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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 19 — Sep. 12, 2011
  • pp: 18529–18542
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Deterministic design of wavelength scale, ultra-high Q photonic crystal nanobeam cavities

Qimin Quan and Marko Loncar  »View Author Affiliations


Optics Express, Vol. 19, Issue 19, pp. 18529-18542 (2011)
http://dx.doi.org/10.1364/OE.19.018529


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Abstract

Photonic crystal nanobeam cavities are versatile platforms of interest for optical communications, optomechanics, optofluidics, cavity QED, etc. In a previous work [Appl. Phys. Lett. 96, 203102 (2010)], we proposed a deterministic method to achieve ultrahigh Q cavities. This follow-up work provides systematic analysis and verifications of the deterministic design recipe and further extends the discussion to air-mode cavities. We demonstrate designs of dielectric-mode and air-mode cavities with Q > 109, as well as dielectric-mode nanobeam cavities with both ultrahigh-Q (> 107) and ultrahigh on-resonance transmissions (T > 95%).

© 2011 OSA

1. Introduction

In this follow-up work, we provide numerical proof of the proposed principles, and systematically optimize the design recipe to realize a radiation limited cavity and waveguide coupled cavity. Furthermore, we extend the recipe to the design of air-mode cavities, whose optical energies are concentrated in the low-index region of the structure.

Nanobeam cavities have recently emerged as a powerful alternative to the slab-based 2-D PhC cavities [17

17. 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]

22

22. Y. Tanaka, T. Asano, and S. Noda, “Design of photonic crystal nanocavity with Q-factor of ∼109,” J. Lightwave Technol. 26, 1532 (2008). [CrossRef]

]. Nanobeams can achieve Qs on par with those found in slab-based geometries, but in much smaller footprints, and are the most natural geometries for integration with waveguides [23

23. M. Notomi, E. Kuramochi, and H. Taniyama, “Ultrahigh-Q nanocavity with 1D Photonic Gap,” Opt. Express , 16, 11095 (2008). [CrossRef] [PubMed]

31

31. P. B. Deotare, M. W. McCutcheon, I. W. Frank, M. Khan, and M. Loncar, “High quality factor photonic crystal nanobeam cavities,” Appl. Phys. Lett. 94, 121106 (2009). [CrossRef]

]. Our deterministically designed cavities have similar structures to the mode-gap cavities proposed by Notomi et al. [23

23. M. Notomi, E. Kuramochi, and H. Taniyama, “Ultrahigh-Q nanocavity with 1D Photonic Gap,” Opt. Express , 16, 11095 (2008). [CrossRef] [PubMed]

], and later demonstrated experimentally by Kuramochi et al. [33

33. E. Kuraamochi, H. Taniyama, T. Tanabe, K. Kawasaki, Y-G. Roh, and M. Notomi, “Ultrahigh-Q one-dimensional photonic crystal nanocavities with modulated mode-gap barriers on SiO2 claddings and on air claddings,” Opt. Express 18, 15859–15869 (2010). [CrossRef]

], as well as our own work [34

34. Q. Quan, I. B. Burgess, S. K. Y. Tang, D. L. Floyd, and M. Loncar, “High-Q/V photonic crystal nanobeam cavities in an ultra-low index-contrast polymeric optofluidic platform,” arXiv:1108.2669 (2010).

]. We note that the same design principle discussed here could be directly applied to realize ultra-high Q cavities based on dielectric stacks that are of interest for realization of vertical-cavity surface emitting lasers (VCSELs) and sharp filters. Finally, it is important to emphasize that while our method is based on the framework of Fourier space analysis [35

35. J. Vuckovic, M. Loncar, H. Mabuchi, and A. Scherer, “Optimization of the Q factor in photonic crystal microcavities,” IEEE J. Quantum Electron. 38, 850–856 (2002). [CrossRef]

37

37. D. Englund, I. Fushman, and J. Vuckovic, “General recipe for designing photonic crystal cavities,” Opt. Express 13, 5961–5975 (2005). [CrossRef] [PubMed]

], alternative approach, based on phase-matching between different mirror segments, could also be used to guide the design, as well as to explain the origin of deterministic ultra-high Q-factors in our devices [38

38. M. Palamaru and P. Lalanne, “Photonic crystal waveguides: Out-of-plane losses and adiabatic modal conversion,” Appl. Phys. Lett. 78, 1466–1468 (2001). [CrossRef]

, 39

39. P. Lalanne, S. Mias, and J. P. Hugonin, “Two physical mechanisms for boosting the quality factor to cavity volume ratio of photonic crystal microcavities,” Opt. Express 12, 458–467 (2004). [CrossRef] [PubMed]

].

2. Numerical verification of the deterministic design approach

In this section, we use finite-difference time-domain (FDTD) simulations to systematically study the design principles proposed in [32

32. Q. Quan, P. B. Deotare, and M. Loncar, “Photonic crystal nanobeam cavity strongly coupled to the feeding waveguide,” Appl. Phys. Lett. 96, 203102 (2010). [CrossRef]

]. Figure 1(a) shows the schematics of the proposed cavity structure [32

32. Q. Quan, P. B. Deotare, and M. Loncar, “Photonic crystal nanobeam cavity strongly coupled to the feeding waveguide,” Appl. Phys. Lett. 96, 203102 (2010). [CrossRef]

]. It consists of an array of air-holes in decreasing radii, etched into a ridge waveguide. The hole-to-hole distances (“periodicity”) are constant. The structure is symmetric with respect to the dashed line in Fig. 1(a). In contrast to the majority of other cavity designs, current structure has no additional cavity length inserted between the two mirrors (L=0), that is the hole-to-hole spacing between the two central holes is the same as the rest of the structure (a). This minimizes the cavity loss and the mode volume simultaneously. The cavity loss is composed of the radiation loss into the free space (characterized by Q rad) and the coupling loss to the feeding waveguide (Q wg). Q wg can be increased simply by adding more gratings along the waveguide. Q rad can be increased by minimizing the spatial Fourier harmonics of the cavity mode inside the lightcone, achieved by creating a Gaussian-like attenuation profile [32

32. Q. Quan, P. B. Deotare, and M. Loncar, “Photonic crystal nanobeam cavity strongly coupled to the feeding waveguide,” Appl. Phys. Lett. 96, 203102 (2010). [CrossRef]

]. The optical energy is concentrated in the dielectric region in the middle of the cavity (Fig. 1(b)). In order to achieve the Gaussian-like attenuation, we proposed to use a linearly increasing mirror strength along the waveguide [32

32. Q. Quan, P. B. Deotare, and M. Loncar, “Photonic crystal nanobeam cavity strongly coupled to the feeding waveguide,” Appl. Phys. Lett. 96, 203102 (2010). [CrossRef]

], which was achieved by tapering the hole radii.

Fig. 1 (a) Schematic of the proposed nanobeam cavity. (b) FDTD simulation of the energy density distribution in the middle plane of the nanobeam cavity.

First, we analyze the ideal tapering profile using plane wave expansion method and verify the results with 3D FDTD simulations. The dielectric profile of the structure in the middle plane of the cavity can be expressed as
1ε(ρ)=1εSi+(1εair1εSi)S(ρ)
(1)
with
S(ρ)={1|ρrj|R0|ρrj|>R
r j = j · ax̂, a is the period, and j = ±1, ±2... are integers. R is the radius of the hole. Using plain wave expansion method [40

40. K. Sakoda, Optical Properties of Photonic Crystals, 2nd Ed. (Springer, 2005).

] in the beam direction (),
1ε(x)=κ0+κ1eiGx+κ1eiGx+
(2)
where G = 2π/a. The zeroth (κ 0) and first (κ 1) order Fourier components can be expressed as [40

40. K. Sakoda, Optical Properties of Photonic Crystals, 2nd Ed. (Springer, 2005).

]
κ0=fεair+1fεSi
(3)
κ1=2f(1εair1εSi)J1(GR)GR
(4)
J 1 is the first order Bessel function. Filling fraction f = πR 2/ab is the ratio of the area of the air-hole to the area of the unit cell. We note that the above expressions are calculated assuming nanobeam cavity has infinite thickness (i.e 2D equivalent case). Better estimation can be obtained by replacing ε air and ε Si with the effective permittivities.

The dispersion relation can be obtained by solving the master equation [41

41. J. D. Joannopoulos, S. G. Johnson, J. N. Winn, and R. D. Meade, Photonic Crystals: Molding the Flow of Light, 2nd Ed. (Cambridge University Press, 2007).

]:
c2ε(x)2Ex2=2Et2
(5)
Inside the bandgap, the wavevector (k) for a given frequency (ω) is a complex number, whose imaginary part denotes the mirror strength (γ). For solutions near the band-edge, of interest for high-Q cavity design [32

32. Q. Quan, P. B. Deotare, and M. Loncar, “Photonic crystal nanobeam cavity strongly coupled to the feeding waveguide,” Appl. Phys. Lett. 96, 203102 (2010). [CrossRef]

], the frequency can be written as ω=(1δ)κ0πc/a (δ is the detuning from the mid-gap frequency) and the wavevector as k = (1 + )π/a. Substituting this into the master equation, we obtain δ2+γ2=κ12/4κ02. The cavity resonance asymptotes to the dielectric band-edge of the center mirror segment: wres(1κ1j=1/2κ0j=1)κ0j=1πc/a (j represents the j th mirror segment counted from the center), at which point the mirror strength γj =1 = 0. γ increases with j. With ε air = 1 and ε Si = 3.462, we calculate in Fig. 2(a) the γj relation for different tapering profiles. It can be seen that quadratic tapering profile results in linearly increasing mirror strengths, needed for Gaussian field attenuation [32

32. Q. Quan, P. B. Deotare, and M. Loncar, “Photonic crystal nanobeam cavity strongly coupled to the feeding waveguide,” Appl. Phys. Lett. 96, 203102 (2010). [CrossRef]

]. To verify this, we perform FDTD simulation and obtain the band diagram (Fig. 2(b)) and γf relation (Fig. 2(c)). As shown in Fig. 2(d), linearly increasing mirror strength is indeed achieved after quadratic tapering.

Fig. 2 (a) Mirror strengths of each mirror segment for different tapering profiles obtained from the plane wave expansion method (‘1’ indicates the mirror segment in the center of the cavity). (b) Band diagram of the TE-like mode for f = 0.2 and f = 0.1. The green line indicates the light line. The circle indicates the target cavity resonant frequency. (c) Mirror strengths for different filling fractions, obtained using 3D band diagram simulation. (d) Mirror strengths as a function of mirror number after quadratic tapering. (e) Radiation-Q factors for nanobeam cavities with different cavity lengths between the two Gaussian mirrors, obtained using 3D FDTD simulations. (f) Resonances of the cavities that have different total number of mirror pair segments in the Gaussian mirror, and their deviations from the dielectric band-edge of the central mirror segment, obtained using both FDTD simulation and perturbation theory. (g) Hz field distribution on the surface right above the cavity, obtained from 3D FDTD simulation. The structure has dimension of a = 0.33μm, b = 0.7μm, the first 20 mirror segments (counted from the center) have fs varying from 0.2 to 0.1, followed by 10 additional mirror segments with f = 0.1. (h) Hz field distribution on the surface right above the cavity, obtained from the analytical formula Hz=sin(πax)exp(σx2)exp(ξy2), with a = 0.33μm, σ = 0.14, ξ = 14. (i) Hz field distribution along the dashed line in (g)&(h). Length unit in (g)–(i) is μm.

Next, with the optimized tapering profile, the cavity is formed by putting two such mirrors back to back, leaving a cavity length L in between (Fig. 1(a)). Figure 2(e) shows the simulated Q-factors for various Ls. Highest Q rad is achieved at zero cavity length (L=0), which supports the prediction in [32

32. Q. Quan, P. B. Deotare, and M. Loncar, “Photonic crystal nanobeam cavity strongly coupled to the feeding waveguide,” Appl. Phys. Lett. 96, 203102 (2010). [CrossRef]

] based on 1D model.

Third, we verify that the cavity mode has a Gaussian-like attenuation profile. Figure 2(g) shows the Hz-field distribution in the plane right above the cavity, obtained from 3D FDTD simulation. As shown in Fig. 2(h), this field distribution can be ideally fitted with Hz = sin(πx/a)exp(−σx 2)exp(−ξy 2), with a = 0.33, σ = 0.14 and ξ = 14. The fitted value a agrees with the “period”, and σ agrees with that extracted value from Fig. 2(d): σ=dγdxπa=0.13. Figure 2(i) shows Hz distribution along the dashed line in Figs. 2(g) and 2(h). Therefore, we conclude that zero cavity length, fixed periodicity and a quadratic tapering of the filling fraction results in a Gaussian field profile, which leads to a high-Q cavity [32

32. Q. Quan, P. B. Deotare, and M. Loncar, “Photonic crystal nanobeam cavity strongly coupled to the feeding waveguide,” Appl. Phys. Lett. 96, 203102 (2010). [CrossRef]

].

Finally, as we have pointed out in [32

32. Q. Quan, P. B. Deotare, and M. Loncar, “Photonic crystal nanobeam cavity strongly coupled to the feeding waveguide,” Appl. Phys. Lett. 96, 203102 (2010). [CrossRef]

], current method results in a cavity whose resonance is asymptotically approaching the dielectric band-edge frequency of the central mirror segment (circled in Fig. 2(b)). The deviation from the band-edge frequency can be calculated using perturbation theory [41

41. J. D. Joannopoulos, S. G. Johnson, J. N. Winn, and R. D. Meade, Photonic Crystals: Molding the Flow of Light, 2nd Ed. (Cambridge University Press, 2007).

, 42

42. S. G. Johnson, M. Ibanescu, M. A. Skorobogatiy, O. Weisbergs, J. D. Joannopoulos, and Y. Fink, “Perturbation theory for Maxwell’s equations with shifting material boundaries,” Phys. Rev. E 65, 066611 (2002). [CrossRef]

]:
δλλ=δε|E|||2δ(ε1)|D|2dV2ε|E|2dV
(6)
E || is the component of E that is parallel to the side wall surfaces of the holes and D is the component of D that is perpendicular to the side wall surfaces of the holes. Under Gaussian distribution, the major field component Dy = cos(π/ax)exp(−σx 2)exp(−ξy 2), δε perturbation occurs at r = ±(j − 1/2)a + Rj, where Rj=fjab/π denotes the radius of the j th hole (counted the center), with j=2,3...N, N is the total number of mirror segments at each side. Since the cavity mode has a Gaussian profile, 1/σ characterizes the effective length of the cavity mode, and scales linearly with N, with a nonzero intercept due to diffraction limit. For large N, the intercept can be neglected, and thus σN=20×0.142/N. Plug the perturbation induced by the quadratic tapering from f = 0.2 to f = 0.1 into Eq. (6), the frequency offset δλ/λ v.s N can be obtained. Figure 2(f) shows the frequency offset for different total number of mirror pairs (N), calculated from the perturbation theory, as well as using FDTD simulations. It can be seen that the deviation decreases as the number of modulated mirror segments increases, and is below 1% for N > 15.

Fig. 3 (a) Schematic of the Gaussian nanobeam cavity, with an air hole in the symmetry plane (dashed line). (b) Energy distribution in the middle plane of the cavity obtained from 3D FDTD simulation. (c)&(d) Hz field distribution on the surface right above the cavity: (c) is obtained from 3D FDTD simulation and (d) is obtained from the analytical formula Hz=cos(πax)exp(σx2)exp(ξy2), with a = 0.33μm, σ = 0.14, ξ = 14. (e) Hz field distribution along the dashed line in (c)&(d). Length unit: μm.

Armed with the analytical field profile of the cavities: Hzodd(x)=sin(πx/a)exp(σx2) (Fig. 2) and Hzeven(x)=cos(πx/a)exp(σx2) (Fig. 3), we can obtain the radiation losses and far fields of the cavities using the Fourier space analysis [35

35. J. Vuckovic, M. Loncar, H. Mabuchi, and A. Scherer, “Optimization of the Q factor in photonic crystal microcavities,” IEEE J. Quantum Electron. 38, 850–856 (2002). [CrossRef]

]. The Fourier transforms can be analytically obtained FT(Hzodd)=(exp((k+π/a)2/4σ)exp((kπ/a)2/4σ))/i8σ and FT(Hzeven)=(exp((k+π/a)2/4σ)+exp((kπ/a)2/4σ))/8σ. Under σa 2 << 1, both distributions have their Fourier components strongly localized at k = ±π/a, as is verified by FDTD simulations in Fig. 4(a) and 4(b). Since Hzodd(x) is an odd function, it always has a zero Fourier component at k = 0. Therefore, dielectric-centered cavities (Fig. 1) should have higher Q-factors. However, in high-Q cavity designs, σa 2 << 1 is satisfied and thus both dielectric-centered and air-centered cavities have comparable Q-factors. FDTD simulation shows that the above Hzodd and Hzeven cavities have Q tot = 3.8 × 108 and Q tot = 3.5 × 108 respectively. The mode volume of the Hzodd cavity is 0.67(λ res/n Si)3, smaller than the Hzeven cavity (V = 0.76(λ res/n Si)3).

Fig. 4 (a)&(b) The distribution of the spatial Fourier components of the cavity mode, obtained from 3D FDTD simulation: (a) for the Hzodd cavity and (b) for the Hzeven cavity respectively. (c)&(d) The far field profile of the cavity mode obtained from 3D FDTD simulation: (c) for the Hzodd cavity and (d) for the Hzeven cavity respectively. The inset cavity structure shows the orientation of the waveguide direction in (c)&(d). Dashed line indicates the symmetry plane.

The far field radiation patterns (obtained using FDTD simulations) of the two cavities are shown in Figs. 4(c) and 4(d). The powers, in both cases, are radiated at shallow angles (> 70° zenith angle) to the direction of the waveguide. The Hodd cavity has even less radiated power at small zenith angles, consistent with the above analysis. By integrating the zenith and azimuth angle dependent far field emission, we found that 32% and 63% of the power emitted to + direction can be collected by a NA=0.95 lens, respectively for Hodd cavity and Heven cavity.

3. Ultra-high Q, dielectric-mode photonic crystal nanobeam cavities

3.1. Radiation-Q limited and waveguide-coupled cavities

Since the dielectric-centered Hzodd cavity has smaller V than the Hzeven one, we focus our discussion in the Hzodd case. Using the above design algorithm, we design the Gaussian mirror and put 10 additional mirrors with the maximum mirror strength on both ends of the Gaussian mirror to obtain the radiation-limited cavity (Q wg >> Q tot). We find in Fig. 5 that Q tot increases exponentially and V increases linearly as the total number of mirror pairs in the Gaussian mirror (N) increases. A record ultra-high Q of 5.0 × 109 is achieved while maintaining the small mode volume of 0.9 × (λ res/n Si)3 at N = 30.

Fig. 5 (a) Total Q-factors (log(10) scale) and effective mode volumes (V/(λ res/n Si)3) of nanobeam cavities for different total number of mirror pair segments in the Gaussian mirror. In each case, 10 additional mirror segments with f=0.1 (maximum mirror strength) are added on both ends of the Gaussian mirror. Therefore, the total-Q of the cavity is limited by radiation-Q. A record ultra-high Q of 5.0 × 109 is achieved with a Gaussian mirror that comprises 30 mirror segments and an additional 10 mirror pairs on both ends. (b) On-resonance transmissions and total Q-factors (log(10) scale) v.s the total number of mirror pair segments in the Gaussian mirror. In this case additional mirror pairs (10 of them) are not included. A record high-T (97%) and high-Q (1.3 × 107) cavity is achieved at N = 25.

Our design strategy has an additional important advantage over other types of photonic crystal cavities [17

17. 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]

31

31. P. B. Deotare, M. W. McCutcheon, I. W. Frank, M. Khan, and M. Loncar, “High quality factor photonic crystal nanobeam cavities,” Appl. Phys. Lett. 94, 121106 (2009). [CrossRef]

], that is: the cavity naturally couples to the feeding waveguide, as the hole radii decrease away from the center of the cavity. High-Q and high transmissions (T) cavities are possible with the above design steps (i)–(ix), with no additional “coupling sections” needed. We study T and Q total dependence on the total number of mirror pair segments in the Gaussian mirror (N) in Fig. 5(b). Partial Q-factors (Q rad, Q wg) are obtained from FDTD simulations, and T is obtained using T=Qtotal2/Qwg2 [41

41. J. D. Joannopoulos, S. G. Johnson, J. N. Winn, and R. D. Meade, Photonic Crystals: Molding the Flow of Light, 2nd Ed. (Cambridge University Press, 2007).

]. As shown in Fig. 5(b), we achieve a nanobeam cavity with Q = 1.3 × 107, T = 97% at N = 25.

3.2. Higher order modes of the dielectric-mode cavity

The ultra-high Q mode that we deterministically designed is the fundamental mode of the cavity. Meanwhile, higher order cavity modes also exist. The number of higher order modes depends on the width of the photonic band gap and total number of mirror segments in the Gaussian mirror. To reduce the simulation time, we study the higher order modes of a waveguide-coupled cavity, that has a total number of 12 mirror pair segments, possessing a moderate Q-factor. Figure 6(a) shows the transmission spectrum obtained from FDTD simulation, by exciting the input waveguide with a waveguide mode, and monitoring the transmission through the cavity at the output waveguide. The band-edge modes are observed at wavelengths longer than 1.6μm and shorter than 1.3μm. Figures 6(b)–6(d) shows the major field-component (Ey) distribution of the three cavity modes. As expected, the eigenmodes alternate between symmetric and anti-symmetric modes. Symmetry plane is defined perpendicular to the beam direction, in the middle of the cavity (dashed line in Fig. 6). The total Q-factors of modes I–III are 10,210, 1,077 and 286 respectively. Effective mode volumes of them are 0.55, 0.85 and 1.06 respectively. We note that transversely odd modes are well separated from the transversely symmetric cavity modes, hence were not considered in Fig. 6.

Fig. 6 (a) Transmission spectrum of the cavity from FDTD simulation. (b)–(d) The Ey field distribution in the middle plain of the nanobeam cavity. Resonances and symmetries of the modes are indicated in the plot. Symmetry plane is indicated by the dashed line. Length unit in (b)–(d) is μm.

4. Ultra-high Q, air-mode photonic crystal nanobeam cavities

4.1. Radiation-Q limited cavity

An air-mode cavity concentrates the optical energy in the low index region of the cavity. Therefore, these cavities are of interest for applications where strong interactions between light and material placed in the low index region of the cavity is required, including nonlinear optics [4

4. J. Leuthold, C. Koos, and W. Freude, “Nonlinear silicon photonics,” Nat. Photonics 4, 535–544 (2010). [CrossRef]

], optical trapping [7

7. D. G. Grier, “A revolution in optical manipulationm,” Nature 424, 21–27 (2003). [CrossRef]

], biochemical sensing [8

8. D. Psaltis, S. R. Quake, and C. Yang, “Developing optofluidic technology through the fusion of microfluidics and optics,” Nature 442, 381–386 (2006). [CrossRef] [PubMed]

] and light-atom interaction [43

43. J. Vuckovic, M. Loncar, H. Mabuchi, and A. Scherer, “Optimization of three-dimensional micropost microcavities for cavity quantum electrodynamics,” Phys. Rev. E 65, 016608 (2001).

]. The ultra-high Q air-mode nanobeam cavity is realized by pulling the air-band mode of photonic crystal into its bandgap, which can also be designed using the same design principles that we developed for dielectric-mode cavities. In contrast to the dielectric-mode case, the resonant frequency of the air-mode cavity is determined by the air band-edge frequency of the central mirror segment. Then, to create the Gaussian confinement, the bandgaps of the mirror segments should shift to higher frequencies as their distances from the center of the cavity increase. This can be achieved by progressively increasing the filling fractions of the mirror segments away from the center of the structure (instead of decreasing in the dielectric-mode cavity case). One way to accomplish this is to increase the size of the holes away from the center of the cavity. While this may be suitable for non-waveguide coupled (radiation-Q limited) cavities, it is not ideal for a waveguide-coupled cavity. For this reason, we employ the design that relies on tapering of the waveguide width instead of the hole size. Similar geometry was recently proposed by Ahn et. al. [44

44. B. H. Ahn, J. H. Kang, M. K. Kim, J. H. Song, B. Min, K. S. Kim, and Y. H. Lee, “One-dimensional parabolic-beam photonic crystal laser,” Opt. Express 18, 5654–5660 (2010). [CrossRef] [PubMed]

] for the design of a dielectric-mode photonic crystal laser.

Fig. 7 (a) TE band diagram for an air-mode nanobeam cavity. Hole radii r = 100nm, a = 330nm, b=1μm (red) and b=0.7μm (black). (b) Mirror strengths for different beam widths. (c) Linearization of mirror strengths after quadratic tapering the beam widths.

Similar in the dielectric-mode cavity cases, Hzodd and Hzeven air-mode cavities can be formed by placing the air and dielectric in the central symmetric plane of the cavity, respectively. Again, we will focus on Hzodd, air-mode cavities and the conclusions will be valid to the Hzeven cavities as well. Figure 8(a) shows the total Q of nanobeam cavities that have different total number of mirror pair segments in the Gaussian mirrors. We have achieved a record ultra-high Q of 1.4 × 109, air-mode nanobeam cavity. As shown in Fig. 8(a), the effective mode volumes of the air-mode cavities are much larger than the dielectric-mode cavities.

Fig. 8 (a) Total Q-factors (log(10) scale) and effective mode volumes (V/(λ res/n Si)3) of the nanobeam cavities for different total number of mirror pair segments in the Gaussian mirror. In each case, 10 additional mirror segments with w=0.7μm are added on both ends of the Gaussian mirror, so that the total-Q of the cavity is limited by radiation-Q. A record ultra-high Q of 1.4 × 109 is achieved with a Gaussian mirror that comprises 30 mirror segments and 10 additional mirror pairs on both ends. (b) On-resonance transmissions and total Q-factors (log(10) scale) v.s the total number of mirror pair segments in the Gaussian mirror. In this case additional mirror pairs (10 of them) are not included. A record high-T (96%) and high-Q (3.0 × 106) cavity is achieved at N = 25.

4.2. Cavity strongly coupled to the feeding waveguide

As we have pointed out, the tapering-width approach (as compared to taping hole radii) offers a natural way of coupling the nanobeam air-mode cavity to the feeding waveguide. Since the width of the beam is decreasing, the cavity naturally couples to the feeding waveguide. We study T and Q total dependence on the total number of mirror pair segments in the Gaussian mirror (N) using FDTD simulations. As shown in Fig. 8(b), we achieve nanobeam cavity with Q = 3.0 × 106, T = 96% at N = 25.

4.3. Higher order modes of the air-mode cavity

The ultra-high Q cavity that we were able to design is the fundamental mode of the cavity. Higher order modes coexist with the fundamental modes inside the band gap. Fig. 9(a) shows the transmission spectrum of a waveguide-coupled air-mode nanobeam cavity, that has 15 mirror pair segments in the Gaussian mirror. The band-edge modes are observed at wavelengths longer than 1.6μm. The modes in the range of 1.2μm to 1.35μm are formed by the higher order band modes in Fig. 7(a). Figures 9(b)–9(c) show the major field-component distribution (Ey) of the two cavity modes inside the bandgap. The total Q-factors of these two modes are 23,935 and 5,525 respectively. The effective mode volumes are 2.32 and 3.01 respectively.

Fig. 9 (a) Transmission spectrum of the cavity from FDTD simulation. (b)&(c) The Ey field distribution in the middle plain of the nanobeam cavity. Resonances and symmetries of the modes are indicated in the plot. Symmetry plane is indicated by the dashed line. Length unit in (b)&(c) is μm.

5. Conclusion

Acknowledgments

We acknowledge numerous fruitful discussions with M.W. MuCutcheon and P. B. Deotare. This work is supported by NSF Grant No. ECCS-0701417 and NSF CAREER grant.

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S. John, “Strong localization of photons in certain disordered dielectric superlattices,” Phys. Rev. Lett. 58, 2486 (1987). [CrossRef] [PubMed]

11.

J. S. Foresi, P. R. Villeneuve, J. Ferrera, E. R Thoen, G. Steinmeyer, S. Fan, J. D. Joannopoulos, L. C. Kimerling, H. I. Smith, and E. P. Ippen, “Photonic-bandgap microcavities in optical waveguides,” Nature 390, 143–145 (1997). [CrossRef]

12.

O. Painter, R. K. Lee, A. Scherer, A. Yariv, J. D. O’Brien, P. D. Dapkus, and I. Kim, “Two-dimensional photonic band-gap defect mode laser,” Science 284, 1819–1821 (1999). [CrossRef] [PubMed]

13.

J. Ctyroky, “Photonic bandgap structures in planar waveguides,” J. Opt. Soc. Am. A 18, 435–441 (2001). [CrossRef]

14.

M. R. Watts, S. G. Johnson, H. A. Haus, and J. D. Joannopoulos, “Electromagnetic cavity with arbitrary Q and small modal volume without a complete photonic bandgap,” Opt. Lett. 27, 1785–1787 (2002). [CrossRef]

15.

J. M. Geremia, J. Williams, and H. Mabuchi, “Inverse-problem approach to designing photonic crystals for cavity QED experiments,” Phys. Rev. E 66, 066606 (2002). [CrossRef]

16.

M. Burger, S. J Osher, and E. Yablonovitch, “Inverse problem techniques for the design of photonic crystals,” IEICE Trans. Electron. E87C, 258–265 (2004).

17.

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]

18.

B. S. Song, S. Noda, T. Asano, and Y. Akahane, “Ultra-high-Q photonic double-heterostructure nanocavity,” Nat. Mater. 4, 207–210 (2005). [CrossRef]

19.

S. Tomljenovic-Hanic, C. M. de Sterke, and M. J. Steel, “Design of high-Q cavities in photonic crystal slab heterostructures by air-holes infiltration,” Opt. Express 14, 12451–12456 (2006). [CrossRef] [PubMed]

20.

E. Kuramochi, M. Notomi, S. Mitsugi, A. Shinya, T. Tanabe, and T. Watanabe, “Ultrahigh-Q photonic crystal nanocavities realized by the local width modulation of a line defect,” Appl. Phys. Lett. 88, 041112 (2006). [CrossRef]

21.

K. Nozaki, S. Kita, and T. Baba, “Room temperature continuous wave operation and controlled spontaneous emission in ultrasmall photonic crystal nanolaser,” Opt. Express 15, 7506–7514 (2007). [CrossRef] [PubMed]

22.

Y. Tanaka, T. Asano, and S. Noda, “Design of photonic crystal nanocavity with Q-factor of ∼109,” J. Lightwave Technol. 26, 1532 (2008). [CrossRef]

23.

M. Notomi, E. Kuramochi, and H. Taniyama, “Ultrahigh-Q nanocavity with 1D Photonic Gap,” Opt. Express , 16, 11095 (2008). [CrossRef] [PubMed]

24.

P. Velha, E. Picard, T. Charvolin, E. Hadji, J. C. Rodier, P. Lalanne, and E. Peyrage, “Ultra-high Q/V Fabry-Perot microcavity on SOI substrate,” Opt. Express 15, 16090–16096 (2007). [CrossRef] [PubMed]

25.

S. Reitzenstein, C. Hofmann, A. Gorbunov, M Strauß, S. H. Kwon, C. Schneider, A. Loffler, S. Hofling, M. Kamp, and A. Forchel, “AlAs/GaAs micropillar cavities with quality factors exceeding 150000,” Appl. Phys. Lett. 90, 251109 (2007). [CrossRef]

26.

A. R. Md Zain, N. P. Johnson, M. Sorel, and R. M. De La Rue, “Ultra high quality factor one dimensional photonic crystal/photonic wire microcavities in silicon-on-insulator (SOI),” Opt. Express 16, 12084 (2008). [CrossRef]

27.

Y. Zhang and M. Loncar, “Ultra-high quality factor optical resonators based on semiconductor nanowires.” Opt. Express 16, 17400–17409 (2008). [CrossRef] [PubMed]

28.

M. W. McCutcheon and M. Loncar, “Design of a silicon nitride photonic crystal nanocavity with a Quality factor of one million for coupling to a diamond nanocrystal,” Opt. Express 16, 19136–19145 (2008). [CrossRef]

29.

L. D. Haret, T. Tanabe, E. Kuramochi, and M. Notomi, “Extremely low power optical bistability in silicon demonstrated using 1D photonic crystal nanocavity,” Opt. Express 17, 21008–21117 (2009). [CrossRef]

30.

J. Chan, M. Eichenfield, R. Camacho, and O. Painter, “Optical and mechanical design of a “zipper” photonic crystal optomechanical cavity”, Opt. Express 17, 3802–3817 (2009). [CrossRef] [PubMed]

31.

P. B. Deotare, M. W. McCutcheon, I. W. Frank, M. Khan, and M. Loncar, “High quality factor photonic crystal nanobeam cavities,” Appl. Phys. Lett. 94, 121106 (2009). [CrossRef]

32.

Q. Quan, P. B. Deotare, and M. Loncar, “Photonic crystal nanobeam cavity strongly coupled to the feeding waveguide,” Appl. Phys. Lett. 96, 203102 (2010). [CrossRef]

33.

E. Kuraamochi, H. Taniyama, T. Tanabe, K. Kawasaki, Y-G. Roh, and M. Notomi, “Ultrahigh-Q one-dimensional photonic crystal nanocavities with modulated mode-gap barriers on SiO2 claddings and on air claddings,” Opt. Express 18, 15859–15869 (2010). [CrossRef]

34.

Q. Quan, I. B. Burgess, S. K. Y. Tang, D. L. Floyd, and M. Loncar, “High-Q/V photonic crystal nanobeam cavities in an ultra-low index-contrast polymeric optofluidic platform,” arXiv:1108.2669 (2010).

35.

J. Vuckovic, M. Loncar, H. Mabuchi, and A. Scherer, “Optimization of the Q factor in photonic crystal microcavities,” IEEE J. Quantum Electron. 38, 850–856 (2002). [CrossRef]

36.

K. Srinivasan and O. Painter, “Momentum space design of high-Q photonic crystal optical cavities,” Opt. Express 10, 670–684 (2002). [PubMed]

37.

D. Englund, I. Fushman, and J. Vuckovic, “General recipe for designing photonic crystal cavities,” Opt. Express 13, 5961–5975 (2005). [CrossRef] [PubMed]

38.

M. Palamaru and P. Lalanne, “Photonic crystal waveguides: Out-of-plane losses and adiabatic modal conversion,” Appl. Phys. Lett. 78, 1466–1468 (2001). [CrossRef]

39.

P. Lalanne, S. Mias, and J. P. Hugonin, “Two physical mechanisms for boosting the quality factor to cavity volume ratio of photonic crystal microcavities,” Opt. Express 12, 458–467 (2004). [CrossRef] [PubMed]

40.

K. Sakoda, Optical Properties of Photonic Crystals, 2nd Ed. (Springer, 2005).

41.

J. D. Joannopoulos, S. G. Johnson, J. N. Winn, and R. D. Meade, Photonic Crystals: Molding the Flow of Light, 2nd Ed. (Cambridge University Press, 2007).

42.

S. G. Johnson, M. Ibanescu, M. A. Skorobogatiy, O. Weisbergs, J. D. Joannopoulos, and Y. Fink, “Perturbation theory for Maxwell’s equations with shifting material boundaries,” Phys. Rev. E 65, 066611 (2002). [CrossRef]

43.

J. Vuckovic, M. Loncar, H. Mabuchi, and A. Scherer, “Optimization of three-dimensional micropost microcavities for cavity quantum electrodynamics,” Phys. Rev. E 65, 016608 (2001).

44.

B. H. Ahn, J. H. Kang, M. K. Kim, J. H. Song, B. Min, K. S. Kim, and Y. H. Lee, “One-dimensional parabolic-beam photonic crystal laser,” Opt. Express 18, 5654–5660 (2010). [CrossRef] [PubMed]

45.

D. W. Vernooy, A. Furusawa, N. P. Georgiades, V. S. Ilchenko, and H. J. Kimble, “Cavity QED with high-Q whispering gallery modes,” Phys. Rev. A 57, R2293–R2296 (1998). [CrossRef]

46.

D. K. Armani, T. J. Kippenberg, S. M. Spillane, and K. J. Vahala, “Ultra-high-Q toroid microcavity on a chip,” Nature 421, 925–928 (2003). [CrossRef] [PubMed]

47.

M. Soltani, S. Yegnanarayanan, and A. Adibi, “Ultra-high Q planar silicon microdisk resonators for chip-scale silicon photonics,” Opt. Express 15, 4694–4704 (2007). [CrossRef] [PubMed]

OCIS Codes
(140.4780) Lasers and laser optics : Optical resonators
(230.5298) Optical devices : Photonic crystals
(230.7408) Optical devices : Wavelength filtering devices

ToC Category:
Photonic Crystals

History
Original Manuscript: May 23, 2011
Revised Manuscript: August 8, 2011
Manuscript Accepted: August 18, 2011
Published: September 8, 2011

Citation
Qimin Quan and Marko Loncar, "Deterministic design of wavelength scale, ultra-high Q photonic crystal nanobeam cavities," Opt. Express 19, 18529-18542 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-19-18529


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References

  1. Quality factor is defined as Q=ω0Energy storedPower loss, and mode volume is defined as V = ∫ dVɛ|E|2/[ɛ|E|2]max.
  2. K. J. Vahala, “Optical microcavities,” Nature424, 839–846 (2003). [CrossRef] [PubMed]
  3. J. L. O’Brien, A. Furusawa, and J. Vuckovic, “Photonic quantum technologies,” Nat. Photonics3, 687–695 (2009). [CrossRef]
  4. J. Leuthold, C. Koos, and W. Freude, “Nonlinear silicon photonics,” Nat. Photonics4, 535–544 (2010). [CrossRef]
  5. M. Eichenfield, J. Chan, R. Camacho, K. J. Vahala, and O. Painter, “Optomechanical crystals,” Nature462, 78–82 (2009). [CrossRef] [PubMed]
  6. D. Van Thourhout and J. Roels, “Optomechanical device actuation through the optical gradient force,” Nat. Photonics4, 211–217 (2010). [CrossRef]
  7. D. G. Grier, “A revolution in optical manipulationm,” Nature424, 21–27 (2003). [CrossRef]
  8. D. Psaltis, S. R. Quake, and C. Yang, “Developing optofluidic technology through the fusion of microfluidics and optics,” Nature442, 381–386 (2006). [CrossRef] [PubMed]
  9. E. Yablonovitch, “Inhibited spontaneous emission in solid-state physics and electronics,” Phys. Rev. Lett.58, 2059–2062 (1987). [CrossRef] [PubMed]
  10. S. John, “Strong localization of photons in certain disordered dielectric superlattices,” Phys. Rev. Lett.58, 2486 (1987). [CrossRef] [PubMed]
  11. J. S. Foresi, P. R. Villeneuve, J. Ferrera, E. R Thoen, G. Steinmeyer, S. Fan, J. D. Joannopoulos, L. C. Kimerling, H. I. Smith, and E. P. Ippen, “Photonic-bandgap microcavities in optical waveguides,” Nature390, 143–145 (1997). [CrossRef]
  12. O. Painter, R. K. Lee, A. Scherer, A. Yariv, J. D. O’Brien, P. D. Dapkus, and I. Kim, “Two-dimensional photonic band-gap defect mode laser,” Science284, 1819–1821 (1999). [CrossRef] [PubMed]
  13. J. Ctyroky, “Photonic bandgap structures in planar waveguides,” J. Opt. Soc. Am. A18, 435–441 (2001). [CrossRef]
  14. M. R. Watts, S. G. Johnson, H. A. Haus, and J. D. Joannopoulos, “Electromagnetic cavity with arbitrary Q and small modal volume without a complete photonic bandgap,” Opt. Lett.27, 1785–1787 (2002). [CrossRef]
  15. J. M. Geremia, J. Williams, and H. Mabuchi, “Inverse-problem approach to designing photonic crystals for cavity QED experiments,” Phys. Rev. E66, 066606 (2002). [CrossRef]
  16. M. Burger, S. J Osher, and E. Yablonovitch, “Inverse problem techniques for the design of photonic crystals,” IEICE Trans. Electron.E87C, 258–265 (2004).
  17. Y. Akahane, T. Asano, B. S. Song, and S. Noda, “High-Q photonic nanocavity in a two-dimensional photonic crystal,” Nature425, 944–947 (2003). [CrossRef] [PubMed]
  18. B. S. Song, S. Noda, T. Asano, and Y. Akahane, “Ultra-high-Q photonic double-heterostructure nanocavity,” Nat. Mater.4, 207–210 (2005). [CrossRef]
  19. S. Tomljenovic-Hanic, C. M. de Sterke, and M. J. Steel, “Design of high-Q cavities in photonic crystal slab heterostructures by air-holes infiltration,” Opt. Express14, 12451–12456 (2006). [CrossRef] [PubMed]
  20. E. Kuramochi, M. Notomi, S. Mitsugi, A. Shinya, T. Tanabe, and T. Watanabe, “Ultrahigh-Q photonic crystal nanocavities realized by the local width modulation of a line defect,” Appl. Phys. Lett.88, 041112 (2006). [CrossRef]
  21. K. Nozaki, S. Kita, and T. Baba, “Room temperature continuous wave operation and controlled spontaneous emission in ultrasmall photonic crystal nanolaser,” Opt. Express15, 7506–7514 (2007). [CrossRef] [PubMed]
  22. Y. Tanaka, T. Asano, and S. Noda, “Design of photonic crystal nanocavity with Q-factor of ∼109,” J. Lightwave Technol.26, 1532 (2008). [CrossRef]
  23. M. Notomi, E. Kuramochi, and H. Taniyama, “Ultrahigh-Q nanocavity with 1D Photonic Gap,” Opt. Express, 16, 11095 (2008). [CrossRef] [PubMed]
  24. P. Velha, E. Picard, T. Charvolin, E. Hadji, J. C. Rodier, P. Lalanne, and E. Peyrage, “Ultra-high Q/V Fabry-Perot microcavity on SOI substrate,” Opt. Express15, 16090–16096 (2007). [CrossRef] [PubMed]
  25. S. Reitzenstein, C. Hofmann, A. Gorbunov, M Strauß, S. H. Kwon, C. Schneider, A. Loffler, S. Hofling, M. Kamp, and A. Forchel, “AlAs/GaAs micropillar cavities with quality factors exceeding 150000,” Appl. Phys. Lett.90, 251109 (2007). [CrossRef]
  26. A. R. Md Zain, N. P. Johnson, M. Sorel, and R. M. De La Rue, “Ultra high quality factor one dimensional photonic crystal/photonic wire microcavities in silicon-on-insulator (SOI),” Opt. Express16, 12084 (2008). [CrossRef]
  27. Y. Zhang and M. Loncar, “Ultra-high quality factor optical resonators based on semiconductor nanowires.” Opt. Express16, 17400–17409 (2008). [CrossRef] [PubMed]
  28. M. W. McCutcheon and M. Loncar, “Design of a silicon nitride photonic crystal nanocavity with a Quality factor of one million for coupling to a diamond nanocrystal,” Opt. Express16, 19136–19145 (2008). [CrossRef]
  29. L. D. Haret, T. Tanabe, E. Kuramochi, and M. Notomi, “Extremely low power optical bistability in silicon demonstrated using 1D photonic crystal nanocavity,” Opt. Express17, 21008–21117 (2009). [CrossRef]
  30. J. Chan, M. Eichenfield, R. Camacho, and O. Painter, “Optical and mechanical design of a “zipper” photonic crystal optomechanical cavity”, Opt. Express17, 3802–3817 (2009). [CrossRef] [PubMed]
  31. P. B. Deotare, M. W. McCutcheon, I. W. Frank, M. Khan, and M. Loncar, “High quality factor photonic crystal nanobeam cavities,” Appl. Phys. Lett.94, 121106 (2009). [CrossRef]
  32. Q. Quan, P. B. Deotare, and M. Loncar, “Photonic crystal nanobeam cavity strongly coupled to the feeding waveguide,” Appl. Phys. Lett.96, 203102 (2010). [CrossRef]
  33. E. Kuraamochi, H. Taniyama, T. Tanabe, K. Kawasaki, Y-G. Roh, and M. Notomi, “Ultrahigh-Q one-dimensional photonic crystal nanocavities with modulated mode-gap barriers on SiO2 claddings and on air claddings,” Opt. Express18, 15859–15869 (2010). [CrossRef]
  34. Q. Quan, I. B. Burgess, S. K. Y. Tang, D. L. Floyd, and M. Loncar, “High-Q/V photonic crystal nanobeam cavities in an ultra-low index-contrast polymeric optofluidic platform,” arXiv:1108.2669 (2010).
  35. J. Vuckovic, M. Loncar, H. Mabuchi, and A. Scherer, “Optimization of the Q factor in photonic crystal microcavities,” IEEE J. Quantum Electron.38, 850–856 (2002). [CrossRef]
  36. K. Srinivasan and O. Painter, “Momentum space design of high-Q photonic crystal optical cavities,” Opt. Express10, 670–684 (2002). [PubMed]
  37. D. Englund, I. Fushman, and J. Vuckovic, “General recipe for designing photonic crystal cavities,” Opt. Express13, 5961–5975 (2005). [CrossRef] [PubMed]
  38. M. Palamaru and P. Lalanne, “Photonic crystal waveguides: Out-of-plane losses and adiabatic modal conversion,” Appl. Phys. Lett.78, 1466–1468 (2001). [CrossRef]
  39. P. Lalanne, S. Mias, and J. P. Hugonin, “Two physical mechanisms for boosting the quality factor to cavity volume ratio of photonic crystal microcavities,” Opt. Express12, 458–467 (2004). [CrossRef] [PubMed]
  40. K. Sakoda, Optical Properties of Photonic Crystals, 2nd Ed. (Springer, 2005).
  41. J. D. Joannopoulos, S. G. Johnson, J. N. Winn, and R. D. Meade, Photonic Crystals: Molding the Flow of Light, 2nd Ed. (Cambridge University Press, 2007).
  42. S. G. Johnson, M. Ibanescu, M. A. Skorobogatiy, O. Weisbergs, J. D. Joannopoulos, and Y. Fink, “Perturbation theory for Maxwell’s equations with shifting material boundaries,” Phys. Rev. E65, 066611 (2002). [CrossRef]
  43. J. Vuckovic, M. Loncar, H. Mabuchi, and A. Scherer, “Optimization of three-dimensional micropost microcavities for cavity quantum electrodynamics,” Phys. Rev. E65, 016608 (2001).
  44. B. H. Ahn, J. H. Kang, M. K. Kim, J. H. Song, B. Min, K. S. Kim, and Y. H. Lee, “One-dimensional parabolic-beam photonic crystal laser,” Opt. Express18, 5654–5660 (2010). [CrossRef] [PubMed]
  45. D. W. Vernooy, A. Furusawa, N. P. Georgiades, V. S. Ilchenko, and H. J. Kimble, “Cavity QED with high-Q whispering gallery modes,” Phys. Rev. A57, R2293–R2296 (1998). [CrossRef]
  46. D. K. Armani, T. J. Kippenberg, S. M. Spillane, and K. J. Vahala, “Ultra-high-Q toroid microcavity on a chip,” Nature421, 925–928 (2003). [CrossRef] [PubMed]
  47. M. Soltani, S. Yegnanarayanan, and A. Adibi, “Ultra-high Q planar silicon microdisk resonators for chip-scale silicon photonics,” Opt. Express15, 4694–4704 (2007). [CrossRef] [PubMed]

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