1. Fabrication and characterization of erbium-doped toroidal microcavity lasers
J.Kalkman, A.Tchebotareva, A.Polman, T.J.Kippenberg,
B.Min and K.J.Vahala, J.Appl.Phys. 99 (2006)
유형숙, 김기현

2. Contents I. Objective II. Introduction III. Fabrication & Analysis
VI. Results
V. Summary & Discussion

3. 과 제 목 표 Er ions 이 doping 된 Si-chip resonator 에 대한 이해
Objective
과 제 목 표
Er ions 이 doping 된 Si-chip resonator 에 대한 이해
Laser flow 를 이용한 Microcavity Fabricaton 에 대한 이해
두 가지 doping 방법 (Postimplanted , Preimplanted) 조사 및 분석, 비교

4. Introduction
Optical Microcavity
Advantage : Confinement of light spatially and temporally, high Q value
Disadvantage : fragile, integration difficulties on a Si substrate
Standard fabrication processes combined with a laser reflow process (toroidal ring resonators on a Si substrate with Q>108)
D.K.Armani, T.J.Kippenberg, S.M.Spillane, and K.J.Vahala,
Nature(London) 421, 925 (2003)

5. III. Fabrication & Analysis

6. Fabrication Process Fabrication
Preimplanted Microtoroids / Preimplanted Microtoroids
Outer diameter : 57um Minor diameter : 6 um
CO2 laser (10.6um, 10W, 1ms)

7. Postimplanted Microtoroid & Preimplantated Microtoroid
Fabrication & Analysis
Postimplanted Microtoroid & Preimplantated Microtoroid
Er implantation
Inhomogeneous implantation profile  good overlap with the optical mode
Er ions are implanted at an angle of 70o relative to the toroid’s axis of symmetry
2 MeV Er + at a fluence of 4.2ⅹ10 15 cm-2
Annealed for 1h at 800 oC in vacuum.
Oxygen plasma etch for romovement carbon contamination
Q > 108
1~2 um thick thermally grown SiO2 films on a Si substrate. The 2um thick SiO2 was implanted with 2MeV Er + ions
Er ions are Symmetric Er distribution

8. VI. Results

9. Er PL emission spectra at 1536 nm
Results
Er PL emission spectra at 1536 nm
Peaks have an equidistant spacing. FSR=8.6nm.
 By whispering gallery mode
An enhanced optical density of states at the cavity resonance
-> an increase of the Er spontaneous emission into these cavity modes
Pump : 488 nm laser top pumping
Step 6
Far field (not affected by coupling)
Angular Free Spectral Range :
FSRl=l2/(2pRneff)
R : the major radius of the toroid, 29.4um
neff : the effective mode index, 1.45
위 그림에 의하면 기존에 implantation 으로 만들어진 경우의 Q=108 이었던 것에 비해 그값이 <15000 정도로 작게 나오는데 이는 probed Er ions 이 low-Q mode 와 couple 되기 때문이다.

10. The relation between cavity modes and Er emission into these modes
Results
The relation between cavity modes and Er emission into these modes
Normalized transmission using a tapered optical fiber
 It is Possible to preferentially excite high-Q, low-order modes, which have their field maxima located close to the perimeter of the microtoroid.
Relative Er PL intensity obtained from the side of the microtoroid and from the top of a planar section of the microtoroid
<by Postimplant Method>
Dips at different wavelength / smaller linewidth
However, both FSR~8.6nm
 The probed Er ions preferentially couple to low-Q whispering gallery modes.

11. Comparison of Postimplantation Method and Preimplantation Method
Results
Comparison of Postimplantation Method and Preimplantation Method
Postimplantation Method
Preimplantation Method
Mode overlap G (Calculated)
0.066
0.02
For both post- and preimplanted microtoroids , the measured Q’s are for a large part determined by Er absorption
Quality factor, Q
Theoretical results
(from mode coupling theory)
1.6ⅹ107
(1442nm)
1.8ⅹ108
(1411nm)
Experimental results
3.9ⅹ107
Lasing around 1.5um
(pump : 1440nm)
Single and multimode lasing
Er microtoroid lasing
(lasing threshold:125uW pump)
(lasing threshold : 4.5uW pump)
: Er absorption-related Q
S.M.Spillane, T.J.Kippenberg, K.J.Vahala, K.W.Goh, E.Wilcut, and H.J.Kimble, Phys.Rev. A 71, (2005)

12. SUMMARY Discussion Summary & Discussion
On-chip Er microlaser fabricatoion by a CO2 laser pulse
Comparison of Two Fabrication Method ( Postimplanted and Preimplanted Methods)
- Er distribution
- Overlap factor
- For both, the Q’s are determined by Er absorption
- Single and Multimode lasing (depending on the pump power)
No macroscopic Er diffusing during the laser reflow
Lowest lasing threshold (4.5uW) in the preimplanted microtroid
An increase of the Er spontaneous emission into cavity mode.
Discussion
Application : low-threshold lasers, optical filters, wavelength multiplexers optical memories and optical swiches.