Small signal modulation of photonic crystal surface emitting lasers.

We report the small-signal characterization of a PCSEL device, extracting damping factors and modulation efficiencies, and demonstrating -3 dB modulation bandwidths of up to 4.26 GHz. Based on modelling we show that, by reducing the device width and improving the active region design for high-speed modulation, direct modulation frequencies in excess of 50 GHz are achievable.

Increase in terahertz-wave intensity in a magnetic field due to difference-frequency mixing by exciton excitation in a GaAs/AlAs multiple quantum well.

Magnetic fields can increase the intensity of terahertz (THz) waves due to changing the dipole moment direction using the Lorentz force. This study reports the increase in the THz-wave intensity generated by differential frequency mixing using commercial permanent magnets under exciton-excitation. While a weak magnetic field applied to a multiple quantum well increases the THz-wave intensity due to excitons, a strong field causes its decrease. According to the calculations, the increase is caused by the electron-hole separation due to the Lorentz force. Furthermore, the calculations suggest the importance of carrier acceleration to enhance the intensity. Importantly, the increase in the THz-wave intensity due to differential frequency mixing does not require a strong magnetic field and can be achieved with inexpensive commercially available magnets.

Cu2O-Based Electrochemical Biosensor for Non-Invasive and Portable Glucose Detection.

Electrochemical voltammetric sensors are some of the most promising types of sensors for monitoring various physiological analytes due to their implementation as non-invasive and portable devices. Advantages in reduced analysis time, cost-effectiveness, selective sensing, and simple techniques with low-powered circuits distinguish voltammetric sensors from other methods. In this work, we developed a Cu2O-based non-enzymatic portable glucose sensor on a graphene paste printed on cellulose cloth. The electron transfer of Cu2O in a NaOH alkaline medium and sweat equivalent solution at very low potential (+0.35 V) enable its implementation as a low-powered portable glucose sensor. The redox mechanism of the electrodes with the analyte solution was confirmed through cyclic voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy studies. The developed biocompatible, disposable, and reproducible sensors showed sensing performance in the range of 0.1 to 1 mM glucose, with a sensitivity of 1082.5 ± 4.7% µA mM-1 cm-2 on Cu2O coated glassy carbon electrode and 182.9 ± 8.83% µA mM-1 cm-2 on Cu2O coated graphene printed electrodes, making them a strong candidate for future portable, non-invasive glucose monitoring devices on biodegradable substrates. For portable applications we demonstrated the sensor on artificial sweat in 0.1 M NaOH solution, indicating the Cu2O nanocluster is selective to glucose from 0.0 to +0.6 V even in the presence of common interference such as urea and NaCl.

Increase in terahertz-wave generation by difference frequency mixing by the overlap of exciton states in different GaAs/AlAs quantum wells and spectroscopic measurements.

Intense terahertz-wave emission in the higher frequency region can result in various applications such as terahertz spectroscopy and ultrafast data communication. In this study, an increase in terahertz waves by the overlap of exciton states in different quantum wells and spectroscopic demonstration are reported. The excitation energy dependence of signal intensity shows the effect of the overlap. The signals measured under the condition of square dependence of intensity on the excitation power indicate interference with the periods corresponding to the laser energy difference. Furthermore, the absorption coefficient of the transparent sheet is obtained at specific frequency. These results indicate that the generation of intense terahertz waves at various frequencies using excitons is possible and that difference frequency mixing is a useful terahertz-wave source.

Near-infrared and mid-infrared semiconductor broadband light emitters.

Semiconductor broadband light emitters have emerged as ideal and vital light sources for a range of biomedical sensing/imaging applications, especially for optical coherence tomography systems. Although near-infrared broadband light emitters have found increasingly wide utilization in these imaging applications, the requirement to simultaneously achieve both a high spectral bandwidth and output power is still challenging for such devices. Owing to the relatively weak amplified spontaneous emission, as a consequence of the very short non-radiative carrier lifetime of the inter-subband transitions in quantum cascade structures, it is even more challenging to obtain desirable mid-infrared broadband light emitters. There have been great efforts in the past 20 years to pursue high-efficiency broadband optical gain and very low reflectivity in waveguide structures, which are two key factors determining the performance of broadband light emitters. Here we describe the realization of a high continuous wave light power of >20 mW and broadband width of >130 nm with near-infrared broadband light emitters and the first mid-infrared broadband light emitters operating under continuous wave mode at room temperature by employing a modulation p-doped InGaAs/GaAs quantum dot active region with a 'J'-shape ridge waveguide structure and a quantum cascade active region with a dual-end analogous monolithic integrated tapered waveguide structure, respectively. This work is of great importance to improve the performance of existing near-infrared optical coherence tomography systems and describes a major advance toward reliable and cost-effective mid-infrared imaging and sensing systems, which do not presently exist due to the lack of appropriate low-coherence mid-infrared semiconductor broadband light sources.

Optical characterization of In-flushed InAs/GaAs quantum dots emitting a broadband spectrum with multiple peaks at ~1 µm.

We investigated optical properties of In-flushed InAs quantum dots (QDs) grown on a GaAs substrate by molecular beam epitaxy. By using the In-flush technique for setting the height of self-assembled InAs QDs, we have tuned the emission wavelength of InAs QDs to the ~1 µm regime, which can be utilized as a non-invasive and deeply penetrative probe for biological and medical imaging systems. The controlled emission exhibited a broadband spectrum comprising multiple peaks with an interval of approximately 30 meV. We examined the origin of the multiple peaks using spectral and time-resolved photoluminescence, and concluded that it is attributed to monolayer step fluctuations in the height of the In-flushed QDs. This feature can be advantageous for realizing a broadband light source centered at the ~1 µm regime, which is especially suitable for the non-invasive cross-sectional biological and medical imaging system known as optical coherence tomography.