Tunable Split-Disk Micro-Lasers

Scanning electron micrograph of a polymeric WGM split-disk cavity: Two opposing half-disks with an intermediate air gap of several microns are structured onto an elastomeric PDMS substrate with direct laser writing. (Source: T. Siegle, KIT)

Whispering gallery mode resonators are used to make tiny micro-lasers, sensors, switches, routers and other devices. These tiny structures rely on a pheno­menon similar to an effect observed in circular galleries, such as in some cathe­drals or museums, where sound waves travel across the gallery and are reflected and refocused tightly enough that a whisper on one side can be heard on the other.

The same pheno­menon applies to light. When light is stored in ring-shaped or spherical active reso­nators, the waves superimpose in such a way that it can result in laser light. Now, investigators report a new type of dye-doped WGM micro-laser that produces light with tunable wavelengths. Not only is the tuning range of the new devices broader than has been possible in the past, it is completely reversible. A unique feature of the design, according to Tobias Siegle of Karlsruhe Insti­tute of Tech­nology in Germany, is that tuning is made possible by changing the size of a flexible substrate. Stretching the substrate changes the distance between the two sides of the split disk in the resonator and thus, the wave­length of the light produced.

The disk itself is typi­cally about 25 microns across, with an initial gap between its two halves of roughly 2.5 microns, just 3 percent the diameter of a typical human hair. The disk is mounted on an elas­tomer, or stretchy plastic substrate, which can be pulled in a direction perpen­dicular to the disk’s split, decreasing the gap size. The light produced by these micro-lasers shifts toward the blue range of the spectrum when the gap size decreases. They observed wave­length shifts of several nano­meters in the visible range.

“Our new design produces a broad tuning range that cannot be easily achieved with other WGM reso­nators,” Siegle said. “Addi­tionally, the tuning mechanism is completely reversible.” This feature allows the device to be used in funda­mental optics research. Another feature of the split-disk tech­nology is enhanced sens­itivity in refractive index sensing. “For a gap width of 1.4 microns, the sensing perfor­mance is increased by 65 percent,” Siegle said, when comparing to a refe­rence disk reso­nator without a gap.

The most useful devices have a low lasing threshold, since this allows the use of small amounts of energy. A low threshold reduces or prevents photo-bleaching of the dye molecules used in the device and increases its expected lifetime. The inves­tigators tested their design and found low-threshold lasing for split disks fabri­cated using a 3-D or electron-beam litho­graphy technique.

Another quantity they studied is the quality, or Q factor, which corre­sponds to the photon storage time in the laser cavity. A high Q value is desirable, and although investi­gators found that their split-disk design reduced Q somewhat, the lasing threshold was within an appro­priate range, making the design valuable. Future work will be focused on developing tunable coupled resonator wave­guides, which can be used as optical delay lines or filters, and in other appli­cations. (Source: AIP)

Reference: T. Siegle et al.: Split-disk micro-lasers: Tunable whispering gallery mode cavities, APL Phot., online 19 September 2017; DOI: 10.1063/1.4985766

Link: Inst. of Applied Physics, Karlsruhe Institute of Technology, Karlsruhe, Germany

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