P.D. Dapkus (1), Kostadin Djordjev (2), Seung-June Choi (3),
and Sang-Jun Choi (4)
Department of Electrical Engineering, University of Southern California
Los Angeles, CA 90089-0243

(1) dapkus@usc.edu, (2) Now at Agilent Laboratories, (3) seungjuc@usc.edu, (4) seungjuc@usc.edu.

Abstract: Active semiconductor microdisk components for photonic integrated circuits have been demonstrated. The devices utilize a free carier injection (FCI) active region to tune the resonant frequency, and electroabsorption and gain active regions to change the quality factor of the cavity.

Introduction
Microdisk resonant couplers are miniature and versatile components that enable frequency selective coupling between waveguides. In this geometry a disk cavity supporting whispering-gallery modes is side or vertically coupled to I/O bus waveguides, Fig.1. If the device is designed properly so that the power balance between the light coupled in and out of the cavity is maintained, it completely extracts the resonant wavelength from the input port and reroutes it to the dropped port. Thus, numerous devices operating on a fixed wavelength, such as add-drop filters, switches, and demultiplexers could be built [1].

FIG.1. A schematic diagram of a vertically-coupled microdisk resonator (left) and a SEM photograph of a real InP microresonator with a post (right).

Active microdisk resonators are even more appealing, because they can compensate for fabrication imperfections and add additional functionality [2]. If we inject free carriers into an intrinsic microcavity, a change in the effective index is achieved owing to the free-carrier Injection (FCI) plasma effect. By positioning the channel of interest at resonance and by shifting the resonant wavelength (as a result of the index change) one can achieve change in the output intensity and reroute the light from the dropped to the transmitted port. Thus, the design of tunable filters, active switches and demultiplexers, routers and modulators from such devices is possible. In addition, the transmission can be changed by varying the quality factor, Q, of the resonator. The introduction of gain or electroabsorption (EA) region is a natural way to vary the cavity loss and Q. Microdisk devices with gain active regions can be used to compensate for losses and fabrication imperfections and can operate as microdisk laser sources and amplifiers. The EA effect has very short response time so that microcavities with EA active regions can be designed to be very fast and small resonant modulators, routers and switches.

Because these microresonators are very small and facetless cavities literally thousands of components could be integrated into a single photonic chip by coupling the microdisks to the same bus waveguide. Circuits containing microdisk laser sources, detectors, switches, routers, and multiplexers are envisioned. We present here results on prototype devices that show the basic functionality required to fabricate these various circuit components by demonstrating microdisk resonators with free carrier, gain and EA active regions.

Fabrication
The vertically coupled device geometry enables the independent optimisation of the epitaxial structure in both the bus waveguides and the disk cavity by placing in them in different layers of the structure. The structure is grown in a single growth process. A 1-mm-thick p-doped (InP) disk cladding layer, followed by a 0.4-mm-thick intrinsic (Q1.25mm) disk core layer. If the device is to employ FC injection only, then the core contains only the bulk quaternary material. If the device utilize gain or EA regions, then 4QWs with emission wavelength of l=1.55mm are incorporated into the core. The growth continues with a 0.8-mm-thick n-doped (InP) coupling layer, followed by a 0.4-mm-thick n-doped (Q1.1mm) bus core layer and a 1-mm-thick n-doped (InP) bus cladding layer.

Wafer-to-wafer thermal bonding is used as integration enabler for this 3D device [3]. After the bus wavegiuide patterned is defined and etched, the growth substrate is flipped over and thermally bonded to a transfer wafer, the original substrate is selectively removed by a wet etchant, and the disk mesa is defined on top of the exposed epi-layers by an optimised RIE dry etch process [4]. Polyimide is used for planarization and metal bond-pad definition. The devices are thinned, cleaved into bars and an AR coating is deposited on both facets.

Results: Active Microdisk Devices
The transmission and dropped coefficients of the active devices were measured with a tunable diode laser while the appropriate bias was applied.

FIG.2. Transmission coefficient of an active microdisk device utilizing a free-carriers injection active region.

The measured responses from a 10mm FCI microdisk device are shown in Fig.2. The active region is a simple p-i-n structure and by injecting carriers into the i region, a change in the modal index by Dn=-2×10^-3 at 1mA is observed. The resonant wavelength shifts at a rate of 1nm/mA, the maximum tuning range being restricted by the cavity heating.

Applying a current shifts the resonance by 1nm/mA.spectral range FSR=10nm, quality factor Q=5500, finess F=40. It may be used as a switch or active router by positioning the channel of interest at resonance. Changing the current by DI =200mA (DV=0.1V) toggles the switch from OFF to ON state.

FIG.3. Transmission coefficient of an active microdisk device utilizing a gain active region. Applying a current increases Q and decreases the transmission at above bandgap wavelengths

Fig.3 shows the transmission characteristics of microdisk with a gain active region. Far from the band gap wavelength the resonance is simply shifted without significant decrease of Q. Above the bangap, as shown in the inset to the graph, by increasing the drive current, a decrease in the loss inside the cavity is observed, which leads to higher Q and lower T. More interesting is the case when I=10mA, where the transmission not only increases, but is even larger than unity, i.e. the device acts as an amplifyer. In the dropped channel the output increases with increasing current and exhibits a net gain of 7 – 10 dB.

Fig.4 shows the transmission characteristics of a microdisk with EA active region. Note that T at resonance increases and Q decreases at shorter wavelengths, where the active region is absorbing and the material loss is larger. Due to the QCSE, applying a reverse bias shifts the absorption edge towards longer wavelengths and thus introduces additional loss into the cavity. This leads to increase of the transmission and decrease of the Q at the resonant wavelengths, as shown in the inset to Fig.4. Due to the electrorefraction effect, the resonant wavelength is red shifted by 0.2nm@-3V bias, corresponding to a modal refractive index change of about Dn=+4×10^-4. The device could be used as a switch, router or fast modulator, because the applied reverse bias increases the transmission at resonance and decreases the dropped power at the same wavelength.

FIG.4. Transmission coefficient of an active microdisk device utilizing an EA active region. Applying a reverse bias decreases the Q and increases the transmission at below bandgap wavelengths.

Conclusions
InP vertically coupled microdisk resonators with active region utilizing the FCI, gain and EA effects are demonstrated for the first time. The devices exhibit single mode operation, large free spectral range (FSR=10nm) and a high Q of 5700. These devices are viewed as building blocks for the future photonic integrated circuits. Miniature active switches, routers and fast modulators amenable to large-scale integration are envisioned as part of a WDM system.

References

[1] Little et al IEEE JLT, 15 (1997), pp.998

[2] Djordjev et al IEEE JLT, 20 (2002), pp.105

[3] Djordjev et al IEEE PTL, 14 (2002), pp.331

[4] Choi et al JVST(B), 20 (2002), pp.3