Cost-Effective Bull's Eye Aperture-Style Multi-Band Metamaterial Absorber at Sub-THz Band: Design, Numerical Analysis, and Physical Interpretation.

Theoretical and numerical studies were conducted on plasmonic interactions at a polarization-independent semiconductor-dielectric-semiconductor (SDS) sandwiched layer design and a brief review of the basic theory model was presented. The potential of bull's eye aperture (BEA) structures as device elements has been well recognized in multi-band structures. In addition, the sub-terahertz (THz) band (below 1 THz frequency regime) is utilized in communications and sensing applications, which are in high demand in modern technology. Therefore, we produced theoretical and numerical studies for a THz-absorbing-metasurface BEA-style design, with N-beam absorption peaks at a sub-THz band, using economical and commercially accessible materials, which have a low cost and an easy fabrication process. Furthermore, we applied the Drude model for the dielectric function of semiconductors due to its ability to describe both free-electron and bound systems simultaneously. Associated with metasurface research and applications, it is essential to facilitate metasurface designs to be of the utmost flexible properties with low cost. Through the aid of electromagnetic (EM) coupling using multiple semiconductor ring resonators (RRs), we could tune the number of absorption peaks between the 0.1 and 1.0 THz frequency regime. By increasing the number of semiconductor rings without altering all other parameters, we found a translation trend of the absorption frequencies. In addition, we validated our spectral response results using EM field distributions and surface currents. Here, we mainly discuss the source of the N-band THz absorber and the underlying physics of the multi-beam absorber designed structures. The proposed microstructure has ultra-high potentials to utilize in high-power THz sources and optical biomedical sensing and detection applications based on opto-electronics technology based on having multi-band absorption responses.

The potential of terahertz sensing for cancer diagnosis.

The terahertz (THz) region lies between the microwave and infrared regions of the electromagnetic (EM) spectrum such that it is strongly attenuated by water and very sensitive to water content. Here, we numerically present what is to our knowledge the detecting system based on THz reflectance spectral responses data in the diagnosis of in vivo and ex vivo of some cancer's samples such as skin, breast and colon cancer tissue samples. The numerical analysis on the use of semiconductor metamaterial design/device as a complex refractive index (CRI) biosensor have been carried out. We demonstrate the application of terahertz pulse detecting (TPD) in reflection geometry for the study of normal and cancerous biological tissues. THz radiation has very low photon energy and thus it does not pose any ionization hazard for biological tissues. The sensitivity of THz radiation to polar molecules, such as water, makes TPD suitable to study the diseases in human body. By studying the THz pulse shape in the time domain, we have been able to differentiate between diseased and normal tissue for the study of basal cell carcinoma (BCC), breast and colon cancers. These results demonstrate the potential of TPD for the study of skin tissue and its related disorders, both in vivo and ex vivo. Findings of this study demonstrate the potential of TPD to depict breast and colon cancers and both in vivo and ex vivo of skin cancer and encourage further studies to determine the sensitivity and specificity of the technique.

Polarization-Independent Perfect Optical Metamaterial Absorber as a Glucose Sensor in Food Industry Applications.

Perfect optical metamaterial absorbers (POMMA) utilize intrinsic loss, with the aid of appropriate structural design, to achieve near unity absorption at a certain wavelength. In all the reported absorbers, the absorption occurs only at a single wavelength or dual/multi-band wavelengths where plasmon resonances are ex-cited in the nanostructure. Here we not only show a single-band perfect absorber but also demonstrate that our proposed design has the ability to be multi-band absorber at the same structure. Furthermore, we numerically demonstrate the proposed POMMA can be utilized as a glucose sensor for refractive index sensing which has more than 225 nm/RIU sensitivity at the infrared frequency regime which is good value. Its polarization-independent absorbance is about 100% at normal incidence for both TE and TM polarization modes. The proposed optical glucose sensor offers great potential to maintain the performance of localized surface plasmon (LSP) sensors in nanostructures in food industry applications.

Time, space, and spectral multiplexing for radiation balanced operation of semiconductor lasers.

Radiation balanced lasing (RBL) is an attractive pathway towards the development of high power and good beam quality lasers because heat removal via anti-Stokes luminescence (optical refrigeration) does not require additional connections and components and the heat is dissipated away from the active medium. Optical refrigeration had been demonstrated in the rare-earth doped laser medium but is far more difficult to achieve it in semiconductors laser medium. The main obstacle to achieve RBL in semiconductors is that the most efficient cooling occurs at relatively low carrier densities, while the gain required to sustain laser operation occurs at much higher densities. In this study, we explore the means of resolving this conundrum by separating the optical refrigeration and lasing in temporal, spatial, and/or spectral domains. Time multiplexing involves modulating the pump and operating the laser in pulse modes with lasing and cooling intervals. Space multiplexing involves having separate regions (quantum wells and dots) for lasing and cooling. The spectral multiplexing involves operating with two separate pumps - one for lasing and one for cooling. These methods will be compared in the study with the goal of selecting the optimal path RBL in semiconductor lasers.

Bandgap engineering and prospects for radiation-balanced vertical-external-cavity surface-emitting semiconductor lasers.

The vertical-external-cavity surface-emitting laser (VECSEL) has shown promise in becoming an efficient source of high power and high beam quality coherent radiation. In order to live up to its true potential, the VECSEL must be thermally managed in order to avoid thermal damage as thermal lensing and filamentation causing preventing it from operating in a single mode regime. For an optically pumped VECSEL, optical cooling presents an elegant solution for thermal management as it does not require electrical or thermal conduction. The goal of optical refrigeration is to achieve radiation balance lasing (RBL) when the active medium is maintained at a steady uniform temperature. In this work, we investigate the active medium characteristics and operating conditions that can lead to RBL in a semiconductor medium and show that to achieve RBL, the gain medium should be engineered to create a density of states that simultaneously allows gain and strong anti-Stokes luminescence. Such a medium may incorporate bandtail states, impurities or quantum dots. We provide a recipe for optimization of such band structure-engineered materials to achieve the lowest threshold and highest output power.

Large group delay in a microwave metamaterial analog of electromagnetically induced reflectance.

Recently reported metamaterial (MM) analogs of electromagnetically induced reflectance (EIR) enable a unique route to endow classical optical structures with aspects of quantum optical systems. This method opens up many fascinating prospects on novel optical components, such as slow light units, highly sensitive sensors, and nonlinear devices. Here we designed and simulated a microwave MM made from aluminum thin film to mimic the EIR system. High reflectance of about 99 percent and also a large group index at the reflectance window of about 243 are demonstrated, which mainly arise from the enhanced coupling between radiative and nonradiative elements. The interaction between the elements of the unit cell, induced directly or indirectly by the incident electromagnetic wave, leads to a reflectance window, resembling the classical analog of EIR. This reflectance window, caused by the coupling of radiative-nonradiative modes, can be continuously tuned in a broad frequency regime. The strong normal phase dispersion in the vicinity of this reflectance window results in the slow light effect. This scheme provides an alternative way to achieve tunable slow light in a broad frequency band and can find important applications in active and reversibly tunable slow light devices.

Slowing down light using terahertz semiconductor metamaterial for dual-band thermally tunable modulator applications.

Compared to the neighboring infrared and microwave regions, the terahertz regime is still in need of fundamental technological advances. We have designed a terahertz (THz) semiconductor metamaterial (MM) waveguide system, which exhibits a significant slow-light effect, based on a classical electromagnetically induced transparency phenomenon. The potential of MMs for THz radiation originates from a resonant electromagnetic response that can be tailored for specific applications. By appropriately adjusting the distance between the two radiative and nonradiative modes, a flat band corresponding to a nearly constant group index (of the order of 4924) in the THz regime can be achieved. Finite-difference time-domain simulations show that the incident pulse can be slowed down. The proposed device from a paucity of naturally occurring materials has useful applications in electronic or photonic properties at terahertz frequencies. This proposed compact configuration may find potential applications in plasmonic slow-light systems, optical buffers, and thermal and electromagnetic modulating applications and temperature sensors.