In highly doped semiconductor quantum wells (QWs), electrons populate various energy states from different subbands and, therefore, several optical intersubband transitions (ISBTs) can occur simultaneously. Coulomb coupling between these ISBTs gathers the strength of all the individual transitions and concentrates all the oscillator strength in a single collective excitation: the multisubband plasmon (MSP). MSPs are an excellent platform for the study of collective and exotic effects in semiconductors and for the demonstration of novel device concepts. Indeed, the high electronic densities involved in the collective excitation greatly enhance the coupling strength and enable the ultra-strong coupling regime between MSPs and either optical modes in a cavity or phonons in the semiconductor. In this Perspectives paper, after addressing the basic physics of MSPs and the state of the art, we outline the most promising paths for the research community in this topic from the point of view of basic physics, material platforms, and applications of MSPs.

/pdf/perspectives-and-opportunities-with-multisubband-plasmonics-f1xlyhdg.pdf

Perspectives and opportunities with multisubband plasmonics

We investigate the effect of quantum confinement on plasmons in highly doped semiconductor layers. The collective response of the electron gas is constructed from the basis of the confined electronic wavefunctions, accounting for dipole-dipole coupling and non-parabolicity. Our work shows how quantum engineering can be applied to semiconductor plasmonics.

Semiconductor quantum plasmonics

Blackbody thermal emission is spatially diﬀuse. Achieving highly directional thermal emission typically requires nanostructuring the surface of the thermally emissive medium. The most common conﬁguration is a subwavelength grating that scatters surface polaritonic modes from the near-ﬁeld to the far-ﬁeld and produces antenna-like lobes of thermal emission. This concept, however, is typically limited to a particular linear polarization, and requires sophisticated lithography. Here, we revisit the simple motif of a planar Salisbury screen. We show analytically how the interplay between the real and imaginary part of the dielectric permittivity of the emitter layer deﬁnes the directional characteristics of emission, which can range from diﬀuse to highly directional. We propose a realistic conﬁguration and show that hexagonal Boron Nitride thin ﬁlms can enable grating-like thermal emission lobes in a lithography-free platform.

pdf/lithography-free-directional-control-of-thermal-emission-2mt3ll97.pdf

Lithography-free directional control of thermal emission

Abstract Blackbody radiation is incoherent and omnidirectional, whereas various novel applications in renewable energy require a degree of directional control of a thermally emitted beam. So far, such directional control has required nano-structuring the surface of a thermally emitting material, typically by forming diffraction gratings. This, however, necessitates lithography and usually results in polarization-dependent properties. Here, we derive analytical conditions for highly directional thermal emission using a planar 3-layer structure analogous to a Salisbury screen. We present design rules for maximizing the directionality of such structures. Notably, these design rules pertain to both linear polarizations, thus generalizing the principles of a grating for unpolarized light. We show that the key requirement to achieve such performance is ultra-high quality factor resonances in materials supporting phonon polaritonic modes, as those found in low-dimensional materials. We propose a realistic device based on hexagonal Boron Nitride and predict performances comparable to lithography-based nano-structures. 

pdf/lithography-free-directional-control-of-thermal-emission-59eap7e9nl.pdf

Materials’ plasmon activity is defined by their electronic structure. Nowadays, the application of plasmonic materials is increasingly determined by the possibilities to control the electronic processes in them. The electronic structure’s design is of particular importance for tuning the plasmon frequency and the excitation of hot electrons, which are important parameters determining the interaction of the nanostructures with the environment. The effective control of these parameters is important for the improvement of the efficiency and sensitivity of various processes, diagnostic methods and technologies in the field of photocatalysis and surface enhancement spectroscopies. This review is focused on the characterization techniques and the approaches for tuning the electronic states of plasmonic media. The diversity of materials and their electronic structure determine the approach for the engineering of the electronic structure. In the case of noble metals, the possibility for tuning the energy for interband transitions from their d band is considered by using intermetallic alloys (between noble metals themselves and with an addition of post-transition metals in them), while in semiconductor materials—the effect of charge transfer is mainly used. Such knowledge is not only essential from a practical point of view, but also contributes to understanding the processes in the field of new materials such as 2D noble metals and intermetallics. 

Review of the Current State of Optical Characterization and Design of Electronic States in Plasmonic Materials—From Noble Metals to Silverene and Goldene

Plasmons in heavily doped semiconductor layers are optically active excitations with sharp resonances in the 5-15 um wavelength region set by the doping level and the effective mass. Here we demonstrate that volume plasmons can form in doped layers of widths of hundreds of nanometers, without the need of potential barrier for electronic confinement. Their strong interaction with light makes them perfect absorber and therefore suitable for incandescent emission. Moreover, by injecting microwave current in the doped layer, we can modulate the temperature of the electron gas. We have fabricated devices for high frequency thermal emission and measured incandescent emission up to 50MHz, limited by the cutoff of our detector. The frequency dependent thermal emission is very well reproduced by our theoretical model that let us envision a frequency cutoff in the tens of GHz.

Semiconductor quantum plasmons for high frequency thermal emission

Plasmons in heavily doped semiconductor layers are optically active excitations with sharp resonances in the 5-15 μm wavelength region set by the doping level and the effective mass. Here, we demonstrate that volume plasmons can form in doped layers of widths of hundreds of nanometers, without the need of potential barrier for electronic confinement. Their strong interaction with light makes them perfect absorbers and therefore suitable for incandescent emission. Moreover, by injecting microwave current in the doped layer, we can modulate the temperature of the electron gas. We have fabricated devices for high frequency thermal emission and measured incandescent emission up to 50 MHz, limited by the cutoff of our detector. The frequency-dependent thermal emission is very well reproduced by our theoretical model that let us envision a frequency cutoff in the tens of GHz.

https://hal.archives-ouvertes.fr/hal-03058307/document

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Semiconductor quantum plasmons for high frequency thermal emission

Semiconductor quantum plasmons for high frequency thermal emission