that the incident light is substantially absorbed. Landy et al. has shown the fi rst PA, operating in microwave frequency range, where the structure consists of an electric resonator and a cut wire, which independently couple to electric and magnetic fi elds. [ 1 ] Later for the higher frequency ranges, Liu et al. demonstrated an infrared PA system. [ 8 ] The structure is composed of metal-dielectric-metal layers, where the top metal layer is patterned with subwavelength antennas serving as a resonator, and the bottom one is an optical mirror which signifi cantly attenuates the transmittance. The coupling of light to the antennas induces an electric fi eld, while the nearfi eld couplings between the antennas and the metal sheet result in mirror-image charges in the bottom layer. This generates a current loop which induces a magnetic fi eld. [9][10][11][12][13][14] Then, tuning the amplitude and resonance frequency of the electric and magnetic responses can be used to match the impedance of PA to freespace, which minimizes the refl ectance. Hence, minimizing refl ection with impedance matching, while attenuating transmission with a metal sheet leads to perfect absorption. Recently, the dependence of absorbance on a critical coupling condition between resonators and optical mirrors has been investigated to provide a universal way for unity absorbance. [ 15 ] Supporting strong absorbance capabilities, PAs are good candidates for surface enhanced infrared absorption (SEIRA) spectroscopy applications. As the infrared region is accompanied with low radiation damping, PAs engineered at this wavelength window could support plasmonic resonances with high Q-factors, which leads to strong nearfi eld enhancements. This feature is highly advantageous for achieving large spectroscopic signals associated with the molecular vibrational modes of interest. [16][17][18][19][20][21][22] In order to reliably identify the targeted molecules, it is crucial to simultaneously monitor different molecular fi ngerprints. However, PAs' unity absorbance is limited within a narrow spectral window where the plasmonic resonances of their subwavelength antennas lie. This problem could be addressed by utilizing nanoparticle or nanoaperture based confi gurations, supporting multiple resonances. [23][24][25][26][27][28][29][30][31] Recently, different multiband PA structures have been introduced to serve for variety of applications from microwave to mid-infrared frequency ranges. [32][33][34][35][36][37][38][39] A dual-resonant perfect absorber (PA) based on multiple dipolar nanoantenna confi guration is introduced. The PA platform exhibits near-unity (95%-98%) absorbance in dual-resonances. A fi ne-tuning mechanism of dual-resonances is determined via geometrical device parameters of the constituting dipolar elements of the compact PA system. It is also shown that the dual plasmonic resonances are associated with easily accessible and large local electromagnetic fi elds. Possessing large absorbance with strong nearfi elds, the PA system is highly a...

No abstract

SiGe quantum wells are promising candidates for the development of intersubband light emitters and photodetectors operating at mid-and far-infrared wavelengths. By virtue of their inherent compatibility with the Si microelectronics platform, these devices may be integrated seamlessly within complex optoelectronic systems for sensing and imaging applications. However, the development of high-quality SiGe intersubband active layers is complicated by the large lattice mismatch between Si and Ge, which limits the number of quantum wells that can be grown on bulk Si substrates before the onset of structural degradation due to inelastic strain relaxation. To address this issue, we investigate the use of lattice matched growth templates consisting of quantum-well nanomembrane stacks that were at one point free-standing, allowing for the internal stress to be relaxed via elastic strain sharing rather than defect formation. SiGe quantum-well infrared photodetectors (QWIPs) based on this approach are developed and characterized. Efficient current extraction from these ultrathin devices is obtained by bonding the nanomembranes directly on a doped Si substrate. Pronounced photocurrent peaks at mid-infrared wavelengths are measured, with improved responsivity compared to otherwise identical devices grown simultaneously on the supporting Si substrate.

Terahertz intersubband photodetectors are developed based on GaN/AlGaN quantum wells grown on a free-standing semi-polar ð20 2 1Þ GaN substrate. These quantum wells are nearly free of the polarization-induced internal electric fields that severely complicate the design of nitride intersubband devices on traditional c-plane substrates. As a result, a promising bound-to-quasi-bound THz photodetector design can be implemented. Pronounced photocurrent peaks at the design frequency near 10 THz are measured, covering frequencies that are fundamentally inaccessible to existing arsenide intersubband devices due to reststrahlen absorption. This materials system provides a favorable platform to utilize the intrinsic advantages of nitride semiconductors for THz optoelectronics.

SiGe/Si quantum wells are of great interest for the development of Group-IV THz quantum cascade lasers. The main advantage of Group-IV over III-V materials such as GaAs is that, in the former, polar phonon scattering, which significantly diminishes the efficiency of intersubband light emission, is absent. However, for SiGe/Si multiple-quantum-well structures grown on bulk Si, the lattice mismatch between Si and Ge limits the critical thickness for dislocation formation and thus the number of periods that can be grown. Similarly, the use of composition-graded SiGe films as a lattice-matched substrate leads to the transfer of dislocations from the graded buffer substrate into the quantum wells, with a consequent decrease in light emission efficiency. Here we instead employ nanomembrane strain engineering to fabricate dislocation-free strain relaxed substrates, with lattice constants that match the average lattice constants of the quantum wells. This procedure allows for the growth of many periods with excellent structural properties. The samples in this work were grown by low-pressure chemical vapor deposition and characterized via high-resolution X-ray diffraction and far-infrared transmission spectroscopy, showing narrow intersubband absorption features indicative of high crystalline quality.