Optothermal tuning of liquid crystal infiltratedInGaAsP photonic crystal nanocavities

Mehmet A. Dündar,1,* Bowen Wang,1,2 Richard Nötzel,1 Fouad Karouta,1 and Rob W. van der Heijden1

1COBRA Research Institute, Eindhoven University of Technology,P.O. Box 513, NL-5600 MB Eindhoven, The Netherlands

2Centre for Optical and Electromagnetic Research, State Key Laboratory of Modern Optical Instrumentation,Zijingang Campus, Zhejiang University, Hangzhou 310058, China

*Corresponding author: m.dundar@tue.nl

Received March 4, 2011; revised April 5, 2011; accepted April 21, 2011;posted April 21, 2011 (Doc. ID 143585); published May 24, 2011

A large and reversible all-optical tuning effect is demonstrated for liquid crystal (LC) infiltrated InGaAsP photoniccrystal membrane nanocavities. The tuning is based on the change in the refractive index of the LC due to the largelocal heating caused by absorption of laser light by the semiconductor. Compared to opto-thermal tuning based onsemiconductor heating alone, the effects with the LC are an order of magnitude larger and can be either redshiftingor blueshifting, depending on the spatial distribution of the cavity mode’s polarization direction. © 2011 OpticalSociety of America

OCIS codes: 230.5298, 230.5750, 230.3720, 160.3710, 250.5230.

1. INTRODUCTIONPhotonic crystal (PhC) nanocavities are important as ultra-small optical devices to realize functions in future photonicintegrated circuits, such as add-drop filters [1] and opticalswitches [2]. Because of their strong light-matter interactionon a wavelength scale, they are attractive as building blocksfor future quantum information systems and to realize crucialcomponents in solid-state cavity quantum electrodynamic sys-tems [3]. For all applications, it is necessary that the deviceson the chip are tunable, either to create the functionality or tocorrect for fabrication imperfections or both. One of the sim-plest ways to achieve the tunability is to integrate local micro-heaters near the device and to exploit the temperaturedependence of the refractive index [4]. This technique hasbeen applied to ring resonators from conventional wave-guides [5] as well as to PhC devices [6].

Of particular interest are optical tuning methods with anexternal laser beam, because they are contactless, do not re-quire elaborate sample processing steps, are very local, andare easily reconfigurable. A focused laser beam has beenwidely employed for tuning. The mechanism relies on non-linear optical effects as carrier injection [7] or Kerr effect [8].In the latter case, the Kerr nonlinearity was realized throughanother material infiltrated in the PhC holes. A simple heatingwas successfully employed by using an extra metal film toabsorb the light and by optimizing the thermal insulation ofthe device from the rest of the chip [9]. Also, direct absorptionin the semiconductor has been used for local laser heating[10,11].

Infiltration of various liquids into the PhC holes has beenadopted to increase the performance of PhC devices, particu-larly their capability for tuning [12–14]. Among the differentliquids, LC has attracted much attention as an infill sourcebecause its refractive index can be varied by temperature orelectric field. So far LC-infiltrated 2D deeply etched photoniccrystals have led to a large tuning of cavity modes and band

edges by varying the temperature [15–17]. In these experi-ments, the entire chip was heated.

The optical tuning of planar photonic crystals using thesame LC has been demonstrated previously by infiltrating aphotoresponsive mixture of this LC and azobenzene deriva-tives [18]. Upon irradiation with UV light, the azobenzene mo-lecules isothermally induce a phase transition of the LC to thenematic state, while the transition could be largely reversedby irradiation with visible light. The photonic crystal cavityresonance wavelength in the isotropic state was always red-shifted with respect to that in the nematic state. To exploit thefull reversibility of this tuning mechanism, two different lightsources at UV and visible frequencies are required. In the pre-sent work, we demonstrate tuning of an LC-infiltrated PhCcavity in an InGaAsP membrane PhC by local heating witha focused laser spot. The heating is induced by the absorptionof radiation in the semiconductor, but the tuning effects aredominated by the temperature dependence of the LC refrac-tive index. The observed tuning rate dλ=dT is large, reversible,and has an opposite sign for the modes with different polar-izations, which is a genuine property of the LC because of itsanisotropic optical properties [19,20].

2. EXPERIMENTALThe fabrication procedure of a 220nm thick InGaAsP mem-brane with a single missing air hole cavity (H1), which hasa single layer of self-assembled InAs quantum dots (QDs, den-sity 3 × 1010 cm−2), [21] can be found in [22]. InGaAsP is thestandard semiconductor for use at the telecommunicationwavelength window near 1:55 μm. InAs QDs can be incorpo-rated as efficient, tunable light emitters during epitaxialgrowth. The QDs have a broad spectrum with a high emissionintensity concentrated around 1:55 μm as shown in Fig. 1(a).Figure 1(b) shows the scanning electron microscope (SEM)image of a fabricated H1 cavity with a lattice spacing (a) of470nm and radius to lattice spacing ratio of 0.35. The radius

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of the six innermost holes is reduced to 0:3a and these holesare radially shifted outward by 0:032a. The H1 type of nano-cavity has been chosen because its characteristics and modesare well known from the literature and it has a reasonablygood quality factor (Q). The overlap of the electric field inten-sity with the holes is near 10%, resulting in a good sensitivity toinfiltration of the holes [23].

The fabricated structures are treated by an O2 plasma andare rinsed in H3PO4:H2O solution to increase hydrophilicityof the surface in order to obtain a good wetting for the infiltra-tions. The LC is infiltrated into the PhC holes under theambient pressure by placing a drop of the LC on top of the sam-ple. The sample and the LCwere heatedwell above the clearingtemperature. Then the sample is blown by dry nitrogen to re-move the excess LC on top of the sample.

The cavity modes are detected by measuring the photolu-minescence (PL) from the QDs, which is strongly peaked atthe cavity resonances. The PL is generated by a CW diode la-ser at a wavelength of 660nmwith an excitation spot diameteraround 2 μm. The optothermal tuning is achieved by usingthe same laser as the heating source by varying its power.The excitation of the cavities and the collection of the PL sig-nal are done by using a 50× microscope objective (NA ¼ 0:5).The collected signal is dispersed in a 50 cm monochromatorand detected by a liquid N2 cooled InGaAs array.

3. RESULTS AND DISCUSSIONAs a reference, we first investigate the effect of the laser-induced heating for empty structures. Figure 2(a) showsthe PL signal collected from the H1 cavity before the infiltra-tion under a low excitation power. The modes at 1455 and1460 nm are split dipole modes, which we refer to as DX andDY, respectively, and which are identified by comparing withthe results of 3D finite difference time domain (FDTD) simu-lation (CrystalWave). The DX and DY modes are ideallydegenerate but split in reality due to the imperfect fabrication.Figure 2(b) shows that the DX and the DY modes both redshift

by more than 5 nm as the laser power on the cavity surfaceincreases up to 410 μW. The power incident on the sampleis estimated from a separate measurement with a power meterat the position of the sample. The redshift of the modes iscaused by the increase in the refractive index of the semicon-ductor due to the thermo-optic effect. Temperature tuning ofInP-based cavities leads to a resonance shift of 0:1 nm=K[4,22], which implies that the observed shift corresponds toa local temperature increase of more than 50 °C. Such a largetemperature increase with the modest laser power occursdue to the poor thermal coupling of the undercut slab to thesemiconductor chip.

Figure 3(a) shows the temperature dependence of the re-fractive indices of the LC (5CB: 4-pentyl-4’cyanobiphenyl,Merck) for 1:5 μm calculated from the parameters given in[24]. In the nematic state, the LC has ordinary (no) and extra-ordinary (ne) refractive indices, and their temperaturechanges are most pronounced at the nematic to isotropicphase transition temperature Tc. The refractive index ni inthe isotropic state is some average of no and ne. The Tc of5CB has a conveniently low value of 35 °C, which can be easilyreached with the low heating power of the laser. Note that theindices vary in opposite directions as the temperature in-creases and at the phase transition temperature. In the isotro-pic state, T > 35 °C, the refractive index does not changemuch with the temperature. Figure 3(b) shows the PL emis-sion from the H1 cavity after the infiltration of the LC atthe nematic state with the low laser power of 50 μW. The spec-trum shows that the DX and DY modes are redshifted by100nm with an increased splitting of 15nm. The redshift ofthe modes is caused by the change in the environment refrac-tive index due to the LC. The 3D FDTD simulation confirmsthat the LC occupies the undercut region, the PhC holes, andthe surface. Figure 3(c) represents the PL signal collectedfrom the infiltrated cavity under three different pump powers.The peak positions of the two modes are plotted as a functionof power in Fig. 3(d). When the laser power is increased up to250 μW, the DX mode redshifts by more than 3 nm while theDY mode gradually blueshifts by more than 5nm. The wave-lengths of the DX and the DY modes show a rapid change near300 μW. Apparently, the Tc (35 °C) of the LC is reached at300 μW. Given the temperature sensitivity of the resonance

Fig. 1. (a) InAs QD’s PL emission and (b) SEM image of a fabricatedH1 cavity.

Fig. 2. (a) Photoluminescence signal from the H1 cavity before theLC infiltration and (b) wavelength dependence on the excitationpower of the DX and DY modes.

Fig. 3. (Color online) (a) Temperature dependence refractive indexchange for the LC calculated from the parameters given in [24], (b) thePL signal collected from the H1 cavity after the infiltration at the ne-matic state, (c) PL data collected from the infiltrated cavity underthree different pump-powers, and (d) optothermal tuning of the DXand DY modes.

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wavelengths of ∼0:1nm=K, and the 3 nm wavelength shift for300 μW power observed for the empty cavity [Fig. 2(b)], thispower is somewhat higher than expected. This is reasonablebecause the LC will cause some additional cooling of thecavity [25]. In the isotropic state above 300 μW, the splitting ofthe two modes is not observable within the experimental line-width resolution. The infiltration increases the losses some-what, so that the linewidths broaden. This implies that theobserved splitting in the nematic LC state is dominated bythe LC optical anisotropy and not by PhC fabrication imper-fections. The fabrication-induced splitting as observed beforeinfiltration [Fig. 2(a)] is not observed in the isotropic LC-filledcase, presumably due to the decreased linewidth resolution. Itcannot be ruled out, however, that strain effects due to thelocal heating also have an effect on the fabrication-inducedsplitting. A maximum tuning rate with a blueshift of about0:1nm=μW is observed between 200 and 300 μW with theLC, whereas from Fig. 2(b), a redshift tuning with a magnitudeabout an order of magnitude smaller is obtained for similarlaser powers with the empty PhC. Increasing the excitationpower above 300 μW up to 500 μW, shows no significantchange in the resonant wavelengths. Upon decreasing the la-ser power, it is found that the tuning is reversible and no hys-teresis is observed. This shows that the LC is not evaporatedunder the high excitation power. In principle, there is no rea-son why the two resonant mode wavelengths would not crossas a function of the laser intensity, though it was not observedso far. The crossing would be very interesting as it would offeran accurate and tunable control of the mode degeneracy withpossible applications in quantum optics.

The redshift and the blueshift of the two modes are attrib-uted to the existence of the ordinary and the extraordinaryrefractive indices of the LC. Previously, this was analyzedin the case of a simple heating for the nearly degenerate quad-rupole modes of the same cavity [20]. There, the blueshift ofthe one quadrupole mode was much smaller than the redshiftof the other, as opposed to the behavior for the nearly degen-erate dipole modes investigated now. Since the modes areTE-polarized, the electric field is almost entirely in the plane

of the PhC, i.e., perpendicular to the hole axis. This in-plane E-field pattern, calculated using 3D FDTD [26] for the twomodes, is given in Figs. 4(a) and 4(b). It is plotted for the cen-ter plane of the slab, but the pattern varies only weakly whenwe move from the center to the top or the bottom surface.Unlike the case for the quadrupole modes [20], there is noclear 90° polarization rotation inside the holes for the twomodes. On the other hand, in the center of the cavity, the twomodes have polarizations that are nearly orthogonal to eachother. Therefore, we attribute the observed splitting to inter-actions of the modes with the LC on the top and bottom sur-faces of the slab, assuming that the LC molecules tend to alignparallel to the surface.

4. CONCLUSIONIn this paper, we demonstrated the optothermal tuning of thedegenerate dipole modes in the presence of the LC. The tuningmechanism is based on the change in the LC alignment due tothe large heating by the excitation laser. The dipole modesthat are nearly degenerate in an isotropic liquid medium splitand show an opposite tuning behavior with the excitationpower in an LC environment due to different interactions withthe two refractive indices of the LC. The tuning is reversibleand no hysteresis observed. The large all-optical tuning effectswith LCs will be of interest for realizing optically tunableoptoelectronic PhC devices.

ACKNOWLEDGMENTSThe authors thank P. A. M. Nouwens, B. Smalbrugge, E. J.Geluk, R. van Veldhoven, and T. de Vries for their help inthe fabrication processes.

REFERENCES1. H. Takano, Y. Akahane, T. Asano, and S. Noda, “In-plane-type

channel drop filter in a two-dimensional photonic crystal slab,”Appl. Phys. Lett. 84, 2226–2228 (2004).

2. H. Takeda and K. Yoshino, “Tunable light propagation in Y-shaped waveguides in two-dimensional photonic crystals utiliz-ing liquid crystals as linear defects,” Phys. Rev. B 67,073106 (2003).

3. A. Kiraz, C. Reese, B. Gayral, L. D. Zhang, W. V. Schoenfeld, B. D.Gerardot, P. M. Petroff, E. L. Hu, and A. Imamoglu, “Cavity-quantum electrodynamics with quantum dots,” J. Opt. B 5,129–137 (2003).

4. B. Wild, R. Ferrini, R. Houdre, M. Mulot, S. Anand, and C. J. M.Smith, “Temperature tuning of the optical properties of planarphotonic crystal microcavities,” Appl. Phys. Lett. 84, 846–848 (2004).

5. L. Chen, N. Sherwood-Droz, and M. Lipson, “Compact band-width-tunable microring resonators,” Opt. Lett. 32, 3361–3363(2007).

6. E. A. Camargo, H. M. H. Chong, and R. M. De la Rue, “2D Photo-nic crystal thermo-optic switch based on AlGaAs/GaAs epitaxialstructure,” Opt. Express 12, 588–592 (2004).

7. S. W. Leonard, H. M. van Driel, J. Schilling, and R. B. Wehrspohn,“Ultrafast band-edge tuning of a two-dimensional silicon photo-nic crystal via free-carrier injection,” Phys. Rev. B 66, 161102(2002).

8. M. Hochberg, T. Baehr-Jones, G. X. Wang, M. Shearn, K.Harvard, J. D. Luo, B. Q. Chen, Z. W. Shi, R. Lawson, P. Sullivan,A. K. Y. Jen, L. Dalton, and A. Scherer, “Terahertz all-opticalmodulation in a silicon-polymer hybrid system,” Nat. Mater.5, 703–709 (2006).

9. A. Faraon, D. Englund, I. Fushman, J. Vuckovic, N. Stoltz, and P.Petroff, “Local quantum dot tuning on photonic crystal chips,”Appl. Phys. Lett. 90, 043102 (2007).

Fig. 4. (Color online) Electric field (E) map of the (a) DX and (b) DYmodes (the asymmetric impression exhibited by the modes is causedby the end-point anchoring of the arrows to the grid points). The mag-netic field (H) profiles of (c) DX and (d) DY modes. Note that theE-field has an antinode where the H-field has a node, sinceE ∝ ∇ ×H.

1516 J. Opt. Soc. Am. B / Vol. 28, No. 6 / June 2011 Dündar et al.

10. J. Pan, Y. Huo, K. Yamanaka, S. Sandhu, L. Scaccabarozzi, R.Timp, M. L. Povinelli, S. Fan, M. M. Fejer, and J. S. Harris,“Aligning microcavity resonances in silicon photonic crystalslabs using laser-pumped thermal tuning,” Appl. Phys. Lett.92, 103114 (2008).

11. S. Vignolini, F. Intonti, L. Balet, M. Zani, F. Riboli, A. Vinattieri,D. S. Wiersma, M. Colocci, L. Li, M. Francardi, A. Gerardino,A. Fiore, and M. Gurioli, “Nonlinear optical tuning of photoniccrystal microcavities by near-field probe,” Appl. Phys. Lett. 93,023124 (2008).

12. C. L. C. Smith, U. Bog, S. Tomljenovic-Hanic, M. W. Lee, D. K. C.Wu, L. O’Faolain, C. Monat, C. Grillet, T. F. Krauss, C. Karnutsch,R. C. McPhedran, and B. J. Eggleton, “Reconfigurable microflui-dic photonic crystal slab cavities,” Opt. Express 16, 15887–15896 (2008).

13. F. Intonti, S. Vignolini, F. Riboli, M. Zani, D. S. Wiersma, L. Balet,L. H. H. Li, M. Francardi, A. Gerardino, A. Fiore, and M. Gurioli,“Tuning of photonic crystal cavities by controlled removalof locally infiltrated water,” Appl. Phys. Lett. 95, 173112(2009).

14. G. Barillaro, S. Merlo, and L. M. Strambini, “Optical character-ization of alcohol-infiltrated one-dimensional silicon photoniccrystals,” Opt. Lett. 34, 1912–1914 (2009).

15. S. W. Leonard, J. P. Mondia, H. M. van Driel, O. Toader, S. John,K. Busch, A. Birner, U. Gosele, and V. Lehmann, “Tunable two-dimensional photonic crystals using liquid-crystal infiltration,”Phys. Rev. B 61, R2389–R2392 (2000).

16. C. Schuller, F. Klopf, J. P. Reithmaier, M. Kamp, and A. Forchel,“Tunable photonic crystals fabricated in III-V semiconductorslab waveguides using infiltrated liquid crystals,” Appl. Phys.Lett. 82, 2767–2769 (2003).

17. R. Ferrini, J. Martz, L. Zuppiroli, B. Wild, V. Zabelin, L. A.Dunbar, R. Houdre, M. Mulot, and S. Anand, “Planar photoniccrystals infiltrated with liquid crystals: optical characterizationof molecule orientation,” Opt. Lett. 31, 1238–1240 (2006).

18. P. El-Kallassi, R. Ferrini, L. Zuppiroli, N. Le Thomas, R. Houdre,A. Berrier, S. Anand, and A. Talneau, “Optical tuning of planar

photonic crystals infiltrated with organic molecules,” J. Opt.Soc. Am. B 24, 2165–2171 (2007).

19. M. A. Du ndar, H. H. J. E. Kicken, A. Y. Silov, R. No tzel, F.Karouta, H. W. M. Salemink, and R. W. van der Heijden,“Birefringence-induced mode-dependent tuning of liquid crystalinfiltrated InGaAsP photonic crystal nanocavities,” Appl. Phys.Lett. 95, 181111 (2009).

20. B. W. Wang, M. A. Du ndar, A. Y. Silov, R. No tzel, F. Karouta, S. L.He, and R. W. van der Heijden, “Controlling mode degeneracy ina photonic crystal nanocavity with infiltrated liquid crystal,”Opt. Lett. 35, 2603–2605 (2010).

21. R. Notzel, S. Anantathanasarn, R. P. J. van Veldhoven, F. W. M.vanOtten, T. J. Eijkemans, A. Trampert, B. Satpati, Y. Barbarin, E.A. J.M. Bente, Y. S. Oei, T. de Vries, E. J. Geluk, B. Smalbrugge,M.K. Smit, and J. H. Wolter, “Self assembled InAs/InP quantum dotsfor telecom applications in the 1:55 μmwavelength range: Wave-length tuning, stacking, polarization control, and lasing,” Jpn. J.Appl. Phys. 45, 6544–6549 (2006).

22. M. A. Du ndar, F. Bordas, T. J. Eijkemans, N. Chauvin, A. Y. Silov,R. Notzel, F. Karouta, A. Fiore, and R. W. van der Heijden,“Lithographic and optical tuning of InGaAsP membrane photo-nic crystal nanocavities with embedded InAs quantum dots,” J.Nanophoton. 3, 031765 (2009).

23. M. A. Du ndar, E. C. I. Ryckebosch, R. No tzel, F. Karouta, L. J.van Ijzendoorn, and R. W. van der Heijden, “Sensitivities ofInGaAsP photonic crystal membrane nanocavities to hole re-fractive index,” Opt. Express 18, 4049–4056 (2010).

24. J. Li and S. T. Wu, “Extended Cauchy equations for the refractiveindices of liquid crystals,” J. Appl. Phys. 95, 896–901 (2004).

25. S. H. Kim, J. H. Choi, S. K. Lee, S. H. Kim, S. M. Yang, Y. H. Lee, C.Seassal, P. Regrency, and P. Viktorovitch, “Optofluidic integra-tion of a photonic crystal nanolaser,” Opt. Express 16, 6515–6527 (2008).

26. A. F. Oskooi, D. Roundy, M. Ibanescu, P. Bermel, J. D. Joanno-poulos, and S. G. Johnson, “MEEP: A flexible free-softwarepackage for electromagnetic simulations by the FDTD method,”Comput. Phys. Commun. 181, 687–702 (2010).

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