Demonstration of an erbium-doped microdisk laser on a silicon chip

An erbium-doped microlaser is demonstrated utilizing $\mathrm{Si}{\mathrm{O}}_2$ microdisk resonators on a silicon chip. Passive microdisk resonators exhibit whispering-gallery-type modes (WGM’s) with intrinsic optical quality factors of up to $6\times {10}^7$ and were doped with trivalent erbium ions (peak concentration $\sim 3.8\times {10}^{20}{\mathrm{cm}}^{-3}$) using MeV ion implantation. Coupling to the fundamental WGM of the microdisk resonator was achieved by using a tapered optical fiber. Upon pumping of the ${}^{4}I_{15∕2}\to {}^{4}I_{13∕2}$ erbium transition at $1450\mathrm{nm}$, a gradual transition from spontaneous to stimulated emission was observed in the $1550\text{−}\mathrm{nm}$ band. Analysis of the pump-output power relation yielded a pump threshold of $43\mu \mathrm{W}$ and allowed measuring the spontaneous emission coupling factor: $\beta \approx 1\times {10}^{-3}$.

Figure 1

(Color online) (a) Scanning electron micrograph image of a $60\text{−}\mu \mathrm{m}$-diameter silica microdisk cavity on a Si post on a silicon wafer. The wedge-shaped cavity boundary is intentionally induced during fabrication using a hydrofluoric acid etch. (b) A finite-element simulation of the intensity profile of the fundamental whispering gallery mode in a microdisk cavity (oxide thickness $1\mu \mathrm{m}$, cavity radius $60\mu \mathrm{m}$, $\lambda =1.54\mu \mathrm{m}$) for three wedge angles: 90°, 45°, and 22°. As can be evidenced in the simulation a progressive increase in the cavity boundary angle leads to radial shift of the mode towards the interior of the microdisk, thereby isolating the mode from the scattering-inducing cavity boundary.

Figure 2

(Color online) Experimentally measured quality factor $\left(Q\right)$ for subsequent WGM’s in the weak pump regime. All modes belong to the same mode family (fundamental radial WGM’s) and are separated by their different angular mode numbers. The solid line is a calculation taking into account wavelength-dependent absorption by Er, assuming $\Lambda =0.3$. Dotted lines show calculations for $\Lambda =0.2$ and $\Lambda =0.5$ for comparison. Lower right graph: a transmission spectrum exhibiting strong mode splitting (from a different sample). The upper right graph: the modal coupling parameter—i.e., the ratio of the splitting frequency $({\gamma }^{-1}∕2\pi )$ normalized with respect to the intrinsic cavity line width $({\tau }^{-1}∕2\pi )$—versus $Q$ for the modes measured in the main figure. The observed linear relationship demonstrates absorption-limited $Q$ behavior (solid line).

Figure 3

(Color online) Upper graph: the subthreshold emission collected from the microdisk through the coupling fiber, when the cavity is resonantly pumped at $1480\mathrm{nm}$. Several fundamental cavity modes are observed (spaced with $\mathrm{FSR}=9.1\mathrm{nm}$). The weak, subsidiary peaks are attributed to fundamental modes of opposite polarization. The luminescence maximum at $1555\mathrm{nm}$ coincides with the peak of the erbium emission spectrum. Lower graph: the above-threshold spectrum in the presence of the $1450\text{−}\mathrm{nm}$ pump. The nonlasing modes are suppressed by more than $15\mathrm{dB}$ in comparison with the lasing mode.

Figure 4

(Color online) Output power vs launched pump power for the Er-doped microdisk in Fig. 2 (pump wavelength $1480\mathrm{nm}$). The transition from spontaneous emission to stimulated emission is gradual, indicative that a sizable fraction of the spontaneous emission is coupled into the microcavity. The solid line is a fit using the model from Ref. 17 yielding a lasing threshold of $43\mu \mathrm{W}$. The inset shows the same data on a double-logarithmic scale with a logarithmic fitting routine which improves fitting for the low-power data and yields a spontaneous emission coupling factor $\beta \approx 1\times {10}^{-3}$.