Engineered second-order nonlinearity in silicon nitride YI ZHANG1 JUNIYALI NAURIYAL2 MEITING SONG 1 MARISSA

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Engineered second-order nonlinearity in

silicon nitride

YI ZHANG,1 JUNIYALI NAURIYAL,2 MEITING SONG, 1 MARISSA

GRANADOS BAEZ, 1 XIAOTONG HE, 1 TIMOTHY MACDONALD3,

AND JAIME CARDENAS1*

1The Institute of Optics, University of Rochester, Rochester, NY 14627, USA

2Department of Electrical and Computer Engineering, University of Rochester, Rochester, NY 14627,

USA

3Department of Electrical and Computer Engineering, California State University, Northridge,

Northridge, CA 91330, USA

*jaime.cardenas@rochester.edu

Abstract: The lack of a bulk second-order nonlinearity (χ(2)) in silicon nitride (Si3N4) keeps

this low-loss, CMOS-compatible platform from key active functions such as Pockels electro-

optic (EO) modulation and efficient second harmonic generation (SHG). We demonstrate a

successful induction of χ(2) in Si3N4 through electrical poling with an externally-applied field to

align the Si-N bonds. This alignment breaks the centrosymmetry of Si3N4, and enables the bulk

χ(2). The sample is heated to over 500°C to facilitate the poling. The comparison between the

EO responses of poled and non-poled Si3N4, measured using a Si3N4 micro-ring modulator,

shows at least a 25X enhancement in the r33 EO component. The maximum χ(2) we obtain

through poling is 0.24pm/V. We observe a remarkable improvement in the speed of the

measured EO responses from 3GHz to 15GHz (3dB bandwidth) after the poling, which

confirms the χ(2) nature of the EO response induced by poling. This work paves the way for

high-speed active functions on the Si3N4 platform.

1. Introduction

Silicon nitride (Si3N4) is a high-performance platform for versatile on-chip photonic devices

[1-4] because of its low propagation loss, broad transparency window (400-6700nm [5]) and

good compatibility with complementary metal-oxide semiconductor (CMOS) processing.

However, Si3N4 lacks an intrinsic second-order nonlinearity (χ(2)) due to its centrosymmetric

structure [6], which limits its applications from some critical active functions, such as electro-

optic (EO) modulation and second harmonic generation (SHG).

Previous works have made great progress in realizing χ(2)-related phenomena on a silicon

nitride platform. Pioneer studies report a SHG signal that originates from symmetry breaking

at the interface of a multilayer stack of amorphous Si1-xNx:H films with various compositions

[7]. An electro-optic response is also observed in a stacked Si3N4 multilayer [8]. Other works

point out the existence of a bulk second-order nonlinearity in deposited Si3N4 films [9-10];

possible explanations include the presence of silicon nanocrystals [9] and a built-in static

electric field [10] that leads to an electric field induced second harmonic generation (EFISHG).

There are reports of SHG in Si-rich silicon nitride [11-12]. The use of high-quality micro-ring

resonators [13] and nano gratings [14], in which the optical field at the pump frequency is

amplified, further improves the efficiency of SHG in silicon nitride. Recently, an all-optical

poling (AOP) method [15-16] has demonstrated record-breaking SHG [17] and sum-frequency

conversion (SFC) [18] in Si3N4. The optical poling process in this approach automatically forms

quasi-phase matching (QPM) gratings in the Si3N4 waveguide [15-20] and the QPM condition

can be tuned by re-poling the device at the corresponding wavelength [19-20]. On the other

hand, to realize Pockels EO modulation on the Si3N4 platform, other works have explored

heterogeneous integration of other materials with a usable EO response, such as lithium niobate

(LN) [21], barium titanate (BTO) [22], lead zirconate titanate (PZT) [23], zinc oxide and zinc

sulfide [24], EO polymers [25], and two-dimensional (2D) materials [26-28], onto the Si3N4

platform. These approaches, however, complicate the fabrication process, introduce extra loss

and/or are not CMOS-compatible. Alternatively, a very recent work by Zabelich et. al. [29]

demonstrates an electro-optic response of tens of kHz on a pure Si3N4 platform induced by

electrical poling at elevated temperatures.

In this paper, we propose and demonstrate an induction of a second-order nonlinearity for

electro-optic modulation in silicon nitride through electrically poling the film and aligning the

Si-N bonds. We show an enhancement of at least 25X of the EO response in a Si3N4 micro-ring

resonator. The 3dB bandwidth of the EO response after poling improves from 3GHz to at least

15GHz. Further analysis supports that this enhancement directly derives from the induction of

χ(2) in Si3N4. Khurgin et al. [30] hypothesized that the Si-N bonds in Si3N4 possess a second-

order hyperpolarizability comparable to the Ga-As bonds in gallium arsenide (GaAs), whose

χ(2) is over 300pm/V [30]. Once the centrosymmetry in Si3N4 is broken, e.g., through electrical

poling, a non-trivial bulk second-order nonlinearity will appear and thus enable the Pockels

electro-optic effect. Electrical poling has been used to engineer material properties, such as to

periodically pole various nonlinear crystals for QPM [6] and to induce a second order

nonlinearity in polymers [31], silica glass [32], optical fibers [33], and very recently in Si3N4

[29].

Fig. 1. Top view (top cladding hidden, not to scale) and typical cross section (not to scale) of the

device (a) before and (b) after removal of Mo electrodes. A voltage is applied between the Mo

and the Pt electrode pair for poling and high-speed modulation, respectively. (c) Fabrication

process of the device.

2. Device design and fabrication

We pole silicon nitride by embedding a removable set of electrodes into the device. We deposit

a pair of Molybdenum (Mo) electrodes sandwiching the Si3N4 ring (Figure 1(a)). The gap

between the electrodes and the ring waveguide is 300nm (edge to edge). The Mo electrodes

generate an average horizontal electric field of 4.23×103V/cm per volt of bias applied in the

Si3N4 waveguide (simulated with COMSOL Multiphysics®). We apply 160V across the Mo

electrode pair and build a poling field of 0.676MV/cm. We choose Mo as the material for the

poling electrodes because it is compatible with the high temperatures reached during the poling

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Engineeredsecond-ordernonlinearityinsiliconnitrideYIZHANG,1JUNIYALINAURIYAL,2MEITINGSONG,1MARISSAGRANADOSBAEZ,1XIAOTONGHE,1TIMOTHYMACDONALD3,ANDJAIMECARDENAS1*1TheInstituteofOptics,UniversityofRochester,Rochester,NY14627,USA2DepartmentofElectricalandComputerEngineering,UniversityofRochester,Rocheste...

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Engineered second-order nonlinearity in silicon nitride YI ZHANG1 JUNIYALI NAURIYAL2 MEITING SONG 1 MARISSA

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