A Nonvolatile Plasmonic Switch Employing Photochromic Molecules Ragip A. Pala, Ken T. Shimizu, Nicholas A. Melosh, and Mark L. Brongersma* Geballe Laboratory for AdVanced Materials, Stanford UniVersity, Stanford, California 94305 Received March 27, 2008 ABSTRACT We demonstrate a surface plasmon-polariton (SPP) waveguide all-optical switch that combines the unique physical properties of small molecules and metallic (plasmonic) nanostructures. The switch consists of a pair of gratings defined in an aluminum film coated with a 65 nm thick layer of photochromic (PC) molecules. The first grating couples a signal beam consisting of free space photons to SPPs that interact effectively with the PC molecules. These molecules can reversibly be switched between transparent and absorbing states using a free space optical pump. In the transparent (signal “on”) state, the SPPs freely propagate through the molecular layer, and in the absorbing (signal “off”) state, the SPPs are strongly attenuated. The second grating serves to decouple the SPPs back into a free space optical beam, enabling measurement of the modulated signal with a far-field detector. In a preliminary study, the switching behavior of the PC molecules themselves was confirmed and quantified by surface plasmon resonance spectroscopy. The excellent (16%) overlap of the SPP mode profile with the thin layer of switching molecules enabled efficient switching with power densities of ∼6.0 mW/cm 2 in 1.5 μm × 8 μm devices, resulting in plasmonic switching powers of 0.72 nW per device. Calculations further showed that modulation depths in access of 20 dB can easily be attained in optimized designs. The quantitative experimental and theoretical analysis of the nonvolatile switching behavior in this letter guides the design of future nanoscale optically or electrically pumped optical switches. The microelectronics industry has witnessed a continual progression toward more compact, high speed, and power efficient devices over the last five decades. Currently, one of the most daunting problems preventing significant further increases in processor speed are thermal and RC delay time issues associated with electronic interconnection. Whereas high-speed optical interconnection schemes may offer in- teresting new solutions around these problems, their imple- mentation is hampered by the large size mismatch between electronic and dielectric photonic components. CMOS found- ries will decrease feature sizes on a Si chip from 65 to 45 nm and, ultimately, 10 nm. This will further increase this size mismatch, as dielectric photonic devices are limited in size to hundreds of nanometers by the fundamental laws of diffraction. The use of nanometallic (plasmonic) structures may help bridge the size gap between the two technologies and enable an increased synergy between chip-scale elec- tronics and photonics. Optical modulators are key components for a chip-scale optical link, 1 and their development has recently experienced a number of important advances toward the realization of efficient, high-speed devices. 2–4 Unfortunately, many of the traditional CMOS modulator approaches run into problems as the weak nonlinear optical effects in Si (e.g., free carrier dispersion) exhibit poor scaling characteristics. For this reason, long interaction lengths or resonant cavity devices are needed to attain required modulation depths. For this reason, the demand is growing for innovative new ap- proaches. Recently, surface plasmon-polariton (SPP) based circuit elements including waveguides, 5–9 ring resonators, 10 modulators, 11–13 and photodetectors 14–16 have been proposed and successfully demonstrated, starting a new era in which seamless integration of electronics and photonics may be enabled by plasmonics. 17,18 In this letter, we analyze the performance of an all-optical switch that combines plas- monics with photochromic (PC) materials. The nonvolatile optical switching behavior seen in PC devices may provide significant power advantages over typical optical switches in plasmonic circuitry, which need a continuous optical supply power. 19 Similar to low-leakage electronic devices, these switches only draw power if there is a logic level transition. Because of its simple layout, the conclusions drawn about this device geometry are quite general and important for future more complex modulator designs. SPPs are electron density waves propagating along the surface of a metal and are coupled to bound transverse magnetic (TM) electromagnetic waves. The electromagnetic field intensity associated with a SPP is highest at the metal surface and decays exponentially into the metal and the adjacent dielectric. The strong confinement of the field to * Corresponding author. E-mail: brongersma@stanford.edu. Telephone: (650) 736-2152. Fax: (650) 725-4034. NANO LETTERS 2008 Vol. 8, No. 5 1506-1510 10.1021/nl0808839 CCC: $40.75  2008 American Chemical Society Published on Web 04/16/2008