Strategies for ultrafast optical control of magnetism have been a topic of intense research for several decades because of the potential impact in technologies such as magnetic memory spintronics and quantum computation, as well as the opportunities for non-linear optical control and modulation in applications such as optical isolation and non-reciprocity. Here we report the first experimental quantification of optically induced magnetization in plasmonic Au nanoparticles due to the inverse Faraday effect (IFE). The induced magnetic moment in nanoparticles is found to be ~1,000x larger than that observed in bulk Au, and ~20x larger than the magnetic moment from optimized magnetic nanoparticle colloids such as magnetite. Furthermore, the magnetization and demagnetization kinetics are instantaneous within the sub-picosecond time resolution of our study.
By controlling the relative polarization difference between the pump and probe beams in an ultrafast time-resolved study, the contribution of the IFE and the optical Kerr effect (OKE) was clearly distinguished. Our experiments measured optical rotation indicative of magnetization that is parallel or anti-parallel with a pump beam depending on the helicity of the excitation. Additionally, we observe optically induced magnetization that is ~1,000 times larger than in bulk Au, and linearly proportional with incident optical power. We anticipate these results may be of great interest in the photonics community for application in ultrafast optical control of magnetic properties, and for all-optical methods of optical isolation that do not require externally applied magnetic fields.
The highest efficiency solar cells are also excellent optical emitters. Band edge luminescence results when no other loss mechanisms compete with power conversion, allowing an ideal maximum efficiency of 33.7%. Constraining the angular range of emitted light, in order to promote light trapping and photon recycling within the semiconductor, increases this maximum to 45.1%. By analogy, here we show how a strategy for integrating highly luminescent aligned semiconductor rod-shaped nanocrystals (nanorods) into luminescent solar concentrators (LSCs) can also improve light trapping in the design and similarly enhance performance in comparison with conventional LSCs. This efficiency improvement relies on an asymmetry in the angular dependence of emission versus absorption that can be provided by the stokes shift of the radiation remitted by the nanorod. Our analysis predicts efficiency increases even when non-radiative loss is comparable to current GaAs cells and nanorod optical performance is consistent with state-of-the-art synthetic preparations.
Metasurfaces with broadband optical absorption and engineered thermal emissivity have gained significant interest for applications that require precise control over optical and thermal energy pathways, for example, as selective absorbers in solar heating schemes, or as thermal emitters in thermophotovoltaics. Our laboratory has also recently explored the use of such metasurface absorbers in thermionic power convertor applications. In contrast with traditional methods of photothermalization, plasmonic metasurfaces can also resonantly promote a large population of photo-excited non-thermal ‘hot’ electrons, so that the photo-induced effective temperature of absorbers is a complex combination of separate phononic and electronic contributions, even under steady-state solar excitation.
Here we show how systematic analysis of the photo-induced surface temperature of metasurface absorbers using anti-stokes raman thermometry can be used to separately analyze the extent of vibrational (phononic) heating versus the effective temperature of the electron gas, as well as provide more detailed insight into the non-equilibrium electron distribution under steady-state CW illumination. The spectral dependence of the luminescent up-conversion of anti-stokes scattered photons reflects contributions from both phonon interactions as well as direct electron scattering, and these contributions can be decoupled by analyzing the dependence on excitation wavelength, intensity, substrate temperature, and other systematic variations of structural features of the metasurface.
Optically excited plasmonic nanostructures display remarkable electron dynamics in the form of coherent electron displacement motion, as well as efficient generation of non-thermal ‘hot electrons’ with large kinetic energy. Here, we provide a theoretical and experimental overview of our studies of photo-induced charge transport across plasmonic tunneling junctions composed of nanoscale metallic gaps, as a strategy for taking advantage of such electron motion for optoelectronic energy conversion.
In symmetric plasmonic tunneling gaps the energetic distribution of electrons due to photo-induced thermalization and hot electron generation is insufficient for significant electrical currents, either through excitation over the interface potential barrier, or via tunneling that exhibits a net preference for the direction of charge transfer. However, asymmetric resonant structures can provide optical absorption, photo-excitation and time-dependent electric fields that induce significant temperature gradients and local variations in the hot electron population. Such asymmetry can be used to promote unidirectional tunneling transport currents with significant enhancement compared with conventional photoelectron and thermionic emission (~ 10^15 enhancement), and thus comprises an intriguing mechanism for providing electrical work. This presentation will introduce the theoretical framework of tunneling phenomena associated with photon-induced hot electrons in plasmonic structures, including principles of electron distribution under photon excitation, strategies for amplifying hot electron generation (e.g. manipulating hot spots in nano-antennas) and provide a mechanistic quantum model of power conversion devices based on unidirectional electron tunneling across nanoscale plasmonic junctions. We also report on initial transport measurements of plasmonic tunnel junctions that exhibit optical power conversion by this method.
In contrast with linearly polarized excitation, which necessarily has zero magnitude electrical field twice during an optical cycle, the electrical field vector of circularly polarized light has constant magnitude. During an optical cycle the electric field vector rotates in the plane normal to the wave propagation. Consequently, if plasmonic structures are resonant with circularly polarized excitation, it is possible for them to exhibit regions of strongly modified carrier density for the duration of the optical cycle.
Here, we study a class of achiral toroid and ‘sun burst’ nano-patterned plasmonic surfaces that show persistent, circulating charge density waves during circularly polarized illumination. The direction of the continuously circulating wave (clockwise or counterclockwise) depends on the handedness of the incident beam. Our interest stems from whether these charge density waves can support circular electric currents (DC) manifest experimentally as static magnetic fields during illumination. Using full-wave optical modeling (FDTD method), and mechanistic calculations of the circulating potential acting on electrons in the toroid resonators, we outline the conditions that maximize optical excitation of both circulating displacement currents and electron transport currents. We show that in the limit of very weak coupling to the solenoid-like electron transport, or when < 1 x 10^-6% of the plasmonically active electron population enters the circular transport modes, relatively strong magnetic fields, > 1 G, can be expected. We discuss scanning probe measurements for monitoring the induced magnetic field, as well as the relationship between this phenomenon and the inverse Faraday effect observed in continuous media.
The resonant plasmonic properties of metallic nanostructures depend strongly on charge carrier density. Stemming from this dependence, we report a theoretical framework and provided experimental evidence for a ‘plasmoelectric effect’, a newly described mechanism for generating electrochemical potentials in plasmonic nanostructures. Systematic electrical and optical characterization of Au nano-hole arrays shows that the magnitude and sign of the plasmoelectric potential depends on the frequency difference between the plasmon resonance and incident narrowband radiation. Our findings guide the development of solid-state power conversion devices based on the plasmoelectric effect, as our samples generate electrochemical potentials 1000x larger than comparable thermocouples.