Ultra-high-Q microresonators are fabricated on silicon chips by the solgel technique. Using wafer-based processing and selective reflow, we create toroid-shaped Er-doped microlasers directly from Er-doped solgel layers and Raman microlasers from undoped silica solgel layers

Min, Bumki

Spillane, Sean M.

Vahala, Kerry J.

Yang, Lan

English

Caltech Authors - Main

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CFMl 
Solgel route to Erbium-doped microlasers 
and Raman microlasers on-a-chip 
Lan Yang, Sean M. Spillane, Bumki Min and Kerry J. Vahala 
Deportment OfAppliedPhysics, Colfornio Imtintte of Technology, Pasadena CA 91125 
lm&oltech.edu 
Abstract: Ultra-high-Q microresonators are fabricated on silicon chips by the solgel technique. 
Using wafer-based processing and selective reflow, we create toroid-shaped Erdoped microlasers 
directly from Erdoped solgel layers and Raman microlasers from undoped silica solgel layers. 
02003 Optical Society of America 
OCISeodes: (140.3500) laser, erbium; (lW.4780) optical resonaton; (160.6060) Solgel 
1. Introduction 
The solgel process for preparing silica and silicate from a metal alkoxide precursor has been an impomt  platform 
for inany applications t1-21. It combines control of composition and microstlucture at the molecular level with the 
ability to shape materials in bulk, fiber, powder and thin film f o m .  On the other hand, photonic devices using silica 
layers deposited on silicon chips are the basis for numerous photonic devices. Recently, a silicabased resonator 
structure in the form o f a  microtoroid has demonstrated unprecedented Q factors in the range of 100 million [3]. In 
this work, the process flow for this resonat01 is modified using the solgel method, In one series of experiments, 
Erbium doped solgel films are used as the base material for microtoroid formation. The subsequently processed 
devices function as low threshold micro lasers. In a second set of experiments, pure silica solgel layers serve as the 
base material and are processed into ultra-high-Q Raman microlasers. Both of these cases demonstrate the ability to 
use spin coating of sol gel films as a processing alternative to deposition or oxidation methods for silica layer 
formation. 
2. Fabrication 
The solgel solution was prepared by hydrolyzing tetmethoxysilane e (TEOS) using water in acidic conditions with 
the molar ratio of water to TEOS around 1-2. Er ions were introduced by adding Ero\rO), to achieve the desired Er 
concentration. After reaction at 70°C for three hours, a viscous solution was formed. The silica solgel film was then 
deposited on a Si substrate by the spin-coating method. Multi-coating was used to build up thickness. After each 
coating cycle, the chip was annealed at 1000°C for three hours. Subsequcnt to the spin coat and anneal step, 
processing of microtoroids proceeded as described in reference [2 ] .  Circular silica pads were created on the Si wafer 
through a combination of lithography and etclung. Subsequently, lhese pads served as an etch mask for isotropic 
silicon etching using XeF2. The silica disks are left supported by narroxzer silicon posts. A CO2 laser is then used to 
selectively reflow the silica disks, during which surface tension causes the disks to collapse into toroids. The 
exceptionally smooth surface of the toroid cavities endolls these stluctures with their ultra-high-Q properties [3]. 
Fig. 1 depicts the fabrication process flow of the microlasers beginning with the solgel layers on the Si wafer. 
(*-top) Wtop) O-top) (d-top) 
Fig. 1. Process flow for creation of solgel microcavities on a Si wafer in schematic side view 
(above) and photomicrograph p h i e  view (below). (a) Solgel layer is s p h  on Si wafer; (b) 
circular pads are etched; (c) XeF2 isotropic silicon etch; (d) CO2 laser reflow. 
3. Experiment 
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The principle diameter of the microtoids studied in this work ranged from 50 to 60 tun, A fiber taper with a waist 
diameter of 1-2 prn is aligned along the equatorial plane of the microtomid to couple light in and out of the 
microcavity. To vary  he air gap between the microcavity and the taper, the sample chips were mounted on a three- 
axis translator for position control. Tunable, single frequency, nanow linewidth (<300 kHz) extemal-cavity lasers at 
both 980 nm band and 1480 nm band were used as the pump source. Fig. 2 shows the typical laser spectrum of an 
Erdoped microtomid laser fabricated using an Erdoped solgel layer spun on the silicon chip. The measured laser 
output power plotted versus the absorbed pump power is presented in the inset of Fig. 2. 
Wavelenm (”rn) 
Fig. 2. Laser emission spectrum of the Erdoped microtoroid laser. Inset: Laser output power versus 
absorbed pump power 
In ultra-high-Q microcavities Raman oscillation at low pump powers is possible even in linear media such as silica 
[I] .  For this purpose, a microtoroid cavity was fabricated from the undoped solgel layer. Fig. 3 shows a typical 
Raman laser spectrum of the cavity. The Raman microtoroid laser was excited well above threshold using a pump 
power in the milliWatt range. In the figure, the pump wave is at 1151 nm and Raman lasing occurs near 1550nm. 
The inset to figure 3 is a micrograph showing the microtoroid-taper coupling system. 
1 0 ,  . , . , , . , . , . , . , . , 
3440 , a m  ,480 I I W  ,120 ,540 l l B 0  1680 (Boo 
Waveiength(nrn) 
Fig. 3. Spectrum of a Raman microtomid laser with principal diameter of 55 pm. The pump powe is 1.2 mW 
at 1451 um and Ramai~ oscillation occurs at 1551 um. Inset: micrograph of a solgel microtoroid coupled to a 
titer taper 
4. Conclusion 
By using solgel process, we could create both Er-doped microlasers and Raman microlasers on a chip. The solgel 
process provides a vemtile way to create silica-based devices with precise control of the composition. 
ReferenFs 
I .  
2. 
3. 
4. 
Rustum Roy, Yeramies by the solution-sol-gel Route,” Science 238, 1664-1669 (1987) 
Lan Ymg and K . J  Vahala, “Gain functionalkalion of silica microsphere; Opt. Lett., 28,592-594 (2003) 
D. Armmi, S. Spillme, T. J. Kippenberg “Ultra-high-Q microcavilyon-a-fhip,”Naturr, 421,925-929 (2003) 
S.hl.Spillane, T.J.Kippenberg and K . J . V a h a l ~  “Ultralow-threshold Raman laser using a spherical dielectric miaocavity? Nature, 415, 
621.624 (zona) 


Solgel route to erbium-doped microlasers and Raman microlasers on-a-chip

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Solgel route to erbium-doped microlasers and Raman microlasers on-a-chip

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