Junction-type photonic crystal waveguides for notch- and pass-band filtering

Junction-type photonic crystal waveguides for

notch- and pass-band filtering

Naeem Shahid,1,*

Muhammad Amin,1,2

Shagufta Naureen,1 Marcin Swillo,

1 and

Srinivasan Anand1

1School of Information and Communication Technology, Royal Institute of Technology (KTH), Electrum 229, 164 40

Kista, Sweden 2Current address: Division of Physical Sciences and Engineering, King Abdullah University of Science and

Technology, Thuwal, 23955-6900, Saudi Arabia

*[email protected]

Abstract: Evolution of the mode gap and the associated transmission mini

stop-band (MSB) as a function of photonic crystal (PhC) waveguide width

is theoretically and experimentally investigated. The change of line-defect

width is identified to be the most appropriate way since it offers a wide

MSB wavelength tuning range. A high transmission narrow-band filter is

experimentally demonstrated in a junction-type waveguide composed of two

PhC waveguides with slightly different widths. The full width at half

maximum is 5.6 nm; the peak transmission is attenuated by only ~5 dB and

is ~20 dB above the MSBs. Additionally, temperature tuning of the filter

were also performed. The results show red-shift of the transmission peak

and the MSB edges with a gradient of dλ/dT = 0.1 nm/°C. It is proposed that

the transmission MSBs in such junction-type cascaded PhC waveguides can

be used to obtain different types of filters.

©2011 Optical Society of America

OCIS codes: (130.2790) Guided waves; (130.3130) Integrated optics materials; (220.4830)

Systems design; (230.7390) Waveguides, planar; (250.5300) Photonic integrated circuits;

(260.2030) Dispersion; (050.5298) Photonic crystals.

References and links

1. T. Baba, “Slow light in photonic crystals,” Nat. Photonics 2(8), 465–473 (2008).

2. M. L. Povinelli, S. G. Johnson, and J. D. Joannopoulos, “Slow-light, band-edge waveguides for tunable time

delays,” Opt. Express 13(18), 7145–7159 (2005).

3. E. Schwoob, H. Benisty, C. Weisbuch, C. Cuisin, E. Derouin, O. Drisse, G. H. Duan, L. Legouézigou, O.

Legouézigou, and F. Pommereau, “Enhanced gain measurement at mode singularities in InP-based photonic

crystal waveguides,” Opt. Express 12(8), 1569–1574 (2004).

4. M. Notomi, K. Yamada, A. Shinya, J. Takahashi, C. Takahashi, and I. Yokohama, “Extremely large group-

velocity dispersion of line-defect waveguides in photonic crystal slabs,” Phys. Rev. Lett. 87(25), 253902 (2001).

5. J. Li, T. P. White, L. O’Faolain, A. Gomez-Iglesias, and T. F. Krauss, “Systematic design of flat band slow light

in photonic crystal waveguides,” Opt. Express 16(9), 6227–6232 (2008).

6. A. Di Falco, L. O’Faolain, and T. F. Krauss, “Dispersion control and slow light in slotted photonic crystal

waveguides,” Appl. Phys. Lett. 92(8), 083501 (2008).

7. B. Corcoran, C. Monat, M. Pelusi, C. Grillet, T. P. White, L. O’Faolain, T. F. Krauss, B. J. Eggleton, and D. J.

Moss, “Optical signal processing on a silicon chip at 640Gb/s using slow-light,” Opt. Express 18(8), 7770–7781

(2010).

8. M. Notomi, A. Shinya, K. Yamada, J. Takahashi, C. Takahashi, and I. Yokohama, “Structural tuning of guiding

modes of line-defect waveguides of silicon-on-insulator photonic crystal slabs,” IEEE J. Quantum Electron.

38(7), 736–742 (2002).

9. A. Shinya, M. Notomi, and E. Kuramochi, “Single-mode transmission in commensurate width-varied line-defect

SOI photonic crystal waveguides,” Proc. SPIE 5000, 125–135 (2003).

10. 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(4), 041112 (2006).

11. K. Inoshita and T. Baba, “Lasing at bend, branch and intersection of photonic crystal waveguides,” Electron.

Lett. 39(11), 844–846 (2003).

12. B. Song, S. Noda, T. Asano, and Y. Akahane, “Ultra-high-Q photonic double-heterostructure nanocavity,” Nat.

Mater. 4(3), 207–210 (2005).

#152344 - $15.00 USD Received 3 Aug 2011; revised 3 Sep 2011; accepted 20 Sep 2011; published 7 Oct 2011(C) 2011 OSA 10 October 2011 / Vol. 19, No. 21 / OPTICS EXPRESS 21074

13. H. Benisty, C. Cambournac, F. Van Laere, and D. Van Thourhout, “Photonic-crystal demultiplexer with

improved crosstalk by second-order cavity filtering,” J. Lightwave Technol. 28(8), 1201–1208 (2010).

14. E. Viasnoff-Schwoob, C. Weisbuch, H. Benisty, C. Cuisin, E. Derouin, O. Drisse, G.-H. Duan, L. Legouézigou,

O. Legouézigou, F. Pommereau, S. Golka, H. Heidrich, H. J. Hensel, and K. Janiak, “Compact wavelength

monitoring by lateral outcoupling in wedged photonic crystal multimode waveguides,” Appl. Phys. Lett. 86(10),

101107 (2005).

15. S. Olivier, M. Rattier, H. Benisty, C. Weisbuch, C. J. M. Smith, R. M. De La Rue, T. F. Krauss, U. Oesterle, and

R. Houdré, “Mini-stopbands of a one-dimensional system: The channel waveguide in a two-dimensional

photonic crystal,” Phys. Rev. B 63(11), 113311 (2001).

16. M. Mulot, S. Anand, M. Swillo, M. Qiu, B. Jaskorzynska, and A. Talneau, “Low-loss InP-based photonic-crystal

waveguides etched with Ar/Cl2 chemically assisted ion beam etching,” J. Vac. Sci. Technol. B 21(2), 900–903

(2003).

17. N. Shahid, N. Speijcken, S. Naureen, M. Y. Li, M. Swillo, and S. Anand, “Ultrasharp ministop-band edge for

subnanometer tuning resolution,” Appl. Phys. Lett. 98(8), 081112 (2011).

18. S. Johnson and J. D. Joannopoulos, “Block-iterative frequency-domain methods for Maxwell’s equations in a

planewave basis,” Opt. Express 8(3), 173–190 (2001).

19. M. Qiu, K. Azizi, A. Karlsson, M. Swillo, and B. Jaskorzynska, “Numerical studies of mode gaps and coupling

efficiency for line-defect waveguides in two-dimensional photonic crystals,” Phys. Rev. B 64(15), 155113

(2001).

20. M. Qiu, F2P software, http://www.imit.kth.se/info/FOFU/PC/F2P/

21. A. Berrier, M. Mulot, S. Anand, A. Talneau, R. Ferrini, and R. Houdré, “Characterization of the feature-size

dependence in Ar/Cl2 chemically assisted ion beam etching of InP-based photonic crystal devices,” J. Vac. Sci.

Technol. B 25(1), 1–10 (2007).

22. K. Ogusu and K. Takayama, “Transmission characteristics of photonic crystal waveguides with stubs and their

application to optical filters,” Opt. Lett. 32(15), 2185–2187 (2007).

23. B. Wild, R. Ferrini, R. Houdré, M. Mulot, S. Anand, and C. J. M. Smith, “Temperature tuning of the optical

properties of planar photonic crystal microcavities,” Appl. Phys. Lett. 84(6), 846–848 (2004).

1. Introduction

Waveguide devices based on photonic crystals (PhC) in the InP/InGaAsP/InP low index

contrast system are of practical importance due to the possibility of integration and

compatibility with optical sources at telecom wavelengths. Two-dimensional (2D) photonic

crystals (PhCs) with line defects offer unique waveguiding properties such as slow light [1–3],

dispersion engineering [4–6], non-linear enhancement and ultra-high bandwidth telecom

systems [7] at optical frequencies.

Structural tuning of single line-defect (W1) PhC waveguide, such as the defect width, has

been employed to change the location of mode gap [4,8,9]. The local width modulation of a

line defect has been utilized to obtain high-Q nanocavities [10]. Other types of mode-gap

confined nanocavities have also been reported. These include a hexagonal cavity terminated

by mode-gap waveguides [11] and a double-heterostructure PhC cavity in which the lattice

constant was changed at the interfaces [12]. In all these designs the mode-gap creates a local

confinement to achieve light trapping. The concept of PhC waveguide defect engineering has

been used for wavelength dispersion based devices and multimode PhC waveguides have

been utilized for applications like demultiplexers [13] and wavelength monitors [14]. These

designs make use of multiple junction PhC waveguides. Along the same lines investigation of

mode-gap in W3 waveguides having single junction across the interface could be interesting

for applications such as coarse wavelength selection, selective mirroring in edge emitting

lasers and fluid sensors.

One of the mode-gaps in the W3 PhC waveguides originates from contra-directional mode

coupling between the fundamental (0th order) and the 4th order mode [15,16], which appears

as a mini-stop band (MSB) in transmission. Recently, the transmission MSB in W3 PhC

waveguides has been shown to exhibit ultrasharp band-edges [17]. The investigation of MSB

by varying the line-defect width in incremental amounts (fraction of the defect width) could

be an efficient way to tune the spectral position without having to change the air-fill factor

appreciably. In this work, we present a theoretical and experimental investigation of the

ministop-bands in PhC waveguides, particularly its tunability for filtering and sensing

applications. Adjusting the line-defect width is used to obtain a wide wavelength tuning range


of the MSB filter. A single junction-type waveguide comprising of distinct combinations of

two line defect waveguides, differing by about ~20 nm in the widths is fabricated to realize a

high-transmission narrow band pass filter. The transmission MSBs in such junction-type

cascaded PhC waveguides may be attractive to design different types of filters. Finally, the

temperature tuning of the junction-type PhC filter device is experimentally demonstrated. The

sensitivity of the MSB in PhC waveguides to refractive index changes makes these devices an

attractive choice for sensing, tuning and modulation applications.

2. Design and simulations

We consider an InP/InGaAsP/InP heterostucture (with refractive indices n = 3.17 (InP) and

3.35 (InGaAsP)) with a 300 nm thick upper InP cladding and a 520 nm thick InGaAsP core.

The effective index for this vertical structure is neff = 3.2. A PhC waveguide created by

removing three rows from a triangular lattice of air-holes in the ΓK direction (typically

referred to as a W3 PhC waveguide) exhibits MSBs in transmission due to contra-directional

coupling between the fundamental mode and higher order even modes [17]. The width of the

PhC waveguide, denoted as dn, can be expressed as dn = (n + 1) × √3a/2 and the waveguide is

denoted as Wn. Here ‘a’ is lattice period and ‘n’ is a fraction which depends on the location of

the inner two rows of holes of the waveguide. For instance, for a W2.936 waveguide the width

d2.936 is 3.409a. PhC waveguides consisting of varied line defect widths are 120 periods long.

The defect widths are systematically reduced in steps of 0.1 from exactly 3 missing rows to

2.6 in a triangular lattice with period ‘a’. The period in case of W3 and W2.9 are 420 nm. For

W2.8, W2.7 and W2.6 the chosen lattice constants were 440, 450 and 460 nm respectively.

Fig. 1. (a) Dispersion diagram of the W3 PhC Waveguide; mode coupling region is indicated.

(b) Ex field profile of the fundamental (0th order) mode. (c) Ex field profile of the 4th order

mode. PWE calculation points are marked on dispersion curve in (a). Change of MSB edge

frequencies with the change in width of line defect for (d) low frequency MSB-edge (e) high

frequency MSB-edge.


Figure 1(a) shows the dispersion diagram calculated by the plane-wave expansion (PWE)

method for a W3 PhC waveguide. The PWE simulations [18] were made for an air-fill factor

of 40%. There are 6 guided modes (3 even modes and 3 odd modes) inside the bandgap.

Contra-directional mode coupling is defined by the overlap integral between the modes. Based

on this, in a PhC waveguide with axial symmetry like ~W3, mode coupling takes place only

between the modes of same symmetry (even or odd) [19] that give rise to a mode-gap. The

mode-gap due to the mode coupling between the 0th and 4th order modes (even modes)

occurs around u = 0.28, where u is the normalized frequency (u = a/λ). In the mode-gap

region, such W3 PhC waveguides exhibit a transmission MSB. Hereafter, the above referred

mode-gap will be simply referred to as MSB; and the frequency region where it occurs is

indicated on Fig. 1(a). The calculated profiles (Ex) of the two even modes, the 0th and 4th

order are shown on Figs. 1(b) and 1(c), respectively. These mode-profiles were obtained at the

points (b) and (c) [indicated on Fig. 1(a)] sufficiently far enough from the mode-gap region to

ensure that the obtained mode profiles represent those of the un-disturbed modes. Figures 1(d)

and 1(e) shows the respective frequency shifts of the lower and higher MSB-edge as a

function of waveguide width. As the waveguide width is decreased, both lower and higher

MSB-edges move to higher frequencies while maintaining a nearly constant mode gap.

Fig. 2. MSB central frequency vs. centre-to-centre distance between side rows, scaled in

multiple of lattice period ‘a’, of PhC waveguides. The dotted line represent MSB shift as

predicted by 2D FDTD calculations while the ( × ) signs mark the experimental positions.

To substantiate the results of PWE, the MSB positions are also deduced from the

transmission spectra simulated by the finite-difference time-domain (FDTD) method. We use

a 2-D FDTD method [20] with Perfect Matched Layer (PML) boundary treatment for

numerical simulation. The shift in MSB central frequency from W3 to W2.6 is shown by solid

line in Fig. 2. An alternate method of shifting the location of MSB inside bandgap is by

changing the air-fill factor. The cross marks indicate the determined central positions of

MSBs. The MSB positions obtained in the experiments are in good agreement with the

calculations performed by PWE and FDTD.

3. Fabrication and characterization

We used an InP-based heterostructure, consisting of a 520 nm GaInAsP core layer and capped

by a 300 nm-thick InP top cladding. This InP/GaInAsP/InP low index contrast slab was grown

by metal organic vapor phase epitaxy (MOVPE) on InP substrate. The fabricated photonic

crystal waveguides with different line defect widths were oriented along ГK direction. The

PhC waveguide section is inserted in between two 1.2 µm wide access-ridge waveguides,

each being about 1 mm long. The PhC waveguides were typically ~50 µm long. The PhC

patterns were made by electron beam lithography using ZEP520 as the resist. The patterns

were then transferred on to a 260 nm thick SiO2 mask using CHF3 based reactive ion etching.


Subsequently the sample was deeply-etched by Ar/Cl2 based chemically assisted ion-beam

etching (CAIBE). Details of the etching process and process conditions are given in [21]. The

air fill factor was about 40%, as determined from scanning electron microscopy (SEM)

images. The PhC waveguides were characterized by the end-fire technique. A tunable laser

source with wavelength range 1460-1580 nm was used as the light source, and was coupled

into the cleaved facet of the input access ridge waveguide through a focusing gradient index

lens. The output light is collected by a microscope objective and split into two beams, one to

an infrared camera for alignment and imaging, and, the other to an optical spectrum analyzer

through a single mode fiber to measure the transmission spectra. Polarizers were used at both

input and output of the sample to ensure that only TE mode is launched and collected

respectively.

Fig. 3. Measured Transmission spectra (normalized) for W3 to W2.6 PhC waveguides.

Normalization is performed with respect to transmission level outside MSB.

Figure 3 shows the measured transmission spectra of the waveguides - W3 to W2.6. The

characteristic dips in transmission due to the MSB effect are visible and the transmission

extinction ratios are more than 20 dB in all the waveguides measured. The MSB widths are

similar for the different waveguides except for W2.6 waveguide, which shows a slightly wider

transmission gap. This could be due to an increase in the coupling strength and due to

dispersion effects. The mode coupling condition for the occurrence of MSB is applicable for a

large range of normalized frequency inside photonic bandgap. A maximum frequency shift of

∆u = 2.82 × 10−2

is demonstrated. By choosing a lattice constant of 420 nm the corresponding

shift of 137 nm is obtained for the operating wavelength. MSB widths measured at minimum

transmission are ~12 nm. The experimentally determined wavelength tunability of the MSB

by changing the line-defect widths can be utilized to design junction-type waveguides to

obtain functions such as (relatively) narrow notch and band-pass filters.


3.1 Junction-type photonic crystal waveguide (JPCW)

Varying the line-defect width is an efficient way to adjust position of MSB over a large

wavelength range. As seen in Fig. 3 the MSB effect in these waveguides can be used as

efficient notch filters, and attenuation in excess of 20 dB can be obtained with just ~50 µm

long waveguides. A junction-type photonic crystal waveguide (JPCW) was designed and

fabricated to demonstrate a band-pass filter. JPCW is 120 period (~50 µm) long and is formed

by juxtaposing W3 and W2.936, each being a 60 period long PhC waveguide. Figure 4(a)

shows a SEM image of the fabricated JPCW at the junction region; the change in the

waveguide width is hard to distinguish owing to fact that the waveguide width shown in right

half part of the image is narrower by only ~20 nm. However, optical characteristics of the

waveguides are very sensitive to the waveguide width (Fig. 3).

Fig. 4. (a) SEM top view of a fabricated JPCW at the junction region; location of the junction is

indicated by a vertical line. (b) Normalized transmission spectrum for JPCW.

Figure 4(b) shows the measured transmission spectra of JPCW which shows two

characteristic dips in transmission due to the MSB effect. The wavelength range where MSBs

occur is in good agreement with results presented in Fig. 3. It is also visible that both

transmission notches have an extinction ratio of > 20dB. A high transmission peak between

two MSBs is evident with the full width at half maximum (FWHM) 5.6 nm. At the peak,

transmission is attenuated only by ~5 dB as compared to power level outside MSB. Even

though the above results are very promising, additional experiments to determine the total loss

of the device have to be performed. Nevertheless, since the present fabrication process is very

similar to that reported in [16], we estimate a propagation loss of about 1 dB/100µm [upper

bound] for the PhC waveguide. The peak transmission level between two MSBs is slightly

lower than outside MSBs due to small overlap between two MSBs. Such filter can be used as

course WDM filtering and also for selecting desired portion of the spectrum from broad-band

spontaneous emission sources. Photonic crystal waveguides with stubs have been studied for

their application to optical filters [22]. We anticipate that, by altering the position and/or

engineering the junction in the JPCW and by cascading to form multi-junction waveguides,

broad bandstop filters and narrow bandpass filters can be obtained. These will be the subject

matter of our future studies.

3.2 Temperature tuning of JPCW

The spectral position of the resonance peak in planar PhC microcavities has been utilized to

demonstrate temperature tuning [23]. Since the bandwidth of the transmission MSB is quite

broad (~12 nm) temperature tuning and spectral sensitivity was demonstrated using the sharp


MSB edge [17]. The notch position is not a sensitive measure of the spectral sensitivity. Here

we investigate the temperature tuning of the narrow transmission peak (Fig. 4(b)). The sample

is fixed onto metallic holder which is mounted on Peltier stage. The device is tested for

temperatures starting from room temperature up to 70 °C. Figure 5(a) shows that the

transmission peak red shifts with increasing temperature due to increase of refractive index.

The temperature coefficient for the peak wavelength ∆λ/∆T = 0.1 nm/°C is calculated by

linear fitting of the experimental point as shown in Fig. 5(b). This result shows potential

applications as tunable filters based on the thermal optical adjustment.

Fig. 5. (a) Transmission spectra for two representative temperatures; showing the red-shift of

the transmission peak with temperature increase. (b) Peak wavelength as a function of

temperature; the solid line is a linear fit to the experimental marks.

4. Conclusions

In conclusion, we have studied theoretically and experimentally ministop-bands in PhC

waveguides, particularly its tunability applications for filtering and sensing. By adjusting the

line-defect width, the wavelength of operation of the MSB could be tuned by 137 nm. MSB

widths measured at minimum transmission are ~12 nm. A single junction-type waveguide

comprising of distinct combinations of two line defect waveguides, differing by ~20 nm in the

widths was fabricated and a narrow band pass filter was demonstrated using this concept. A

5.6 nm FWHM is observed for the narrowband filter with transmission extinction ratio ~20

dB to the signal level corresponding to the MSBs. In addition, the peak transmission is

appreciably high; attenuated by only ~5 dB with respect to the transmission outside MSB.

Temperature tuning of the filter was experimentally investigated. Linear temperature

dependence is observed with gradient ∆λ/∆T = 0.1 nm/°C. High sensitivity for the line-defect

width and efficient mode coupling suggest the possibility of dispersion engineering. Since the

transmission characteristics are extremely sensitive to position of PhC holes, it can be used to

determine the precision and accuracy for defining these circles in an electron beam

lithography process. The sensitivity of the MSB in PhC waveguides to refractive index

changes makes these devices an attractive choice for sensing, tuning and modulation

applications.

Acknowledgments

This work was supported by the Swedish Research Council and the Swedish Strategic

Research Foundation. N. Shahid and S. Naureen acknowledge Higher Education Commission,

Pakistan for partially supporting their PhD studies.


Date post:	05-Oct-2016
Category:	Documents
Upload:	srinivasan
View:	214 times
Download:	1 times

Junction-type photonic crystal waveguides for notch- and pass-band filtering

Documents