LATEX template Sub-1 Volt and High-Bandwidth Visible to Near-Infrared Electro-Optic Modulators

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Sub-1 Volt and High-Bandwidth Visible to

Near-Infrared Electro-Optic Modulators

Dylan Renaud1*†, Daniel Rimoli Assumpcao1†, Graham

Joe1, Amirhassan Shams-Ansari1, Di Zhu1,2, Yaowen

Hu1,3, Neil Sinclair1,4 and Marko Loncar1*

1John A. Paulson School of Engineering and Applied Sciences,

Harvard University, Cambridge, 02139, MA, United States.

2Institute of Materials Research and Engineering, Agency for

Science, Technology and Research (A*STAR), 138634, Singapore.

3Department of Physics, Harvard University, Cambridge, 02139,

MA, USA.

4Division of Physics, Mathematics and Astronomy, and Alliance

for Quantum Technologies (AQT), California Institute of

Technology, Pasadena, 91125, MA, USA.

*Corresponding author(s). E-mail(s): renaud@g.harvard.edu;

loncar@g.harvard.edu;

†These authors contributed equally to this work.

Abstract

Integrated electro-optic (EO) modulators are fundamental photonics

components with utility in domains ranging from digital commu-

nications to quantum information processing. At telecommunication

wavelengths, thin-ﬁlm lithium niobate modulators exhibit state-of-the-

art performance in voltage-length product (VπL), optical loss, and

EO bandwidth. However, applications in optical imaging, optogenet-

ics, and quantum science generally require devices operating in the

visible-to-near-infrared (VNIR) wavelength range. In this work, we real-

ize VNIR amplitude and phase modulators featuring VπL’s of sub-1

V·cm, low optical loss, and high bandwidth EO response. Our Mach-

Zehnder modulators exhibit a VπLas low as 0.55 V·cm at 738 nm,

on-chip optical loss of ∼0.7 dB/cm, and EO bandwidths in excess

of 35 GHz. Furthermore, we highlight the new opportunities these

high-performance modulators oﬀer by demonstrating integrated EO

arXiv:2210.13521v2 [physics.optics] 9 Feb 2023

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2Sub-1 Volt and High-Bandwidth VNIR EO-Modulators

frequency combs operating at VNIR wavelengths, with over 50 lines

and tunable spacing, and frequency shifting of pulsed light beyond its

intrinsic bandwidth (up to 7x Fourier limit) by an EO shearing method.

Keywords: Integrated Photonics, Frequency Comb, Frequency Shifting

1 Introduction

Integrated photonics at visible and near-infrared (VNIR) wavelengths is impor-

tant for applications ranging from sensing [1–3] and spectroscopy [4] to

communications [5] and quantum information processing [6,7]. For example,

visible integrated photonic platforms can be combined with any of the large

variety of atomic or atomic-like systems with transitions in the VNIR such

as alkali and alkaline-earth metal atoms [8–10], rare-earth ions [11], diamond

color centers [12,13] and quantum dots [14–16]. Concerning quantum appli-

cations, VNIR photonics enables photon routing [17,18], spectral shifting for

interfacing disparate quantum emitters [11,19], or realizing higher-dimensional

encoded quantum states [20,21], all in a scalable and compact approach.

A variety of visible integrated photonic platforms have been demonstrated,

including silicon nitride [2,22–25], aluminum ntiride [26,27], diamond [28,29],

and lithium niobate (LN) [30–32]. LN is particularly compelling due to its

large electro-optic (EO) coeﬃcient, low optical loss, and wide transparency

window, making it the workhorse material for the modern day telecommu-

nications industry. Recent work has shown the promise of thin-ﬁlm lithium

niobate (TFLN) at telecommunications wavelengths [33]. Beyond the inher-

ent miniaturization and integratability achievable with TFLN, the strong

optical conﬁnement and increased tailorability have enabled performance not

achievable with bulk LN, including CMOS compatible drive voltages and high

bandwidth operation [34–36]. As a result of LN’s large transparency window,

EO TFLN devices in the visible regime have been demonstrated [30–32]. How-

ever, half-wave voltages (Vπ) and large bandwidths beyond that realized in

visible bulk devices has yet to be demonstrated in VNIR TFLN. In particular,

the combination of high-bandwidth and low drive-voltage optical modulation

would enable on-chip routing and spectral control: a critical requirement for

quantum applications.

In this work, we realize VNIR TFLN amplitude and phase modulators

(ﬁgure 1a) operating with VπLof sub-1 V·cm (ﬁgure 1b), extinction ratios

beyond 20 dB, and 3-dB EO bandwidths in excess of 35GHz. We perform two

demonstrations to highlight applications of these devices. We demonstrate an

integrated and tunable EO frequency comb source in the VNIR, showing over

50 lines in a single comb at 638, 738, and 838 nm, and displaying ﬂat-top

spectra with less than 10 dB power variation. Furthermore, we use our devices

to demonstrate spectral shearing of optical pulses over 7 times their intrinsic

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Sub-1 Volt and High-Bandwidth VNIR EO-Modulators 3

(b)(a)

(c)

Normalized Transmission

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.6 0.8

0.4 1.0

Vπ = 0.55 V

Optical Input

Frequency Shifting

Frequency Comb

Frequency

Amplitude Modulation

Time

(d)

SiO

(1 um)

TFLN

SiO

(2 um)

300 nm 600 nm

180 nm

120 nm

800 nm

Optical Output

< 1 V

Voltage (V)

Time

1 mm

gap

Contact

Region

Interaction

Region Y-splitter

Waveguides

Fig. 1 Ultra-low Vπmodulators operating at visible-to-near-infrared wave-

lengths. a, In the time domain, VNIR amplitude modulators with ultra-low drive voltages

(<1 V) can modulate continuous-wave optical inputs at CMOS voltages. Similarly, sub-

volt phase modulators enable VNIR frequency comb generation and frequency shifting over

multiple pulse bandwidths. b, Normalized optical transmission of a 10 mm long amplitude

(Mach–Zehnder) modulator as a function of the applied voltage. At λ= 738 nm, the Vπ

is 0.55 V at 1 MHz. c, Cross section illustrations of modulator waveguide and electrode

regions. d, Optical micrograph of a 5 mm long VNIR TFLN amplitude modulator. The

micrograph additionally shows the unbalanced amplitude modulator waveguides, along with

the Y-splitters, probe contact region of the electrodes, and the electrode gap taper along the

probe contact region to the interaction region.

spectral bandwidth. Together these demonstrations highlight the widespread

utility of TFLN modulators operating in the VNIR spectrum.

2 Results

2.1 Design and Fabrication

Figure 1c illustrates the design of our TFLN VNIR modulators. We fabri-

cate devices on 300 nm thick X-cut TFLN on 2 µm of thermally grown silicon

dioxide on Si (NanoLN). For complete device fabrication details, see methods.

Outside of the electrode region, the waveguides are designed to be single mode

(support transverse-electric, TE00, and transverse-magnetic, TM00) at 740 nm.

We choose this constraint to minimize excitation of higher-order modes, which

can lead to a reduction in EO performance in the electrode region. Using

ﬁnite-diﬀerence eigenmode simulations (Lumerical), the required waveguide

top width for single mode operation is determined to be approximately 300

nm. A disadvantage of this width is that it reduces mode conﬁnement. This

leads to higher optical loss due to mode overlap with sidewalls and cladding,

and absorption loss from electrodes. For this reason, we adiabatically increase

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4Sub-1 Volt and High-Bandwidth VNIR EO-Modulators

the waveguide width to 600 nm in the electrode region. Finally, our ampli-

tude modulators feature Y-splitters with excess losses of approximately 0.2

dB/splitter [30]. An optical micrograph of a fabricated 5 mm long amplitude

modulator is provided in ﬁgure 1d.

For the electrodes, we employ a push-pull conﬁguration with co-planar

waveguide (CPW) travelling-wave electrodes. Finite element method (COM-

SOL) simulations are used to design electrodes with impedance close to 50 Ω

and a simulated microwave phase index of nRF = 2.22 at 50 GHz. Due to the

relatively large optical group index at VNIR wavelengths compared to telecom-

munication wavelengths (nvis ≈2.38, ntel ≈2.25), perfect velocity matching

requires a reduction in bottom oxide (BOX) thickness and/or gold thickness,

which comes at the expense of increased optical and RF loss. To avoid this,

our devices have an index mismatch between the microwave phase and optical

group index of the TE00 mode of ∆n∼0.17. For an impedance matched, 1

cm long lossless modulator, this index mismatch corresponds to a theoretical

bandwidth of ∼80 GHz.

2.2 Visible-to-Near-Infrared Mach Zehnder Modulators

We fabricate 1 cm long Mach-Zehnder modulators (MZMs) with varying gap

sizes and experimentally evaluate their performance across both wavelength

and electrode gap parameter spaces. The experimental setup is shown in the

inset of ﬁgure 2a (see methods for measurement details).

As shown in ﬁgure 2a, the Vπof our 1 cm long, 3 µm gap devices is as

low as 0.42 V at 532 nm, and increases only slightly to 0.45,0.55, and 0.85 V

at 638, 738, and 838 nm, respectively. The increase in VπLfor longer wave-

lengths follows from the smaller phase accumulation for the same modulator

length. Our VπLis a factor of 2-3 smaller, depending on wavelength consid-

ered, than the best previously reported values for VNIR TFLN modulators,

without compromising bandwidth or device insertion loss [30–32]. We note

that our improvement stems predominantly from the reduction in electrode

gap, i.e., enhancement in optical-microwave ﬁeld overlap.

We perform the same measurements at 738 nm, but with MZMs of var-

ied gap sizes, seeing an increasing VπLfor larger gap sizes (ﬁgure 2b). For

comparison, we also theoretically calculate VπLas a function of gap. The sim-

ulated response shows excellent agreement with our measured results. Notably,

we measure VπL < 1 V·cm for gap sizes as large as 5 µm. For comparison,

recent work on VNIR devices with smaller gaps (2 µm) have reported larger

low frequency VπL[31].

We extract the on-chip modulator loss by fabricating and measuring 3 µm

gap modulators of varying electrode length. From this we obtain an on-chip

loss of ∼0.7±0.2 dB/cm (see supplementary for details). This value is over an

order of magnitude smaller than that reported in other recent demonstrations

for VNIR LN modulators [32,37]. Including the lensed ﬁber-to-chip coupling

loss (∼7 dB/facet), the total device insertion loss comes to ∼15 dB. We

note that because the total device insertion loss is dominated by coupling

loss (mismatch between the lensed ﬁber and rib waveguide mode), it can be

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LATEXtemplateSub-1VoltandHigh-BandwidthVisibletoNear-InfraredElectro-OpticModulatorsDylanRenaud1*y,DanielRimoliAssumpcao1y,GrahamJoe1,AmirhassanShams-Ansari1,DiZhu1,2,YaowenHu1,3,NeilSinclair1,4andMarkoLoncar1*1JohnA.PaulsonSchoolofEngineeringandAppliedSciences,HarvardUniversity,Cambridge,02139,MA,U...

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