The fabrication of the Er+ sol-gel microtoroid fabrication process is outlined in Figure 1.   Figure 2 shows a laser emission spectrum for CW (continuous wave) operation of an Er3+-doped microtoroid laser (optical spectrum analyzer resolution of 0.5 nm) with an optical micrograph of the fiber-coupled microlaser presented in the inset. Single-mode operation could be obtained by proper choice of the coupling condition. The green ring around the toroid periphery is due to upconverted photoluminescence from Er3+.

A theoretical analysis shows that the threshold power is minimized at a certain erbium ion concentration. This optimal erbium ion concentration depends, in turn, on the intrinsic quality factor of the pump mode. In the low concentration limit, the threshold power increases sharply because Er3+ ions are not able to give sufficient gain required for loss compensation; while in high concentration limit, the threshold again increases due to increases of concentrationdependent loss mechanism, such as upconversion and ionpair induced quenching.  We found that for a toroid with a principal diameter of 60 mm and intrinsic quality factor (pump mode) between 5x10⁶ and 1x10⁷, thresholds in the range of 400-600 nW can be achieved with an Er3+ concentration of 2x10¹⁹ cm^-3. Figure 3 shows the measured laser output power plotted versus the absorbed pump power from a microtorid with properties as described above. The threshold was estimated to be as low as 660 nW, which is about three times lower than that of the most recently reported Er-implanted microtoroid resonators.

This ultralow threshold originates from the small mode volume, high quality factor of the microcavity, and homogeneous distribution of the Er3+ inside the cavity, which enable the optimized overlap between the active region and the pump modes. Above threshold, the laser output power increases linearly with the absorbed pump power, as expected.

By varying the erbium concentration in the starting solgel solution, we could adjust the final doping concentration in the microcavities, which ultimately modifies the laser dynamics as previously described for the case of microsphere lasers. In particular, for a heavily doped (Er₂O3 ~0.15 mol%) microcavity, pulsation behavior, as opposed to CW operation, is observed. The pulse repetition rate is 0.9 MHz at a laser output power of 3.8 mW and is attributed to the presence of saturable absorption in the cavity owing to unpumped or incompletely pumped erbium regions and ion-pair induced quenching.

More information can be found in the following papers:

L. Yang, T. Carmon, B. K. Min, S. M. Spillane, and K. J. Vahala
“Erbium-doped and Raman microlasers on a silicon chip fabricated by the sol-gel process”
Applied Physics Letters, Volume 86, 091114, February 2005.

Lan Yang, Deniz Armani and Kerry Vahala
"Fiber-coupled Erbium Microlasers on a chip"
Applied Physics Letters, vol. 83, No. 5, 825-826, August 2003.

Deniz Armani, Tobias Kippenberg, Sean Spillane and Kerry Vahala
"Ultra-high-Q toroid microcavity on a chip"
Nature, vol. 421, pp. 925-929, 27 February 2003.

Figure 1:  Schematic process flow for creation of solgel microcavities on a Si wafer (left) and photomicrograph plan view (right) after each step. (a) Solgel layer is spun on Si wafer; (b) circular pads are etched; (c) XeF₂ isotropic silicon etch; (d) CO₂ laser reflow.

Figure 2: Typical laser spectrum of an Er3+-doped solgel silica microtoroid laser. The inset is a photomicrograph top view of an Er3+-doped solgel silica microtoroid laser with principal diameter of 60 mm coupled by a fiber taper. The green ring is due to upconverted photoluminescence from Er3+.

Figure 3: Measured laser output power plotted vs the absorbed pump power for a microtoroid laser with principal diameter of 60 mm. The lasing threshold is 660 nW with pump wavelength at 1442 nm and lasing wavelength at 1553 nm.