32-channel wide-bandwidth massive RF photonic combiner based on distributed groups of arrayed photodetectors.

We propose and experimentally demonstrate a low-loss, radio frequency (RF) photonic signal combiner with flat response from 1 GHz to 15 GHz and low group delay variation of 9 ps. The distributed group array photodetector combiner (GAPC) is implemented in a scalable Si photonics platform and has applications in RF photonic systems that rely on combining massive numbers of photonic signals.

Frequency behavior of AlInAsSb nBn photodetectors and the development of an equivalent circuit model.

We report the frequency response of Al0.3InAsSb/Al0.7InAsSb nBn photodetectors. The 3-dB bandwidth of the devices varies from ∼ 150 MHz to ∼ 700 MHz with different device diameters and saturates with bias voltage immediately after the device turn on. A new equivalent circuit model is developed to explain the frequency behavior of nBn photodetectors. The simulated bandwidth based on the new equivalent circuit model agrees well with the bandwidth and the microwave scattering parameter measurements. The analysis reveals that the limiting factor of the bandwidth of the nBn photodetector is the large diffusion capacitance caused by the minority carrier lifetime and the device area. Additionally, the bandwidth of the nBn photodetector is barely affected by the photocurrent, which is found to be caused by the barrier structure in the nBn photodetector.

Room-temperature bandwidth of 2-µm AlInAsSb avalanche photodiodes.

We investigate the room-temperature bandwidth performance of AlInAsSb avalanche photodiodes under 2-µm illumination. Parameter characterization denotes RC-limited performance. While measurements indicate a maximum gain-bandwidth product of 44 GHz for a 60-µm-diameter device, we scale this performance to smaller device sizes based on the RC response. For a 15-µm-diameter device, we predict a maximum gain-bandwidth product of approximately 144 GHz based on the reported measurements.

Efficient absorption enhancement approaches for AlInAsSb avalanche photodiodes for 2-µm applications.

Recently, advances in imaging and LIDAR applications have stimulated the development of high-sensitivity receivers that operate at wavelengths of ≥ 2 µm, which has driven research on avalanche photodiodes (APDs) that operate in that spectral region. High quantum efficiency is a key performance parameter for these photodetectors. Increasing the thickness of the absorption region is a straightforward approach to increase the quantum efficiency. However, the primary source of dark current is the narrow-bandgap material used for 2-µm detection. Increasing its thickness results in higher noise. In this paper, we describe two approaches to enhance the quantum efficiency, both of which are superior to a conventional anti-reflection (AR) coating. For normal incidence at 2 µm, finite-difference time-domain (FDTD) simulations show the absorption can be enhanced by more than 100% with a triangular-lattice photonic crystal, and nearly 400% by applying a metal grating. This is achieved by coupling normal incidence light into the laterally propagating modes in the device. Moreover, the significantly higher absorption of the metal grating compared to the photonic crystal is due to the high coupling efficiency provided by the metal grating. This work provides promising methods and physical understanding for enhancing the quantum efficiency for 2-µm detection without increasing absorber thickness, which also enables low dark current and high bandwidth.

Photonic generation of pulsed microwave signals in the X-, Ku- and K-band.

Photonic microwave generation of high-power pulsed signals in the X-, Ku- and K-band using charge-compensated MUTC photodiodes is demonstrated. The impulse photoresponse without modulation showed a maximum peak voltage of 38.3 V and full-width at half-maximum of 30 ps. High power pulsed microwave signals at 10, 17 and 22 GHz with peak power up to 44.2 dBm (26.3 W), 41.6 dBm (14.5 W) and 40.6 dBm (11.5 W) were achieved, respectively.

Heterogeneous photodiodes on silicon nitride waveguides.

Heterogeneous integration through low-temperature die bonding is a promising technique to enable high-performance III-V photodetectors on the silicon nitride (Si3N4) photonic platform. Here we demonstrate InGaAs/InP modified uni-traveling carrier photodiodes on Si3N4 waveguides with 20 nA dark current, 20 GHz bandwidth, and record-high external (internal) responsivities of 0.8 A/W (0.94 A/W) and 0.33 A/W (0.83 A/W) at 1550 nm and 1064 nm, respectively. Open eye diagrams at 40 Gbit/s are demonstrated. Balanced photodiodes of this type reach 10 GHz bandwidth with over 40 dB common mode rejection ratio.

40 Gbit/s waveguide photodiode using III-V on silicon heteroepitaxy.

Low-dark-current waveguide modified uni-traveling carrier photodiodes (PDs) are demonstrated by direct heteroepitaxy of InGaAs/InAlGaAs on silicon templates. The PDs have a dark current of 0.1 µA at -3V bias and an internal (external) responsivity of 0.78 A/W (0.27 A/W). The 3 dB bandwidth is 28 GHz, and open eye diagrams are detected at 40 Gbit/s.

Triple-mesa avalanche photodiodes with very low surface dark current.

The dark current of a photodetector is a key parameter for high-sensitivity optical receivers. We report low-dark-current, triple-mesa avalanche photodiodes that have ~50 times lower dark current than conventional single-mesa devices, and suppress surface leakage. The tolerances of triple-mesa avalanche photodiode parameters are presented.

III-V on silicon avalanche photodiodes by heteroepitaxy.

We demonstrate a III-V avalanche photodiode (APD) grown by heteroepitaxy on silicon. This InGaAs/InAlAs APD exhibits low dark current, gain >20, external quantum efficiency >40%, and similar low excess noise, k∼0.2, as InAlAs APDs on InP.

Segmented waveguide photodetector with 90% quantum efficiency.

We demonstrate a novel InGaAsP/InP segmented waveguide photodetector based on directional couplers. By matching the imaginary parts of the propagation constants of the even and odd modes, we designed a photodetector with 6 elements, each with an absorber volume of only 19 µm3 and a bandwidth of 15 GHz, that has an internal quantum efficiency (QE) of 90% at 1550 nm wavelength corresponding to 1.13 A/W.

Low dark current III-V on silicon photodiodes by heteroepitaxy.

Top-illuminated PIN and modified uni-traveling carrier (MUTC) photodiodes based on InGaAs/InAlAs/InP were epitaxially grown on Si templates. Photodiodes with 30-µm diameter have dark currents as low as 10 nA at 3 V corresponding to a dark current density of only 0.8 mA/cm2. The responsivity, 3-dB bandwidth, output power and third-order output intercept point (OIP3) were 0.79 A/W, 9 GHz, 2.6 dBm and 15 dBm, respectively.

Design of emitter structures based on resonant perfect absorption for thermophotovoltaic applications.

We report a class of thermophotovoltaic emitter structures built upon planar films that support resonant modes, known as perfectly-absorbing modes, that facilitate an exceptional optical response for selective emission. These planar structures have several key advantages over previously-proposed designs for TPV applications: they are simple to fabricate, are stable across a range of temperatures and conditions, and are capable of achieving some of the highest spectral efficiencies reported of any class of emitter structure. Utilization of these emitters leads to exceptionally high device efficiencies under low operating temperature conditions, which should open new opportunities for waste heat management. We present a theoretical framework for understanding this performance, and show that this framework can be leveraged as a search algorithm for promising candidate structures. In addition to providing an efficient theoretical methodology for identifying high-performance emitter structures, our methodology provides new insight into underlying design principles and should pave way for future design of structures that are simple to fabricate, temperature stable, and possess exceptional optical properties.

Dynamic-quenching of a single-photon avalanche photodetector using an adaptive resistive switch.

One of the most common approaches for quenching single-photon avalanche diodes is to use a passive resistor in series with it. A drawback of this approach has been the limited recovery speed of the single-photon avalanche diodes. High resistance is needed to quench the avalanche, leading to slower recharging of the single-photon avalanche diodes depletion capacitor. We address this issue by replacing a fixed quenching resistor with a bias-dependent adaptive resistive switch. Reversible generation of metallic conduction enables switching between low and high resistance states under unipolar bias. As an example, using a Pt/Al2O3/Ag resistor with a commercial silicon single-photon avalanche diodes, we demonstrate avalanche pulse widths as small as ~30 ns, 10× smaller than a passively quenched approach, thus significantly improving the single-photon avalanche diodes frequency response. The experimental results are consistent with a model where the adaptive resistor dynamically changes its resistance during discharging and recharging the single-photon avalanche diodes.

Self-organized 2D periodic arrays of nanostructures in silicon by nanosecond laser irradiation.

We report a phenomenon of spontaneous formation of self-organized 2D periodic arrays of nanostructures (protrusions) by directly exposing a silicon surface to multiple nanosecond laser pulses. These self-organized 2D periodic nanostructures are produced toward the edge as an annular region around the circular laser spot. The heights of these nanostructures are around 500 nm with tip diameter ~100 nm. The period of the nanostructures is about 1064 nm, the wavelength of the incident radiation. In the central region of the laser spot, nanostructures are destroyed because of the higher laser intensity (due to the Gaussian shape of the laser beam) and accumulation of large number of laser pulses. Optical diffraction from these nanostructures indicates a threefold symmetry, which is in accordance with the observed morphological symmetries of these nanostructures.

Towards high-power, high-coherence, integrated photonic mmWave platform with microcavity solitons.

Millimetre-wave (mmWave) technology continues to draw great interest due to its broad applications in wireless communications, radar, and spectroscopy. Compared to pure electronic solutions, photonic-based mmWave generation provides wide bandwidth, low power dissipation, and remoting through low-loss fibres. However, at high frequencies, two major challenges exist for the photonic system: the power roll-off of the photodiode, and the large signal linewidth derived directly from the lasers. Here, we demonstrate a new photonic mmWave platform combining integrated microresonator solitons and high-speed photodiodes to address the challenges in both power and coherence. The solitons, being inherently mode-locked, are measured to provide 5.8 dB additional gain through constructive interference among mmWave beatnotes, and the absolute mmWave power approaches the theoretical limit of conventional heterodyne detection at 100 GHz. In our free-running system, the soliton is capable of reducing the mmWave linewidth by two orders of magnitude from that of the pump laser. Our work leverages microresonator solitons and high-speed modified uni-traveling carrier photodiodes to provide a viable path to chip-scale, high-power, low-noise, high-frequency sources for mmWave applications.