High-Speed Silicon Optical Modulator Developed
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RESEARCH/RESEARCHERS Microscale Cantilever Beam Resonators “Weigh” Individual Virus Particles Nanotechnology researchers are pursuing the fabrication of a lab-on-a-chip device to detect biochemical entities such as airborne virus particles. As reported in the March 8 issue of Applied Physics Letters, A. Gupta and co-workers at Purdue University have taken a mechanical approach and fabricated an array of microscale resonant cantilever beams, which act as mass detectors. The frequency of oscillation of each cantilever is measured using a microscope scanning laser
Doppler vibrometer under ambient conditions. The cantilevers are small enough that they do not require an external source, but are driven by thermal and ambient fluctuations. The researchers dispersed a solution of vaccinia virus particles (vaccinia is a member of the Poxviridae family and forms the basis for the smallpox vaccine) in deionized water over the cantilever array and allowed the particles to incubate for 30 min before drying the array. They showed that the attachment of a single vaccinia virus particle, with a mass of about 10 fg, shifts a 1.27 MHz resonant frequency by 5%. The researchers
have demonstrated that the shift in the natural frequency is proportional to the effective number of virus particles on the beam, as expected from a mechanical analysis, by imaging the cantilevers with a scanning electron microscope and counting the number of virus particles attached. The scientists started with p-type siliconon-insulator wafers and photolithographically patterned and etched the arrays of 5-µm-long cantilevers in the silicon. The cantilevers are 1–2 µm wide and 20–30 nm thick. After protecting the cantilevers with an oxide etch-stop layer, the research team used a xenon difluoride
High-Speed Silicon Silicon Optical Optical Modulator Modulator Developed Developed High-Speed Silicon, the material of choice for electronics, has not seen wide consideration as an optical material for photonics. A key limitation has been the relatively low speed of silicon optical modulators compared with other materials such as III–V compounds and lithium niobate. Researchers from Intel, led by A. Liu, have now reported the fabrication of an all-Si optical modulator with a modulation bandwidth exceeding 1 GHz. This represents a nearly two orders of magnitude improvement over previous Si devices, which have modulation frequencies in the range of ~20 MHz. Liu and co-workers described their approach, based on a metal oxide semiconductor (MOS) capacitor structure embedded in a Si waveguide, in the February 12 issue of Nature. In this device, the modulation of light for encoding data as changes in intensity is achieved by modulating the refractive index. In Si, this is achieved by the free-carrier dispersion effect; the introduction of free carriers results in absorption and also a change in the refractive index. The researchers used a MOS capacitor phase shifter for charge-density modulation that induces a phase shift in the optical mode. Their device consists of an
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