The influences of contact electrode shape and vertical illumination direction on the performance of p-i-n photodiodes (PDs) are studied by theory and experiment. The PDs with ring-covered p-contact electrode (RCC-PD) and fully-covered p-contact electrode (FCC-PD) were fabricated and tested. For the RCC-PD, the influences of p-side and n-side illumination on the bandwidth also analysed. The results show that for the PDs with 40 μm mesa diameter under −3 V bias and 7.0 mA output photocurrent, a 40.6 and 31.6% increase of bandwidth and responsivity in the FCC-PD compared with RCC-PD. The 3 dB bandwidth and responsivity of the FCC-PD with a 40 μm mesa diameter at −3 V bias can reach 33.6 GHz and 0.5 A/W, respectively.

To improve the bandwidth efficiency of free-space optical systems and at the same time to reduce the impact of the background noise, a differential M-ary pulse-amplitude modulation (M-PAM) signalling scheme that uses two laser transmitters is proposed. The authors first consider the condition that the receiver perfectly knows the instantaneous channel coefficient and compare the performance of the proposed differential PAM with the conventional PAM signalling and show the improved performance when the background noise level is relatively high. Second, the practical situation where the receiver has to estimate the channel for signal detection is considered. The authors propose an estimation scheme based on the characteristics of the differential PAM signalling while requiring no pilot symbol transmission. The proposed data-aided channel estimation is performed on a sequence of received PAM symbols. The authors also show that for a sufficiently large observation window, the proposed estimation method allows achieving a performance close to the perfect channel knowledge.

A compact plasmonic bandpass filter based on metal–insulator–metal plasmonic waveguides and a coupled U-shaped cavity is proposed in this study. Air and silver are used as insulator and metal materials and silver is characterised by the Drude model in the simulations. By tuning the height of the designed cavity, it is found that the resonance wavelength can be easily adjusted. Furthermore, the quality factor of the designed filter can be increased by increasing the cavity numbers. One, two or three cavities can be used to generate single-mode filters. Finally, by sweeping different structural parameters, the effect of them on transmittance spectrum is investigated. All results are obtained by the finite-difference time domain method. The proposed structure due to its remarkable properties such as being a single-mode filter, having a compact size, tunablity of the resonance frequency and its variable quality factor can be used in complex integrated optical structures such as demultiplexers and so on.

Synthesis of indium (III) oxide (In₂O₃) and chromium (Cr)-doped In₂O₃ thin film (TF) by spin-on technique was reported. The device showed the Schottky nature under dark and light illumination condition. The ideality factor of the diode showed an exponential increase with the decrease in temperature. The low ideality factor of ∼4 was calculated at 300 K. The barrier height of the device increased with temperature. The Cr-doped In₂O₃ TF device possessed high sensitivity compared with undoped In₂O₃ TF at −2 V all over the temperature range from 10 to 300 K. The Cr-doped In₂O₃ TF-based device showed extremely high sensitivity toward ultraviolet (320 nm) light as compared with visible light.

Light is widely used to control a variety of microwave devices, including switches, antennas, and detectors. Here, the authors present a photoconductive device integrated into a coplanar waveguide to tune complex impedances at microwave frequencies with applied light. The authors measured the current–voltage characteristics of the device as a function of the applied light intensity and fit the behavior to a known metal-semiconductor-metal junction model with two Schottky barriers. The authors also measured the frequency-dependent scattering parameters and extracted the device impedances in a Pi-network model. The devices showed three orders of magnitude impedance tunability from 100 MHz to 40 GHz, and the authors developed a circuit model to fit the frequency-dependent impedances. This simple device has applications in microwave electronics, microwave metrology, and multi-state calibrations.

In this study, free space optical communication has been studied for different atmospheric conditions. Spatial modulation (SM) has been used to improve the performance of two-way relay cooperative communication. The proposed system employs decode and forward (DF) relaying technique along with physical layer network coding, hence it has been named as optical SM based DF two-way relay. It enables full-duplex communication thereby enhancing the system efficiency. The transmit optical source index as well as the information are exchanged between the two bidirectional nodes. Performance analysis of the system has been carried out in terms of lower and upper bounds of outage probability and average data rate for N- cascaded fading channels. The analytical results have been validated by means of Monte Carlo simulations. The variation of the performance due to change in different parameters has also been reported in this study.

A new design of 1-to-2 line all-optical logic decoder has been proposed based on a two-dimensional photonic crystal (PhC) structure. The decoder operates based on the principle of optical interference within the PhC waveguides. An optical bias has been used to generate high-logic states at zero input conditions. Simulations explore a high contrast ratio, in the order of 11.3 dB, for both the output ports of the device. Response times have been calculated from the transient responses of the output ports, and are found as 0.65 and 0.77 ps for the ports 1 and 2, respectively. These results in a bit rate of ≃625 Gbps, considering the worst-case scenario. Phase sensitivity analysis has also been performed which shows that the device output is quite sensitive to the differences in phases of the bias and signals. However, a strategy has been proposed to make the same insensitive to the phase changes. Finally, the small footprint (in the order of 234 μm²); standard operating wavelength; and silicon-based design of the decoder, along with its said merits, make the device highly potential for on-chip photonic-integrated-circuit applications.

The proposed configuration of surface plasmon resonance (SPR) biosensor consists of layers of the black phosphorus (BP), transition metal dichalcogenides (TMDC), i.e. WS₂, WSe₂ nanomaterials and silicon (Si). Sensitivity, minimum reflectivity (R _min) and full-width half maxima are calculated as performance parameters. The maximum sensitivity of 184.6°/RIU is achieved without TMDCs. After incorporation of single-layer TMDCs as a protective layer for BP, the sensitivity of 163.1°/RIU is obtained for a single layer of WS₂. This decrement in sensitivity is due to absorption properties of TMDCs. The proposed result shows that the sensitivity of proposed biosensor having Si, BP with TMDCs is better in comparison to results obtained for Si-graphene and Si-TMDCs-based SPR biosensor in the existing literature.