Design of Amorphous Silicon Photonic Crystal-based M-Z Modulator

Operating at 1.55 µm

Sandro Rao

, Maurizio Casalino

, Giuseppe Coppola

, Rifat Kisacik

Tolga Tekin

and Francesco G. Della Corte

1,2

Università degli Studi “Mediterranea”, Dipartimento di Ingegneria dell'Informazione, delle Infrastrutture e dell'Energia

Sostenibile (DIIES), Via Graziella Feo di Vito, 89122, Reggio Calabria, Italy

Institute for Microelectronics and Microsystems – Consiglio Nazionale delle Ricerche (IMM-CNR)

Via Castellino, 111, 80132, Napoli, Italy

Fraunhofer Institute for Reliability and Microintegration (IZM), System Integration and Interconnection Technologies,

Gustav-Meyer-Allee 25, 13355, Berlin, Germany

Keywords: Photonic Crystal, Electro-optic Modulator, Amorphous Silicon.

Abstract: The design of an amorphous silicon-based Mach–Zehnder electro-optic modulator including two guiding p-

i-n structures integrated inside a two-dimensional (2-D) photonic crystal (PhC) working at 1.55 µm, is

reported. Electrically induced free carrier dispersion effect in this photonic material with a very cost-effective

technology, is investigated for modulation. Our numerical analysis, performed by a time-domain (FDTD)-

based software, proves that the voltage-length product can be remarkably reduced by taking advantage of both

the strong PhC confinement and the wide refractive index tunability of amorphous silicon.

1 INTRODUCTION

The incorporation of optical phase modulators into

slow wave structures can offer advantages in terms of

both reduced device length and low power

consumption. This is due to the interaction

enhancement between the refractive index variation

mechanism and the propagating optical mode (Vlasov

et al., 2005). The latter property is particularly

interesting to develop highly-compact photonic

devices based on the engineered change of the signal

optical phase within the waveguide. Among the

different devices, the Mach–Zehnder interferometer

(MZI) acquires a fundamental importance because it

can be used as a basic building block of more complex

photonic devices such as optical filters, wavelength

multiplexers, intensity modulators, switches and

optical gates (Reed et al., 2010).

The technology of low-absorption hydrogenated

amorphous silicon (a-Si:H) has been recently

demonstrated to be a promising low cost and CMOS

compatible platform for combining electronic

integrated circuits with active optical functions, that

could make feasible the three-dimensional (3D)

integration of complex photonic-electronic integrated

circuits (PEICs) (Della Corte and Rao,2013).

Recently, an a-Si:H based MZI electro-optic (EO)

modulator has been experimentally demonstrated

(Rao et al., 2012). In such device we exploited the

free carrier depletion approach, within a reverse

biased waveguide integrated p-i-n diode, in order to

reduce the transient characteristics with respect to the

field-induced accumulation-type devices (Zelikson et

al., 1992; Della Corte et al., 2008; Rao et al., 2010;

Rao et al., 2013). Electrons and holes are in fact

quickly swept from the “active” layer where light

propagates when a bias is applied across the device.

Such configuration allowed to reach for the first

time a modulation bandwidth larger than 150 Mbps in

as-deposited a-Si:H-based devices (Rao et al., 2014).

The Factor of Merit (FoM) V

×L has been

measured to be about 40 V×cm, although in an

optimized setup with a reduced thickness of the

waveguide core, i.e. down to 400 nm-thick, the

modulating pulse amplitude, V

, is expected to

decrease by a factor of ~10 (Della Corte et al., 2011).

On the other hand, in order to get a modulation

efficiency enhancement, for a given fixed geometry,

the relative overlap between the optical field

distribution within the waveguide core and the

164

Rao, S., Casalino, M., Coppola, G., Kisacik, R., Tekin, T. and Corte, F.

Design of Amorphous Silicon Photonic Crystal-based M-Z Modulator Operating at 1.55 µm.

DOI: 10.5220/0005745101620168

In Proceedings of the 4th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2016), pages 164-170

ISBN: 978-989-758-174-8

depletion region might be maximized (Rao et al.,

2014).

It is well-known that an increase of the

confinement of the optical field inside a guiding

structure can be achieved by using photonic crystal

waveguides (PhCW) which prohibit the light

propagation for specific frequencies within a band-

gap, enabling therefore new ways to carry light to and

from components of a PEIC (Brimont et al., 2011;

Brimont et al. 2009). PhCW, moreover, exhibits near-

zero reflection, very low propagating losses through

sharp bends (Mekis et al., 1996) and reduced

radiation losses (Johnson et al., 1998).

In this paper, in order to take advantage from the

strong confinement provided by the PhC structures,

the performances of a novel MZI consisting of two

coupled p-i-n vertical integrated waveguides

surrounded by a two-dimensional (2D) PhC, are

performed by a finite-difference time-domain

(FDTD)-based software (RSoft).

In such design, we have taken full advantage of

the wide tunability of the a-Si:H refractive index,

achievable acting on the process parameters during

the Plasma Enhanced Chemical Vapour Deposition

(PECVD) (Cocorullo et al., 1996), a non-common

feature among materials used in photonics.

A schematic representation of the proposed device

is shown in Fig.1. The proposed MZI starts with a

single mode rib waveguide which splits into two

symmetric branches by means of an input splitter.

Hence, the two arms become parallels and are

surrounded by a proper PhC structure to increase the

propagating wave confinement in the waveguide

core. One arm acts as an optical phase shifter. To

achieve this, a transparent conductive oxide, e.g.

sputtered Al-doped ZnO (ZnO/Al) (Della Corte et al.,

2011; Rao, 2013), is deposited on the waveguide top

to apply a reverse bias to the integrated diode.

At the output, light beams interfere between each

other by means a directional coupler.

Figure 1: Schematic representation of the proposed device.

Dimensions are not in scale.

2 PHOTONIC CRYSTAL

WAVEG U ID ES DE SI G N

For the following analysis, a 2D PhC forming a

hexagonal lattice of air rods in an a-Si:H background

layer will be considered. It is known that the

hexagonal lattice pattern of a PhCW provides the

largest band gap among all PhCW geometries

allowing to obtain guiding structures which are single

mode (SM) and polarization independent (PI) at the

wavelength of 1.55 μm (Chen et al., 2009). Moreover,

a single-line defect waveguide (SLDW), created by

removing a full row of “cylinders”, leads to photonic

band gaps (PBGs) where the propagating

electromagnetic modes are forbidden inside the

structure. In order to obtain a PhC geometry with a

complete gap (Mekis et al., 1996) and PI (TE and

TM), the two materials, in this case air and a-Si:H,

should have a high refractive index contrast.

In Fig. 2 we report a generic schematic PhC

structure, with hexagonal lattice pattern, in which the

circular rods are filled with air. The bulk refractive

index was measured in our previous work and used

for the realization of a MZI EO modulator (Zelikson

et al., 1996; Della Corte 2011 et al.; Rao et al., 2013)

a-Si:H

=3.46).

Figure 2: Schematic PhC structure, with hexagonal lattice

pattern, and corresponding refractive index colour bar.

In our geometry, the r and a indicate the rod radius

and lattice constant, respectively. The lattice constant

was set to 1 μm and subsequently the rod radius

varied from 0.1 μm to 0.6 μm in order to find out the

optimum value resulting in the widest band-gap for

both TE and TM modes. The results of parametric

simulation are shown in Fig. 3.

Design of Amorphous Silicon Photonic Crystal-based M-Z Modulator Operating at 1.55 µm

165

Figure 3: Gap map for the hexagonal lattice PhC.

The photonic bandgap map shown in Fig. 3

introduces red, blue and green marked areas where

the TE, TM and both of those modes are not allowed

to propagate, respectively. The rod radius values

forbidding the propagation of both TE and TM modes

(green area) can be seen in detail in the up side of

Fig.3.

A rod radius of 0.48 μm provides the widest band

gap for our structure presented in Fig.2.

In Fig. 4, the resulting PhC structure is reported

together with its refractive index.

Figure 4: PhC structure, with hexagonal lattice pattern, and

corresponding refractive index colour bar.

Subsequently, the PhC structure presented in

Fig.4 is simulated through BandSOLVE [17] tool in

order to obtain the a/λ ratio and, therefore, the

forbidden wavelength range for an optical signal

propagating through the PhC. The simulation output

is reported in Fig. 5.

Fig. 5 reveals that the simulated PhC structure

does not allow the propagation of both TE and TM

modes for the hashed area where the a/λ ratio lies

between 0.448 to 0.530.

The achieved a/λ ratio values have been also

verified through the Fullwave [17] tool by analyzing

the stop band range for both TE and TM modes, as

reported in Fig. 6, where the red circles highlight the

normalized frequency ranges at which both TE (up)

and TM (down) modes cannot propagate.

By considering the a/λ ratio varying between

0.448 and 0.53, the forbidden wavelength area is

Figure 5: Photonic band structure for both TE and TM modes.

PHOTOPTICS 2016 - 4th International Conference on Photonics, Optics and Laser Technology

166

Figure 6: Stop band ranges. The red circles highlight the

normalized frequency ranges for TE (up) and TM (down)

modes.

calculated by the product of a/λ and r/a (0.48). The

resulting forbidden wavelength area for both TE and

TM modes covers the wavelengths between 1886 nm

and 2232 nm, out of around 1550 nm.

However, for the designed geometry, as shown in

Fig. 7, a very thin diaphragm (Delta=40 nm) between

two adjacent rods has been obtained, implying

consequently a careful and expensive process for the

fabrication of the PhC-based active device.

Figure 7: Air rods. “Delta” is the distance between two

adjacent rods.

To fully exploit the advantage of the refractive

index tunability of a-Si:H by changing the PECVD

process parameters (Cocorullo et al., 1996; Rao et al.,

2012), parametric simulations have been performed

starting from different values of the a-Si:H refractive

index in order to investigate if more “relaxed”, low

cost structures, i.e., less dependent on the fabrication

tolerances and technological constraints, could be

realized. Moreover, the other goal of the parametric

simulations is to evaluate if the resulting structures

exhibit polarization dependency.

Polarization dependency and the corresponding

forbidden wavelength area of various structures are

listed in Table 1.

From Table 1, it can be seen that for an a-Si:H

with refractive index n=3.58 (already measured as

reported in Ref. (Rao et al., 2010; Rao et al., 2014;

Rao et al., 2010; Rao et al., 2010; Rao et al., 2010;

Rao et al. 2012; Rao et al., 2012)) the spacing Delta

is increased by a factor of five with respect to the

value considered in the first simulation. Due to its

simple fabrication process and calculated forbidden

wavelength area, the PhC structure - radius r=0.4 μm,

lattice constant a =1 μm – has been chosen to

integrate in the MZI structure. The photonic bands of

such structure are shown in Fig.8.

Table 1: Parametric simulation results for different refractive index values.

n a[µm] r[µm] Delta[nm] TE TM λ

min

[µm] λ

max[

µm]

3.46 1 0.48 40 Yes Yes 1886 2232

3.46 0.76 0.36 40 Yes Yes 1496 1715

3.46 0.59 0.25 90 Yes Yes 1512 1573

3.46 0.47 0.18 110 No Yes 1245 1993

3.52 1 0.39 220 Yes No 1511 1586

3.58 1 0.4 200 Yes Yes 1531 1565

Design of Amorphous Silicon Photonic Crystal-based M-Z Modulator Operating at 1.55 µm

167

Figure 8: Photonic band structure for both TE and TM modes.

3 PHC MZI DESIGN

AND SIMULATION RESULTS

The designed PhCW has been integrated inside a MZI

in order to evaluate the phase shift, ΔΦ, induced in

the active arm by monitoring the output intensity,

which is given by (Chen et al., 2009):

where I

and I

out

are the MZI input and output

light beam intensities, respectively.

One can further derive the effective index change

Δneff of the waveguide mode using the equation:

where L is the active length of the phase shifter,

and λ is the wavelength in the free space. A schematic

of the PhCW structure, for Δn

eff

estimation, is

depicted in Fig. 9.

Figure 9: Schematic structure of the PhCW integrated

inside a MZI.

Parametric simulations were performed to

calculate the optimum values of the overall geometric

parameters reported in Fig. 9. In particular the

intersecting angle

for the input and output MZI

)cos(

Δ⋅+

inin

out

eff

Δ=Δ

PHOTOPTICS 2016 - 4th International Conference on Photonics, Optics and Laser Technology

168

splitter has been determined in order to achieve a

trade-off between the need for a compact device and

low insertion losses. The device geometries have

been optimized in order to achieve birefringence-free,

SM propagation and acceptable coupling losses to

standard SM fibers with cleaved termination.

Simulation results are well summarized in the

following table:

Table 2: Parametric simulation results. The reported device

geometries have been optimized in order to achieve

birefringence-free, SM propagation and low coupling

losses.

[μm]

a [μm] b [μm]

[°]

[μm]

1 100 4 573 0.8 100

Moreover, the impact of the refractive index

change induced in only one arm of the MZI

(surrounded by the PhC structure) was investigated

for different lengths. We designed, in fact, MZIs with

arm lengths of L=0.5 mm, 1 mm and 1.5 mm.

As already mentioned, the refractive index change

can be induced through the free carrier dispersion

effect and therefore it is controlled by the carrier

concentration profile. By reverse biasing the device,

we can induce a modulation of the space charge

volume that, in turn, modifies the refractive index

profile of the waveguide and therefore the optical

phase of the 1.55 μm wavelength light passing

through it (Zelikson, 1992).

Fig. 10 (a) shows the normalized optical signal at

the MZI output as a function of the effective

refractive index change (Δn

eff

) induced in MZI arms

of different lengths, while Fig. 10 (b) reports the

effective refractive index variation required to induce

a phase shift of ΔΦ=π, between the two optical

beams, for arms of different lengths.

By using the same Δn/ΔV calculated and

experimentally measured for the a-Si:H MZI reported

in our previous work (Zelikson et al., 1992),

characterized by an arm length of 13 mm, we can

conclude that with the newly designed PhC-based

device we can obtain a full π-shift in a 1.5 mm-long

arm by applying the same driving signal (~30V), with

a consequent reduction of the V

×L FoM from 40

V×cm to 4.5 V×cm.

It should be finally considered that a thinning of

the waveguide core can allow a further reduction of

the driving signal amplitude necessary for a π-shift,

as the bias to drive in full depletion a p-i-n device

scales with a square law of the thickness of the

intrinsic region, as reported in Fig. 11.

(a)

(b)

Figure 10: (a) Normalized transmitted MZI optical power

vs. effective refractive index variation (Δneff) and (b)

effective refractive index variation (Δneff(π)) for inducing

a phase shift of ΔΦ=π for the three considered MZI arm-

lengths.

Figure 11: Calculated full depletion bias of a p-i-n

waveguide as a function the i-layer doping. The three

curves refer to different i-layer thicknesses (Della Corte et

al., 2011).

The proposed device, therefore, can even work

with lower driving voltage allowing a FoM as low as

of that observed in performing electro-optical

modulators in silicon (Reed et al., 2010).

a-Si doping concentration [cm-3]

Full-depletion bias [V]

t = 500 nm

t = 1000 nm

t = 2000 nm

Design of Amorphous Silicon Photonic Crystal-based M-Z Modulator Operating at 1.55 µm

169

4 CONCLUSIONS

In this work, numerical simulations of a PhC MZ

modulator based on a-Si:H and working at 1.55 µm,

are reported. Our FDTD numerical simulations show

that taking advantage from both the tunability

property of the a-Si:H physical parameters and the

strong optical beam confinement within a PhCW, a

more efficient phase shifting can be obtained in the

interferometric structure. Our results show that the

FoM is enhanced of an order of magnitude with

respect to our previously realized active device based

on the electrically induced free carrier dispersion

effect. In fact, by reverse biasing the vertical p-i-n

diode integrated into the PhCWs, 1.5 mm-long, we

achieved a reduction of the V

×L voltage-length from

40 to 4.5 V×cm and we predicted as much reduction

for sub-micron waveguide core thickness. Moreover,

the corresponding PhC fabrication process does not

require sophisticated technological facilities leading

therefore to a truly low cost technology.

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