We report on a waveguide composed of a cascade of gain-enabled quantum dots with subwavelength dimensions. Fabrication is demonstrated through DNA-mediated self-assembly and a two-layer molecular self-assembly process that enables rapid prototyping. The device, which is identified with fluorescence microscopy and tested by optical near field detection, allows optical signal transfer at a well-defined wavelength in flexible routing geometry, such as straight paths and 90 degrees bends. The structure serves as a critical building block for nanophotonic systems with high integration density.
While 32 nm lithography technology is on the horizon for integrated circuit (IC) fabrication, matching the pace for miniaturization with optics has been hampered by the diffraction limit. However, development of nanoscale components and guiding methods is burgeoning through advances in fabrication techniques and materials processing. As waveguiding presents the fundamental issue and cornerstone for ultra-high density photonic ICs, we examine the current state of methods in the field. Namely, plasmonic, metal slot and negative dielectric based waveguides as well as a few sub-micrometer techniques such as nanoribbons, high-index contrast and photonic crystals waveguides are investigated in terms of construction, transmission, and limitations. Furthermore, we discuss in detail quantum dot (QD) arrays as a gain-enabled and flexible means to transmit energy through straight paths and sharp bends. Modeling, fabrication and test results are provided and show that the QD waveguide may be effective as an alternate means to transfer light on sub-diffraction dimensions.
We present cross-talk calculations for a subdiffraction nanophotonic waveguide that consists of a colloidal quantum dot (QD) array 10 nm in diameter and compare the results with conventional continuous dielectric waveguides, assuming the same 10 nm size as well as a 200 nm cutoff diameter for guided mode. We find that the QD cascade has much lower cross talk than 10 nm dielectric waveguides at an identical separation >30 nm. Moreover, results for 200 nm dielectric waveguides at a 280 nm gap are comparable with those of QD structures spaced 110 nm apart. Hence the proposed QD device is potentially superior to conventional waveguides in achieving lower cross talk in the subdiffraction regime and provides a new route to achieving high-density photonic integrated circuits.
We model and demonstrate the behavior of two-dimensional (2D) self-assembled quantum dot (QD) sub-diffraction waveguides. By pumping the gain-enabled semiconductor nanoparticles and introducing a signal light, energy coupling of stimulated photons from the QDs enables light transmission along the waveguide. Monte Carlo simulation with randomized inter-dot separation reveals that the optical gain necessary for unity transfer is 3.1 × 10(7) m(-1) for a 2D (2 µm length by 500 nm width) array compared to 11.6 × 10(7) m(-1) for a 1D (2 µm length) given 8 nm diameter quantum dots. The theoretical results are borne out in experiments on 2D arrays by measurement of negligible crosstalk component with as little as 200 nm waveguide separation and is indicative of near-field optical coupling behavior. The transmission loss for 500 nm wide structures is determined to be close to 3 dB/4 µm, whereas that for 100 nm width is 3 dB/2.3 µm. Accordingly, higher pump power and gain would be necessary on the narrower device to create similar throughput. Considering existing nanoscale propagation methods, which commonly use negative dielectric materials, our waveguide shows an improved loss characteristic with comparable or smaller dimensions. Thus, the application of QDs to nanophotonic waveguiding represents a promising path towards ultra-high density photonic integrated circuits.
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