Integrated Buried Heaters for Ecient Spectral Control of Air-Clad Microresonator Frequency Combs Gr egory Moille1 2aDaron Westly2Edgar F. Perez1 2Meredith Metzler3Gregory Simelgor2and Kartik

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Integrated Buried Heaters for Eﬃcient Spectral Control of Air-Clad

Microresonator Frequency Combs

Gr´egory Moille,1, 2, a) Daron Westly,2Edgar F. Perez,1, 2 Meredith Metzler,3Gregory Simelgor,2and Kartik

Srinivasan1, 2

1)Joint Quantum Institute, NIST/University of Maryland, College Park, USA

2)Microsystems and Nanotechnology Division, National Institute of Standards and Technology, Gaithersburg,

USA

3)Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg,

USA

(Dated: October 6, 2022)

Integrated heaters are a basic ingredient within the photonics toolbox, in particular for microresonator fre-

quency tuning through the thermo-refractive eﬀect. Resonators that are fully embedded in a solid cladding

(typically SiO2) allow for straightforward lossless integration of heater elements. However, air-clad res-

onators, which are of great interest for short wavelength dispersion engineering and direct interfacing with

atomic/molecular systems, do not usually have similarly low loss and eﬃcient integrated heater integration

through standard fabrication. Here, we develop a new approach in which the integrated heater is embedded

in SiO2below the waveguiding layer, enabling more eﬃcient heating and more arbitrary routing of the heater

traces than possible in a lateral conﬁguration. We incorporate these buried heaters within a stoichiometric

Si3N4process ﬂow that includes high-temperature (>1000 ◦C) annealing. Microring resonators with a 1 THz

free spectral range and quality factors near 106are demonstrated, and the resonant modes are tuned by nearly

1.5 THz, a 5×improvement compared to equivalent devices with lateral heaters.Finally, we demonstrate

broadband dissipative Kerr soliton generation in this platform, and show how the heaters can be utilized to

aid in bringing relevant lock frequencies within a detectable range.

Since their discovery, frequency combs have yielded a

plethora of applications – spectroscopy1, optical clocks2,

frequency synthesis3, and distance ranging4. Their inte-

gration on chip through the use of dissipative Kerr soliton

(DKS) states in diﬀerent platforms5–10 has led to a focus

on low power11 and low footprint12 devices for deployable

metrology outside of the laboratory13. The realization of

octave-spanning microcombs has been made possible by

harnessing unprecedented control of integrated microres-

onators dispersion13, that additionally overlap with the

atomic optical transition frequency14 has helped spur in-

terest in their use in portable optical atomic clocks2,15.

In such applications, the frequency comb acts as a gear

box13 translating the optical frequency stability of a

comb tooth locked to an atomic transition to a microwave

frequency through the DKS repetition rate [Fig. 1(a)].

However, the comb needs to be fully stabilized when real-

izing such a frequency division scheme. In the clock case,

the carrier envelope oﬀset fceo (i.e. the shift from the

zero frequency) [Fig. 1(b)] needs to be locked along with

a comb tooth close enough from the optical atomic tran-

sition frequency, here called flock [Fig. 1(c)].Their lock-

ing makes the system in Fig. 1(a) stiﬀ, in the sense that

only a single set of geometric parameters (ring width and

thickness combination) will provide for enough power en-

hancement at the frequencies of interest (often at the lo-

cation of dispersive waves (DWs)) while bringing a comb

tooth suﬃciently close to the atomic transition frequency.

a)Electronic mail: gmoille@umd.edu

Yet, each of these goals are essentially driven by two dif-

ferent parameters: the DW spectral position is deﬁned by

the cavity dispersion while flock (the beat note between

a comb tooth and the optical atomic transition) and fceo

can be controlled by a simple uniform spectral shift of

the comb. Therefore, integrated heaters appear as a suit-

able solution, leveraging spectral tuning via the thermo-

refractive eﬀect16 and compatibility with χ(3) microcomb

platforms. Frequency tuning ranging up to hundreds of

gigahertz has been demonstrated17–20, though mostly in

resonators fully embedded in a silica cladding.

The use of air-clad devices is, however, desirable for

dispersion engineering to reach atomic transition frequen-

cies in the short near-infrared and near-visible21 and al-

lows for post-fabrication processing to tune the geomet-

rical dispersion if needed22. Direct integration of the

heater to the side of the ring results in poor heat build-up

at the ring core [Fig. 1(d.i)], caused by the air trenches

that act as an insulator but are essential to create the ring

during the lithography and etch fabrication steps. The al-

ternative approach we propose in this paper is to bury the

integrated heater below the ring resonator [Fig. 1(d,ii)],

where a few micrometer gap of silica separates the metal

from the optical layer, resulting in no change in the ring

losses. Due to the continuous path for heat transfer be-

tween the heater and ring layers, the eﬃciency of such

a buried heater is much higher than for the lateral one,

conﬁrmed by thermal simulation [Fig. 1(d)]. We demon-

strate the fabrication of such unique integrated heaters,

which are wire-bonding ready for integration in optical

clock systems. In addition, fully embedding the heater in

silica and subsequently planarizing the silica layer makes

arXiv:2210.01865v1 [physics.optics] 4 Oct 2022

Locking

Clock Output

fceo+frep

locked comb

= divider

Strontium

⇒ visible

comb

440 K

360

320

400

480

20 30

r (µm)

−4

z (µm)

20 30

r (µm)

Si3N4

360

400

320 K

480

440

SiO2

Heater

Ring

SiN

Air

698 nm

clock

transition

(d) Need for tunability: integrated heater for air-clad resonator

(i) (ii)

SHG

(a) Optical-atomic clock principle (b) Frequency comb locking

frep

fceo as small

as possible

flock as small

as possible

Sr clock

Fig. 1 –Optical atomic clock principle and stiﬀness of the frequency comb locking frequency – (a) Concept of an optical

frequency comb acting as a frequency divider, depicted as a gear box and inspired by Ref.13 where the stability of a comb tooth in the

optical domain is divided down to the microwave domain with the clock output being the microcomb repetition rate. For the division to

be as low noise as possible, the frequency comb needs to be locked. This means that the carrier envelope oﬀset (fceo) and a single comb

tooth frequency need to be locked. (b) Concept of fceo detection and locking. A tooth from an octave-spanning frequency comb and the

second harmonic of a low frequency tooth from the same comb are interfered with each other to retrieve fceo, which can in principle

range from 0 to frep/2 and thus potentially be outside the photodetector bandwidth when frep is large (typical for microcombs). The

octave span of the microcomb is obtained thanks to dispersive waves (DWs), where the cavity dispersion allows for comb tooth resonance

enhancement at the targeted frequencies. (c) The clock relies on the locking of one comb tooth to the optical transition of an atomic

reference, for instance 85Sr at 698 nm. Therefore, one comb tooth must be close enough to this optical transition to be able to be locked.

As for fceo, this locking frequency flock can be between 0 and frep/2. (d) As dispersion mostly impacts the position of a comb tooth

that is ampliﬁed through DW enhancement, a control knob to shift the whole frequency comb is needed. Integrated heaters that leverage

the thermo-refractive eﬀect can provide this functionality. However, an air-clad ring is needed to support the dispersion required to reach

the 698 nm wavelength of the 85Sr reference. Ergo, two possibilities exist: lateral heating (i) and the new buried heater technology

introduced here. (ii). Finite element method simulations show that the heat build-up is much more eﬃcient in the case of the buried

heater, with a temperature inside the ring reaching 420 K against only 365 K for the same heating power.

the remainder of the fabrication process compatible with

a typical Si photonics process ﬂow 23. We experimentally

show the high tuning eﬃciency of the system, where a

resonator mode shift of 1.5 THz (equal to 1.5×the res-

onator free spectral range (FSR)) is achieved without

degradation of the resonator quality factor and with lim-

ited cross-talk. While tuning of air-clad resonators has

numerous potential applications, including in sensing and

coupling to quantum emitters, we showcase the utility of

these heaters in the context of broadband DKS micro-

combs. We experimentally demonstrate tunability of the

microcomb with an estimated shift of fceo of 15 % of the

FSR and an estimated shift of the locked optical clock

frequency flock of close to 2 FSR, while little modiﬁca-

tion to dispersion – and hence the DW position – occurs.

The fabrication of the system [Fig. 2(a)] starts with a

standard commercial silicon substrate with a 3 µm thick

thermal oxide. We then deﬁne the heater pattern us-

ing a direct-write lithography maskless aligner (MLA)24

[Fig. 2(a-i)]. The platinum is then deposited (electorn

beam evaporation) and lifted oﬀ. Another layer of 2.8 µm

of SiO2is deposited above the heater layer using a 180 °C

low temperature High Density Plasma Chemical Vapor

Deposition (HDPCVD) process [Fig. 2(a-ii)]. To allow for

the best material and the lowest absorption, we proceed

to anneal the wafer at 1000 °C for three hours. Thanks

to the low diﬀusivity of metal into silica25, the buried

heater remains geometrically intact and the silica layer

exhibits low absoprtion after the annealing. A chemical-

mechanical polishing (CMP) step [Fig. 2(a-ii)] is done to

create a ﬂat surface before low-pressure chemical vapor

deposition (LPCVD) of a 430 nm thick ﬁlm of silicon ni-

tride26 [Fig. 2(a-iii)]. From this point on, the process is

the same as our regular nano-patterning and fabrication

of air-clad ring resonators22. The ring resonator patterns

are created through electron-beam lithography, with the

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IntegratedBuriedHeatersforEcientSpectralControlofAir-CladMicroresonatorFrequencyCombsGregoryMoille,1,2,a)DaronWestly,2EdgarF.Perez,1,2MeredithMetzler,3GregorySimelgor,2andKartikSrinivasan1,21)JointQuantumInstitute,NIST/UniversityofMaryland,CollegePark,USA2)MicrosystemsandNanotechnologyDivision,Nat...

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Integrated Buried Heaters for Ecient Spectral Control of Air-Clad Microresonator Frequency Combs Gr egory Moille1 2aDaron Westly2Edgar F. Perez1 2Meredith Metzler3Gregory Simelgor2and Kartik

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