U.S. Patent Sep. 20, 2022 Sheet 3 of 3 US 11,451,306 B2

i

4

i

Output SM waveguide

6 20

x{microns}
‘®) Quantum emitter-MM WG modal coupling

€

TE

MM WG width (nm)

FIG. 3


US 11,451,306 B2

9

invention has been described by way of illustrations and not
limitation, Embodiments herein can be used independently
or can be combined.

All ranges disclosed herein are inclusive of the endpoints,
and the endpoints are independently combinable with each
other. The ranges are continuous and thus contain every
value and subset thereof in the range. Unless otherwise
stated or contextually inapplicable, all percentages, when
expressing a quantity, are weight percentages. The suflix
“(s)” as used herein is intended to include both the singular
and the plural of the term that it modifies, thereby including
at least one of that term (e.g., the colorant(s) includes at least
one colorants). “Optional” or “optionally” means that the
subsequently described event or circumstance can or cannot
occur, and that the description includes instances where the
event occurs and instances where it does not. As used herein,
“combination” is inclusive of blends, mixtures, alloys, reac-
tion products, and the like.

As used herein, “a combination thereof” refers to a

combination comprising at least one of the named constitu- 2

ents, components, compounds, or elements, optionally
together with one or more of the same class of constituents,
components, compounds, or elements.

All references are incorporated herein by reference.

The use of the terms “a” and “an” and “the” and similar
referents in the context of describing the invention (espe-
cially in the context of the following claims) are to be
construed to cover both the singular and the plural, unless
otherwise indicated herein or clearly contradicted by con-
text. “Or” means “and/or.” It should further be noted that the
terms “first,” “second,” “primary,” “secondary,” and the like
herein do not denote any order, quantity, or importance, but
rather are used to distinguish one element from another. The
modifier “about” used in connection with a quantity is
inclusive of the stated value and has the meaning dictated by
the context (e.g., it includes the degree of error associated
with measurement of the particular quantity). The conjune-
tion “or” is used to link objects of a list or alternatives and
is not disjunctive; rather the elements can be used separately
or can be combined together under appropriate circum-
stances.

What is claimed is:

1. A single quantum emitter single photon source for
producing a single photon stream, the single quantum emit-
ter single photon source comprising:

‘a single-mode excitation waveguide that receives a pump
light, propagates pump light as resonant excitation
light, and produces evanescent excitation light from
resonant excitation light;

a first separation gap that separates the single-mode 5

excitation waveguide and a multi-mode intermediate
waveguide and that receives the evanescent excitation
light from the single-mode excitation waveguide;

the multi-mode intermediate waveguide in evanescent

communication with the single-mode excitation wave- 5

guide and that:

comprises a single quantum emitter disposed in the
multi-mode intermediate waveguide,

evanescently receives the evanescent excitation light
from the single-mode excitation waveguide across
the first separation gap,

propagates the evanescent excitation light as resonant
excitation light, and

subjects the single quantum emitter to the resonant
excitation light such that the single quantum emitter
produces emitted single photons from the resonant
excitation light;

0

&

0

45

0

10

a second separation gap that separates the multi-mode
intermediate waveguide and a single-mode output
waveguide and that receives the emitted single photons
from the multi-mode intermediate waveguide; and
the single-mode output waveguide in evanescent commu-
nication with the multi-mode intermediate waveguide
and that:
evanescently receives the emitted single photons from
the multi-mode intermediate waveguide across the
second separation gap, and

propagates the emitted single photons as emitted single
photon stream.

2. The single quantum emitter single photon source of
claim 1, further comprising a substrate on which the single-
mode excitation waveguide, the multi-mode intermediate
waveguide, and the single-mode output waveguide are dis-
posed.

3. The single quantum emitter single photon source of
claim 2, further comprising a laser disposed on the substrate
in optical communication with the single-mode excitation
waveguide and that provides the pump light to the single-
mode excitation waveguide.

4. The single quantum emitter single photon source of
claim 1, wherein the single quantum emitter comprises a
fluorescent molecule, a quantum dot, a sub-wavelength
scattering center, or a combination comprising at least one of
the foregoing single quantum emitters.

. The single quantum emitter single photon source of
claim 1, wherein a wavelength of the pump light is the same
as photon emitted from the quantum emitter.
6. The single quantum emitter single photon source of
claim 1, wherein a wavelength of the emitted single photons
is from 900 nm to 1000 nm.
7. A single quantum emitter single photon source for
producing a single photon stream, the single quantum emit-
ter single photon source comprising:
a single-mode excitation waveguide that receives a pump
light, propagates pump light as resonant excitation
light, and produces evanescent excitation light from
resonant excitation light;
a separation gap that separates the single-mode excitation
waveguide and a multi-mode intermediate waveguide
and that receives the evanescent excitation light from
the single-mode excitation waveguide;
the multi-mode intermediate waveguide in evanescent
communication with the single-mode excitation wave-
guide and that:
comprises a single quantum emitter disposed in the
multi-mode intermediate waveguide,

evanescently receives the evanescent excitation light
from the single-mode excitation waveguide across
the separation gap,

propagates the evanescent excitation light as resonant
excitation light, and

subjects the single quantum emitter to the resonant
excitation light such that the single quantum emitter
produces emitted single photons from the resonant
excitation light;
an adiabatic mode transformer that separates the multi-
mode intermediate waveguide and a single-mode out-
put waveguide and that:
is in optical communication with the multi-mode inter-
mediate waveguide,

receives the emitted single photons from the multi-
mode intermediate waveguide, and

optically couples the multimode intermediate wave-
guide and the single-mode output waveguide; and



US 11,451,306 B2

3

It has been discovered that a single quantum emitter
single photon source herein provides an integrated photonic
device that produces pure, waveguide-bound single photon
streams via resonance fluorescence from a single quantum
emitter from optical excitation through on-chip waveguide-
bound resonant laser light

A single quantum emitter can be a light source that
produces single photons in an on-chip waveguide for inte-
grated quantum photonic articles and processes. Illumina-
tion with laser light resonant of a single quantum emitter
disposed in a waveguide produces indistinguishable wave-
guide-bound single photons. Leakage of light from the
excitation laser into the waveguide must be suppressed.
Conventionally, illumination with a free-space laser beam
that propagates perpendicular to a length of the waveguide
has limited efficacy and limited controllability. The single
quantum emitter single photon source overcomes these
issues through illumination of the single quantum emitter
with a high-order confined mode of the waveguide that is

evanescently coupled another waveguide that introduces 2

excitation photons to the single quantum emitter from which
is extracted a single-photon stream via a third waveguide.

It has been discovered that single quantum emitter single
photon source 200 produces emitted single photon stream
210. In an embodiment, with reference to FIG. 1, single
quantum emitter single photon source 200 including: single-
mode excitation waveguide 201 that receives pump light
202, propagates pump light 202 as resonant excitation light
204, and produces evanescent excitation light 203 from
resonant excitation light 204; first separation gap 213.1 that
separates single-mode excitation waveguide 201 and multi-
mode intermediate waveguide 205 and that receives eva-
nescent excitation light 203 from single-mode excitation
waveguide 201; multi-mode intermediate waveguide 205 in
evanescent communication with single-mode excitation
waveguide 201 and that: comprises single quantum emitter
207 disposed in multi-mode intermediate waveguide 205,
evanescently receives evanescent excitation light 203 from
single-mode excitation waveguide 201 across first separa-
tion gap 213.1, propagates evanescent excitation light 203 as
high-order mode resonant excitation light 206, and subjects
single quantum emitter 207 to resonant excitation light 206
such that single quantum emitter 207 produces first order
mode emitted single photons 208 from high-order mode
resonant excitation light 206; second separation gap 213.2
that separates multi-mode intermediate waveguide 205 and
ingle-mode output waveguide 209 and that receives emitted
single photons 208 from multi-mode intermediate wave-
guide 205; and single-mode output waveguide 209 in eva-
nescent communication with multi-mode
waveguide 205 and that: evanescently receives emitted
single photons 208 from multi-mode intermediate wave-
guide 205 across second separation gap 213.2, and propa-
gates emitted single photons 208 as emitted single photon
stream 210.

In an embodiment, with reference to FIG. 2, single
quantum emitter single photon source 200 includes: single-
mode excitation waveguide 201 that receives pump light
202, propagates pump light 202 as resonant excitation light
204, and produces evanescent excitation light 203 from
resonant excitation light 204; separation gap 213 that sepa-
rates single-mode excitation waveguide 201 and multi-mode
intermediate waveguide 205 and that receives evanescent
excitation light 203 from single-mode excitation waveguide
201; multi-mode intermediate waveguide 205 in evanescent
communication with single-mode excitation waveguide 201
and that: includes single quantum emitter 207 disposed in

intermediate 5

&

&

s

4

multi-mode intermediate waveguide 205, evanescently
receives evanescent excitation light 203 from single-mode
excitation waveguide 201 across separation gap 213, propa-
gates evanescent excitation light 203 as resonant excitation
light 206, and subjects single quantum emitter 207 to
resonant excitation light 206 such that single quantum
emitter 207 produces emitted single photons 208 from
resonant excitation light 206; adiabatic mode transformer
211 that separates multi-mode intermediate waveguide 205
and single-mode output waveguide 209 and that: is in optical
communication with multi-mode intermediate waveguide
205, receives emitted single photons 208 from multi-mode
intermediate waveguide 205, rejects resonant excitation
light, and optically couples multi-mode intermediate wave-
guide 205 and single-mode output waveguide 209; and
single-mode output waveguide 209 in optical communica-
tion with adiabatic mode transformer 211 and that: receives
emitted single photons 208 from adiabatic mode transformer
211, and propagates emitted single photons 208 as emitted
single photon stream 210.

According to an embodiment, single quantum emitter
single photon source 200 further includes a substrate on
which single-mode excitation waveguide 201, multi-mode
intermediate waveguide 205, and single-mode output wave-

5 guide 209 are disposed. The substrate can include a material

that mechanically and optically support waveguides and
optical communication. Exemplary substrates include glass,
ceramic, plastic, meta icon dioxide, and the like.

‘A laser can be disposed on the substrate in optical
communication with single-mode excitation waveguide 201
to provide pump light 202 to single-mode excitation wave-
guide 201. Exemplary lasers include semiconductor diode
lasers, titanium-sapphire lasers, and the like. A wavelength
of pump light 202 can be from 300 nm to 2000 nm,
specifically from 700 nm to 1000 nm, and more specifically
from 900 nm to 950 nm. It is contemplated that the wave-
length of pump light 202 coincides with an emission wave-
length of single quantum emitter 207, ie., the wavelength of
pump light 202 can be the same as the emission wavelength
of single quantum emitter 207, ¢.g., from 400 nm to 1000
nm.

In an embodiment, single quantum emitter 207 includes a
fluorescent molecule, a quantum dot, a color center, or a
combination comprising at least one of the foregoing single
quantum emitters. Exemplary fluorescent molecules include
terrylene, dibenzoterrylene, and the like. Exemplary quan-
tum dots include InAs, GaAs CdTe, and the like. Exemplary
color centers include nitrogen-vacancies in a diamond, a
vacancy in SiC, and the like. A wavelength of emitted single
photons 208 can be from 300 nm to 2000 nm, specifically
from 700 nm to 1000 nm, and more specifically from 900 nm
to 950 nm.

In an embodiment, adiabatic mode transformer 211 filters
free-space propagation light 212 by evanescently coupling
resonant excitation high-order mode light 206 out of mul-
timode intermediate waveguide 205 while communicating
emitted single photons 208 from multi-mode intermediate
waveguide 205 to single-mode output waveguide 209
through adiabatic mode transformer 211. For this purpose,
adiabatic mode transformer 211 converts first order mode
light from multimode intermediate waveguide 205 to single-
mode output waveguide 209. Adiabatic mode transformer
211 can include a waveguide with a longitudinally-varying
profile that transitions from a multi-mode regime to a
single-mode regime. At a specific point along a length of
adiabatic mode transformer 211, adiabatic mode transformer
211 ceases to support high-order waveguide-bound modes



US 11,451,306 B2

1
SINGLE QUANTUM EMITTER SINGLE
PHOTON SOURCE AND PRODUCING A
SINGLE PHOTON STREAM.

CROSS REFERENCE TO RELATED,
APPLICATIONS

The application claims priority to U.S. Provisional Patent
Application Ser. No. 62/861,343 filed Jun. 14, 2019, the
disclosure of which is incorporated herein by reference in its
entirety.

STATEMENT REGARDING FEDERALLY,
SPONSORED RESEARCH

‘This invention was made with United States Government
support from the National Institute of Standards and Tech-
nology (NIST), an agency of the United States Department
of Commerce. The Government has certain rights in the
invention. Licensing inquiries may be directed to the Tech-
nology Partnerships Office, NIST, Gaithersburg, Md.,
20899; voice (301) 975-2573; email tpo@nist.gov.

BRIEF DESCRIPTION

Disclosed is a single quantum emitter single photon
source for producing a single photon stream, the single
quantum emitter single photon source comprising: a single-
mode excitation waveguide that receives a pump light,
propagates pump light as resonant excitation light, and
produces evanescent excitation light from resonant excita-
tion light; a first separation gap that separates the single-
mode excitation waveguide and a multi-mode intermediate
waveguide and that receives the evanescent excitation light
from the single-mode excitation waveguide: the multi-mode
intermediate waveguide in evanescent communication with
the single-mode excitation waveguide and that: comprises a
single quantum emitter disposed in the multi-mode interme-
diate waveguide, evanescently receives the evanescent exci-
tation light from the single-mode excitation waveguide
across the first separation gap, propagates the evanescent
excitation light as resonant excitation light, and subjects the
single quantum emitter to the resonant excitation light such
that the single quantum emitter produces emitted single
photons from the resonant excitation light; a second sepa-
ration gap that separates the multi-mode intermediate wave-
guide and a single-mode output waveguide and that receiv
the emitted single photons from the multi-mode intermediate
waveguide; and the single-mode output waveguide in eva-

nescent communication with the multi-mode intermediate s

waveguide and that: evanescently receives the emitted single
photons from the multi-mode intermediate waveguide
across the second separation gap, and propagates the emitted
single photons as emitted single photon stream.

Disclosed is a single quantum emitter single photon s

source for producing a single photon stream, the single
quantum emitter single photon source comprising: a single-
mode excitation waveguide that receives a pump light,
propagates pump light as resonant excitation light, and
produces evanescent excitation light from resonant excita-
tion light; a separation gap that separates the single-mode
excitation waveguide and a multi-mode intermediate wave-
guide and that receives the evanescent excitation light from
the single-mode excitation waveguide; the multi-mode inter-
mediate waveguide in evanescent communication with the
single-mode excitation waveguide and that: comprises a
single quantum emitter disposed in the multi-mode interme-

20

&

at

2

diate waveguide, evanescently receives the evanescent exci-
tation light from the single-mode excitation waveguide
across the separation gap, propagates the evanescent exci-
tation light as resonant excitation light, and subjects the
single quantum emitter to the resonant excitation light such
that the single quantum emitter produces emitted single
photons from the resonant excitation light; an adiabatic
mode transformer that separates the multi-mode intermedi-
ate waveguide and a single-mode output waveguide and
that: is in optical communication with the multi-mode
intermediate waveguide, receives the emitted single photons
from the multi-mode intermediate waveguide, and optically
couples the multi-mode intermediate waveguide and the
single-mode output waveguide, and eliminates the resonant
excitation light, preventing it from entering the single-mode
output waveguide; and the single-mode output waveguide in
optical communication with the adiabatic mode transformer
and that: receives the emitted single photons from the
adiabatic mode transformer, and propagates the emitted
single photons as emitted single photon stream.

Disclosed is a process for producing a single photon
stream with a single quantum emitter single photon source,
the process comprising: producing a pump light; receiving,
by a single-mode excitation waveguide, the pump light:
propagating the pump light as resonant excitation light in the
single-mode excitation waveguide; producing evanescent
excitation light from the resonant excitation light; receiving
the evanescent excitation light in a separation gap that
separates the single-mode excitation waveguide from a
multi-mode intermediate waveguide; propagating the eva-
nescent excitation light through the separation gap; evanes-
cently receiving, by the multi-mode intermediate wave-
guide, the evanescent excitation light from the single-mode
excitation waveguide through the separation gap; propagat-
ing the evanescent excitation light as resonant excitation
light through the multi-mode intermediate waveguide: sub-
jecting a single quantum emitter disposed in the multi-mode
intermediate waveguide to the resonant excitation light;
producing, by the single quantum emitter, emitted single
photons from the resonant excitation light; propagating the
emitted single photons through the multi-mode intermediate
waveguide: communicating the emitted single photons from
the multi-mode intermediate waveguide to a single-mode
output waveguide; receiving, by the single-mode output
waveguide, the emitted single photons from the multi-mode
intermediate waveguide; and propagating, through the
single-mode output waveguide, the emitted single photons at
emitted single photon stream to produce the emitted single
photon stream.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description should not be considered lim-
iting in any way. With reference to the accompanying
drawings, like elements are numbered alike.

FIG. 1 shows a single quantum emitter single photon
source:

FIG. 2 shows a single quantum emitter single photon
source; and

FIG. 3 shows values determined by FDTD for modal
coupling factor B between a single emitter located at the
center of a multi-mode intermediate waveguide and guided
modes thereof, labeled TE, and TE;9, as a function of a
width of the multi-mode intermediate waveguide.

DETAILED DESCRIPTION,

A detailed description of one or more embodiments is
presented herein by way of exemplification and not limita-
tion.


US 11,451,306 B2

7

receiving, by single-mode output waveguide 209, emitted
single photons 208 from multi-mode intermediate wave-
guide 205 through second separation gap 213.2.

In an embodiment, the process for producing a single
photon stream with single quantum emitter single photon
source 200 also includes receiving emitted single photons
208 in adiabatic mode transformer 211 that separates multi-
mode intermediate waveguide 205 from single-mode output
waveguide 209; propagating emitted single photons 208
through adiabatic mode transformer 211; and evanescently
receiving, by single-mode output waveguide 209, emitted
single photons 208 from multi-mode intermediate wave-
guide 205 through adiabatic mode transformer 211.

It should be appreciated that single quantum emitter
single photon source 200 provides an integrated photonic
device that produces a pure stream of on-chip waveguide-
bound indistinguishable single-photons as emitted single
photon stream 210 from resonance fluorescence of single

quantum emitter 207 disposed in multi-mode intermediate 2

waveguide 205 that is an on-chip waveguide. Here, single
quantum emitter 207 is excited by on-chip waveguide-bound
resonant laser light as evanescent excitation light 203 from
resonant excitation light 204.

Single quantum emitter single photon source 200, rather 2

than performing excitation of single quantum emitter 207
via a free-space resonant laser beam, single quantum emitter
207 is excited through a high-order guided wave mode as
resonant excitation light 206, which is injected into multi-
mode intermediate waveguide 205 in which is disposed
single quantum emitter 207 by single-mode excitation wave-
guide 201 evanescently-coupled to multi-mode intermediate
waveguide 205. Emitted single photons 208 are extracted
from multi-mode intermediate waveguide 205 into single-
mode output waveguide 209 evanescently coupled to multi-
mode intermediate waveguide 205. Single quantum emitter
207 produces a single photon that is coupled with high
probability to the first-order mode of multi-mode interme-
diate waveguide 205 and with very low probability to
higher-order modes of multi-mode intermediate waveguide
205. Emitted single photons 208 is then transferred to
single-mode output waveguide 209.

With regard to transfer of photons among waveguides in
single quantum emitter single photon source 200, FIG. 3A
shows an electromagnetic finite-difference time-domain
simulation of evanescently coupled multi-mode intermedi-
ate waveguide 205 and single-mode output waveguide 209.
The scale indicates light intensity. Light is initially carried
by a high-order TE,, mode of multi-mode intermediate
waveguide 205 from position x:
209) are phase matched so that efficient (e.g., 91% efficient)
power transfer via propagation of emitted single photons
208 occurs from the TE, waveguide mode of multi-mode
intermediate waveguide 205 to the single TE, mode of

single-mode output waveguide 209 at x-35 jum. A reverse s

propagation is also valid, wherein light initially carried in
single-mode output waveguide 209 by TEo mode is trans-
ferred with 91% efliciency into TE, mode of multi-mode
intermediate waveguide 205. Single quantum emitter single
photon source 200 provides insertion of resonant excitation
light 204 into single-mode excitation waveguide 201 and
extraction of emitted single photons 208 from multi-mode
intermediate waveguide 205 as shown in FIG. 1. A power
transfer efficiency between waveguides can be controlled
though dimensions of single-mode output waveguide 209
and spatial separation between multi-mode intermediate
waveguide 205 and single-mode output waveguide 209.

. The waveguides (208, s

&

&

8

FIG. 3B shows values calculated with finite-difference
time-domain (FDTD) for modal coupling factor B between
single quantum emitter 207 disposed at a center of multi-
mode intermediate waveguide 205 and guided modes of
multi-mode intermediate waveguide 205 that is labeled as
TEo9 and TE as a function of width of multi-mode inter-
mediate waveguide 205. Modal coupling factor B indicates
efliciency with which single quantum emitter 207 is brought
into an excited state by resonant excitation light 206 carried
by different modes supported by multi-mode intermediate
waveguide 205 and also indicates a probability with which
emitted single photons 208 couple to different modes of
multi-mode intermediate waveguide 205. Accordingly, the
optical power to excite single quantum emitter 207 into the
excited state can be minimized by modifying modal cou-
pling factor B via the width of multi-mode intermediate
waveguide 205.

Single quantum emitter single photon source 200 and
processes disclosed herein have numerous beneficial uses,
including simple operation for resonant excitation, indepen-
dence from free-space optical pumping, independence from
free-space optics pump suppression apparatus, and produc-
tion of waveguided single-photon stream. Advantageously,
single quantum emitter single photon source 200 overcomes
limitations of technical deficiencies of conventional com-
positions such as technically demanding operation, free-
space outcoupling of single-photon stream, pump suppres-
sion dependent on quality of free-space optical pumping and
free-space optical pump suppression apparatus. Further,
optimization of pump suppression relies on guided-wave
optics and geometrical feature control, which is simpler to
rationalize than free-space optical schemes.

Beneficially, single quantum emitter single photon source
200 can include single InAs quantum dots that produce
single-photon streams on-demand into on-chip silicon
nitride waveguides, wherein the single quantum emitters are
optically excited with pulsed laser light and produce single
photons at each pulse, on-demand. Moreover, single quan-
tum emitter single photon source 200 produces streams of
highly indistinguishable single photons from excitation with
laser light that is at the same wavelength as the emitted
single photons. Single quantum emitter single photon source
200 overcomes technical issues with conventional articles
that introduce excitation laser light into a waveguide where
the single-photons are launched such as quantum emitter
excitation through a free-space beam that is orthogonal to a
waveguide into Which the single-photons are launched. Such
orthogonal excitation can have significant leakage into the
waveguide. In contrast, single quantum emitter single pho-
ton source 200 overcomes leakage problems from free-space
excitation laser.

Single quantum emitter single photon source 200 and
processes herein unexpectedly provide excitation of quan-
tum emitter 207 by resonant excitation light 206 that is
evanescently injected into multimode intermediate wave-
guide 205; and provides extraction of single-photon stream
208 into output waveguide 209, either evanescently through
separation gap 213, or through adiabatic mode transformer
211, while preventing excitation pump light 206 from being
injected into output waveguide 209. Moreover, single quan-
tum emitter single photon source 200 provides a single
photon stream directly in an on-chip waveguide.

While one or more embodiments have been shown and
described, modifications and substitutions may be made
thereto without departing from the spirit and scope of the
invention. Accordingly, it is to be understood that the present



a2) United States Patent

Davanco

US0114513'

(06B2

US 11,451,306 B2
Sep. 20, 2022

(10) Patent No.:
(45) Date of Patent:

(54) SINGLE QUANTUM EMITTER SINGLE
PHOTON SOURCE AND PRODUCING A
SINGLE PHOTON STREAM

(58) Field of Clas: mn Search
CPC HO04B 10/70

See application file for complete search history.

(71) Applicant: Government of the United States of (56) References Cited
America, as represented by the U.S. PATENT DOCUMENTS
Secretary of Commerce, Gaithersburg,
MD (US) 8,842,949 B2 9/2014 Schroder et al.
10,074,771 B2* 9/2018 Hardtdegen HO4L 9/0852
(72) Inventor: Marcelo Ishihara Davanco, Rockville, 2020/0209656 A1* 7/2020 Young HO4L 9/0852
MD (US)
OTHER PUBLICATIONS
(73) Assignee: GOVERNMENT OF THE UNITED
STATES OF AMERICA, AS Davanco, M., et al., “Heterogeneous integration for on-chip quan-
REPRESENTED BY THE tum photonic circuits with single quantum dot devices”, Nature
SECRETARY OF COMMERCE, Communications, 2017, p. 1-12, DOI: 10.1038/s41467-017-00987-
Gaithersburg, MD (US) 6
(Continued)
(*) Notice: Subject to any disclaimer, the term of this — . oe
patent is extended or adjusted under 35 Primary Examiner —Daniel G Dobson
US.C. 154(b) by 302 days, (74) Attorney, Agent, or Firm — Office of Chief Counsel
~ for National Institute of Standards and Technology
(21) Appl. No.: 16/875,201 67) ABSTRACT
(22) Filed: May 15, 2020 A single quantum emitter single photon source produces a
oo" single photon stream and includes: a single-mode excitation
(65) Prior Publication Data waveguide that receives a pump light, propagates pump light
as resonant excitation light, and produces evanescent exci-
US 2020/0396006.A1 Dee. 17, 2020 tation light; a separation gap between single-mode excitation
waveguide and a multi-mode intermediate waveguide; a
. — multi-mode intermediate waveguide that: includes a single
Related U.S. Application Data quantum emitter, evanescently receives the evanescent exci-
(60) Provisional application No. 62/861,343, filed on Jun, ‘ation light from the single-mode excitation waveguide,
14, 2019. . propagates the evanescent excitation light as resonant exci-
tation light, and subjects the single quantum emitter to the
(51) Int. CL. resonant excitation light such that the single quantum emit-
HO4B 10/00 (2013.01) ter produces emitted single photons; and a single-mode
‘HOAB 10/70 (2013.01) output waveguide that receives the emitted single photons
(Continued) from the multi-mode intermediate waveguide and propa-
(52) gates the emitted single photons as an emitted single photon
HO4B 10/70 (2013.01); GO2B 6/29337 Seam.
(2013.01) 18 Claims, 3 Drawing Sheets
200
201 204 203 213
202 i )
=z
; {|
i \
{ 207-208

205


US 11,451,306 B2

11

the single-mode output waveguide in optical communi-
cation with the adiabatic mode transformer and that:
receives the emitted single photons from the adiabatic
mode transformer, and
propagates the emitted single photons as emitted single
photon stream.

8. The single quantum emitter single photon source of
claim 7, further comprising a substrate on which the single-
mode excitation waveguide, the multi-mode intermediate
waveguide, and the single-mode output waveguide are dis-
posed.

9. The single quantum emitter single photon source of
claim 8, further comprising a laser disposed on the substrate
in optical communication with the single-mode excitation
waveguide and that provides the pump light to the single-
mode excitation waveguide.

10. The single quantum emitter single photon source of
claim 7, wherein the single quantum emitter comprises a
fluorescent molecule, a quantum dot, a sub-wavelength

scattering center, or a combination comprising at least one of 2

the foregoing single quantum emitters.

11. The single quantum emitter single photon source of
claim 7, wherein a wavelength of the pump light is the same
as that emitted from the quantum emitter.

12. The single quantum emitter single photon source of
claim 7, wherein a wavelength of the emitted single photons
is from 900 nm to 1000 nm.

13. A process for producing a single photon stream with
a single quantum emitter single photon source, the process
comprising:

producing a pump light;

receiving, by a single-mode excitation waveguide, the

pump light;

propagating the pump light as resonant excitation light in

the single-mode excitation waveguide;

producing evanescent excitation light from the resonant

excitation light;
receiving the evanescent excitation light in a separation
gap that separates the single-mode excitation wave-
guide from a multi-mode intermediate waveguide:

propagating the evanescent excitation light through the
separation gap;

evanescently receiving, by the multi-mode intermediate

waveguide, the evanescent excitation light from the
single-mode excitation waveguide through the separa-
tion gap:

propagating the evanescent excitation light as resonant

excitation light through the multi-mode intermediate
waveguide:

0

30

&

12

subjecting a single quantum emitter disposed in the multi-
mode intermediate waveguide to the resonant excita-
tion light;

producing, by the single quantum emitter, emitted single

photons from the resonant excitation light;
propagating the emitted single photons through the multi-
mode intermediate waveguide;

communicating the emitted single photons from the

multi-mode intermediate waveguide to a single-mode
output waveguide;

receiving, by the single-mode output waveguide, the

emitted single photons from the multi-mode interme-
diate waveguide: and

propagating, through the single-mode output waveguide,

the emitted single photons at emitted single photon
stream to produce the emitted single photon stream.
14. The process of claim 13, farther comprising:
receiving the emitted single photons in a second separa-
tion gap that separates the multi-mode intermediate
waveguide from the single-mode output waveguide;

propagating the emitted single photons through the sec-
ond separation gap; and

evanescently receiving, by the single-mode output wave-

guide, the emitted single photons from the multi-mode
intermediate waveguide through the second separation
Bap.
15. The process of claim 13, further comprising:
receiving the emitted single photons in an adiabatic mode
transformer that separates the multi-mode intermediate
waveguide from the single-mode output waveguide;

propagating the emitted single photons through the adia-
batic mode transformer; and

evanescently receiving, by the single-mode output wave-

guide, the emitted single photons from the multi-mode
intermediate waveguide through the adiabatic mode
transformer,

16. The process of claim 13, wherein the single quantum
emitter comprises a fluorescent molecule, a quantum dot, a
sub-wavelength scattering center, or a combination compris-
ing at least one of the foregoing single quantum emitters.

17. The process of claim 13, wherein a wavelength of the
pump light is the same as that emitted from the quantum
emitter.

18. The process of claim 13, wherein a wavelength of the
emitted single photons is from 900 nm to 1000 nm.

+e eee


US 11,451,306 B2

5

so that pump laser power in resonant excitation light 206
propagated by multi-mode intermediate waveguide 205 is
launched into free-space. Adiabatic mode transformer 211
adiabatically converts single photons initially carried by the
fundamental mode of multi-mode intermediate waveguide
205 to the fundamental mode of single-mode output wave-
guide 209. Adiabatic mode transformer 211 provides spatial
mode filtering of photons and selectively communicates
emitted single photons 208 to single-mode output wave-
guide 209.

Waveguides (201, 205, 211, 209) independently can
include a core and a cladding to transversely confine and
guide light over a length of more than one. Exemplary
waveguides include GaAs suspended channel waveguides,
InGaAsP ridge-loaded waveguides, and silicon-on-insulator
ridge waveguides. A transmittivity of waveguides (201, 205,
211, 209) independently can be from 0 to 100% at a
wavelength from 300 nm to 2000 nm.

Single quantum emitter single photon source 200 can be

made in various ways. In an embodiment, a process for 2

making single quantum emitter single photon source 200
includes forming single-mode excitation waveguide 201 on
a substrate; forming multi-mode intermediate waveguide
205 on the substrate proximate to and in evanescent com-

munication with single-mode excitation waveguide 201; 2:

disposing single quantum emitter 207 in multi-mode inter-
mediate waveguide 205 by drop-casting; forming single-
mode output waveguide 209 on the substrate proximate to
and in communication with multi-mode intermediate wave-
guide 205. In some embodiments, the process includes
forming single-mode output waveguide 209 in evanescent
communication with multi-mode intermediate waveguide
205. In some embodiments, the process includes forming
adiabatic mode transformer 211 in optical communication
with multi-mode intermediate waveguide 205 and single-
mode output waveguide 209 so that adiabatic mode trans-
former 211 is optically interposed between multi-mode
intermediate waveguide 205 and adiabatic mode transformer
211. It is contemplated that forming waveguides on substrate
can include microfabrication or nanofabrication processing
that include deposition, lithography, etching, and the like.
Single-mode excitation waveguide 201 and multi-mode
intermediate waveguide 205 are formed on the substrate so
that a length of single-mode excitation waveguide 201 and
multi-mode intermediate waveguide 205 overlap for eva-
nescent coupling between single-mode excitation wave-
guide 201 and multi-mode intermediate waveguide 205. It is
contemplated that a length of overlap between single-mode
excitation waveguide 201 and multi-mode intermediate

waveguide 205 can be from 1 um to 1000 yum, specifically s

from 10 um to 100 jm, and more specifically from 15 um to
20 jum. The length of overlap can be from 0.5 to 1000
wavelengths, specifically 1 wavelength to 100 wavelengths,
5 wavelengths to 10 wavelengths.

In some embodiments, multi-mode intermediate wave- s

guide 205 and single-mode output waveguide 209 are
formed on the substrate so that a length of multi-mode
intermediate waveguide 205 and single-mode output wave-
guide 209 overlap for evanescent coupling between multi-
mode intermediate waveguide 205 and single-mode output
waveguide 209. It is contemplated that a length of overlap
between multi-mode intermediate waveguide 205 and
single-mode output waveguide 209 can be from 1 um to
1000 pm, specifically from 10 um to 100 um, and more
specifically from to 15 pm to 20 um.

In some embodiments, multi-mode intermediate wave-
guide 205 and single-mode output waveguide 209 are

&

6

formed on the substrate so that adiabatic mode transformer
211 optically couples multi-mode intermediate waveguide
205 and single-mode output waveguide 209. It is contem-
plated that a length of adiabatic mode transformer 211
through which emitted single photons 208 propagates can be
from 0 to 1000 jum, specifically from 1 jum to 100 pm, and
more specifically from to 2 um to 10 pm. A width of
adiabatic mode transformer 211 parallel to the substrate and
substantially orthogonal to a direction of propagation of
emitted single photons 208 through adiabatic mode trans-
former 211 can be from 0.3 jum to 100 um, specifically from
0.5 um to 10 jum, and more specifically from 1 jum to 2 um.

The process for making single quantum emitter single
photon source 200 also can include epitaxial growth of
quantum emitters, quantum emitter localization, and the
like.

Inan embodiment, a process for operating single quantum
emitter single photon source 200 includes injecting pump
laser light 202 into single-mode excitation waveguide 201 at
a wavelength closely resonant with that of quantum emitter
207; controlling excitation laser power so that it is less than
10% of the power to saturate single-photon emission from
quantum emitter 207; tuning the laser wavelength slowly
towards that of quantum emitter 207; observing an increase
of flux of emitted single-photon stream 210 at output wave-
guide 209; and optionally observing, e.g., with a single-
photon detector or single-photon-sensitivity spectrometer,
emitted single-photon stream 210 at output waveguide 209.

Single quantum emitter single photon source 200 has
numerous advantageous and unexpected benefits and uses.
In an embodiment, a process for producing a single photon
stream with single quantum emitter single photon source
200 includes: producing pump light 202; receiving, by
single-mode excitation waveguide 201, pump light 202:
propagating pump light 202 as resonant excitation light 204
in single-mode excitation waveguide 201; producing eva-
nescent excitation light 203 from resonant excitation light
204; receiving evanescent excitation light 203 in separation
gap 213 that separates single-mode excitation waveguide
201 from multi-mode intermediate waveguide 205; propa-
gating evanescent excitation light 203 through separation
gap 213; evanescently receiving, by multi-mode intermedi-
ate waveguide 205, evanescent excitation light 203 from
single-mode excitation waveguide 201 through separation
gap 213; propagating evanescent excitation light 203 as
resonant excitation light 206 through multi-mode interme-
diate waveguide 205; subjecting single quantum emitter 207
disposed in multi-mode intermediate waveguide 205 to
resonant excitation light 206; producing, by single quantum
emitter 207, emitted single photons 208 from resonant
excitation light 206; propagating emitted single photons 208
through multi-mode intermediate waveguide 205; commu-
nicating emitted single photons 208 from multi-mode inter-
mediate waveguide 205 to single-mode output waveguide
209; receiving, by single-mode output waveguide 209, emit-
ted single photons 208 from multi-mode intermediate wave-
guide 205; and propagating, through single-mode output
waveguide 209, emitted single photons 208 at emitted single
photon stream 210 to produce emitted single photon stream
210.

In an embodiment, the process for producing a single
photon stream with single quantum emitter single photon
source 200 also includes receiving the emitted single pho-
tons 208 in second separation gap 213.2 that separates
multi-mode intermediate waveguide 205 from single-mode
output waveguide 209; propagating emitted single photons
208 through second separation gap 213.2; and evanescently



US 11,451,306 B2

Sheet 2 of 3

Sep. 20, 2022

U.S. Patent

806

6 Dla

L086

206

Sls



U.S. Patent Sep. 20, 2022 Sheet 1 of 3 US 11,451,306 B2

213.2

210

2

206

me
\
209

(
207
FIG. 1

4
205

213.1

200
202


US 11,451,306 B2
Page 2

(51) Int. Cl.

GO2B 6/293 (2006.01)
HOAJ 14/00 (2006.01)
(56) References Cited

OTHER PUBLICATIONS,

Davanco, M., et al., “Efficient quantum dot single photon extraction
into an optical fiber using a nanophotonic directional coupler”,
Applied Physics Letters, 2011, p. 121101, vol. 99.

Davanco, M., et al., “Fiber-coupled semiconductor waveguides as
an eflicient optical interface to a single quantum dipole”, Optics
Letter, 2009, p. 2542-2544, vol. 34 Vo.16.

Davanco, M., et al., “Efficient spectroscopy of single embedded
emitters using optical fiber taper waveguides” Optics Letters, 2009,
p. 10542-10563, vol. 17 No. 13.

Uppu, R., et al., “On-chip deterministic operation of quantum dots
in dual-mode waveguides for a plug-and-play dingle-photon source”,
2020, DOK: arXiv:2001.10716.

* cited by examiner