Important New Research
Phase Analysis for Frequency Standards in the Microwave and Optical Domains
Phase measurement of the microwave interrogation signal of CSF1 averaged over 60 000 cycles. The two Ramsey interactions are marked by the gray-shaded stripes.
Coherent manipulation of atomic states is a key concept in high-precision spectroscopy and used in atomic fountain clocks and a number of optical frequency standards. Operation of these standards can involve a number of cyclic switching processes, which may induce cycle-synchronous phase excursions of the interrogation signal and thus lead to shifts in the output of the frequency standard. We have built a field- programmable gate array (FPGA)-based phase analyzer to investigate these effects and conducted measurements on two kinds of frequency standards. For the caesium fountains PTB- CSF1 and PTB-CSF2, we were able to exclude phase variations of the microwave source at the level of a few microradians, corresponding to relative frequency shifts of less than 10−16. In the optical domain, we investigated phase variations in PTB’s Yb+ optical frequency standard and made detailed measurements of acousto-optic modulator (AOM) chirps and their scaling with duty cycle and driving power. We ascertained that cycle-synchronous as well as long-term phase excursion do not cause frequency shifts larger than 10−18.
(Michael Kazda, Vladislav Gerginov, Nils Huntemann, Burghard Lipphardt, and Stefan Weyers, IEEE Trans. Ultrasonics, Ferroelectrics, and Frequency Control, 63, 970 (2016).)
Ultrasound-mediated drug delivery
Therapeutic effects of hyperthermia-mediated chemotherapy. Two-photon microscopy images of a tumor during treatment illustrate increased drug release when tissue is heated (hyperthermia) by focused ultrasound. The blood vessels are green and the released drug is red.
Thanks to advances in transducer technology and imaging-based guidance systems and the expanding body of research on the therapeutic benefits, ultrasound techniques are poised to improve the treatment of cancer and other diseases in the clinic. To many, the term medical ultrasound conjures snowy images of nascent life in the womb. Indeed, its relatively low cost, portability, and impeccable safety profile have propelled diagnostic ultrasound into becoming one of the most widespread imaging methods in the world. So it may seem somewhat contradictory that ultrasound can also induce a broad spectrum of bioeffects in tissue.
Ultimately, ultrasound is a form of energy. It interacts with the body’s tissues during propagation, and in the process it can deposit heat, displace tissues, and initiate cavitation events in which gas bubbles form and violently oscillate or collapse. Those interactions can in turn elicit bioeﬀects, albeit at exposure levels that are typically orders of magnitude higher than those used for imaging purposes.From the inception of the ﬁeld of biomedical ultrasound in the mid 20th century, researchers have appreciated the potential implications of ultrasound-induced bioeﬀects—both from the perspective of designing imaging systems to avoid tissue damage and from the perspective of harnessing those eﬀects for therapeutic pur-poses. In the ensuing decades, foundational research advanced the understanding of ultrasound as asource of energy that could destroy target tissues in the body. Early therapeutic ultrasound typically employed thermal mechanisms; the beam of a highly focused transducer was mechanically translated over a site of interest to ablate (essentially cook) tissue along its path. (David Goertz and Kullervo Hynynen, Phys. Today 69 (3), 30 (2016)). (Contributed by Lori Bridal).
Microwave ac conductivity of ferroelectric domain walls
(a) An out-of-plane piezoresponse force microscopy image showing an artificially created stripe domain structure in an 100 nm-thick epitaxial thin film of Pb(Zr0.2Ti0.8)O3. Ferroelectric polarization P is orientated up ⨀ and down ⨂ in the domains as indicated in the image. (b) Near-field microwave microscopy image of conduction clearly reveals conductivity in the walls of the stripe domains.
This study reveals that domain walls in thin films of two different conventional ferroelectrics—lead zirconate titanate and bismuth ferrite—exhibit large conductance at microwave frequencies. In ferroelectrics, domains and domain walls can be created and reconfigured by electric fields. The thickness of the domain walls is only a few nm. Combination of these two properties makes the walls attractive candidates for building blocks in future logic and memory devices. So far in nanoscale measurements at dc, ferroelectric domain walls were seen as highly insulating preventing their nondestructive readout. The measurements performed in the paper yield an estimate of the wall ac conductivity ~100 times higher at 3 GHz compared to direct current. The ac conduction is immune to large contact resistance and enables a completely non-destructive electronic read-out of domain walls. The large ac conductivity was found in stable, nominally uncharged domain walls, which is unexpected within today's models. The authors, concluded that a new conduction regime has been detected. The effect of the large ac conductance is explained by existence of localized clouds of charge carriers along the domain walls. The domain walls in this model are morphologically roughened by intrinsic lattice disorder. Charge clouds provide ac conduction even if they do not form a continuous path necessary for dc conductivity, because charge carriers localized by energy barriers within the clouds can contribute to ac conduction by oscillating between the barriers at high frequencies. Therefore, the work demonstrates a strong and presently unrealized potential of high-frequency conduction for basic nanoscale physics, materials science, and a great number of future applications revealing that new opportunities and device paradigms can be envisioned in the microwave domain.
A Tselev & et al. "Microwave a.c. conductivity of domain walls in ferroelectric thin films". Nature Communications 7 (2016); http://dx.doi.org/10.1038/ncomms11630. (Contributed by A. Kholkin)