Nonvolatile All-Optical 1 × 2 Switch for Chipscale Photonic Networks

individual switching events occur with transition times below 200 ps and thus hold promise for ultrafast light routing on chip. The approach offers a reliable and simple route toward hybrid reconfigurable photonic devices without the need for electrical contacting.

Integrated chip-level photonics has the potential to revolutionize future computer systems by eliminating the "von-Neumann information bottleneck" and the power losses resulting from the use of electrical interconnects. Yet, the need for optical-to-electrical conversion has so far hindered the implementation of chip-level all-optical routing schemes, which remain operational without continuous power consumption. Here, a crucial component to successful implementation of such all-photonic networks is demonstrated -an effective, practicable all-optical nonvolatile switch. Current integrated all-optical switches require constant bias power to operate, and lose their state when it is removed. By contrast, our switch is entirely nonvolatile, with the direction of light flow altered by switching the phase state of an embedded phase-change cell using 1 ps optical pulses. High on/off switching contrast devices are achieved that are fully integrated and compatible with existing photonic circuits. It is shown that individual switching events occur with transition times below 200 ps and thus hold promise for ultrafast light routing on chip. The approach offers a reliable and simple route toward hybrid reconfigurable photonic devices without the need for electrical contacting.

Introduction
Optical data communication is the most viable solution to overcoming the bandwidth limitations of conventional electronic signaling. While in the past light has been used predominantly for long-distance information transfer, rapid progress in the fabrication of on-chip photonic circuits has now paved FULL PAPER FULL PAPER FULL PAPER properties such as high switching contrast in both the optical and electronic domain, phase transition times down to subnanoseconds, [17] data retention for years, and high cyclability up to 10 12 switching cycles, [18] which make them highly attractive for nonvolatile applications, exploited in particular to date in memory devices. [16,18] In addition, the optical and electrical properties of PCMs have been demonstrated to enable so-called mixed-mode applications, [19] where the PCM is switched electrically affecting the optical domain or vice versa.
Here, we demonstrate an on-chip nonvolatile all-optical switch based on the PCM Ge 2 Sb 2 Te 5 (GST). The device does not require constant power supply to keep its state, in contrast to previously reported all-optical switches. Furthermore, it is fully integrated such that it can be combined with other on-chip photonic circuitry in the future. We demonstrate a nonvolatile switching contrast of 5 dB and provide guidelines on how to increase it further with optimized geometries.

Device Principle
The operation principle of our all-optical switch is presented in Figure 1. The device, as shown in Figure 1a, consists of a nanoscale GST element on top of an on-chip ring resonator that is evanescently coupled to two waveguides. Probe light with off-resonance wavelength is directed to the "THROUGH" port, cf. Figure 1b, while on-resonance light is partially coupled into the ring and exits from there into the "DROP" port. The ratio of power coupled to the DROP and THROUGH port depends on the coupling and attenuation parameters of the ring cavity. For light with on-resonance wavelength (orange marker in Figure 1b) the direction of the light flow can thus be changed by adjusting one of these two parameters. Here, we predominantly change the attenuation within the cavity by switching the GST between its low-loss amorphous and highly absorptive crystalline state. Evanescent interaction between the guided mode and the GST directly influences the electromagnetic phase and the amplitude of the intracavity light. [20] The phase transformations of the GST element, amorphization and crystallization, are initiated with 1 ps optical Write pulses (red marker in Figure 1b), which are also sent into the device via the on-chip photonic circuitry. Since the spatial length of these optical pulses is shorter than the cavity itself, intra-cavity interference does not take place. Therefore, the amount of optical power coupled into the ring does not change when the wavelength of the picosecond pulse is on or off-resonance.

Results
We control the direction of the light flow in our device by switching the GST between the amorphous and crystalline phase state. The transmission spectra of the device in these two states are presented in Figure 2. In the highly absorptive crystalline state (black trace), the loss within the ring resonator exceeds the coupling loss to the waveguide. [21] In this weakly coupled regime, the resonance dip observed in the THROUGH port (upper panel) has a relatively broad full width half maximum (FWHM) of 1.18 nm (corresponding to a Q-factor of 1300) and a moderate extinction ratio of 4.3 dB. Upon amorphization (red trace), the attenuation within the cavity decreases drastically (by about 3 dB) resulting in both a more narrow (FWHM of 0.69 nm, thus Q-factor of 2400) and a deeper (extinction ratio of 9.7 dB) resonance. This drastic change in the resonance shape is due to the fact that now the loss within the cavity matches the coupling loss, i.e., the ring is designed to be critically coupled in the low-loss amorphous state. The resonance dip observed in the DROP port (lower panel) shows the same behavior. While in the crystalline state, due to high attenuation of the intra-cavity light, the height of the resonance is relatively small (−11 dB peak transmission) it considerably increases upon amorphization (−5.1 dB peak www.advopticalmat.de  is embedded in a ring resonator, which is evanescently coupled to two waveguides. b) Depending on the laser wavelength, all optical power is fully directed to the THROUGH port (off-resonance) or divided between DROP and THROUGH port (on-resonance). Therefore, adjusting the probe signal on-resonance (orange laser symbol) enables control of the light flow by changing the phase-state of the GST and thus the characteristics of the optical cavity. transmission). Therefore, light with on-resonance wavelength (here 1562.3 nm) is directed to the THROUGH/DROP port with an insertion loss of −4.3/−5.1 dB by setting the GST in its crystalline/amorphous state. The on-off switching contrast exceeds 5 dB in both ports. Note that in Figure 2 the observed resonance shift upon switching is small, despite the large refractive index contrast between amorphous and crystalline GST states. This is, as explained in more detail in the discussion, mainly due to the fact that the length of the GST-cell is much smaller than the circumference of the cavity. All-optical operation of the switch is presented in Figure 3. The device is probed with 500 ps optical pulses with 2.5 pJ energy and with on-resonance wavelength to achieve maximum on/off switching contrast. Three identical probe pulses are sent into the device (Figure 3a, black trace) separated by an amorphization (I) and crystallization (II) event. In the initial (crystalline) state most of the light is directed to the THROUGH port (central, blue trace). Amorphization (I) of the GST, however, results in a redirection of the light flow to the DROP port (lower, green trace): The optical power transmitted to the DROP port increases by 4.9 dB, while the respective power sent into the THROUGH port decreases by 4.7 dB. The subsequent crystallization (II) of the GST recovers the initial state. This measurement with 500 ps pulses demonstrates that packets with bit rates exceeding 1 Gb s -1 can be routed with our nonvolatile switch. However, due to a photon lifetime of ≈2 ps, derived from the quality factor of the ring in the amorphous state, we expect that considerably higher bit rates (exceeding 100 Gb s -1 ) should be possible.
As sketched in the upper panel of Figure 3b, for amorphization we use two 1 ps pulses of 95 pJ energy (peak pulse power ≈100 W, separated by 25 ns, given by the repetition rate of the pulsed laser). This two-step amorphization scheme enables high switching contrast and avoids any possible damage to the GST layer. The reason for the latter is the strong exponential decay of the optical field along the cell due to the high absorption coefficient of crystalline GST. Thus, a single amorphization pulse strong enough to amorphize the back end of the cell can easily damage its front end. By contrast, crystallization is induced stepwise by a train of pulses with decreasing energy, ranging from 38 down to 19 pJ, as shown in the lower panel of Figure 3b. The pulses are grouped into sequences of 100 pulses each with a fixed repetition time of 25 ns between consecutive pulses.
We monitor the transient behavior of the all-optical switch upon amorphization with an ultra-fast 12 GHz photodetector (New Focus, Model 1554B) and a 6 GHz oscilloscope (Agilent Infiniium 54855A). The transmitted light is first sent through a fiber-coupled optical bandpass filter (Pritel, TFA-1550) to suppress the transmitted pump power. Afterward, the signal is amplified by an erbium-doped fiber amplifier to increase the signal-to-noise ratio encountered at the photodetector. Finally, the signal is again filtered to further suppress the pump power and to remove unwanted amplifier noise. The resulting signal of the THROUGH port is plotted in Figure 3c for an on-resonance probe wavelength and a switching event occurring at time delay 0 ps. Overall, the signal drops within a few hundred picoseconds to a lower transmission level, as expected from an amorphization event. Since this transition takes place within less than 200 ps we expect our measurement to be limited by the timing resolution of the oscilloscope used (6 GHz), in consistency with reported amorphization times of tens of picoseconds. [22] Nevertheless, this measurement demonstrates that our switch can be closed on a sub-nanosecond timescale. Furthermore, we note that thermo-optical effects, [23] which have been shown to relax within ≈1 ns, [14] do not limit the switching time. The reason for this is that upon heating not only the real part of GST's refractive index, but also its imaginary part changes. While a change of the real part results in a shift of the resonance, the respective increase of the imaginary part affects the depth and the width of the resonance dip. Since these two contributions have different (wavelength-dependent) effects on the transmitted probe power, at probe wavelengths slightly off-detuned from (here ≈100 pm below) the resonance wavelength, their respective effects cancel each other out. Note that the initial increase of the signal at time t = 0 is due to freecarrier effects [22,24] and a not completely suppressed pump pulse (whose power exceeds the probe power by seven orders of magnitude).
Finally, we demonstrate high reproducibility of the all-optical operation of our device. In Figure 4a, the transmitted power at the THROUGH port is plotted upon completing ten full switching cycles. Each switching cycle consists, as explained in Figure 3b, of one amorphization step (two 1 ps pulses) and six crystallization steps (100 ps pulses each). As can be seen from the observed transmission values, our switching scheme allows highly reproducible operation between the on-and off state. Furthermore, the data suggest that, similarly to our previous results, [14] intermediate states can also reliably be accessed. We have further carried out successive operation up to 1000 cycles, as shown in Figure 4b, without considerable reduction in switching contrast. Here, contrast is defined as the ratio of the transmitted powers in the two states. We observe a conditioning process within the first few switching cycles during which the reproducibility of the device operation stabilizes.

Discussion
Our results demonstrate that a single picosecond laser source with adjustable output power is sufficient to reliably redirect the light flow within the presented nonvolatile all-optical switch, i.e., to induce both amorphization and crystallization. The 1 ps pulses employed in our experiments simplify device operation because matching of the laser wavelength to a cavity resonance is not required. This is in particular interesting for cavities which exhibit a considerable resonance shift upon switching. The achieved device performances in terms of transition time, switching energy, and operational contrast, demonstrate the feasibility of using PCMs for nonvolatile photonic devices. While at the current stage of development the switching contrast may not be high enough for implementation into real photonic communication systems, performance in this respect, and indeed in terms of switching energy and speed, could be improved by various design and operational adjustments, as we outline below. The demonstrated accumulated switching times of 25 ns (amorphization) and 15 μs (crystallization) can be reduced by increasing the repetition rate of the ps-laser system (here 40 MHz). By doing so, our two-step amorphization scheme can be carried out on a sub-nanosecond timescale due to the fast 200 ps transition time of a single event (Figure 3c). By contrast, the crystallization time can be reduced to the nanosecond regime. Here, the high number of pulses, which is not ideal in terms of switching time and energy consumption, is necessary since the heat generated by a single picosecond pulse diffuses away too fast to induce more than a small partial recrystallization of the GST-cell. [24] Thus, the number of recrystallization pulses can be reduced by increasing the repetition rate, because with decreasing pulse separation the heat accumulation within the GST-cell is increased. Furthermore, proper heat management within the device can prolong the thermal relaxation process such that less crystallization steps are required, even single-shot crystallization with picosecond pulses can become possible. [25] An alternative route toward shorter crystallization times is to use picosecond pulses for amorphization only and nanosecond pulses [14] to induce crystallization instead.
The measured switching contrast of ≈5 dB is currently limited by the extinction ratio in the amorphous state (≈10 dB) and the small resonance shift upon switching. By using a modified device design, e.g., a slightly different GST cell length, the ring resonator can hit the point of critical coupling more closely and thus provide an improved extinction ratio (up to 20 dB or even more). The resonance shift upon switching, Δλ r , could also be increased (since the probing is performed at a single wavelength, an increase of the resonance shift can dramatically increase the switching contrast, and at the same time reduce the insertion loss), by, for example, using longer GST cells and/ or shorter cavities to further improving the switching contrast. This can be understood by studying the resonance condition of the ring resonator Here, λ r,0 denotes the resonance wavelength before the switching, n eff,GST,0 the effective refractive index of the GSTcell in the initial phase state, Δn eff,GST the respective change upon switching, n g,eff the effective group index of the bare waveguide, L GST the length of the GST-cell, and R the radius of the ring. Longer GST-cells can be directly realized following our fabrication scheme. Alternatively, high-Q ring resonators can particularly be fabricated in high refractive index materials such as silicon, where ring radii down to a few μm have been demonstrated. [26] Furthermore, small mode volume can be achieved in photonic crystal cavities regardless of the material.
A major benefit of our device here is its nonvolatility -once set into a state no further power supply is required to maintain that state. To the best of our knowledge, this is the first fully integrated on-chip all-optical switch with this property. Our switching scheme requires accumulated 180 pJ and 17 nJ of pulse energy to induce amorphization and crystallization, respectively. In particular, the energy for crystallization can in the future significantly be reduced by using longer pulses or higher repetition rates to enable efficient heat accumulation within the cell during the pulse sequence. Both of these energies can be reduced by using longer (at least a few picoseconds) pulses with on-resonance wavelength, which are more efficiently coupled into the ring and thus absorbed by the PCM cell. With a critical coupling configuration, which is also desirable to maximize the switching contrast, perfect absorption of the incident pulse can be achieved. In accordance with our previous results, we expect that switching energies down to tens of pJ are possible. [14] In photonic crystal cavities the small mode www.advopticalmat.de Adv. Optical Mater. 2017, 5, 1600346 www.advancedsciencenews.com volume would enhance the resonance shift upon switching and thus the volume of the GST-cell could be further reduced to reach switching energies on the order of a few pJ. [20] In comparison, all-optical volatile switches have been demonstrated to consume just a few fJ per bit. [10] However, they also require a bias power of typically a few μW to keep a state after switching. Therefore, in applications with relatively low-rate switching operation, such as packet-switching or reconfigurable on-chip and chip-chip routing, our nonvolatile device enables considerable lower overall power consumption while still supporting prospective bit-rates up to 100 Gb s −1 .

Conclusion
We have demonstrated, for the very first time, that phase-change materials can be used in an on-chip photonic cavity to reliably operate a nonvolatile all-optical 1 × 2 switch with a single 1 ps laser source. Within the device, the light-flow is directed to one of two ports with low insertion loss and high switching contrast in both ports. Transition times of less than 200 ps hold promise for ultrafast, down to sub-nanosecond, switching times. Since, in contrast to previously reported all-optical switches, the device keeps its state after switching without an optical bias power, it offers significant lower power consumption for all-optical packet-switching, routing to multicore architectures and reconfigurable on-chip photonic networks. Therefore, these results hold promise for future on-chip all-optical networks for highspeed data communication.

Experimental Section
Fiber-Optic Setup: The on-chip device was characterized with an off-chip optical fiber setup. Light was coupled into and out of the chip with focusing grating couplers which were optimized for light around 1550 nm. The transmission spectra were recorded with a tunable continuous wave (CW) laser (Santec, TSL-510C) in combination with low-noise photodetectors (New Focus, Model 2011). For pulsed probing of the device 500 ps pulses were generated from the CW laser with an electrooptical modulator (Lucent Technologies, 2623CS), which was controlled by a 500 MHz electrical pulse generator (HP, 8131A). The GST was switched with a fiber-coupled femtosecond laser source, which emitted 1 ps optical pulses at a repetition rate of 40 MHz (Pritel, FFT). A homemade pulse picker, an acousto-optical modulator (Gooch & Housego, Fiber-Q T-M040) connected to a 500 MHz electrical pulse generator (HP, 8131A), was used to control the pulse sequence sent into the device. The pulse generator, which was synchronized to the modelocked fiber laser via its trigger input, enabled precise control of the opening level and opening time of the acousto-optical modulator and thus the pulse number and pulse power. Before the pulses were sent into the device they were further amplified with an erbium-doped fiber amplifier (Pritel, LNHPFA-33).
Device Fabrication: Our device was fabricated in a multistep electron-beam lithography procedure [27] followed by GST sputter deposition and lift-off. [28] First, the photonic circuits were fabricated from a 330 nm thick silicon nitride-on-insulator layer using electronbeam lithography and subsequent dry etching. The waveguides were fully etched down to the oxide buffer layer. To ensure single-mode (TE-like) operation all waveguides were designed to be 1.1 μm wide. The ring had a radius of 30 μm and the gaps between the ring and the waveguides were designed to be 150 and 200 nm for the lower and upper gap (see Figure 1b), respectively. The corresponding cross-coupling coefficients were estimated from reference devices to be κ (gap = 150 nm) ≈ 0.46 and κ (gap = 200 nm) ≈ 0.34. This imbalance between the two gaps resulted in critical coupling in the amorphous phase-state. [21] While the unloaded Q-factor of the ring resonator is expected from reference measurements to be on the order of 10 5 , the loaded quality-factor of the device without GSTelement was measured to be 3900 due to strong coupling with the two adjacent waveguides. After dry etching, opening windows with a footprint of 600 × 750 nm 2 for the PCM-sections were defined on top of the waveguides in a further lithography step with positive tone Polymethylmethacrylat resist. Subsequently, 20 nm of GST and 10 nm of indium tin oxide (ITO) were sputter deposited and lift-off. The ITO was used to cap the GST in order to prevent oxidation. Eventually, the chip was placed for 10 min on a hotplate at 200 °C to crystallize the as-deposited amorphous GST.