Control Of Optical Fields Research Articles

Large elasto-optic effect and reversible electrochromism in multiferroic BiFeO3.

The control of optical fields is usually achieved through the electro-optic or acousto-optic effect in single-crystal ferroelectric or polar compounds such as LiNbO3 or quartz. In recent years, tremendous progress has been made in ferroelectric oxide thin film technology—a field which is now a strong driving force in areas such as electronics, spintronics and photovoltaics. Here, we apply epitaxial strain engineering to tune the optical response of BiFeO3 thin films, and find a very large variation of the optical index with strain, corresponding to an effective elasto-optic coefficient larger than that of quartz. We observe a concomitant strain-driven variation in light absorption—reminiscent of piezochromism—which we show can be manipulated by an electric field. This constitutes an electrochromic effect that is reversible, remanent and not driven by defects. These findings broaden the potential of multiferroics towards photonics and thin film acousto-optic devices, and suggest exciting device opportunities arising from the coupling of ferroic, piezoelectric and optical responses.

Electromechanical control of nitrogen-vacancy defect emission using graphene NEMS.

Despite recent progress in nano-optomechanics, active control of optical fields at the nanoscale has not been achieved with an on-chip nano-electromechanical system (NEMS) thus far. Here we present a new type of hybrid system, consisting of an on-chip graphene NEMS suspended a few tens of nanometres above nitrogen-vacancy centres (NVCs), which are stable single-photon emitters embedded in nanodiamonds. Electromechanical control of the photons emitted by the NVC is provided by electrostatic tuning of the graphene NEMS position, which is transduced to a modulation of NVC emission intensity. The optomechanical coupling between the graphene displacement and the NVC emission is based on near-field dipole–dipole interaction. This class of optomechanical coupling increases strongly for smaller distances, making it suitable for nanoscale devices. These achievements hold promise for selective control of emitter arrays on-chip, optical spectroscopy of individual nano-objects, integrated optomechanical information processing and open new avenues towards quantum optomechanics.

Assembling nanoantennas

Metallic nanoparticles enable the control of optical fields at the nanometre scale, enhancing the absorption and emission of local emitters. Now, using the self-assembling properties of DNA, functional nanoantennas have been developed that comprise a versatile and robust assembly of gold nanoparticles and emitters.

Optical nano-imaging of gate-tunable graphene plasmons

The ability to manipulate optical fields and the energy flow of light is central to modern information and communication technologies, as well as quantum information processing schemes. However, because photons do not possess charge, a way of controlling them efficiently by electrical means has so far proved elusive. A promising way to achieve electric control of light could be through plasmon polaritons—coupled excitations of photons and charge carriers—in graphene. In this two-dimensional sheet of carbon atoms, it is expected that plasmon polaritons and their associated optical fields can readily be tuned electrically by varying the graphene carrier density. Although evidence of optical graphene plasmon resonances has recently been obtained spectroscopically, no experiments so far have directly resolved propagating plasmons in real space. Here we launch and detect propagating optical plasmons in tapered graphene nanostructures using near-field scattering microscopy with infrared excitation light. We provide real-space images of plasmon fields, and find that the extracted plasmon wavelength is very short—more than 40 times smaller than the wavelength of illumination. We exploit this strong optical field confinement to turn a graphene nanostructure into a tunable resonant plasmonic cavity with extremely small mode volume. The cavity resonance is controlled in situ by gating the graphene, and in particular, complete switching on and off of the plasmon modes is demonstrated, thus paving the way towards graphene-based optical transistors. This successful alliance between nanoelectronics and nano-optics enables the development of active subwavelength-scale optics and a plethora of nano-optoelectronic devices and functionalities, such as tunable metamaterials, nanoscale optical processing, and strongly enhanced light–matter interactions for quantum devices and biosensing applications.

Optothermal sample preconcentration and manipulation with temperature gradient focusing

In this article, we present an optothermal analyte preconcentration method based on temperature gradient focusing. This approach offers a flexible, noninvasive technique for focusing and transporting charged analytes in microfluidics using light energy. The method uses the optical field control provided by a digital projector as established for particle manipulation, to achieve analogous functionality for molecular analytes for the first time. The optothermal heating system is characterized and the ability to control of the heated zone location, size, and power is demonstrated. The method is applied to concentrate a sample model analyte, along a microcapillary, resulting in almost 500-fold local concentration increase in 15 min. Optically controlled upstream and downstream transport of a focused analyte band is demonstrated with a heater velocity of ∼170 μm/min.

Visualizing the Optical Interaction Tensor of a Gold Nanoparticle Pair

The control of optical fields on the nanometer scale is a central theme of plasmonics and nanophotonics. Methods for characterizing localized optical field distributions are necessary to validate theoretical predictions, to test nanofabrication procedures, and to provide feedback for design improvements. Typical methods of probing near fields (e.g., single molecule fluorescence and near-field microscopy) cannot probe both the complex-valued and vectorial nature of the field distributions. We demonstrate that a nanoparticle probe with isotropic polarizability in combination with polarization control of excitation and detection beams provides access to this information through the interaction tensor. For a sample consisting of a single nanoparticle we show that the recorded images correspond to maps of the local Green's function tensor elements that couple the probe and sample. The tensorial mapping of interacting nanoparticles is of interest for optical sensing, optical antennas, surface-enhanced Raman scattering, nonlinear optics, and molecular rulers.

Optical field control of charge transmission through a molecular wire. II. Photoinduced removal of the Franck-Condon blockade

Charge transmission through a single molecule attached to two nanoelectrodes is discussed for the case of an additional optical excitation of electronic transitions in the neutral as well as the charged state. $I\text{\ensuremath{-}}V$ and current radiation field-strength characteristics are presented. For large values of nuclear rearrangement upon charging and in the low-voltage region, the current suppression known as the Franck-Condon blockade is completely removed. This suggests optical current switching via the opening of additional transmission channels and the formation of strongly nonequilibrium vibrational distributions. While the triggered current flow sensitively depends on the strength of intramolecular vibrational energy redistribution, it is rather robust with respect to molecular deexcitation caused by electron-hole pair formation in the leads.

Optical field control of charge transmission through a molecular wire. I. Generalized master equation description

A generalized master equation description of charge transmission through a single molecule that goes beyond a perturbative treatment of electron-vibrational coupling and accounts for nonequilibrium vibrational distributions is presented. By utilizing a projection superoperator formalism, exact expressions for the generalized master equation kernels that include intramolecular vibrational energy redistribution, excited electronic levels, as well as their optical excitation and de-excitation are derived. This enables one to study optical current switching in the case of weak electron-lead coupling and large reorganization energy upon charging. Additionally, it is possible to quantify the influence of intramolecular vibrational energy redistribution and excited state de-excitation on the current switching.

Control of Optical Field by Using a Photonic Crystal.

In the last decade, photonic crystals have attracted an increasing interest, mainly because the optical field, i.e., radiation field and light propagation characteristics can be thereby controlled in principle. This is important not only in the basic science such as physics, but also in developing new devices in optoelectronics. For the latter, it is expected, for example, that using a variety of novel unique features of photonic crystals, an extremely-miniaturized planar light circuit will be developed in future. In this review article, the present status of researches in this field is highlighted. First, several pronounced features of two- and three-dimensional photonic crystals are summarized. Second, some aspects of recent development of the crystals are presented with special concern to two-dimensional ones. Third, examples of controlling optical field are presented, which include laser action, enhancement of second-harmonic generation signal, photonic crystal waveguides, and light delay component.