This paper reports tunable integrated photonic devices in the visible and near-infrared (NIR) regions using a phase change material, Germanium Telluride (GeTe), within sub-wavelength layered optical cavity structures. GeTe exhibits two distinct index of refraction values at its amorphous and crystalline states in this spectral range. Utilizing this property, we demonstrate a multi-color filter working in visible range (400 nm-750 nm), achieving four colors through novel optical and thermal engineering of a thin film stack that includes two GeTe layers with only a single integrated joule heater element. Specifically, ultra-thin GeTe films were sandwiched between a bottom metallic mirror and a top high-index dielectric (titanium dioxide). It is shown that the crystallization temperature (Tx) of GeTe is dependent on the film thickness when less than ~20 nm. The refractive index of GeTe only changes significantly in the NIR region when it undergoes phase transitions when heated. To enhance the color contrast, a 250-nm thick silicon dioxide layer is placed under GeTe to create an optical cavity between GeTe and a bottom metal reflector. Using this optimized design, the tunable color filter shows four distinct colors using the integrated heater. In addition to a color filter, we demonstrate an electrically tunable multi-mode optical shutter in near infrared range of 1.1 μm and 800 nm with more than 20 dB modulation depth. The low static power consumption of these devices achieved through reliable memory-base phase transitioning of GeTe makes them prime candidates for a number of portable consumer electronic applications.
This paper reports for the first time a high-contrast, low-power reflective color filter employing a phase change material in a waveguide grating with different periods. Refractive index modulation is produced by crystallographic phase transition of Germanium Telluride (GeTe) using an external stimulus, such as heat. Nanostrips of GeTe are used due to their reliable, fast, and reversible phase transitions. These thin nanostrips reduce the total light absorption and yield a vivid, bright reflected color. Moreover, a buried optical waveguide consisting of a silicon nitride (Si3N4) core, which was grown between two layers of silicon dioxide (SiO2) cladding, is placed under these gratings to enhance the color contrast. The waveguide resonance is enhanced using a bottom palladium reflector and GeTe gratings above with different periods. Selective absorption within visible-NIR region that depend on the period of the grating and the phase of the GeTe is introduced within these devices for the first time. A unique anti-reflective coating was also re-introduced to suppress the surface reflection, thus enhancing the color contrast. Vivid reflected red and green colors have been shown for a device with active area of 400 μm2. These colors were electrically transitioned to blue and yellow, respectively, for several cycles. We further report that GeTe nanostrips with different cross-sectional areas demonstrate different phase transition behaviors. Thus, several colors were achieved within the same area of the device employing GeTe nanostrips with different periods.
This paper introduces tunable color reflectors for three primary colors (red, blue, and green) for use in low power display systems employing a phase change material (PCM) using interference resonance in an optical cavity. Optical index tunability of the PCM sitting on top of the cavity, results in tuning the device reflection spectrum and thus permits vivid color tuning. The phase change material used to achieve these results was Germanium Telluride (GeTe) due to its high stability. Specifically, ultra-thin films of GeTe was grown on top of a SiO2 cavity with a bottom palladium reflector in a Fabery-Perot type design. This enhances the color tuning when a double-layer anti-reflection coating with high and low refractive indexes are used on top of the GeTe film. Low sensitivity to incident light angle and polarization without the need of sub-micron lithography, provide the potential for this device to be very useful for portable device applications. The devices with different thickness of GeTe were fabricated to demonstrate green, red, blue colors. Electrical pulses with different periods and duty cycles were used to switch the phase of the GeTe locally using joule heating method for several cycles. After transition, darker green, blue, and purple colors were shown for devices with 8 to 20 nm thickness of GeTe films.
This paper presents a high-contrast electro-optical modulator with record-breaking amplitude modulation index of 27 dB and forward loss of < 3 dB at 1.5 μm. The high contrast is achieved by utilizing slit and surface plasmon polariton resonances in an array of gold lines filled with a phase change material (Germanium Telluride-GeTe). Stacking multiple layers of the phase change plasmonic grating enabled development of the high-index modulator in chip scale dimensions at telecommunication wavelength. Coupling the optical modes of multiple layers results in such a high contrast when GeTe goes through crystallographic phase transition. Phase transition through joule-heating is achieved by employing a matching circuit.
In this paper, we present an ultra-fast and high-contrast optical shutter with applications in atomic clock assemblies, integrated photonic systems, communication hardware, etc. The shutter design exploits the total light absorption phenomenon in a thin phase change (PC) material placed over a metal layer. The shutter switches between ON and OFF states by changing PC material phase and thus its refractive index. The PC material used in this work is Germanium Telluride (GeTe), a group IV-VI chalcogenide compound, which exhibits good optical contrast when switching from amorphous to crystalline state and vice versa. The stable phase changing behavior and reliability of GeTe and GeSbTe (GST) have been verified in optical memories and RF switches. Here, GeTe is used as it has a lower extinction coefficient in near-IR regions compared to GST. GeTe can be thermally transitioned between two phases by applying electrical pulses to an integrated heater. The memory behavior of GeTe results in zero static power consumption which is useful in applications requiring long time periods between switching activities. We previously demonstrated a meta-surface employing GeTe in sub-wavelength slits with >14 dB isolation at 1.5 μm by exciting the surface plasmon polariton and localized slit resonances. In this work, strong interference effects in a thin layer of GeTe over a gold mirror result in near total light absorption of up to 40 dB (21 dB measured) in the amorphous phase of the shutter at 780 nm with much less fabrication complexity. The optical loss at the shutter ON state is less than 1.5 dB. A nickel chrome (NiCr) heater provides the Joule heating energy required to achieve the crystallographic phase change. The measured switching speed is 2 μs.
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