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MAY 2018 I PCB007 MAGAZINE 59 lowed by an acid cleaner (to remove the chro- mate conversion coating) be used. One must still have a clean copper surface to facilitate re- sist adhesion. PCB007 References 1. Mitutoyo website. Michael Carano is VP of technology and business development for RBP Chemical Technology. To reach Carano, or read past columns, click here. file. In addition, during the past several years, low profile ED copper foils have been intro- duced with an enhancement known as reverse treat. Essentially, the shiny side of the copper foil (also known as drum side) has been treat- ed with material to enhance the bond of the photoresist to the copper foil (Figure 1). It is important to note that if one employs RTF foil, there should be minimal if any micro- etching or scrubbing of the RTF. Any aggres- sive etching or cleaning will negatively impact the topography one is paying for with RTF. It is suggested however that an alkaline cleaner fol- Wi-Fi and cellular data traffic are increasing exponen- tially, but unless the capacity of wireless links can be in- creased, all that traffic is bound to lead to unacceptable bottlenecks. Upcoming 5G networks are a temporary fix, not a long- term solution. For that, researchers have focused on tera - hertz frequencies, the submillimeter wavelengths of the electromagnetic spectrum. Data traveling at terahertz fre- quencies could move hundreds of times faster than to- day's wireless. In 2017, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) dis- covered that an infrared frequency comb in a quantum cascade laser could offer a new way to generate terahertz frequencies. Now, those re- searchers have uncovered a new phenomenon of quantum cascade laser frequency combs, which would allow these devices to act as inte- grated transmitters or receivers that can effi- ciently encode information. "This work represents a complete paradigm shift for the way a laser can be operated," said Federico Capasso, the Robert L. Wallace Profes- sor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering and senior author of the paper. "This new phenom- enon transforms a laser—a device operating at optical frequencies—into an advanced modula- tor at microwave frequencies, which has a tech- nological significance for efficient use of band- width in communication systems." Laser Frequency Combs May Be the Future of Wi-Fi Unlike conventional lasers, which emit a single frequen- cy, these emit multiple frequencies simultaneously, even- ly spaced to resemble the teeth of a comb. Today, optical frequency combs are used for everything from measuring the fingerprints of specific molecules to detecting distant exoplanets. This research, however, wasn't interested in the optical output of the laser. "We were interested in what was going on inside the laser, in the laser's electron skeleton," said Marco Piccardo, a postdoctoral fellow at SEAS and first author of the paper.