Multiple Exciton Generation in Si and Ge Nanocrystals: An ab Initio Comparative Study

We have simulated multiexciton generation (MEG) processes in Si and Ge nanocrystals, employing the equation of motion coupled cluster single and double as a high-level ab initio approach. Simulations, consistent with the experimental results reported so far, reveal the key role of the d-polarized valence component in the chosen basis set on the accuracy and reliability of the results. Moreover, the MEG thresholds calculated with def2SVP basis set are ∼8.23(8.07) eV for seven (eight)-atom Si clusters and ∼7.58(6.84) eV for similar Ge clusters. The normalized MEG thresholds of Ge nanocrystals are 8% smaller with respect to Si. Thus, in contrast to Si, they are more appealing to the optical device designers for enhancing the device quantum efficiency. Furthermore, the resemblance of the symmetry of the simulated seven-atom clusters to those of the experimentally domelike grown nanocrystals makes the behavior of their MEG quantum probability similar

A new bandwidth-voltage trade-off paradigm in low-loss LNOI electro-optic modulators, using equalizer configuration

As a crucial element for various applications, including optical communications and analog photonic links, electro-optic modulators have a small size, low voltage, and wide bandwidth. Electro-optic modulators based on the thin-film lithium niobate (TFLN) are highly efficient and have a large modulation bandwidth. However, there is a tradeoff between driving voltage and modulation bandwidth. In this study, we present the electro-optical modulator based on the TFLN platform by using the electro-optic equalizer. Using the electro-optic equalizer causes that the proposed modulator has a broad bandwidth while the driving voltage is not compromised. In this proposed modulator, to compensate for the velocity mismatch and minimize interference of the electrode with the waveguide, a two-layer metallization method is used. This proposed modulator results in minimal optical and microwave losses and allows for a large electro-optic bandwidth of up to 300GHz for a 5 mm-long device. Additionally, an efficiency of 4.5 V.cm

Nonidealities and dark current in IR photodetector based on silicide-nanolayer schottky barrier integrated into a Si microring resonator

\u3cp\u3eUsing a Z-transfer model applicable to a microring resonator enhanced internal photoemission-based photodetector (MRRE-IPE-PD); we develop a model for evaluating the effect of reflections in the microring resonator (MRR) on the quantum efficiency (QE). Simulations show that a 5% increase in the reflection from an ideal case QE and MRR quality factor reduces by ∼30%. We also show that further increase in the reflection can result in a dramatic decrease in QE and break the degenerate resonant frequency down into two frequencies. Considering the detuning condition in an MRR, we extend an existing model suitable for the MRRE-IPE-PD. We evaluate the effect of this nonideality on the PD QE. A detuning equal to free spectral range (FSR)/3 reduces the QE to half of its value for an ideal case. Nonetheless, for a detuning equal to FSR/2, the QE of the ideal case is retained. We also evaluate the dependence of the QE bandwidth product (QE × BW) on the MRR radius in under-critical, critical, and over-critical coupling conditions. Simulations reveal that the latter is the optimized condition, wherein the QE ×BW for a PD with 2-nm PtSi decreases from 32 to 30 GHz, when the radius is increased from 7 to 10μm.\u3c/p\u3

Properties of Bilayer Graphene Quantum Dots for Integrated Optics: An Ab Initio Study

Due to their bandgap engineering capabilities for optoelectronics applications, the study of nano-graphene has been a topic of interest to researchers in recent years. Using a first-principles study based on density functional theory (DFT) and thermal DFT, we investigated the electronic structures and optical properties of bilayer graphene quantum dots (GQDs). The dielectric tensors, absorption spectra, and the refractive indexes of the bilayer GQDs were obtained for both in-plane and out-of-plane polarization. In addition, we calculated the absorption spectra via time-dependent DFT (TD-DFT) in the linear response regime. The TDDFT results show that a blue shift occurs in the absorption spectrum, which is consistent with the experimental results. In this investigation, we consider triangular and hexagonal GQDs of various sizes with zigzag and armchair edges. Our simulations show that unlike monolayer GQDs, for which light absorption for out-of-plane polarization occurs in the ultraviolet wavelength range of 85–250 nm, the out-of-plane polarization light absorption peaks in the bilayer GQDs appear in the near-infrared range of 500–1600 nm, similar to those in bilayer graphene sheets. The out-of-plane polarization light absorption peaks in the near-infrared range make bilayer GQDs suitable for integrated optics and optical communication applications

Multiple Exciton Generation in Si and Ge Nanocrystals: An ab Initio Comparative Study

We have simulated multiexciton generation (MEG) processes in Si and Ge nanocrystals, employing the equation of motion coupled cluster single and double as a high-level ab initio approach. Simulations, consistent with the experimental results reported so far, reveal the key role of the d-polarized valence component in the chosen basis set on the accuracy and reliability of the results. Moreover, the MEG thresholds calculated with def2SVP basis set are ∼8.23(8.07) eV for seven (eight)-atom Si clusters and ∼7.58(6.84) eV for similar Ge clusters. The normalized MEG thresholds of Ge nanocrystals are 8% smaller with respect to Si. Thus, in contrast to Si, they are more appealing to the optical device designers for enhancing the device quantum efficiency. Furthermore, the resemblance of the symmetry of the simulated seven-atom clusters to those of the experimentally domelike grown nanocrystals makes the behavior of their MEG quantum probability similar

The second-order coherence analysis of number state propagation through dispersive non-Hermitian multilayered structures

Abstract To examine the second-order coherence of light propagation of quantum states in arbitrary directions through dispersive non-Hermitian optical media, we considered two sets of non-Hermitian periodic structures that consist of gain/loss unit cells. We show that each batch can satisfy the parity-time symmetry conditions at a distinct frequency. We then varied the gain/loss strength in the stable electromagnetic regime to evaluate the transmittance of N-photon number states through each structure. The results show both sets preserve their antibunching characteristics under specific incident light conditions. Furthermore, s(p)-polarized light exhibits higher (lower) second-order coherence at larger incident angles. In addition, the antibunching features of the transmitted states degrade with an increase in the number of unit cells in multilayered structures for both polarizations

Theoretical investigation of metal Schottky barrier detector on Si microring resonator

We propose a silicon microring detector for 1. 55 μm wavelength, working at room temperature by means of internal photoemission absorption effect (IPE). To analyze the device, we model the microring waveguide in presence of a thin metal film, using the Z-transform method. Moreover, to calculate the quantum efficiency of the photodetector, we have used the extended analytical model of the IPE for thin metal film based on the Fowler theory. Since the proposed device benefits from both the resonant-cavity-enhanced and waveguide photodetectors; it will enjoy from the high efficiency and wavelength selectivity in a broad spectral range. We also calculate the dependency of efficiency and bandwidth characteristics of the microring based photodiode on the device parameters and coupling conditions. Simulations show that the critical coupling and over coupling conditions are suitable for high efficiency and high speed applications, respectively. Besides, we show that the efficiency of proposed structure is much higher than its resonant-cavity-enhanced photodetector counterparts. The results also show that there is a trade off between the 3dB bandwidth and efficiency of the proposed photodetector

Internal photoemission-based photodetector on Si microring resonator

We propose a photodetector (PD) based on the internal photoemission effect over a Schottky barrier on a CMOS-compatible Si microring resonator for 1.55 μm. To analyze the device, we model the microring waveguide partially covered by a metal/silicide nanolayer, using the Z-transform method. The proposed structure benefits from the resonant-cavity-enhanced (RCE) waveguide PDs enjoying high efficiency and wavelength selectivity. Simulations show that the maximum value of the bandwidth-efficiency product for the proposed structure is in the order of 10 GHz, which is much higher than those reported for other RCE-based PDs