Search

Sub-ambient daytime radiative cooling (SA-DRC) has been demonstrated by various advanced emitters in recent studies, which have attracted considerable attention from fundamental sciences and potential applications. Apart from emitter's unique radiative properties, the cooling loss of the emitter affects the performance of SA-DRC in actual applications. Although the emitter is fixed in an enclosure space to decrease the effect of ambient wind on the performance of cooling, the cooling loss of the emitter still exists, and an effective heat transfer coefficient between the emitter and ambient air has directly been widely used to predict the magnitude of the cooling loss power. However, this mathematical description is limited, which will lead to a controversial prediction in some cases. In this study, the cooling loss of the emitter was investigated based on a local environment. The simulation was conducted on the basis of four advanced emitters that were reported in recent studies, and the results show that the cooling loss of the emitter increases with the increasing interface temperature of the enclosure space that surrounds the emitter, which weakens the cooling effect of the emitter. A case study shows that the temperature reduction of an infrared-spectral-selective emitter could be degraded from −3.2 °C to 0.1 °C when the interface temperature increases from ambient temperature to 6 °C higher than ambient temperature, which implies that the cooling effect has been entirely destroyed. [ABSTRACT FROM AUTHOR]

This work presents a flexible manipulation solution on droplet formation based on the modified T-junction with a rectangular rib to reduce the droplet size and improve monodispersity. The droplet formation in the ordinary and modified T-junctions is numerically investigated using the verified three-dimensional volume of fluid method. The results reveal that the modified T-junction can significantly enlarge the dripping regime and droplet-generable regimes while decreasing the jetting regime. In the modified T-junction, the droplet detachment is much easier as the detachment driving forces are strengthened, while the resistance forces are weakened. By investigating the droplet formation in the ordinary and different modified T-junctions with change in viscosity, surface tension, and wall wettability, it is found that the dominant geometric factor affecting the droplet formation is the rib height, not the rib width. Based on the rib height, two modified scaling laws are proposed to predict droplet size in squeezing and dripping regimes. The wall wettability can deteriorate the droplet formation in the ordinary T-junction, while the rib in the modified T-junction can weaken this adverse effect. [ABSTRACT FROM AUTHOR]

Time- and Angle-resolved photoelectron spectroscopy from surfaces can be used to record the dynamics of electrons and holes in condensed matter on ultrafast time scales. However, ultrafast photoemission experiments using extreme-ultraviolet (XUV) light have previously been limited by either space-charge effects, low photon flux, or limited tuning range. In this article, we describe XUV photoelectron spectroscopy experiments with up to 5 nA of average sample current using a tunable cavity-enhanced high-harmonic source operating at 88 MHz repetition rate. The source delivers >1011 photons/s in isolated harmonics to the sample over a broad photon energy range from 18 to 37 eV with a spot size of 58 × 100 μm2. From photoelectron spectroscopy data, we place conservative upper limits on the XUV pulse duration and photon energy bandwidth of 93 fs and 65 meV, respectively. The high photocurrent, lack of strong space charge distortions of the photoelectron spectra, and excellent isolation of individual harmonic orders allow us to observe laser-induced modifications of the photoelectron spectra at the 10−4 level, enabling time-resolved XUV photoemission experiments in a qualitatively new regime. [ABSTRACT FROM AUTHOR]

We present a detailed description of the design, construction, and performance of high-power ultrafast Yb:fiber laser frequency combs in operation in our laboratory. We discuss two such laser systems: an 87 MHz, 9 W, 85 fs laser operating at 1060 nm and an 87 MHz, 80 W, 155 fs laser operating at 1035 nm. Both are constructed using low-cost, commercially available components, and can be assembled using only basic tools for cleaving and splicing single-mode fibers. We describe practical methods for achieving and characterizing low-noise single-pulse operation and long-term stability from Yb:fiber oscillators based on nonlinear polarization evolution. Stabilization of the combs using a variety of transducers, including a new method for tuning the carrier-envelope offset frequency, is discussed. High average power is achieved through chirped-pulse amplification in simple fiber amplifiers based on double-clad photonic crystal fibers. We describe the use of these combs in several applications, including ultrasensitive femtosecond time-resolved spectroscopy and cavity-enhanced high-order harmonic generation.