We demonstrated a novel two-dimensional photonic crystal (PC) based Symmetric Mach Zehnder type all-optical switch (PC-SMZ) with InAs quantum dots (QDs) acting as a nonlinear phase-shift source. The 600-µm-long PC-SMZ having integrated wavelength-selective PC-based directional couplers and other PC components exhibited a 15-ps-wide switching-window with 2-ps rise/fall time at a wavelength of 1.3 µm. Nonlinear optical phase shift in the 500-µm-long straight PC waveguide was also achieved at sufficiently low optical-energy (e.g., π phase shift at ~100-fJ control-pulse energy) due to the small saturation energy density of the QDs, which is enhanced in the PC waveguide, without using conventional measures such as SOAs with current-injected gain. The results pave the way to novel PC- and QD-based photonic integrated circuits including multiple PC-SMZs and other novel functional devices.
©2004 Optical Society of America
Two-dimensional photonic crystal (2DPC) waveguides are promising devices for novel applications in miniaturized photonic integrated circuits (PICs) [1–3] because of their unique features, namely, strong dispersion and the appearance of a band-gap [4, 5]. However, until now most work has concentrated on micro-lasers and passive components such as optical filters and couplers [6, 7]. All-optical switches for future photonic networks networks are likely to be a promising candidate for 2DPC applications, because the switches require not only ultrafast response and low optical-energy consumption but also the ability to integrate multiple channels [8, 9]. Although some types of 2DPC switches based on nonoptical processes have been reported recently, their switching speeds were relatively low, on the order of milliseconds to microseconds, due to thermo-optic effect  or an infiltrated nematic liquid crystal . On the other hand, ultrafast response phenomena (~10 ps) based on nonlinear optical (NLO) processes in 2DPCs have been reported due to strong nonradiative recombination of the photocarriers at etched holes  or due to the fast shifting of the wavelength of a PC resonance via carrier induced nonlinear index . However, their experimental configurations may not be appropriate for practical use as actual switching devices.
Recently, we have proposed a novel 2DPC-based all-optical switch aimed at applications, in future photonic networks, e.g. an ultrafast time-division demultiplexer. The switch is a GaAs-based 2DPC Symmetric Mach Zehnder switch (PC-SMZ)  with InAs quantum dots (QDs) acting as a nonlinear phase-shift source . The purpose of this paper is to experimentally demonstrate the low-energy operation and the fast switching response of the PC-SMZ.
2. Proposed PC-SMZ
Figure 1(a) shows a proposed PC-SMZ interferometric all-optical switch with InAs QDs in nonlinear optical (NLO) waveguides (WGs) . Figure 1(b) schematically shows a control pulse (CP) inducing saturation absorption in the assembled QDs, as shown in Fig. 1(d), where the origin of the NLO-induced phase shift is shown . A signal pulse (SP) detuned from the CP experiences a corresponding refractive index change, resulting in a phase shift in the interferometer. The SP appears or disappears at the output port depending on the phase difference between the two NLO-WGs, thus exhibiting demultiplexing behaviour. The PC-SMZ has four 2DPC-based directional couplers (DCs) : Two of them (indicated by a- PCDC in Fig. 1 (b)) are identical 3-dB couplers at the beam-splitting and beam-joining points. Another two DCs (b-PCDC) are located at the junctions of the SP and CP. The b-PCDCs are designed as wavelength-selective couplers that sufficiently couple both SP and CP into the NLO-WG. The ultra-fast response is based on the same principle as in the conventional SMZ switches ; that is, the time-differential phase modulation induced by two CPs (ON-CP and OFF-CP in the figure) cancels a slow phase-decay component originating from the long carrier lifetime, as shown in Fig. 1(c). Indeed, demultiplexing from 332 Gbit/s down to 42 Gbit/s has been demonstrated in a conventional SMZ switch . In addition, low switching-energy operation is also expected for the PC-SMZ because of the small saturation energy density (13 fJ/µm2) of the InAs QDs . In addition, the NLO-induced phase shift is enhanced in the PC-WG because of a strong confinement and the effect of an intentionally selected slow wave with a low group velocity. These enhancement effects contribute to the reduction of both the CP energy and the NLO-WG length that generates the phase shift necessary for the switch . Moreover, due to the naturally small size of the PC structure, the switch enables monolithic integration of multiple switches in a region smaller than 1 mm2, two orders of magnitude smaller than conventional multiple switches hybrid-integrated on planar lightwave circuits [8, 9].
For the purpose to experimentally demonstrate the low-energy operation and the fast switching response of the PC-SMZ, we prepared two samples: one was a 500-µm-long straight PC-WG to measure the NLO-induced phase shift, and the other was a 600-µm-long PC-SMZ chip to demonstrate the switching operation. The samples were composed of air-bridge-type 2DPC slabs with a single missing line in a hexagonal triangular lattice (lattice constant, a=350 nm) of air holes (radius, r=110 nm). Here, a 250-mm-thick GaAs core layer with three stacked layers of InAs QDs was grown on top of a 2-µm-thick Al0.6Ga0.4As sacrificial clad layer on a GaAs substrate by molecular beam epitaxy. The QDs were formed in Stranski-Krastanov mode growth by a two-step growth technique . The QD sheet density of each layer was 3.5×1010 cm-2. The peak wavelength and the full-width half-maximum of the QDs in the measured photoluminescence spectrum were 1285 nm and 30 meV, respectively. Air-bridge PC structures were fabricated using high-resolution electron-beam lithography, dry etching, and selective wet-etching techniques . They were cleaved to lengths of 500–600 µm. This fabrication technique could reduce the propagation loss to 0.76 dB/mm even for a 10-mm-long 2DPC sample without QDs ; this length is sufficient to integrate multiple switching elements, as shown in Fig. 1(a), in future photonic integrated circuits.
The NLO-induced properties of concern here depend on the CP energy, and the detuning energy and delay time between CP and SP. We assembled a two-color pump/probe system having two electrically synchronized short-pulse lasers at 80-MHz repetition rate and individual delay lines [15, 19]. An SP and a CP were collinearly coupled into a waveguide sample. The NLO-induced phase shift was measured by lock-in-based heterodyne detection in cross correlation between an SP with an NLO-induced change and a reference pulse without a change, individually frequency-modulated by an acoustic optical modulator (AOM) . The group velocity was also measured by the heterodyne detection based on a time-of-flight technique. Then, the group velocity was derived from the ratio of the pulse propagation time in the sample to that in air. The switching-window was measured based on the two-color pump/probe system, and then the CP was divided into two optical paths with individual delay lines and attenuators to form ON-CP and OFF-CP. One SP and two CPs were simultaneously coupled into each input port by using a zoom lens system.
4. Results and discussion
4.1 NLO-induced phase shift
First, the enhancement effect of the NLO-induced phase shift in the PC-WG with QDs is shown. Figure 2 shows the decay characteristics of the NLO-induced phase shift (red circles) and amplitude change (blue circles) of the SP. The peak wavelength and pulse width were respectively set to 1285 nm and 2 ps for the CP, for pumping, and to 1295 nm and 0.2 ps for the SP. Net pulse energies coupled into the waveguide were determined to be 20 fJ/pulse for the CP and 0.6 fJ/pulse for the SP, estimated by the monitored input and output energies at these wavelengths under transparent excitation by an intense pulse. A maximum phase shift of 55 degree was obtained. It should be noted that both decay characteristics exhibited a similar exponential time constant of 60 ps, as indicated by the broken line, which suggests that the phase shift originates from the saturation absorption in the InAs QDs , as schematically shown in Fig. 1(d).
Figure 3 shows the NLO-induced phase shift as a function of the peak wavelength of the SP measured under similar conditions. Here, the red and green closed circles indicate the phase shifts for the PC waveguide and the slab region without PC structure shown in the inset, respectively. The red and green solid curves show the corresponding group indices which defined by the reciprocal of the measured group velocity . As estimated from the band calculation, the group index rapidly increased due to the large dispersion near 1325 nm at the band edge of the single-missing-line defect mode. A similar dependence was reported in Fabry-Perot interference measurements . The large group index for the SP enhanced the phase shift due to the intensified electric field, as seen in the PC waveguide case. Furthermore, the strong optical-beam confinement in the vertical direction (~0.9) induced by the high-index contrast in the air-bridge structure also enhances the phase shift, which affects both SP and CP. Thus, the enhanced electric field of the pulses intensifies the interaction with the electronic state in the QDs.
Figure 4 shows the CP energy dependence of the NLO-induced phase shift (red circles) for the SP at 1325 nm. The blue circles show a similar dependence for a 1-mm-long non-PC ridge-waveguide with similar QDs . The CP energy to produce a 0.5π phase shift in the PC waveguide is more than three orders of magnitude lower than that in the non-PC ridge waveguide. This is due not only to the enhancement effects mentioned above but also to the narrow cross-section of the PC waveguide. It should be noted that a π phase shift in the PC-WG was obtained at a CP energy less than 100 fJ. To the best of our knowledge, this is the best efficiency obtained in an all-optical process without the help of semiconductor optical amplifiers (SOAs) with current-injected gain. This means that a structure combining QDs with a PC-WG results in a promising NLO device for novel applications.
4.2 Optical switching responses of the PC-SMZ
Next, optical switching responses of the PC-SMZ are shown. Figures 5(a) and 5(b) show a schematic diagram and an optical micrograph of the fabricated PC-SMZ, respectively. The structure includes two types of PC-DCs, 300-µm-long NLO WGs, 60-degree bends, three input ports, and two output ports in the 600-µm-long sample. In this experiment, the InAs QDs were grown in the entire area. The ‘bar’ output port is a normally OFF port for the SP and acts as a demultiplexing port, while the ‘cross’ port is a normally ON port. Figures 5(c) and 5(d) show SEM (scanning electron microscope) photographs of the input DCs and output cleaved edge, respectively.
Figure 6 shows the decay characteristics of the SP transmission measured at the bar port as a function of the time delay between the SP and CP. The peak wavelengths of the CP and SP were 1285 nm and 1305 nm, respectively. The ON-CP and OFF-CP energies coupled into the waveguide were estimated to be ~100 fJ. In the case of only ON-CP excitation, the transmitted SP energy slowly decayed due to the slow carrier relaxation in the upper NLO-WG shown in Fig. 3(a). It should be noted that, after the OFF-CP was introduced at the CP-off port 27 ps after the ON-CP, the slow decay component was successfully cancelled as a result of the SMZ switching principle , as shown in the red curve indicated by ‘ON-CP/ OFF-CP’. This is the first known observation of a switching-window demonstrated using a PC-based optical switch. Since the switching-window works as a filter in the time domain, the SP intensity can be modulated depending on the switching-window shape, resulting in demultiplexing. Here, the measured rise and fall times were ~2 ps, which was limited by timing jitter between the electrically synchronized SP and CP.
Figure 7(a) shows the switching-window measured at the bar and cross ports. It was confirmed that about half the energy in the cross port was transferred to the bar port in the switching-window, as expected by Mach-Zehnder interference, resulting in an extinction ratio of ~5 dB at the bar port. The transferred energy was smaller than that expected from Figs. 3 and 4, which suggest almost full energy transfer. This is mainly because a large amount of the CP energy was absorbed by the QDs before reaching the NLO-WG. Therefore, the extinction ratio and the switching energy can be improved by using a sample with the QDs formed only in the NLO-WG region .
Finally, we will refer to the tunability of the switching-window width, which directly limits the available transmission bit rate when used as a demultiplexer. Figure 7(b) shows measured switching-windows with widths of 35, 25 and 15 ps. The width of 15 ps corresponds to more than 40 Gbit/s. This figure also verifies that the width of the switching window can be intentionally controlled by changing the time delay between the ON-CP and OFF-CP. The switching windows with abrupt rise/fall times mean that all of the PC-based functional elements, including the PC-DCs designed to have wavelength selectivity for the SP and CP, were successfully operated.
We demonstrated a novel two-dimensional PC-SMZ with InAs QDs acting as a nonlinear phase-shift source. The 600-µm-long PC-SMZ having integrated wavelength-selective PC-based directional couplers and other PC components exhibited a 15-ps-wide switching-window with 2-ps rise/fall time at a wavelength of 1.3 µm. Nonlinear optical phase shift in the 500-µm-long straight PC waveguide was also achieved at sufficiently low optical-energy (e.g., π phase shift at ~100-fJ control-pulse energy) due to the small saturation energy density of the QDs, which is enhanced in the PC waveguide, without using conventional measures such as SOAs with current-injected gain. The switch enables monolithic integration of multiple switches in a region smaller than 1 mm2, two orders of magnitude smaller than conventional switches. Taking into account the fine fabrication technology used, which has already enabled low propagation loss (~0.76 dB/mm) and the successful integration of several PC elements, these results indicate the feasibility of PC- and QD-based monolithically integrated photonic circuits for multi-channel demultiplexers and other functional devices in future photonic networks.
This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan within the framework of the Femtosecond Technology Project.
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