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Organic semiconductors have the unique ability to conduct both ionic and electronic charge carriers in thin films, an emerging advantage in applications such as light-emitting devices, transistors, and electrochromic devices, among others. Evidence suggests that the profiles of ions and electrochemical doping in the polymer film during operation significantly impact the performance and stability of the device. However, few studies have directly characterized ion profiles within LECs. Here, we present an in-depth study of the profiles of ion distributions in LECs following application of voltage, via time-of-flight secondary ion mass spectrometry. Ion distributions were characterized with regard to film thickness, salt concentration, applied voltage, and relaxation over time. Results provide insight into the correlation between ion profiles and device performance, as well as potential approaches to tuning the electrochemical doping processes in LECs.

The invention of optical frequency combs (OFCs) based on femtosecond (fs) mode-locked lasers has merged laser based spectroscopy with fs-laser technology. OFCs have triggered quantum leaps of advancement in photonics, physics, astronomy, and engineering, while also lending themselves to applications in medical and environmental fields. The key feature of OFCs is their ultrahigh frequency and time resolution. In this review, we introduce the concept behind OFCs and their sources, with a focus on stability and phase-noise performance. We conclude with a discussion of recent progress of a monolithic OFC, which provides ultralow free-running phase noise and an unprecedented frequency stability of 1 part in 10 19 at a 1 s gate time.

We demonstrate the use of two dual-output Mach-Zehnder modulators (DO-MZMs) in a direct comparison between a femtosecond (fs) pulse train and a microwave signal. Through balanced detection, the amplitude-to-phase modulation (AM-PM) conversion effect is suppressed by more than 40 dB. A cross-spectrum technique enables us to achieve a high-sensitivity phase noise measurement (−186 dBc/Hz above 10-kHz offset), which corresponds to the thermal noise of a +9 dBm carrier. This method is applied to compare a 1-GHz fs monolithic laser to a 1-GHz microwave signal generated from photodetection of a free-running 500 MHz mode-locked laser. The measured phase noise is −160 dBc/Hz at 4-kHz, −167 dBc/Hz at 10-kHz, and −180 dBc/Hz at offset frequencies above 100-kHz. The measurement is limited by the free-running 500-MHz laser’s noise, the flicker noise of the modified uni-traveling carrier photodiode and the thermal noise floor, not by the method itself. This method also has the potential to achieve a similar noise floor even at higher carrier frequencies.

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