$_2$$\cdot$6H$_2$O, in frequencies of up to 1.1 THz. Spin sensitivity was estimated to be the order of $10^{11}$-$10^{12}$ spins/gauss above 1 THz. This technique will not only broaden the scope of ESR spectroscopy application but also lead to high-spectral-resolution ESR imaging.\\ $\left[ 1 \right]$H. Takahashi, E. Ohmichi and H. Ohta, Appl. Phys. Lett. {\bfseries 107}, 182405 (2015).   

Ultrafast nano-imagining of the photoinduced phase transition dynamics in VO$_{\mathrm{2}}$

Many quantum phase transitions in correlated matter exhibit spatial inhomogeneities with expected yet unexplored effects on the associated ultrafast dynamics. Here we demonstrate the combination of ultrafast non-degenerate pump-probe spectroscopy with scattering scanning near-field optical microscopy ($s$-SNOM) for ultrafast nano-imaging. In a femtosecond near-field non-degenerate near-IR (NIR) pump and mid-IR (MIR) probe experiment, we study the photoinduced insulator-to-metal (IMT) transition in nominally homogeneous VO$_{\mathrm{2}}$ micro-crystals using far-from equilibrium excitation. We observe spatial heterogeneity on 50-100 nm length scales in the fluence dependent IMT dynamics, ranging from sub-100 fs to 1 ps. With pump fluences as high as nominally 10 mJ/cm$^{\mathrm{2}}$ we can reach distinct excitation and saturation regimes. These results suggest a large sensitivity of the IMT with respect to local variations in strain, doping, or defects difficult to discern microscopically.   

Nanoscale dynamics of the Insulator-to-Metal transition in VO$_{2}$

We have improved upon the technique of time resolved scanning near-field optical microscopy to study the development of inhomogeneous phase transitions in the time domain with 20 nanometer spatial resolution and 100 femtosecond temporal resolution. In our present work, we study Vanadium Dioxide (VO$_{2})$, which is a canonical correlated electron system that exhibits an insulator-to-metal transition (IMT) above room temperature. We observe inhomogeneous dynamics that are related to mesoscopic strain variations. Our measurement resolves the dynamical evolution of the IMT on length scales that are short compared with the typical sizes of metallic domains in VO$_{2}$. By using Near-Infrared radiation, measured on a pulse-to-pulse basis, we are able to achieve an unprecedented Signal-to-Noise ratio. Our advances pave a pathway to study a wide range of systems with inhomogeneities properties on the nanoscale with high sensitivity, nanoscopic spatial, and ultrafast temporal resolution.   

s-SNOM based IR and THz spectroscopy for nanoscale material characterization

Scattering-type Scanning Near-field Optical Microscopy (s-SNOM) allows to overcome the diffraction limit of conventional light microscopy enabling optical measurements at a spatial resolution of 10nm. \\ s-SNOM employs an externally-illuminated sharp metallic AFM tip to create a nanoscale hot-spot at its apex. The optical tip-sample near-field interaction is determined by the local dielectric properties (refractive index) of the sample and detection of the elastically tip-scattered light yields nanoscale resolved near-field images simultaneous to topography. \\ Development of a dedicated Fourier-transform detection module for analyzing light scattered from the tip which is illuminated by a broadband laser source enables IR spectroscopy of complex polymer nanostructures. Applications presented further demonstrate characterization of embedded structural phases in biominerals (bone), organic semiconductors or functional semiconductor nanostructures.\\ Furthermore, by extending the concept of broadband-s-SNOM spectroscopy to the THz-spectral range, we demonstrate optical near-field imaging and spectroscopy at THz-frequencies (0.5-2.5 THz) by coupling the free space beam of a dedicated THz-TDS to the s-SNOM system.   

Novel combination of near-field s-SNOM microscopy with peak-force tapping for nano-chemical and nano-mechanical material characterization with sub-20 nm spatial resolution

Heterogeneity in material systems requires methods for nanoscale chemical identification. Scattering scanning near-field microscopy (s-SNOM) is chemically sensitive in the infrared fingerprint region while providing down to 10 nm spatial resolution. This technique detects material specific tip-scattering in an atomic force microscope. Here, we present the first combination of s-SNOM with peak-force tapping (PFT), a valuable AFM technique that allows precise force control between tip and sample down to 10s of pN. The latter is essential for imaging fragile samples, but allows also quantitative extraction of nano-mechanical properties, e.g. the modulus. PFT can further be complemented by KPFM or conductive AFM for nano-electrical mapping, allowing access to nanoscale optical, mechanical and electrical information in a single instrument. We will address several questions ranging from graphene plasmonics to material distributions in polymers. We highlight a biological application where dental amelogenin protein was studied via s-SNOM to learn about its self-assembly into nanoribbons. At the same time PFT allows to track crystallization to distinguish protein from apatite crystals for which amelogenin is supposed to act as a template.   

Towards optically-integrated scanning tunneling microscopy studies of defects in semiconductors

As electronic devices approach the nanoscale, their function is increasingly dependent on the local environment of individual defects. We are developing a combination of optical illumination and scanning tunneling microscopy techniques to study how the properties of individual defects depend on aspects of the local environment, such surface or defect proximity, applied electric fields, and illumination. Here we present studies of individual Zn and Er impurities in GaAs(110).\par We use controlled motion of the STM tip during voltage sweeps to resolve previously hidden in-gap states of Zn acceptors and probe Zn further from the surface than previously accessible. We discovered two classes of Zn acceptors, one with defect states that did not shift with tip-induced band bending (TIBB), and one with states that do. Similar behavior was observed for above-gap illumination, consistent with the surface photovoltage effect (SPV). For Er on GaAs(110), we discovered three different adsorption states sharing two different sites. We found defect states near the conduction band edge, which shifted with TIBB as well as IR illumination resonant with the Er f-shell transitions.