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We develop a gallium arsenide (GaAs) photonic crystal nanocavity device capable of capturing and releasing a pulse of light by dynamic control of the Q factor through free carrier photoexcitation. Unlike silicon-based devices where the performance of this dynamic optical control is limited by absorption from free carriers with nanosecond-order lifetimes, the short carrier lifetime (∼ 7 ps) of our equivalent GaAs devices enables dynamic control with negligible absorption losses. We capture a 4 ps optical pulse by briefly cycling the Q factor from 40,000 to 7900 and back just as the light couples to the nanocavity and confirm that the captured energy can be subsequently released on demand by a second injection of free carriers. Demonstrating dynamic control with negligible loss in a GaAs nanophotonic device also opens the door to dynamic control of cavity quantum electrodynamics with potential application towards quantum information processing.
© 2014 Optical Society of America
1. Introduction
Chip-based nanophotonic devices, such as 2D photonic crystals (PCs), can confine light to within wavelength-order dimensions with extremely low loss [1–5]. As the photon lifetime or group delay in silicon-based photonic devices has become longer, there have been several efforts to dynamically change their properties while interacting with light in order to realize novel concepts of adaptive bandwidth and adiabatic transition [6–9]. These concepts have led to demonstrations of catch and release of pulses [10–12], wavelength conversion [13–15], optical switching [16] and the manipulation of strongly coupled states [17]. These rapid changes of the device’s refractive index are performed by photo-excitation of free carriers using short optical pulses, which change the plasma frequency and thus the refractive index of the medium [18]. As free carriers are excited extremely quickly and have nanosecond order lifetimes in silicon slabs, this produces a rapid change of index that can then thereafter be approximated as constant unless the photon interaction time approaches the carrier lifetime. Therefore, this method can ‘turn on’ the desired dynamic change. Unfortunately, the excitation of free carriers also ‘turns on’ loss due to free carrier absorption, which persists throughout the storage or transfer of light. While such losses can be mitigated by spatially isolating the carrier injected region from the light storing region [10,12], carrier losses remain the limiting factor on many such systems [11]. The electrical injection of carriers into photonic crystals by p-i-n junction [19] can also achieve dynamic control over photons provided the interaction time is long enough (on the order of nanoseconds). However, the same carrier absorption problem persists for these devices as well.
Clearly there is need for a dynamic control technique that preserves the concept of changing refractive index within the photon-device interaction time while suppressing the impact of carrier absorption. Beyond the applications described above, this would permit dynamic control at the single photon regime, enabling systems for solid-state photonic quantum computation [20]. In this work, we investigate dynamic control by a ‘momentary’ refractive index change, where a short duration refractive index change is utilized to efficiently capture a signal pulse in a nanocavity. A material with very short carrier lifetime excited by a short light pulse can realize this by temporally increasing the coupling bandwidth at the instance of signal pulse arrival while suppressing the impact of carrier absorption to negligible levels during the following storage time.
2. Controlling Q with short-lived carriers
Consider a photonic crystal nanocavity device used to demonstrate dynamic control over the total Q factor (QTotal) (schematic in Fig. 1(a)) by manipulating the phase difference (θ) between the two optical paths by which light couples back from the nanocavity into the waveguide according to
where Qin and Qv quantify the coupling between the nanocavity and the waveguide in the absence of any interference and the coupling between the nanocavity and to out of plane free space modes, respectively [7]. Control of the Q factor through θ is achieved as follows: the initial Q condition is thermally tuned by a GaN continuous laser source (405 nm) shone onto the waveguide between the nanocavity and the heterointerface, while dynamic changes are achieved by rapid photo-excitation of free carriers in the same waveguide region by a 4 ps control pulse (775 nm). When properly timed, this control pulse can dynamically adjust the cavity Q factor such that a 4 ps signal pulse (1550 nm) can couple easily into the nanocavity in the low Q state and then be held there in the high Q state [7, 9, 10]. In silicon, where the carrier lifetime is long (Fig. 1(b)), the system initially has a low Q, then carriers are photo-excited by the control pulse to increase the Q factor through interference just as the signal pulse enters the nanocavity. Light that coupled to the nanocavity in the low Q state is suddenly captured in the high Q state, but is also subject to absorption. We propose instead beginning with the system in a high Q state (Fig. 1(c)) and quickly decreasing the Q factor for only as long as necessary to couple the signal pulse to the nanocavity, then having the system rapidly return to the initial high Q state as the carriers rapidly decay. The dynamic event would lower Q rather then increase it and carrier absorption would only occur during the short-lived low Q condition.
Fig. 1 a) Schematic of interference-based dynamic Q control using photo-excited free-carriers and thermal tuning. b) When the free carrier lifetime is long, initially the in-plane Q is set to be low, then is increased by free carriers, thus carrier losses are a factor throughout the high Q capture state. c) If the free carrier lifetime is short, an initially high Q system could be lowered by free carriers briefly then return to the high Q state, limiting carrier losses to only a short window when Q was low.
To realize this functionality, we considered materials with short carrier lifetimes that could be used as a medium for our 2D PC slab dynamic Q design. Gallium Arsenide (GaAs) is a sound medium for 2D slab PCs [21] with an established fabrication process. Moreover, the integration of quantum dots to produce cavity quantum electrodynamic systems [22–24] could lead to photonic quantum computation [20]. GaAs can have a significantly higher surface recombination rate for excited carriers than silicon. While the carrier recombination rate at the surface of silicon is only ∼ 104 cm/s [25] for the carrier densities of 1017 – 1018 cm−3 excited by our dynamic control system, GaAs has a recombination rate of ∼ 106 cm/s at similar densities [26]. Given the high surface to volume ratio of 2D PC membranes, the higher surface recombination rate should markedly reduce the lifetime of excited carriers, and could provide the short-lived temporal control desired. Furthermore, as the Q factor has been shown to manipulate the emission of quantum dots coupled to a PC nanocavity [27], this could also lead to incorporating dynamic Q control into cavity quantum electrodynamics.
3. Sample fabrication
Transferring this PC design from Si to GaAs is straightforward as the refractive index of GaAs at 1550 nm is 3.4, very close to that of silicon. The direct electronic bandgap of GaAs is of similar magnitude to the indirect band gap of silicon (1.4 eV and 1.1 eV, respectively), suggesting that the same 775 nm pulse laser and 405 nm continuous laser will still be effective for dynamic and static control.
The sample substrates were fabricated by molecular beam epitaxy and consist of 250 nm of GaAs on a 1000 nm Aluminium Gallium Arsenide sacrificial layer (Al0.7Ga0.3As) above a thick GaAs substrate. The PC pattern was written on a coat of resist by electron beam lithography, then transferred to the GaAs thin film using Hydrogen Iodide and Xenon plasma [28]. The base was polished to thin the total sample to 150 ∼ 200 μm then cleaved. Finally the sacrificial layer was undercut by a Hydrochloric acid wet etch. GaAs is less rigid than silicon and the surface stresses on the air-suspended samples led to the PC region frequently bowing or breaking. To relieve strain on the device, slits were etched around along the PC boundaries. Figure 2(a) shows the PC device bordered by slits to prevent surface strain from damaging the suspended device.
Fig. 2 a) SEM image of a GaAs single waveguide dynamic Q sample bordered by slits to reduce surface strain. Lattice constant a = 430 nm and hole radius of 0.29a. The line defect is a W1 waveguide and the cavity is an L3 with shifted edge holes. The hetero-interface mirror is 80a past the cavity along the waveguide (not shown). b) Thermal tuning of Q factor. A CW GaN laser thermally tunes the static Q factor ranging from 4000 to > 12, 000 by changing θ.
4. Dyamic Q control in a GaAs PC
To characterize the dynamic Q range, a GaN continuous laser source (405 nm) was shone onto the waveguide between the nanocavity and heterointerface in order to confirm that the Q factor’s initial condition could be thermally tuned. The vertically emitted spectrum from the cavity as a function of GaN power (Fig. 2(b)) exhibits the representative cyclical change of Q for this device and suggests a Q factor range from 4000 to beyond 12,000 for this particular sample. The resonance wavelength also changes cyclically with the phase difference [3, 27]. Using a sample of similar design but with resonant wavelength closer to 1550 nm, we set θ to π (Q high) by thermal tuning, then performed pump-probe measurements to catch a signal pulse (4 ps, 1557 nm, spectral width 1.1 nm) within the cavity. A control pulse (4 ps, 778nm) was set to a pulse energy of 30 pJ to produce a phase shift of ∼ π with the intention of lowering Q to permit photons to enter the cavity and then be held there as the free carriers quickly decayed. The time-integrated vertical emission from the cavity as a function of the relative pulse timing is shown in Fig. 3(a). When the control pulse was early or late relative to the signal pulse, the amount of light observed vertically escaping the cavity was small, while simultaneous arrival produced a pronounced increase of the vertical emission. The decay of the vertical emission peak as the signal pulse is delayed confirms that the low Q state is short-lived after pump pulse irradiation. Coupled mode theory simulations of the device were fitted to this response (blue curve), suggesting the carrier lifetime in the device was ∼ 7 ps, almost 3 orders of magnitude faster than that the 1.6 ns observed in silicon [11] and short enough to capture photons dynamically in the cavity with this alternative dynamic Q approach.
Fig. 3 a) Nanocavity vertical emission response to pump-probe measurements with signal and control pulses. The spike and rapid drop-off of the emission indicates that by lowering Q the carriers improve the signal coupling to the cavity and that their short (7 ps) lifetime allow the cavity to quickly return to its high Q value. b) Emission spectra for different Q conditions suggest that the signal pulse is better captured by the well-timed, rapid lowering of Q than in the static high or low state.
To confirm that this increase in vertical emission was indeed caused by dynamic Q control, we measured the spectrum of the signal pulse being vertically emitted from the nano cavity under various conditions (Fig. 3(b). When the Q value was thermally set to be static and low (black), Q value was estimated from the vertical emission to be ∼ 2200. When Q was tuned to a high value and the control pulse was absent (blue), the resonant wavelength shifted and the peak appeared to narrow, though it was difficult to measure the high Q. When the control pulse is correctly timed to capture the signal pulse in the nanocavity(red), the vertical emission increased significantly, with Q reaching 11, 100. This suggests that the 4 ps signal pulse is captured in the cavity by the dynamic Q factor control and its 1.1 nm spectral width is adiabatically compressed to 0.14 nm.
Having established that pulse capture could be performed with a short carrier lifetime, we turned to time-resolved measurements of the cavity energy behaviour to examine the capture state more closely and to investigate the consequent impact of carrier losses. We employed a homodyne system [10] to take time-resolved measurements of the cavity vertical emission and consequently the evolution of the cavity energy. These experiments were performed with a sample of the same design as shown in Fig. 2(a), however additional precautions were taken to limit the oxidation of the sample, such as flushing the device with N2 gas during characterisation. Figure 4(a) shows time-resolved measurements of the device taken when thermo-optically tuned to a static low Q (red), high Q (blue) and when dynamically changing to execute pulse capture (black). These results provide a compelling demonstration of successful pulse capture by dynamic Q control. Under the static high Q state, little of the pulse can effectively couple to the cavity mode because of spectral mismatch, but the light exhibits a long photon lifetime of 33 ps (Q=40, 000). Conversely, the static low Q state shows much better coupling to the cavity mode because its 6.5 ps photon lifetime (Q=7900) is better matched to the 4 ps signal pulse, but the light escapes the cavity just as easily. By dynamically lowering the Q factor from 40, 000 to 7900 briefly, we combine the advantages of both static states: The coupling efficiency of the low Q state and the long cavity lifetime of the high Q state. The relative magnitudes of the vertical emission signifies that four times more signal pulse energy is captured in the dynamic Q case than in the static, high Q case. The small, initial drop in emission in the dynamic catch caseis entirely due to the roundtrip time of the light to travel to the hetero-interface and back to establish the capture condition (∼ 10 ps) and is unrelated to absorption [10]. Within this time, the free carriers have decayed to the point that absorption cannot be detected, as made evident by the photon lifetime being identical to the static, high Q case.
Fig. 4 a) Time-resolved vertical emission under three cases: Static low Q (red), static high Q (blue) and dynamically lowered Q achieving pulse capture (black). b) Light captured in nanocavity released at various times by a second control pulse
Next we ensure that even with this short carrier lifetime, captured light can also be subsequently released. Provided the Q factor can be dynamically lowered far enough to have light couple out of the cavity faster than the 7 ps carrier lifetime, dynamic release on demand should also be possible. We confirm this in Fig. 4(b) by applying a second control pulse to captured light to lower Q again at various times, which releases the energy held in the cavity back into the waveguide. These time-resolved measurements of pulse catch and release in the PC nanocavity are equivalent to those demonstrated in silicon-based structures [10, 11] with the notable absence of free carrier absorption limiting the Q factor, and thus photon lifetime in the cavity, during pulse capture.
5. Summary
In conclusion, we have developed a method for dynamically capturing light in a PC nanocavity using the photo-excitation of short-lived free carriers and experimentally demonstrated its success in GaAs based 2D PCs with carrier lifetimes of only 7 ps. Optical characterization shows that a device’s initial conditions can be thermally tuned to a high Q condition (at least 40,000) then temporarily lowered just long enough to capture a 4 ps optical pulse for as long as 33 ps. The short-lived carriers remove the limitation of carrier absorption suffered by silicon-based dynamic optical nanostructures. Carriers can be excited to briefly lower the Q factor for optimal coupling of an input pulse then quickly decay to returns to maximum Q without measurable losses. Unlike silicon devices that are limited by absorption and the long carrier lifetime, in equivalent GaAs-based devices only the maximum achievable Q factor dictates how long light can be dynamically held before release on demand. Improving the performance of these GaAs-based dynamic Q factor PC devices must involve the incorporation of higher Q nano cavities and the capability to directly observe the light dynamically released from the system. Exploiting short-lived carriers removes a major performance limitation for dynamic nanophotonics devices. Furthermore, its demonstration in GaAs PCs could enable dynamic control of nancavities coupled to quantum dots, bringing dynamic control with minimal losses to cavity quantum electrodynamics for potential application to quantum information processing.
Acknowledgments
This work was supported mainly by Grant-in-Aid for Scientific Research (S) and partially by the Global COE Program of MEXT, Japan, and also partly by JSPS through the FIRST Program initiated by CSTP.
References and links
1. S. Noda, A. Chutinan, and M. Imada, “Trapping and emission of photons by a single defect in a photonic bandgap structure,” Nature 407, 608–610 (2000). [CrossRef] [PubMed]
2. Y. Akahane, T. Asano, B.S. Song, and S. Noda, “High-Q photonic nanocavity in a two-dimensional photonic crystal,” Nature 425, 944–947 (2003). [CrossRef] [PubMed]
3. B.S. Song, T Asano, Y. Akahane, and S. Noda, “Role of interfaces in heterophotonic crystals for manipulation of photons,” Phys. Rev. B 71, 195101 (2005). [CrossRef]
4. T. Baba, “Slow light in photonics crystals,” Nat. Photon. 2, 465–473 (2008). [CrossRef]
5. L. O’Faolain, S.A. Schulz, D.M. Beggs, T.P. White, M. Spasenović, L. Kuipers, F. Morichetti, A. Melloni, S. Mazoyer, J.P. Hugonin, P. Lalanne, and T. Krauss, “Loss engineered slow light waveguides,” Opt. Express 18, 27627–27638 (2010). [CrossRef]
6. V. R. Almeida, C. A. Barios, R. R. Panepucci, and M. Lipson, “All-optical control of light on a silicon chip,” Nature 431, 1081–1084 (2004). [CrossRef] [PubMed]
7. Y. Tanaka, J. Upham, T. Nagashima, T. Sugiya, T. Asano, and S. Noda, “Dynamic control of the Q factor in a photonic crystal nanocavity,” Nature Mater. 6, 862–865 (2007). [CrossRef]
8. Q. Xu, P. Dong, and M. Lipson, “Breaking the delay-bandwidth limit in a photonic structure,” Nature Phys. 3, 406–410 (2007). [CrossRef]
9. J. Upham, Y. Tanaka, T. Asano, and S. Noda, “Dynamic increase and decrease of photonic crystal nanocavity Q factors for optical pulse control,” Opt. Express 16, 21721–21731 (2008). [CrossRef] [PubMed]
10. J. Upham, Y. Tanaka, Y. Kawamoto, Y. Sato, T. Nakamura, B.S. Song, T. Asano, and S. Noda, “Time-resolved catch and release of an optical pulse from a dynamic photonic crystal nanocavity,” Opt. Express 19, 23377–23385 (2011). [CrossRef] [PubMed]
11. J. Upham, Y. Fujita, Y. Kawamoto, Y. Tanaka, B.S. Song, T. Asano, and S. Noda, “The capture, hold and forward release of an optical pulse from a dynamic photonic crystal nanocavity,” Opt. Express 21, 3809–3817 (2013). [CrossRef] [PubMed]
12. A. W. Elshaari, A. Aboketaf, and S. F. Preble, “Controlled storage of light in silicon cavities,” Opt. Express 18, 3014–3022 (2010). [CrossRef] [PubMed]
13. S. Preble, Q. Xu, and M. Lipson, “Changing the colour of light in a silicon resonator,” Nature Photon. 1, 293–296 (2007). [CrossRef]
14. J. Upham, Y. Tanaka, T. Asano, and S. Noda, “On-the-fly wavelength conversion of photons by dynamic control of photonic waveguides,” Appl. Phys. Express 3, 062001 (2010). [CrossRef]
15. D. Beggs, I. Rey, T. Kampfrath, N. Rotenberg, L. Kuipers, and T. Krauss, “Ultrafast tunable optical delay line based on indirect photonic transitions,” Phys. Rev. Lett. 108213901 (2012). [CrossRef] [PubMed]
16. M. Belotti, J. F. Galisteo-López, S. De Angelis, M. Galli, I Maksymov, L. C. Andreani, D Peyrade, and Y. Chen, “All-optical switching in 2D silicon photonic crystals with low loss waveguides and optical cavities,” Opt. Express 16, 11624–11636 (2008). [PubMed]
17. Y. Sato, Y. Tanaka, J. Upham, Y. Takahashi, T. Asano, and S. Noda, “Strong coupling between distant photonic nanocavities and its dynamic control,” Nature Photon. 6, 56–61 (2012). [CrossRef]
18. T. Tanabe, H. Taniyama, and M. Notomi, “Carrier diffusion and recombination in photonic crystal nanocavity optical switches,” J. Lightwave Technol. 26, 1396–1403 (2008). [CrossRef]
19. T. Tanabe, E. Kuramochi, H. Taniyama, and M. Notomi, “Electro-optic adiabatic wavelength shifting and Q switching demonstrated using a p-i-n integrated photonic crystal nanocavity,” Opt. Lett. 35, 3895–3897 (2010). [CrossRef] [PubMed]
20. M. Yamaguchi, T. Asano, Y. Sato, and S. Noda, “Photonic quantum computation with waveguide-linked optical cavities and quantum dots,” Preprint at arXiv:1101.3508v1 (2011).
21. Y. Sugimoto, Y. Yanaka, N. Ikeda, Y. Nakamura, and K. Asakawa, “Low propagation loss of 0.76 dB/mm in GaAs-based single-line-defect two-dimensional photonic crystal slab waveguides up to 1 cm in length,” Opt. Express 12, 1090–1096 (2004). [CrossRef] [PubMed]
22. T. Yoshie, A. Scherer, J. Hendrickson, G. Khitrova, H.M. Gibbs, G. Rupper, C. Ell, O.B. Shchekin, and D.G. Deppe, “Vacuum Rabi splitting with a single quantum dot in a photonic crystal nanocavity,” Nature 432, 200–203 (2004). [CrossRef] [PubMed]
23. K. Hennessy, A. Badolato, M. Winger, D. Gerace, M. Atatüre, S. Gulde, S. Fält, E. L. Hu, and A. Imamoğlu, “Quantum nature of a strongly coupled single quantum dot-cavity system,” Nature 445, 896–899 (2007). [CrossRef] [PubMed]
24. I. Fushman, D. Englund, A. Faraon, N. Stoltz, P. Petroff, and J. Vuckovic, “Controlled phase shifts with a single quantum dot,” Science 320, 769–772 (2008). [CrossRef] [PubMed]
25. D. Dimitropoulos, R. Jhaveri, R. Claps, J.C.S. Woo, and B. Jalai, “Lifetime of photogenerated carriers in silicon-on-insulator rib waveguides,” Appl. Phys. Lett. 86, 071115 (2005). [CrossRef]
26. D.E. Aspnes, “Recombination at semiconductor surfaces and interfaces,” Surface Sci. 132, 406–421 (1983). [CrossRef]
27. T. Nakamura, T. Asano, K. Kojima, T. Kojima, and S. Noda, “Controlling the emission of quantum dots embedded in photonic crystal nanocavity by manipulating Q-factor and detuning,” Phys. Rev. B 84, 24309 (2011). [CrossRef]
28. M. Fujita, A. Sugitatsu, T. Uesugi, and S. Noda, “Fabrication of Indium Phosphide compound photonic crystal by Hydrogen Iodide/Xenon inductively coupled plasma etching,” Jpn J. Appl. Phys. 43, L1400–L1402 (2004). [CrossRef]
