Highly unidirectional & ultralow-threshold lasing from on-chip ultrahigh-Q optical resonator

Yun-Feng Xiao (肖云峰)

Peking University, Beijing 100871, P. R. China 北京大学 物理学院 人工微结构和介观物理国家重点实验室

Email: yfxiao@pku.edu.cn

Tel: (86)10-62765512

Advantages of microcavities

Q

22

cav

in

P QB

P nD

Cavity power build-up factor:

Q ~1×108, D ~ 50m, Vm ~ 600 m3 B ~ 105

Cavity photon lifetime:

WHY ultra-high-Q whispering gallery resonator?

Pin = 1 mW

Pcav ~ 100 W, Icav ~ 2.5 GW/cm2,

~ 100 ns, # of round trip ~ 2105.

Experimental

data in our

group

> 100 W

“Strongly enhanced light-matter interaction”

2

Cavity QED for implementation of quantum computation, quantum routers, ...

Quantum optics, quantum information processing

3

Nature 443, 671 (2006); Science 319, 1062 (2008); PRL 95, 067401 (2005); Nano Lett. 6, 2075 (2006)

g/2~50 MHz

k/2~18 MHz

g/2~2.6 MHz

Highly sensitive optical biosensing

4

F. Vollmer & S. Arnold, Nature Methods 5, 591 (2008)

V. R. Dantham et al., APL 101, 043704 (2012)

J. Zhu, S. K. Ozdier, Y. F. Xiao et al.

Nature Photonics 4, 46 (2010)

X. Yi, Y.-F. Xiao* et al., Appl. Phys. Lett. 97, 203705 (2010); Phys.

Rev. A 83 , 023802 (2011); J. of Appl. Phys. 111 (11), 114702 (2012)

Mode shift Mode splitting

Cooling of mechanical motion

Radiation pressure induced

mechanical oscillation

Cavity optomechanics

5

Amplification of mechanical motion

Mechanical ground state cooling

Nature 475, 359 (2011); Nature 478, 89 (2011)

~ Quantum limited measurement

~ Test of quantum mechanics at increasingly large mass and length scales

~ Implementations in quantum information processing

m

m

Low-threshold lasing

6

Kippenberg group, PRL (2008)

Vahala group, Nature (2002)

Applications of whispering gallery microresonator

Important platform for

Fundamental physics:

Quantum optics

Cavity quantum electrodynamics (QED)

Quantum information

Classical and quantum chaos…

Cavity Optomechancs

Nano/micro-photonics:

Highly sensitive bio/chemical sensing

All-optical low-threshold switching

On-chip microlasing

Filtering… Ultrahigh Q, very small V,

and mass production on a chip

Vahala

, Natu

re, 2

003

7

8

Traditional whispering gallery microcavities

High Q factor & small mode volume

Unfortunately, isotropic emission

Excitation and collection difficulties

Evanescent couplers: tapered fiber, prism,

Strict phase-matching

Coupling losses

Coupling instability

9

From circular to deformed microcavities

10

Nature

385, 45 (1997)

Quadruple “flattened”

Quadruple

Science

280, 1556 (1998) Appl. Phys. Lett.

83, 9 (2003)

Spiral

Phys. Rev. Lett.

100, 033901 (2008)

Limacon

Stadium

Phys. Rev. Lett.

90, 063901 (2003)

Phys. Rev. Lett.

104, 163902 (2010)

PNAS

107, 22404 (2010)

Phys. Rev. Lett.

105, 103902 (2010)

Typical deformed microcavities

Threshold for microlaser: Q2/V

Purcell effect: Q/V

Strong coupling for cavity QED: Q/V1/2

c(3) optical nonlinearities: Q2/V

Optical forces and trapping: Q/V

Optical biosensing: Q/V

IMPORTANT quality factor

11

12

Outline

Physics in ultrahigh-Q, unidirectional microcavities

Realization of ultrahigh-Q, unidirectional microcavities

Ultralow-threshold and highly unidirectional behavior

in Er3+-doped microcavities

Summary

Really?

An intuitive expectation: emit from the points of highest

curvature (j=0, ) in the tangent direction

lead to peaks at q=± /2 in the far-field

Emission points & directions?

Hailin Wang group

13

Ray dynamics......Poincaré surface of section

14

• KAM curves

• Islands

• Chaos

Disjoined nature classically

2-D:

f

sin

c

1/n

f

sin

c

Mechanism for the unidirectional emission

15

Regular mode→chaos

Dynamical tunneling

Chaos→refractive emission

Short-time ray dynamics

16

Summary: Mechanism for the unidirectional emission

Clockwise and counter-clockwise modes

At least 2 × n emission points

Deformed cavity with only one symmetric axis

Emission towards the symmetric axis

Far-field unidirectionality

17

Summary: The theoretical design

Deformed microcavity with only one symmetric axis

Support regular KAM curves or islands located on upper SOS (high-Q)

Dynamical tunneling between regular and chaotic modes

Refractive emission obeying short-term dynamics of chaos (directionality)

CCW

18

0

0 ss

=

1 11 1 1 1 1 1

ext mat rad ext

QQ Q Q Q Q Q

Quality factors

Ultrahigh-Q modes:

• Locate in the upper in SOS: reduce radiation loss

• Minimized material absorption and surface scattering losses

Low loss medium

Small deformation

CO2 reflow process

Materials loss

Radiation loss

Surface scattering

Challenges Our solutions：

deformed microtoroid

𝑸𝒆𝒙𝒕 → ∞,𝑸 = 𝑸𝟎 Coupling loss

19

Two-step dry etching and laser reflow

Fabrication of ultra-high-Q silica toroidal microcavity

• Precise control the shape of the deformed toroid

• Atomic-scale surface: ultralow scattering losses

Fabrication

20

White boundary: circle

Black boundary: ellipse

Red boundary: our design

Nanometer-scaled control

21

Transmission

Free-space

Fiber taper

Q>108

Excitation

22

Experimental Q factors

Lasing action

23

24

Unidirectional-emission lasing

Summary

25

We experimentally realized on-chip deformed microcavities with

experimental Q factor exceeded 108, four orders of magnitude higher

than previous chip-based deformed microcavities.

By doping erbium into the deformed microcavity, the unidirectional-

emission lasing was observed in 1,550 nm band with the threshold as

low as 2 W and a narrow divergence angle about 10 degrees.

The work may open up new possibilities for investigations of fundamental

physics and applied photonics, such as single-photon sources, strong

coupling physics, and cavity optomechanics, by using a convenient free-

space coupling in chip-based ultrahigh-Q microcavities.

Thank you for your attention! www.phy.pku.edu.cn/~yfxiao/index.html

X.-F. Jiang, Y.-F. Xiao*, C.-L. Zou , L. He , C.-H. Dong ,

B.-B. Li , Y. Li , F.-W. Sun , L. Yang , and Q. Gong*

Advanced Materials 2012, 24, OP260-OP264