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Mixed-mode operation of hybrid phase-change nanophotonic circuits Yegang Lu, Matthias Stegmaier, Pavan Nukala, Marco A. Giambra, Simone Ferrari, Alessandro Busacca, Wolfram HP Pernice, and Ritesh Agarwal Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b03688 • Publication Date (Web): 13 Dec 2016 Downloaded from http://pubs.acs.org on December 14, 2016

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Mixed-mode operation of hybrid phase-change nanophotonic circuits †,＃

Yegang Lu,

Matthias Stegmaier,

§,＃

‖

Pavan Nukala,

Marco A. Giambra,

§

∗,§

Ferrari, Alessandro Busacca, & Wolfram H. P. Pernice, †

‡,&

Ritesh Agarwal

Simone ∗,‖

Faculty of Electrical Engineering and Computer Science, Key Laboratory of

Photoelectric Materials and Devices of Zhejiang Province, Ningbo University Zhejiang, 315211, China ‡

Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT),

Hermann-von-Helmholtz-Platz 1, Eggenstein-Leopoldshafen 76344, Germany ‖

Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States §

Institute of Physics, University of Muenster, Muenster 48149, Germany &

Università degli Studi di Palermo, 90133 Palermo, Italy

＃Y. L. and M. S. contributed equally to this work Corresponding authors: [email protected], [email protected]

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ABSTRACT: Phase change materials (PCMs) are highly attractive for nonvolatile electrical and all-optical memory applications because of unique features such as ultrafast and reversible phase transitions, long-term endurance, and high scalability to nanoscale dimensions. Understanding their transient characteristics upon phase transition in both the electrical and optical domains is essential for using PCMs in future multi-functional optoelectronic circuits. Here, we use a PCM nanowire embedded into a nanophotonic circuit to study switching dynamics in mixed-mode operation. Evanescent coupling between light traveling along waveguides and a phase-change nanowire enables reversible phase transition between amorphous and crystalline states. We perform time-resolved measurements of the transient change in both the optical transmission and resistance of the nanowire and show reversible switching operation in both the optical and electrical domains. Our results pave the way towards on-chip multi-functional optoelectronic integrated devices, waveguide integrated memories, and hybrid processing applications.

KEYWORDS: All-optical switching, GeTe nanowires, nanophotonic circuits, phase change Nanophotonic integrated components joined with electrical integrated devices allow for realizing on-chip multi-functional optoelectronic circuits (MOC) to achieve efficient data storage, communication and processing within a single system, with small footprint and 2


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low power consumption. MOCs allow device operation both in the electrical and optical domains, which facilitates management of massive amounts of information, as well as to lower the complexity of multi-functional integration. However, it has been challenging to identify suitable multi-functional materials as core building blocks of an optoelectronic framework that responds to both electrical and optical stimuli. Recently, phase change materials have been proposed for both electrical access memory and reconfigurable nanophotonic devices to enable all-optical switching,1-3 photonic memories4-6, and solid-state displays7,8. The physical properties of phase change materials, including the electrical resistivity and the refractive index, show high contrast between the amorphous and crystalline states. The transitions between the two phases can be triggered reversibly on a sub-ns timescale. The phase state remains stable over long time periods, e.g. 10 years without power supply, which provides attractive nonvolatile attributes for an optoelectronic framework. Moreover, the desirable properties of PCMs are expected to remain when scaled down below 2 nm.9 Compared with continuous films, phase change nanowires possess higher crystallization temperature, lower melting point and lower thermal conductivity.10,11 Thus, PCM nanowires are attractive for thermal stability improvement, power reduction, and superior surface properties combined with a facile fabrication process.11 Indeed, phase change nanowires are promising alternatives to fabricate nanodevices owing to their controllable sizes scaling down to at least 30 nm in diameter.10 These attributes give PCM nanowires great interest for application in MOCs. 3


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The real-time observation of the transient phase change in both the electrical and optical domains is the fundamental way to understand the switching dynamics of the PCMs. For instance, the reported operation speed of a phase change memory is usually assessed by the external width of the Write (Erase) pulse.12,13 Estimation results are not sufficient to achieve ultimate switching speed because of the intrinsic switching time between the covalently and resonantly bonded phases.14 PCMs are also widely utilized for storage media in respective electric or optical domains and have emerged as promising building blocks for ultrahigh-resolution display devices which exploit both domains.7,15 However, to date there has been little study on the transient features and correlation between electrical and optical domains upon phase transition. Here, we employ an integrated mixed-mode measurement platform to study the switching dynamics in both the optical and electrical domains. Our hybrid phase-change nanophotonic platform can serve as framework for realizing novel multifunctional optoelectronic devices such as optoelectronic nonvolatile memories, optoelectronic displays, and reconfigurable optoelectronic circuits. The phase-change mixed-mode device is schematically depicted in Figure 1a. A GeTe nanowire (silver colored) is placed across an on-chip photonic waveguide (blue colored) by putting it on top of two adjacent gold electrodes (golden colored). This way, the nanowire is both coupled to the on-chip photonic waveguide, via evanescent interaction, and electrically contacted which enables simultaneous access with optical and 4


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RF-signals. The used GeTe nanowires offer high-speed, reversible phase-change operation with high optical and electrical contrast between the amorphous (aGeTe) and crystalline (cGeTe) state.16,17 The impact of the nanowire onto a guided optical wave can be seen in the finite-element simulation (COMSOL Multiphysics) depicted in Figure 1b, for both the amorphous (upper panel) and crystalline (lower panel) state of the nanowire (radius of 140 nm). Details on the simulation are provided in the supplementary information. Due to evanescent interaction, the guided light is partially absorbed and scattered out of the waveguide. A significant change in the transmitted optical power between the two phase-states can be observed. The optical transmission increases from 0.7 (aGeTe) to 0.89 (cGeTe) upon crystallization. This change in optical transmission results from the change of the refractive index of GeTe upon phase-transition, increasing from ݊aGeTe = 4.06 + ݅ ∙ 0.14 to ݊cGeTe = 6.86 + ݅ ∙ 1.1818 at a wavelength of 1550 nm. While this refractive index contrast enables optical monitoring of the processes within the nanowire, the absorbed power (11/8 % in case of aGeTe/cGeTe) also allows exciting the GeTe nanowire with nanosecond optical pulses. In contrast, the electronic domain is directly governed by the resistance of the GeTe which changes upon switching by up to 3 orders of magnitude, as can be seen in Figure 1c where the IV-curve of such a nanowire is plotted. Therefore, an optically or electrically induced phase-transition can result in a drastic change of the overall resistance of the device.

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The aforementioned evanescent interaction can be quantified by studying the reflection and transmission coefficients of the system shown in Figure 1b. From the optical reflection parameter one can conclude that reflections are smaller than -30 dB and therefore negligible. In contrast, the optical transmission coefficients give directly the attenuation and the phase shift of the guided optical wave due to the presence of the nanowire. In Figure 1d, the spectral dependence of the respectively derived optical transmission is plotted for different dimensions of the nanowire. It can be clearly seen that at certain wavelengths resonant interaction significantly reduces the transmitted power in the amorphous phase state. The reason for this is enhanced Mie-scattering.19 However, not only the scattered power but also the power absorbed within the nanowire is enhanced, as shown in Figure S1 in the supplementary material, which reduces the input power required to initiate a phase transition. This resonant interaction enables switching with high contrast which can be tuned to the spectral regime of interest by selecting the nanowire diameter appropriately. The photonic circuitry for our mixed-mode measurements was fabricated by two electron beam lithography (EBL) steps from a 330 nm Si3N4, 3300 nm buried SiO2 layer stack on top of silicon substrate (see Figure S2 in Supporting Information).20, Single-crystalline GeTe nanowires were synthesized using metal catalyst mediated vapor-liquid-solid (VLS) process (see Figure S3 in Supporting Information).17,21 Subsequently, these nanowires were transferred mechanically onto the fabricated photonic chip using the 6


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transfer process schematically depicted in Figure 2a. First, a contact printing method was employed to assemble GeTe nanowires on a bare layer of polydimethylsiloxane (PDMS).22 A PDMS stamp was brought into contact with the sample with deposited GeTe nanowires, and consequently some sparse GeTe nanowires were transferred onto the surface of the PDMS due to surface adhesion. The PDMS stamp was affixed to a transfer plate with a hole in the middle, such that the GeTe nanowire can be observed through the hole. With nanowires on the downside, the transfer plate was placed movably on a 2D-stage under an optical microscope. The prefabricated nanophotonic chip was fixed on the microscope 1D-stage below the sample. This way the transfer plate, through its hole the GeTe nanowire, and the chip could be observed via a microscope objective above the 2D-stage. Prior to performing the nanowire transfer, the nanophotonic chip was covered with PMMA and exposed via EBL to pattern opening windows across waveguide and electrodes. The selected GeTe nanowire was aligned to the electrode in the window of the PMMA layer, and then the chip was raised slowly until it touched the GeTe nanowire above. After heating to 120 oC for 30 min, the chip was lowered down to lift off the PDMS on the transfer plate from the chip. The nanowire remains in contact with the electrodes because of van der Waals adhesion.23 Subsequently, the chip was annealed at 150 oC for 10 min in order to enhance the contact between GeTe nanowire and electrodes. Finally, the chip was immersed upside down in the acetone in order to remove the PMMA, as well as the unwanted GeTe nanowires out of the PMMA window. 7


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In Figure 2b we present an optical microscope image of the final device. In order to improve the electrical contact between the Au electrodes and GeTe nanowire, additional Pt was selectively deposited on both ends of the nanowire, as can be seen in Figure 2c, by using focused ion beam (FIB) deposition. To study the mixed-mode properties of the device, we employed the measurement setup schematically depicted in Figure 3. The GeTe nanowire (colored red) is connected to two on-chip electrodes (colored yellow) and at the same time evanescently coupled to the nanophotonic waveguide (colored green). The off-chip fiber-optical setup (red shaded area) enables simultaneous optical excitation and readout of the nanowire, while the electronic setup (green shaded area) is used for monitoring of the device impedance. Light is coupled in and out of on-chip photonic circuitry by means of focusing grating couplers.6 For RF access, the nanowire is electrically contacted by an ultra-fast, 50 Ohm matched RF-probe to minimize signal distortion and maximize temporal resolution. The GeTe nanowire is optically investigated using a two-color measurement scheme for Write, Erase, and readout operations. Pump and probe light are sent contra-directionally into the photonic circuit, facilitated by two fiber-coupled optical circulators, to enable convenient separation of pump and probe light. Back optical reflections within the setup are filtered out by using two different colors for pump (1560 nm) and probe (1570 nm) and a color-selective bandpass filter (Pritel TFA-1550). The continuous wave (CW) probe light is generated from a tunable laser source (Santec 8


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TSL-510C) and the transmitted probe power is detected by both a low-noise (New Focus 2018) and a high-speed (125 MHz, New Focus 1811) photodetector. The pump pulses are generated from a second CW tunable laser source (New Focus 6427) with an electro-optical modulator (Lucent Technology 2623CS) which is controlled by a 500 MHz electrical pulse generator (HP 8131A). Before the pulses are sent into the device, they are further amplified by an erbium-doped fiber-amplifier (Pritel LNHPFA-33 and Pritel LNHPFA-30). While the optical measurements are carried out, the device impedance was monitored time-resolved with a RF setup in a reflection-type configuration. The GeTe nanowire was biased with a DC current using a stable source meter (Keithley 2400, current mode) which was simultaneously used to monitor the DC device resistance. The dynamics in this circuit upon optical excitation are described by the transmission line model, due to the fast changes of the resistance of the nanowire (ZGeTe). Therefore, this terminology is also used before the optical excitation, i.e. while the nanowire is biased by a DC-voltage. In this model, the bias voltage UBias is expressed in terms of the voltage amplitude of the incoming wave Uin and the one of the reflected wave ܷref = Γܷ௜௡ , i.e. ܷBias = ܷ௜௡ + ܷ௥௘௙ ~2ܷ௜௡ . Here, Γ denotes the voltage reflection coefficient which arises from the impedance mismatch at the RF probe/device interface. Upon optical excitation, the impedance of the nanowire changes and thus also Γ. The corresponding change in

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reflected voltage ∆Uref can be described in terms of the resistance variation ∆ZGeTe of the nanowire as follows: ∆ܷref = ∆Γ ∙ ܷ௜௡ =

௓బ ∆௓GeTe

′

ሺ௓GeTe ା௓బ ሻ∙൬௓GeTe ା௓బ ൰

ܷBias

(1)

Here, Z0 denotes the impedance of the transmission line (50 Ω). The resulting changes in reflected voltage are then separated from the DC-biasing circuit by a 6 GHz Bias-Tee (Mini-Circuit ZFBT-6GW+), further amplified (Mini-Circuit ZFL-1000LN+) and are eventually recorded by an ultra-fast 6 GHz oscilloscope (Agilent infiniium 54855A). All-optical operation of a GeTe nanowire with a diameter of about 250 nm is presented in Figure 4. The optical transmission of the device was monitored for every excitation of the pump pulse whose energy increased step by step. The as-prepared crystalline nanowire was initially amorphized (Write operation applied at time instant 0) by a 50 ns long optical pulse with an energy of 8 nJ of which ~122 pJ were absorbed. The generated heat was high enough to partially melt the nanowire. After the excitation, thermal diffusion towards the electrodes resulted in fast cool-down and thus enabled quenching into the amorphous state. The change in transmitted probe signal due to this excitation is presented in Figure 4a (black trace). The observed decrease in optical transmission is ascribed to the resonant Mie-scattering effect which causes strong absorption of the pump pulse by the aGeTe (see Figure 1(d)). Crystallization, on the other hand, was initiated by a 50 ns long pulse with 6.2 nJ, cf. blue trace in Figure 4a. Here, 10


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lower pulse energies were required for the recrystallization than for the amorphization since this process does not require the GeTe to be heated above the melting temperature and additionally the absorption within the nanowire increased upon amorphization. We note that the resistance of the device still maintained its original value after both Erase and Write transmission operations. This is due to the fact that the cross-sectional area of the GeTe nanowire is only partially amorphized. Reversible switching between amorphous and crystalline states with several cycles can be achieved with the device, as shown in Figure 4b, which illustrates that the evanescent field of the waveguide can controllably and reliably initiate phase transitions in the GeTe nanowire. Compared with devices where the PCM (Ge2Sb2Te5 or GST) was directly deposited on a waveguide,6 the GeTe nanowire-based device provides lower contrast between the “0” (crystalline) and “1” (amorphous) states. This can be explained by smaller differences in both optical transmission and absorption coefficients upon phase transition for the GeTe compared with GST.24 The smaller cross-section of the GeTe nanowire also contributes to a lower optical transmission contrast which is proportional to the programming volume. In addition, the GeTe nanowires in contact with the metal electrodes are suspended above the top surface of the waveguide (see Supporting Information, Figure S4). The spacing (about 110 nm) between the free-standing GeTe nanowire and the waveguide weakens the evanescent-field absorption of the crystalline GeTe nanowire and thus contributes to the smaller value of the contrast. Higher switching contrast could be obtained by reducing 11


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the spacing, by enhancing the resonant interaction and by embedding the nanowire into an on-chip optical cavity. A key requirement to observe a considerable resistance increase upon amorphization is that a whole cross-sectional area of the GeTe nanowire is amorphized. Otherwise, the current can still flow through the remaining conductive crystalline material and thus the resistance is only enhanced by the relative decrease of the crystalline area. Here, we achieved such a full cross-sectional amorphization of a nanowire with about a diameter of roughly 300 nm using 50 ns long optical pump pulses with a slightly higher (~10.7 nJ) pulse energy as compared to the thinner nanowire used for the measurements in Figure 4. As shown in Figure 5a, this enabled repeatable operation between the high and low resistance state. The obtained resistance contrast of three orders of magnitude provides substantial sensor margin for electrical read-out. We note that optical pulses with low pump energy can be employed to complete Write/Erase transmission operations (see Figure 4a), and with high pump energy to realize Write/Erase resistance operations. Therefore, asynchronous jumps of the optical transmission and resistance can be controlled precisely by the pump energy. Single read-out mode in optical or electrical domain can identify only two distinct states of optical transmission or resistance. Combining electrical with optical read-out modes in this hybrid phase-change nanophotonic circuits, the device demonstrates the promising feasibility to detect multilevels of the storage. The main benefit of the employed 12


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mixed-mode setup is, however, that the optically triggered phase-transition can also be monitored time-resolved in the electronic domain. Upon crystallization, we observe a considerable decrease of reflected voltage as provided in Figure S5 of the supplementary material. From the measured voltage signal the resistance of the nanowire can be calculated with Equation 1 by using the known initial DC resistance and the bias voltage. Upon excitation with a 50 ns optical pulse, the resulting resistance curve and corresponding change of optical transmission upon optically-induced switching are presented in Figure 5b. Overall, the resistance drops from a few MΩ down to 6 kΩ, in consistency with the DC measurements presented in Figure 5a. The initial decrease of the resistance, as further discussed in the Supporting Information, is a pure thermo-resistive effect, i.e. a decrease of the resistance with temperature.25 The effect of crystallization itself is mainly observed at the end and in particular after the end of the optical excitation where the temperature is high enough to enable fast recrystallization. The resistance does not recover after the end of the optical pulse, indicating the crystallization process, and finally drops, at around 55 ns, within ~300 ps to a stable value of 6 kΩ. This last drop is expected to correspond to the time instant where a full crystallized path across the amorphized cross-section has formed. In Figure 5b we also present the simultaneous change in optical device transmission. During the optical excitation an increase in transmission is observed which, however, starts to relax back right after the end of the pulse. Thus, no nonvolatile change of the optical properties is observed upon this 13


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transition, only a result of the thermo-optical effect, cf. Figure S5. We attribute this to the aforementioned fact that the guided light is most sensitive to the lower part of the nanowire and thus is only negligibly affected by crystallization of the upper part. To prevent damage to the nanowire, in particular to prevent immediate reswitching of the nanowire upon amorphization, we measured in current mode using 0.2 μ A. The corresponding voltage drop across the crystalline nanowire of about 0.6 mV was, however, not sufficient for the observation of the transition from the crystalline to the amorphous state. In summary, we have demonstrated an optical and electrical mixed-mode measurement platform for operating GeTe nanowire-based integrated nonvolatile photonic circuits. Reversible and repeatable switching between amorphous and crystalline states of the GeTe nanowire was realized reliably by evanescent coupling to an optical excitation pulse propagating within the integrated waveguide. The inscribed states of the GeTe nanowire can be identified in both the optical transmission and resistance of the device which demonstrates the potential for the multilevel storage through combination the electrical with optical read-out modes. These results open the way for investigating the interplay between optical and electrical domain for future on-chip optoelectronic integrated circuits. For instance, with new energy efficient routes for crystal-amorphous transformation being discovered,17,21,27 mixed-mode operation of PCM

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can become very attractive for optoelectronic applications in image displays, data storage and logic computing.

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ASSOCIATED CONTENT

Supporting Information Figures S1-S5. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author *E-mail: [email protected] (W.H.P.W) *E-mail: [email protected] (R. A.)

Author Contributions ＃Y. L. and M. S. contributed equally to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by the Deutsche Forschungsgemeinschaft (DFG) through grant PE 1832/2-1 and the National Natural Science Foundation of China (61675107, 61306147), and sponsored by K. C. Wong Magna Fund. This work was supported by the Office of Naval Research (grant N00014-16-1-2350) and National Science Foundation

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(DMR-1210503 and 1505127). The authors want to thank Torsten Scherer for FIB assisted deposition of Pt and Silvia Diewald for assistance in device fabrication.

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(19) Abujetas, D. R.; Paniagua-Domínguez, R.; Sánchez-Gil, J. A. ACS Photonics 2015, 2, 921-929. (20) Gruhler, N.; Benz, C.; Jang, H.; Ahn, J. H.; Danneau, R.; Pernice, W. H. P. Optics Express 2013, 21, 31678-31689. (21) Nukala, P.; Agarwal, R.; Qian, X.; Jang, M. H.; Dhara, S.; Kumar, K.; Johnson, A. T. C.; Li, J.; Agarwal, R. Nano. Lett. 2014, 14, 2201-2209. (22) Fan, Z.; Ho, J. C.; Jacobson, Z. A.; Yerushalmi, R.; Alley, R. L.; Razavi, H.; Javey, A. Nano. Lett. 2008, 8, 20-25. (23) Wang, L.; Meric, I.; Huang, P. Y.; Gao, Q.; Gao, Y.; Tran, H.; Taniguchi, T.; Watanabe, K.; Campos, L. M.; Muller, D. A.; Guo, J.; Kim, P.; Hone, J.; Shepard, K. L.; Dean, C. R. Science 2013, 342, 614-617. (24) Park, J.-W.; Baek, S. H.; Kang, T. D.; Lee, H.; Kang, Y.-S.; Lee, T.-Y.; Suh, D.-S.; Kim, K. J.; Kim, C. K.; Khang, Y. H.; Da Silva, J. L. F.; Wei, S.-H. Appl. Phys. Lett. 2008, 93, 021914-3. (25) Siegrist, T.; Jost, P.; Volker, H.; Woda, M.; Merkelbach, P.; Schlockermann, C.; Wuttig, M. Nat. Mater. 2011, 10, 202-208. (26) Stegmaier, M.; Rı́os, C.; Bhaskaran, H.; Pernice, W. H. P. ACS Photonics 2016, 3, 828-835. 20


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(27) Nam, S. W.; Chung, H. S.; Lo, Y. C.; Qi, L.; Li, J.; Lu, Y.; Johnson, A. T. C.; Jung, Y.; Nukala, P.; Agarwal, R. Science 2012, 336, 1561-1566.

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Figure 1. Operation principle of the phase-change nanowire mixed-mode measurement. (a) Sketch of the on-chip mixed-mode device. The GeTe nanowire is both electrically contacted and evanescently coupled to a photonic waveguide. This enables simultaneous measuring with optical and RF signals. (b) Simulated interaction between the GeTe nanowire and the guided optical light for both the amorphous (upper panel) and crystalline (lower panel) phase state. The high refractive index contrast between the two states results in a significant change of transmitted optical power upon switching which enables sensitive monitoring of the nanowire phase-state by optical means. (c) Measured current-voltage (IV) characteristics of a GeTe nanowire. Upon crystallization, the device resistance drops by over three orders of magnitude. (d) Simulated optical transmission spectrum of the GeTe photonic device for different dimensions of the GeTe nanowire. Resonant Mie-scattering is observed which drastically enhances the evanescent interaction at on-resonance wavelengths in the amorphous phase state. The enhancement of the electromagnetic energy density in the nanowire (inset) additionally increases the absorbed energy (cf. Figure S1) within the nanowire. 22


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Figure 2. GeTe nanowire mixed mode operation nanophotonic device fabrication. (a) Schematic of the GeTe nanowire transfer process: 1) A PDMS stamp is used to mechanically pick up nanowires; 2) Alignment of the chip with the wanted nanowire is achieved using a homemade transfer microscope; 3) The PDMS stamp is lifted off the chip after an annealing process; 4) The PMMA protection layer is removed in acetone. (b) Optical microscope image and (c) tilted view false-color SEM image of the device. The GeTe nanowire is in electrical contact with the Au electrodes with the aid of Pt deposition at both ends of the nanowire.

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Figure 3. Sketch of the mixed-mode measurement setup. The on-chip mixed-mode device, depicted in the center, is characterized simultaneously with an off-chip fiber-coupled pump-probe setup (depicted on the left) and a RF-setup in reflection-type configuration (depicted on the right). While sub-threshold excitation and switching of the nanowire is carried out with nanosecond optical pump pulses, the optical properties are measured by detecting the transmitted probe light. Simultaneously, the device impedance is monitored by DC-biasing the nanowire and measuring the reflected voltage signal time-resolved.

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Figure 4. The time-resolved observation of the switching dynamics and repeatability of the switching operation in the optical domain. The transient transmitted power of the device for (a) Write and Erase optical transmission operations induced by evanescent coupling to 50 ns long pump pulses propagating within the waveguide; (b) Repeated cycles of Write/Erase operation show strong contrast in the measured optical transmission.

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Figure 5. Mixed-mode measurement results. (a) Measured DC resistance during several consecutive Write/Erase-resistance operation cycles. (b) Time-resolved observation of the resistance drop and change of optical transmission during an optically-induced crystallization.

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Table of content figure 94x35mm (300 x 300 DPI)


Mixed-Mode Operation of Hybrid Phase-Change ... - ACS Publications

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