The rapid developments in information technology make it hard to predict what chip-scale information processing systems will look like in future, but we can offer a few preliminary thoughts based on our observations of current technical trends. First, photons will continue to play a growing role as information carriers, thanks to the enormous bandwidth and operating speed afforded by light waves. Second, the submicron-scale device footprint required by the need to reduce power consumption makes plasmonics the preferred choice over dielectric photonics. That is because plasmonics uses metallic nanostructures that can beat the size constraint in conventional optics governed by the law of diffraction.1 Moreover, given the prevailing status of electric signals used in digital devices and computers, we believe it is crucial to enable transient and reversible control of functional plasmonic elements using chip-level voltage signals.
Figure 1. Electrical modulation of photoluminescence from quantum dots. (a) A plasmonic device in which colloidal cadmium selenide/zinc sulfide core/shell nanoparticles are placed within the nanoslit cavity. An optical pump beam at 514nm was directed from the back side, and photoluminescence was collected from the top. The measurement was performed at liquid nitrogen temperature. The inset shows a scanning electron microscopy image of a cross section of the sample, with a scale bar representing 2μm. (b) Emission spectra measured at a series of different applied DC bias voltages across the nanogap. (c) The Stark shift of the emission as a function of externally applied voltage. The dashed line corresponds to a quadratic fitting curve, and the shaded region indicates a saturation effect for large voltages. The inset shows the transient response of the emission intensity when the voltage was switched on. QD: Quantum dot. (Figures modified from References 8 and 9.)
Plasmonic devices are naturally suited for electrical control because these nanometallic structures can serve as electrodes for current injection or application of electric fields. Thus far, demonstrations of electrical manipulations in nanoplasmonics are relatively sparse in the literature. A mainstream practice of electrically switchable plasmonic responses is to interface the metal with electrorefractive media such as liquid crystals, ferroelectric materials, or carrier-injected semiconductors.2–4 Sometimes nanoscale cavities are formed in the metallic architecture to enhance the light-matter interaction and the resultant modulation depth.5 Use of semiconductor gain media has led to a number of notable demonstrations, such as electrical amplification of surface plasmon propagation6 and electrically pumped plasmonic nanolasers.7
Figure 2. Electrical control of second harmonic generation with plasmonics. (a) Nonlinear device where a plasmonic nanocavity contains a commonly used polymer, poly(methyl methacrylate). In the room-temperature experiment, the fundamental wave at ∼1.5μm illuminated the metallic structure from the top. The inset shows a dark-field optical microscope image with a scale bar of 10μm. (b) The normalized change in the frequency-doubled output as a function of the control voltage. Error bars represent standard deviations from five measurements. A nonlinear modulation depth of 140% is observed at an external bias of 20V. (c) The dependence of the voltage-induced harmonic output of the fundamental light's intensity. The dashed line represents least-squares fit indicating a slope of 2.06. (Figures modified from References 8 and 9.)
We demonstrated multiple active plasmonic devices based on a nanocavity consisting of a slit carved in a metal film surrounded by a periodic grating. This structure brings several distinct advantages to combined electrical/photonic applications. The slit that defines the plasmonic cavity is less than 100nm wide. Low voltages across the slit result in very high electric fields. A properly designed nanogroove array works as an optical antenna that enables either the focusing of impinging waves or the beaming of emerging light from the nanoslit region, depending on the experiment's configuration. The property of emitted light is further modified with an increased radiative decay rate through the Purcell effect (spontaneous emission-rate enhancement), thanks to the substantially enhanced optical density of states within the plasmonic cavity.
When we place fluorescent emitters within the metallic nanocavity, their emission properties, including the spectral peak location and radiative strength, can be electrically tuned by an externally applied voltage.8 We showed this phenomenon by filling the cavity with colloidal quantum dots: see Figure 1(a). The voltage-dependent emission spectra from the quantum dots exhibits a noticeable spectral shift toward the red along with a decreased intensity magnitude when the control field increases: see Figure 1(b). We attributed these observations to the combined contributions from the quantum-confined Stark effect (spectral line splitting and shifting) and luminescence quenching. As expected for the Stark effect, the spectral red shift scales quadratically with the applied voltage until saturation occurs: see Figure 1(c). To test the dynamic response of the electrically controlled spectral shift of the emitters, we applied a 100kHz alternating voltage signal across the plasmonic device and obtained time-resolved traces: see inset, Figure 1(c). We expect the maximum modulation speed to be orders of magnitude higher than this test value, considering the fast response of the quantum-confined Stark effect.
Electrical manipulations of plasmonic nanophotonics have progressed beyond the scope of tuning the refractive index, the absorption/gain coefficient, and emission behavior. They have enabled an intriguing opportunity for active nonlinear optics at the deep-subwavelength scale. We recently demonstrated electrically controlled harmonic generation of light in a plasmonic cavity similar to the one used previously, where electrical signals can be encoded in the frequency-doubled light originating from a third-order nonlinear susceptibility χ(3) or 2ω; ω; ω,0: see Figure 2(a).9 Unlike conventional second harmonic generation, which imposes a rigorous requirement on the nonlinear medium's lattice asymmetry, third-order nonlinear responses are ubiquitous in all materials. The micrometer-scale footprint of the proposed device also eases the strict phase-matching condition and makes it suitable for chip-scale applications where dense integration is necessary.
The experimental data for the second harmonic signal change as a function of applied DC voltage indicates a linear dependence of the frequency-doubled output on the driving voltage. The magnitude of the normalized change is more than 7% per volt: see Figure 2(b). When 20V is applied across the nanocavity, the normalized tunability of the frequency-doubled output is more than 140%. We collectively polarized all pertinent electric fields along the control field's direction, which is also the required incident polarization necessary for plasmonic resonance excitation. Much like conventional second harmonic generation, the voltage-induced, frequency-doubled signal is quadratic with the fundamental wave's intensity: see Figure 2(c).
These results are just two examples that represent a broad range of electrically controlled plasmonic devices, a fascinating topic that has just begun to broach the horizon. With more elaborate designs of plasmonic components combining metallic nanostructures and electrically active dielectric media, we anticipate a whole new set of dynamic photonic devices controlled by electric signals at reduced dimensions, enabling novel applications for electro-optic modulation, optical switching, nanoscale spectroscopy, signal processing, light-emitting devices, and electrochemical reactions.
Wenshan Cai, Mark L. Brongersma
Geballe Laboratory for Advanced Materials
Palo Alto, CA
Wenshan Cai, postdoctoral fellow, is researching nanophotonic materials and devices. In January 2012, he will join the faculty of the Georgia Institute of Technology as an associate professor in electrical and computer engineering and jointly in materials science and engineering.
Mark Brongersma, associate professor, received the International Raymond and Beverly Sackler Prize in the Physical Sciences for his work on plasmonics. He is a Fellow of SPIE, the Optical Society of America, and the American Physical Society. He received his PhD from the Foundation for Fundamental Research on Matter Institute in Amsterdam and was a postdoctoral research fellow at the California Institute of Technology.
Young Chul Jun
Sandia National Laboratories
Young Chul Jun received his PhD from Stanford and was in the laboratory of Mark Brongersma. He is currently a postdoctoral scholar at Sandia National Laboratories, where he is conducting research on active plasmonics and metamaterials.
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