The ability to detect and size individual nanoparticles with high resolution is crucial to understanding behaviours of single particles and effectively using their strong size-dependent properties to develop innovative products. We report real-time, insitu detection and sizing of single nanoparticles, down to 30 nm in radius, using mode-splitting in a monolithic ultra-high-Q whispering-gallery-mode (WGM) microtoroid resonator. Particle binding splits a WGM into two spectrally shifted resonance modes, forming a self-referenced detection scheme. This technique provides superior noise suppression and enables extracting accurate size information in a single-shot measurement. Our method requires neither labelling of the particles nor apriori information on their presence in the medium, providing an effective platform to study nanoparticles at single particle resolution.

Detection and characterization of individual nano-scale particles, virions, and pathogens are of paramount importance to human health, homeland security, diagnostic and environmental monitoring 1 .There is a strong demand for high-resolution, portable, and cost-effective systems to make label-free detection and measurement of individual nanoparticles, molecules, and viruses 2-6 . Here, we report an easily accessible, real-time and label-free detection method with single nanoparticle resolution that surpasses detection limit of existing micro-and nano-photonic devices. This is achieved by using an ultra-narrow linewidth whispering gallery microlaser, whose lasing line undergoes frequency splitting upon the binding of individual nano-objects. We demonstrate detection of polystyrene and gold nanoparticles as small as 15 nm and 10 nm in radius, respectively, and Influenza A virions by monitoring changes in self-heterodyning beat note of the split lasing modes. Experiments are performed in both air and aqueous environment. The built-in self-heterodyne interferometric method achieved in a microlaser provides a self-reference scheme with extraordinary sensitivity 7,8 , and paves the way for detection and spectroscopy of nano-scale objects using micro-and nano-lasers.Most of the biological agents and synthetic particles of interest have low polarizability due to their small size and low refractive index contrast with the surrounding medium, leading to weak light-particle interactions which make their label-free optical detection at single particle resolution difficult. Micro-and nano-photonic resonant devices have emerged as highly sensitive platforms for detection of individual virions and nanoparticles due to the significantly enhanced light-matter interactions originating from the high ratio of their quality-factor (Q) to mode volume (V) 7,9-15 . For example, detection of single Influenza virions and polystyrene nanospheres as small as 30 nm in radius have been demonstrated in whispering gallery mode (WGM) microspheres using reactive shift 12 and in microtoroids using mode splitting technique 7 , respectively. In both techniques, wavelength of a tunable laser is scanned to obtain the transmission spectrum of a resonant mode from which specific information, either the resonance shift and/or the mode splitting, is extracted to detect and measure nanoparticles. The ultimate detection limit strongly relies on Q/V which not only determines the light-matter interaction strength but also sets the smallest resolvable changes in the WGM spectrum 16 . Higher Q, limited by material absorption, implies narrower resonance linewidth and better resolution.Here we report the first microlaser-based detection scheme with single particle resolution surpassing the detection capabilities of existing micro-and nano-photonic devices. The ultimate detection limit is set by the laser linewidth, which can be as narrow as a few Hertz for a WGM microlaser, and is certainly much narrower than the resonance linewidth of any passive resonators 17 ...

In this paper, we present the vertical magnetoresistive random access memory (VMRAM) design based on micromagnetic simulation analysis. The design utilizes the vertical giant magnetoresistive effect of the magnetic multilayer. By making the memory element into a ring-shaped magnetic multilayer stack with orthogonal paired word lines, magnetic switching of the memory device becomes very robust. The design also adopts the readback scheme in pseudo spin valve MRAM so that only one transistor is needed for each bit line which can connect hundreds of memory elements, yielding a very high area density. It is estimated that the ultimate area density for the VMRAM is 400 Gbits/in.2. It is suggested that this memory design has the potential to not only replace the present semiconductor memory devices, such as FLASH, but also the potential to replace DRAM, SRAM, and even disk drives.

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