Jesse Lu

Room temperature 1.6 µm electroluminescence from Ge light emitting diode on Si substrate

We report the room temperature electroluminescence (EL) at 1.6 microm of a Ge n+/p light emitting diode on a Si substrate. Unlike normal electrically pumped devices, this device shows a super linear luminescence enhancement at high current. By comparing different n type doping concentrations, we observe that a higher concentration is required to achieve better efficiency of the device. Thermal enhancement effects observed in temperature dependent EL spectra show the capability of this device to operate at room temperature or above. These detailed studies show that Ge can be a good candidate for a Si compatible light emitting device.

Inverse design and demonstration of a compact and broadband on-chip wavelength demultiplexer

An on-chip integrated wavelength demultiplexer designed using an inverse computational algorithm is experimentally demonstrated. 1,300 and 1,550 nm wavelength light is sorted in a device area of just 2.8 × 2.8 μm2.

Nanophotonic computational design

In contrast to designing nanophotonic devices by tuning a handful of device parameters, we have developed a computational method which utilizes the full parameter space to design linear nanophotonic devices. We show that our method may indeed be capable of designing any linear nanophotonic device by demonstrating designed structures which are fully three-dimensional and multi-modal, exhibit novel functionality, have very compact footprints, exhibit high efficiency, and are manufacturable. In addition, we also demonstrate the ability to produce structures which are strongly robust to wavelength and temperature shift, as well as fabrication error. Critically, we show that our method does not require the user to be a nanophotonic expert or to perform any manual tuning. Instead, we are able to design devices solely based on the users desired performance specification for the device.

Direct band Ge photoluminescence near 1.6 μm coupled to Ge-on-Si microdisk resonators

We fabricate and optically characterize germanium microdisks formed out of epitaxial germanium grown on silicon. Resonators coupled to fiber tapers display clear whispering gallery modes in transmission and photoluminescence with quality factors limited by germanium’s material absorption. Continuous wave pumping of the cavities resulted in a dominant heating effect for the cavity modes in both transmission and photoluminescence. Pulsed optical pumping proved to be more effective in minimizing heating, but was not sufficient to observe material gain or lasing. We believe that significantly higher doping levels are critical in order to achieve lasing at reasonable pump conditions.

Plasmonic enhancement of emission from Si-nanocrystals

Plasmonic gratings of different periodicities are fabricated on top of silicon nanocrystals embedded in silicon dioxide. Total enhancements of up to 2 were observed, which matches the value from simulations. Plasmonic enhancements are observed for the first three orders of the plasmonic modes, with the peak enhancement wavelength varying with the periodicity. Biharmonic gratings are also fabricated to extract the enhanced emission from the first order plasmonic mode, resulting in enhancements with quality factors of up to 16.

Cavity-enhanced direct band electroluminescence near 1550 nm from germanium microdisk resonator diode on silicon

We electrically and optically characterize a germanium resonator diode on silicon fabricated by integrating a germanium light emitting diode with a microdisk cavity. Diode current-voltage characteristics show a low ideality factor and a high on/off ratio. The optical transmission of the resonator features whispering gallery modes with quality factors of a few hundred. Direct band gap electroluminescence under continuous current injection shows a clear enhancement of emission by the cavity. At this stage, the pumping level is not high enough to cause linewidth narrowing and invert the material. A higher n-type activated doping of germanium is necessary to achieve lasing.

Inverse design of nanophotonic structures using complementary convex optimization

Fig. 1 shows the result of applying our method to a one-dimensional problem. The algorithm was executed for 100 iterations, which took 3 minutes on a generic desktop computer. The target field, which started as a sinusoid in a Gaussian-envelope, closely matches the field obtained from simulating the dielectric structure using FDTD. Additionally, the values of Y were strictly constrained to be between 1 and 10, which resulted in a nearly binary dielectric structure. This is important, since feasible nanophotonic devices are almost universally binary and discrete in values of ∈

Tunable-wavelength second harmonic generation from GaP photonic crystal cavities coupled to fiber tapers

We demonstrate up to 30 nm tuning of gallium phosphide photonic crystal cavities resonances at aproximately 1.5 microm using a tapered optical fiber. The tuning is achieved through a combination of near-field perturbations and mechanical deformation of the membrane, both induced by the fiber probe. By exploiting this effect, we show fiber-coupled second harmonic generation with a tuning range of nearly 10 nm at the second harmonic wavelength of approximately 750 nm. By scaling cavity parameters, the signal could easily be shifted into other parts of the visible spectrum.

Coupled fiber taper extraction of 1.53 μm photoluminescence from erbium doped silicon nitride photonic crystal cavities

Optical fiber tapers are used to collect photoluminescence emission at approximately 1.5 microm from photonic crystal cavities fabricated in erbium doped silicon nitride on silicon. In the experiment, photoluminescence collection via one arm of the fiber taper is enhanced 2.5 times relative to free space collection, corresponding to a net collection efficiency of 4%. Theoretically, the collection efficiency into one arm of the fiber-taper with this material system and cavity design can be as high as 12.5%, but the degradation of the experimental coupling efficiency relative to this value mainly comes from scattering loss within the short taper transition regions. By varying the fiber taper offset from the cavity, a broad tuning range of coupling strength and collection efficiency is obtained. This material system combined with fiber taper collection is promising for building on-chip optical amplifiers.

Inverse design of a three-dimensional nanophotonic resonator

The inverse design of a three-dimensional nanophotonic resonator is presented. The design methodology is computationally fast (10 minutes on a standard desktop workstation) and utilizes a 2.5-dimensional approximation of the full three-dimensional structure. As an example, we employ the proposed method to design a resonator which exhibits a mode volume of 0.32(λ/n)3 and a quality factor of 7063.