The extreme electro-optical contrast between crystalline and amorphous states in phase-change materials is routinely exploited in optical data storage and future applications include universal memories, flexible displays, reconfigurable optical circuits, and logic devices. Optical contrast is believed to arise owing to a change in crystallinity. Here we show that the connection between optical properties and structure can be broken. Using a combination of single-shot femtosecond electron diffraction and optical spectroscopy, we simultaneously follow the lattice dynamics and dielectric function in the phase-change material Ge2Sb2Te5 during an irreversible state transformation. The dielectric function changes by 30% within 100 fs owing to a rapid depletion of electrons from resonantly bonded states. This occurs without perturbing the crystallinity of the lattice, which heats with a 2-ps time constant. The optical changes are an order of magnitude larger than those achievable with silicon and present new routes to manipulate light on an ultrafast timescale without structural changes.
A novel optical switch operating at a wavelength of 1.55 µm and showing a 12 dB modulation depth is introduced. The device is implemented in a silicon microring resonator using an overcladding layer of the phase change data storage material Ge 2 Sb 2 Te 5 (GST), which exhibits high contrast in its optical properties upon transitions between its crystalline and amorphous structural phases. These transitions are triggered using a pulsed laser diode at λ = 975 nm and used to tune the resonant frequency of the microring resonator and the resultant modulation depth of the 1.55 µm transmitted light.The ever-increasing demand for high speed optical communication networks is driving the development of new photonic devices that can process optical signals in a reliable, low-cost manner. Among competing technologies, Si-based devices have emerged as one of the main candidates for such applications, and several devices, including modulators 1-5 , add-drop filters 6 and wavelength division multiplexers (WDM) 7 have already been demonstrated. An important branch of this technology is the ability to program reconfigurable optical circuits. Indeed, a reprogrammable optical circuit that can hold its configuration without an external continuous source is extremely desirable for a multitude of applications ranging from photonic routers to optical cognitive networks. Recently, new solutions for non-volatile photonic memories have been proposed, involving the use of phase-change materials (PCM) and microring resonators 8,9 .Herein, a non-volatile Si microring resonator optical switch is demonstrated. A thin film of the phasechange material 10 (PCM) Ge 2 Sb 2 Te 5 (GST), which is commonly encountered in optical and electrical data storage applications [11][12][13][14] , is used to switch the resonant frequency and Q-factor of the microring resonator. GST shows high optical contrast between its amorphous, covalently bonded, and crystalline, resonantly bonded, structural phases 15-18 (n cryst − n amorph = 2.5 ; k cryst − k amorph = 1 at 1.55 µm) 19 . Moreover, transitions between the two phases can take place on a sub-ns timescale 20,22 while the resulting final state is stable for several years. These characteristics deem this material appropriate for application in reconfigurable optical circuits.The device, shown in Fig. 1, consists of a Si microring resonator with a bend radius of 5 µm and a coupling region of 3 µm, on top of which a GST thin film with an area of 3×1.5 µm 2 has been deposited. A second Si a) miquel.rude@icfo.es microring with identical dimensions but free of GST is used as a reference during the measurements. A 200 nm gap separates both microrings from a Si strip waveguide (220×440 nm 2 ) with grating couplers 23 at both ends, which are used to deliver light into the device and monitor the transmitted spectrum using single-mode fibers (SMF).
The functionalities of a wide range of optical and opto-electronic devices are based on resonance effects and active tuning of the amplitude and wavelength response is often essential. Plasmonic nano-structures are an efficient way to create optical resonances, a prominent example is the extraordinary optical transmission (EOT) through arrays of nano-holes patterned in a metallic film. Tuning of resonances by heating, applying electrical or optical signals has proven to be more elusive, due to the lack of materials that can induce modulation over a broad spectral range and/or at high speeds. Here we show that nano-patterned metals combined with phase change materials (PCMs) can overcome this limitation due to the large change in optical constants which can be induced thermally or on an ultrafast timescale. We demonstrate resonance wavelength shifts as large as 385 nm --an order of magnitude higher than previously reported--by combining properly designed Au EOT nanostructures with Ge2Sb2Te5 (GST). Moreover, we show, through pump-probe measurements, repeatable and reversible, large-amplitude modulations in the resonances, especially at telecommunication wavelengths, over ps time scales and at powers far below those needed to produce a permanent phase transition. Our findings open a pathway to the design of hybrid metal-PCM nanostructures with ultrafast and widely tuneable resonance responses, which hold potential impact on active nanophotonic devices such as tuneable optical filters, smart windows, bio-sensors and reconfigurable memories.* These authors made equal contribution †valerio.pruneri@icfo.eu 2 Nanophotonic devices incorporating metallic elements can support plasmons, which are collective oscillations of conduction band electrons driven by an external electromagnetic field 1 . Plasmons can confine and guide light well below the diffraction limit, and when supported by suitably engineered nanostructures, they enable the design of disruptive devices for a wide range of applications, including perfect lenses . Plasmons also play an important role in the phenomenon of extraordinary optical transmission (EOT) of visible and infrared light through periodic arrays of subwavelength nanoholes drilled in metallic films. The observation of transmission resonances in these arrays is attributed to the resonant interaction between holes mediated by surface plasmons propagating on the film surfaces 7 . More precisely, transmission peaks emerge close to the Wood anomalies 8 and are well explained in terms of geometrical resonances in the periodic lattice 9,10,11 . An important challenge in the design of plasmonic nanostructures is the precise control of their optical responses in order to meet the requirements of specific device applications. This can be accomplished by casting nanostructures with appropriate materials and geometries. However, such an approach is static and limited by material inhomogeneity and fabrication tolerances. More critically, many applications (e.g., optical switching and modulation) ...
The ability to manipulate light propagation at the nanoscale is of vital importance for future integrated photonic circuits. In this work we exploit the high contrast in the optical properties of the phase change material Ge 2 Sb 2 Te 5 to control the propagation of surface plasmon polaritons at a Au/SiO 2 interface. Using grating couplers, normally incident light at λ = 1.55 μm is converted into propagating surface plasmons on a Au waveguide. Single laser pulses (λ = 975 nm) are applied to a thin film of Ge 2 Sb 2 Te 5 placed on top of the device, which, upon transition from its amorphous to crystalline structural phase, dramatically increases both its refractive index and absorption coefficient, thus inhibiting propagation of the plasmonic mode. This effect is investigated for different interaction lengths between the phase change material and the Au waveguide, and contrast values in the transmitted intensity up to several tens of percents are demonstrated.
Perfect light absorption over wide angles is possible in a multilayer structure, including Au or Ni metal and Ge2Sb2Te5 (GST) phase change material, without the need of sophisticated lithography. The GST layer also permits deep modulation of the absorption when it undergoes a phase transition.
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