The decay dynamics of excited carriers in graphene have attracted wide scientific attention, as its gapless Dirac electronic band structure opens up relaxation channels that are not allowed in conventional bulk materials. We report bright mid-infrared emission from laser-pumped graphene, originating from a previously unobserved decay channel: hot plasmon emission. The observed Fermi-level dependence of radiation rules out all possible mid-infrared Planckian light emission mechanisms. Calculations for our experimental conditions indicate that conditions for plasmon gain exist on the sub-100 fs timescale due to the net positive emission processes. As the population inversion of excited carriers depletes at longer times, spontaneous plasmon emission dominates stimulated plasmon emission, and the cumulative observed emission per excitation pulse is dominated by spontaneous plasmon emission. The calculated time-integrated plasmon-assisted spontaneous emission spectra corroborate with the experimentally measured Fermi-level-dependent emission spectra from a planar graphene. The plasmon emission processes produce non-Planckian behavior. The spectral flux of spontaneously generated plasmons is found to be several orders of magnitude higher than that of spontaneous photon emission. Evidence for bright hot plasmon emission is further supported by a large emission enhancement detected from graphene decorated with gold NDs, which promote localized plasmon excitation and serve as outcoupling scatterers. These observations pave a new avenue for the exploration of ultrafast and ultrabright mid-infrared stimulated and spontaneous emission processes and bright infrared light sources.
Laser-driven light sails need to withstand very high intensities of incident light, and therefore must comprise low-loss materials that remain low loss with increasing temperatures. We will describe our measurements of temperature-dependent optical properties of materials (oxides, nitrides, semiconductors) for the development of metasurfaces for laser-driven light sails. We use oscillator-based models to fit ellipsometry data at different temperatures in the wavelength region where a precise measurement can be made, and revise these models with datapoints in the low-absorption region measured using photo-thermal common-path interferometry. We also demonstrate how metasurface performance is affected by the temperature-dependent properties of constituent materials.
Bound states in the continuum (BICs) in dielectric metasurfaces have gained significant interest for their narrow linewidths and potential applications to high-powered lasers, filters, and sensors. The resonant frequency of a BIC may be tuned by altering its dielectric environment either by changing the substrate or introducing a tunable material such as graphene. In this work, we use a gradient-descent based optimization approach to design 1D BIC metasurfaces with optical modes that exhibit extreme resonant frequency dependence on their dielectric environment. We then experimentally test our predictions by fabricating the structures, measuring their optical response, and comparing to our predictions.
Knowledge of temperature-dependent optical properties of materials is required for photonics applications in extreme conditions, i.e., at high temperatures. In this talk, we will describe our latest measurements of temperature-dependent optical properties of materials (oxides, nitrides, semiconductors) for the development of metasurfaces for high-temperature applications that include thermal radiators and light sails. We use oscillator-based models to fit ellipsometry data at different temperatures in the wavelength region where a precise measurement can be made, and extrapolate to get broadband temperature-dependent optical properties. We also demonstrate using simulations how metasurface performance is affected by the temperature-dependence of constituent materials.
Plasmons in graphene are known to be tunable and to exhibit extreme field confinement, making them useful for optoelectronic devices, and for exploring extreme light matter interactions. Thus far, these effects have been demonstrated at Thz to mid-IR frequencies, with the upper frequency limit set by limits of electron beam lithography, which can make graphene nanostructures as small as 15nm. In this talk, I will show that bottom-up block co-polymer lithography methods can create nanostructures with characteristic lengthscales as small as 12nm, and that in this regime the non-local interactions in graphene become strong, creating a significant blue shift of the plasmonic resonances. This allows for the creation of plasmonic cavities with resonances at wavelengths as short as 2.2um. The confinement factors of these cavities reach 135, which is exceedingly large, but less than what has been predicted by theory.
Patterning graphene into nanostructures enables the coupling of free space radiation to plasmons in graphene. These plasmons are highly tunable and have been used in such applications as tunable filters and chemical sensors in the THz and mid-infrared ranges, with graphene structures with characteristic dimensions of 15 nm of greater. Here, we will demonstrate that block copolymer based fabrication can create sub 15 nm plasmonically active graphene nanostructures in a scalable, efficient, and repeatable manner. Furthermore, we will report the first measurement of a near-infrared graphene plasmon resonance and discuss some implications for next-generation optoelectrical devices.
Active metasurfaces have received remarkable attention due to the advantage of adjusting their functions without changing physical structures. However, the previous active metasurfaces suffer from an inevitable correlation between amplitude and phase modulation of light. They inherently lack the degrees of freedom to independently control the amplitude and phase of light due to their single resonant design. We introduce a metamolecule which incorporates two consecutive graphene plasmonic nano-resonators. The metasurface using active metamolecules can be free from correlation problems and independently control the amplitude and phase of the scattered wave. A generalized graphical approach has been developed for an intuitive design guideline. Furthermore, dynamic beam steering and holographic wavefront reconstruction are demonstrated by full-wave simulation.
In this presentation, it will be shown that the plasmonic absorption of a graphene sheet can be enhanced and perturbed in controllable ways by controlling the thickness and permittivity of the supporting substrate. We will show the results of recent experiments where 25% absorption is achieved in the plasmonic modes of a graphene sheet by carefully selecting the properties of an underlying silicon nitride substrate. We also demonstrate how additional absorption pathways can be created by modifying the surrounding dielectric environment to have optical resonances that can couple to the graphene plasmons.
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