Sean Molesky

Quantum nanophotonics using hyperbolic metamaterials

Engineering optical properties using artificial nanostructured media known as metamaterials has led to breakthrough devices with capabilities from super-resolution imaging to invisibility. In this paper, we review metamaterials for quantum nanophotonic applications, a recent development in the field. This seeks to address many challenges in the field of quantum optics using advances in nanophotonics and nanofabrication. We focus on the class of nanostructured media with hyperbolic dispersion that have emerged as one of the most promising metamaterials with a multitude of practical applications from subwavelength imaging, nanoscale waveguiding, biosensing to nonlinear switching. We present the various design and characterization principles of hyperbolic metamaterials and explain the most important property of such media: a broadband enhancement in the electromagnetic density of states. We review several recent experiments that have explored this phenomenon using spontaneous emission from dye molecules and quantum dots. We finally point to future applications of hyperbolic metamaterials, using the broadband enhancement in the spontaneous emission to construct single-photon sources.

Broadband super-Planckian thermal emission from hyperbolic metamaterials

We develop fluctuational electrodynamics of hyperbolic metamaterials (HMMs) and establish broadband near-field thermal emission beyond the black-body limit. We predict thermal topological transitions in phonon-polaritonic HMMs paving the way for near-field thermal engineering using metamaterials.

High temperature epsilon-near-zero and epsilon-near-pole metamaterial emitters for thermophotovoltaics

We introduce a class of artificial media: high temperature Epsilon-near-Pole metamaterials consisting of plasmonic materials with high melting point and show that they can be used as efficient narrowband omnidirectional thermal emitters in thermophotovoltaic systems.

Controlling thermal emission with refractory epsilon-near-zero metamaterials via topological transitions

Control of thermal radiation at high temperatures is vital for waste heat recovery and for high-efficiency thermophotovoltaic (TPV) conversion. Previously, structural resonances utilizing gratings, thin film resonances, metasurfaces and photonic crystals were used to spectrally control thermal emission, often requiring lithographic structuring of the surface and causing significant angle dependence. In contrast, here, we demonstrate a refractory W-HfO2 metamaterial, which controls thermal emission through an engineered dielectric response function. The epsilon-near-zero frequency of a metamaterial and the connected optical topological transition (OTT) are adjusted to selectively enhance and suppress the thermal emission in the near-infrared spectrum, crucial for improved TPV efficiency. The near-omnidirectional and spectrally selective emitter is obtained as the emission changes due to material properties and not due to resonances or interference effects, marking a paradigm shift in thermal engineering approaches. We experimentally demonstrate the OTT in a thermally stable metamaterial at high temperatures of 1,000 °C.

Ideal near-field thermophotovoltaic cells

We ask the question, what are the ideal characteristics of a near-field thermophotovoltaic cell? Our search leads us to a reformulation of near-field radiative heat transfer in terms of the joint density of electronic states of the emitter-absorber pair in the thermophotovoltaic system. This form reveals that semiconducting materials with narrowband absorption spectra are critical to the energy conversion efficiency. This essential feature is unavailable in conventional bulk semiconductor cells but can be obtained using low dimensional materials. Our results show that the presence of matched van Hove singularities resulting from quantum-confinement in the emitter and absorber of a thermophotovoltaic cell boosts both the magnitude and spectral selectivity of radiative heat transfer; dramatically improving energy conversion efficiency. We provide a model near-field thermophotovoltaic system design making use of this idea by employing the van Hove singularities present in carbon nanotubes. Shockley-Queisser analysis shows that the predicted heat transfer characteristics of this model device are fundamentally better than existing thermophotovoltaic designs. Our work paves the way for the use of quantum dots, quantum wells, two-dimensional semiconductors, semiconductor nanowires and carbon nanotubes as future materials for thermophotovoltaic cells.

Thermal excitation of plasmons for near-field thermophotovoltaics

The traditional approaches of exciting plasmons consist of using electrons (eg: electron energy loss spectroscopy) or light (Kretchman and Otto geometry) while more recently plasmons have been excited even by single photons. A different approach: thermal excitation of a plasmon resonance at high temperatures using alternate plasmonic media was proposed by S. Molesky et.al., Opt. Exp. 21.101, A96-A110, (2013). Here, we show how the long-standing search for a high temperature narrow band near-field emitter for thermophotovoltaics can be fulfilled by high temperature plasmonics. We also describe how to control Weins displacement law in the near-field using high temperature epsilon-near-zero metamaterials. Finally, we show that our work opens up an interesting direction of research for the field of slow light: thermal emission control.

Corrigendum: Quantum nanophotonics using hyperbolic metamaterials (2012 J. Opt. 14 063001)

Engineering optical properties using artificial nanostructured media known as metamaterials has led to breakthrough devices with capabilities from super-resolution imaging to invisibility. In this paper, we review metamaterials for quantum nanophotonic applications, a recent development in the field. This seeks to address many challenges in the field of quantum optics using advances in nanophotonics and nanofabrication. We focus on the class of nanostructured media with hyperbolic dispersion that have emerged as one of the most promising metamaterials with a multitude of practical applications from subwavelength imaging, nanoscale waveguiding, biosensing to nonlinear switching. We present the various design and characterization principles of hyperbolic metamaterials and explain the most important property of such media: a broadband enhancement in the electromagnetic density of states. We review several recent experiments that have explored this phenomenon using spontaneous emission from dye molecules and quantum dots. We finally point to future applications of hyperbolic metamaterials, using the broadband enhancement in the spontaneous emission to construct single-photon sources.

Thermal graphene metamaterials and epsilon-near-zero high temperature plasmonics

The key feature of a thermophotovoltaic (TPV) emitter is the enhancement of thermal emission corresponding to energies just above the bandgap of the absorbing photovoltaic cell and simultaneous suppression of thermal emission below the bandgap. We show here that a single layer plasmonic coating can perform this task with high efficiency. Our key design principle involves tuning the epsilon-near-zero frequency (plasma frequency) of the metal acting as a thermal emitter to the electronic bandgap of the semiconducting cell. This approach utilizes the change in reflectivity of a metal near its plasma frequency (epsilon-near-zero frequency) to lead to spectrally selective thermal emission and can be adapted to large area coatings using high temperature plasmonic materials. We provide a detailed analysis of the spectral and angular performance of high temperature plasmonic coatings as TPV emitters. We show the potential of such high temperature plasmonic thermal emitter coatings (p-TECs) for narrowband near-field thermal emission. We also show the enhancement of near-surface energy density in graphene-multilayer thermal metamaterials due to a topological transition at an effective epsilon-near-zero frequency. This opens up spectrally selective thermal emission from graphene multilayers in the infrared frequency regime. Our design paves the way for the development of single layer p-TECs and graphene multilayers for spectrally selective radiative heat transfer applications.

Imaging ultrafast dynamics on the nanoscale with a THz-STM

We detail a new ultrafast scanning tunneling microscopy technique called THz-STM that uses terahertz (THz) pulses coupled to the tip of a scanning tunneling microscope (STM) to directly modulate the STM bias voltage over subpicosecond time scales [1]. In doing so, THz-STM achieves ultrafast time resolution via a mode complementary to normal STM operation, thus providing a general ultrafast probe for stroboscopic pump-probe measurements. We use THz-STM to image ultrafast carrier trapping into a single InAs nanodot and demonstrate simultaneous nanometer (2 nm) spatial resolution and subpicosecond (500 fs) temporal resolution in ambient conditions. Extending THz-STM to vacuum and low temperature operation has the potential to enable studies of a wide variety of subpicosecond dynamics on materials with atomic resolution.

Ultrafast THz-pulse-induced tunneling dynamics in an STM

We have recently developed an ultrafast terahertz-pulse-coupled scanning tunneling microscope (THz-STM) that can image nanoscale dynamics with simultaneous 0.5 ps temporal resolution and 2 nm spatial resolution under ambient conditions. Broadband THz pulses that are focused onto the metallic tip of an STM induce sub-picosecond voltage transients across the STM junction, producing a rectified current signal due to the nonlinear tunnel junction currentvoltage (I-V) relationship. We use the Simmons model to simulate a tunnel junction I-V curve whereby a THz pulse induces an ultrafast voltage transient, generating milliamp-level rectified currents over sub-picosecond timescales. The nature of the ultrafast field emission tunneling regime achieved in the THz-STM is discussed.