Ex situ characterization techniques in molecular beam epitaxy (MBE) have inherent limitations, such as being prone to sample contamination and unstable surfaces during sample transfer from the MBE chamber. In recent years, the need for improved accuracy and reliability in measurement has driven the increasing adoption of in situ characterization techniques. These techniques, such as reflection high-energy electron diffraction, scanning tunneling microscopy, and X-ray photoelectron spectroscopy, allow direct observation of film growth processes in real time without exposing the sample to air, hence offering insights into the growth mechanisms of epitaxial films with controlled properties. By combining multiple in situ characterization techniques with MBE, researchers can better understand film growth processes, realizing novel materials with customized properties and extensive applications. This review aims to overview the benefits and achievements of in situ characterization techniques in MBE and their applications for material science research. In addition, through further analysis of these techniques regarding their challenges and potential solutions, particularly highlighting the assistance of machine learning to correlate in situ characterization with other material information, we hope to provide a guideline for future efforts in the development of novel monitoring and control schemes for MBE growth processes with improved material properties.

A modulator is an essential building block in the integrated photonics, connecting the electrical with optical signals. The microring modulator gains much attention because of the small footprint, low drive voltage and high extinction ratio. An ultra-low V_pp and high-modulation-depth indium phosphide-based racetrack microring modulator is demonstrated in this paper. The proposed device mainly comprises one racetrack microring, incorporating a semiconductor amplifier, and coupling with a bus waveguide through a multimode interference coupler. Traveling wave electrodes are employed to supply bidirectional bias ports, terminating with a 50-Ω impedance. The on/off extinction ratio of the microring reaches 43.3 dB due to the delicately tuning of the gain. An 11 mV V_pp, a maximum 42.5 dB modulation depth and a 6.6 GHz bandwidth are realized, respectively. This proposed microring modulator could enrich the functionalities and designability of the fundamental integrated devices.

An optoelectronic oscillator (OEO) is a microwave photonic system that produces microwave signals with ultralow phase noise using a high-quality-factor optical energy storage element. This type of oscillator is desired in various practical applications, such as communication links, signal processing, radar, metrology, radio astronomy, and reference clock distribution. Recently, new mode control and selection methods based on Fourier domain mode-locking and parity-time symmetry have been proposed and experimentally demonstrated in OEOs, which overcomes the long-existing mode building time and mode selection problems in a traditional OEO. Due to these mode control and selection methods, continuously chirped microwave waveforms can be generated directly from the OEO cavity and single-mode operation can be achieved without the need of ultranarrowband filters, which are not possible in a traditional OEO. Integrated OEOs with a compact size and low power consumption have also been demonstrated, which are key steps toward a new generation of compact and versatile OEOs for demanding applications. We review recent progress in the field of OEOs, with particular attention to new mode control and selection methods, as well as chip-scale integration of OEOs.