The use of optics to make connections within and between electronic chips has been the subject of research for over 20 years because it could solve many of the problems experienced in electrical systems. A critical challenge for the convergence of optics and electronics is that the micrometre scale of optics is significantly larger than the nanometre scale of modern electronic devices. In the conversion from photons to electrons by photodetectors, this size incompatibility often leads to substantial penalties in power dissipation, area, latency and noise. A photodetector can be made smaller by using a subwavelength active region which, however, could result in very low responsivity because of the diffraction limit of the light.

In our first approach to tackle this problem, we use a C-shaped nano-aperture antenna in a thin metal layer to enhance the photocurrent response of a subwavelength photodetector. The single C-shaped aperture, without any other supporting surface structures, can collect light from a large area and concentrate it into a tiny volume of semiconductor. We demonstrated the first nanometallic-enhanced photodetector at near-infrared wavelengths [1].

In our second approach, we exploit the idea of a dipole antenna from radio waves, but at near infrared wavelengths (~ 1.3 um), to concentrate radiation into a nanometre-scale Ge photodetector [2]. Despite the small antenna size (~ 380 nm long) and the different properties of metals at such high frequencies (~ 230 THz), the antenna has qualitatively similar behavior to the common radio-frequency half-wave (i.e., half wavelength long) Hertz dipole. It gives a relative enhancement of 20 times in the resulting photocurrent in the subwavelength Ge detector element, which has an active volume of 0.00072 um3, two orders of magnitude smaller than previously demonstrated detectors at such wavelengths. Photodetectors are one of the most critical components in optoelectronic integration, and decreasing their size may enable novel chip architectures and ultra-low electrical and optical power operations.

Strong optical fields associated with nanometallic structures have been studied extensively in recent years, but we are among the very first in studying the interaction of these strong fields with semiconductors and the further transformation of the optical energy into electricity. Please feel free to write to me if you have any comments on my research: luke_tang@stanford.edu.

References:
[1]. L. Tang, D. A. B. Miller, A. K. Okyay, J. A. Matteo, Y. Yuen, K. C. Saraswat, and L. Hesselink, Opt. Lett. 31, 1519 (2006).
[2]. L. Tang, S. E. Kocabas, S. Latif, A. K. Okyay, D. Ly-Gagnon, K. C. Saraswat, and D. A. B. Miller, Nature Photon. 2, 226–229 (2008).

Low-voltage surface-normal electroabsorption modulators are attractive transmitter devices for optical interconnections between CMOS electronic chips. Our quasi-waveguide angled-facet electroabsorption modulator (QWAFEM) architecture offers high contrast ratio over a wide wavelength range for a low, CMOS-compatible voltage in addition to misalignment tolerance of the input optical beam relative to the device.

Quantum dots have quantum confinement in all three dimensions, which open up novel applications for optics in the nano regime. Germanium quantum dots which are epitaxially grown on silicon substrate show optical absorption in the C-Band. In this project we address optical properties of Ge quantum dots with the possibility of making ultra small optical devices including detectors, optical modulators, and emitters.

We are investigating different geometries of surface-normal optical modulators in the SiGe material system. The goal is to find solutions for modulating light in photonic integrated chips.

The side-entry modulator, designed in the SiGe material system, CMOS compatible. Special features are operation under 1V and unique architecture. Because light enters through the side of the chip, the top is free for heat dissipation and the bottom is free for electrical contacts.

Understanding the carrier dynamics of Ge/SiGe quantum wells has recently been motivated by possible device applications such as modulators and saturable absorbers.

CMOS photodetectors need to be low-voltage/low-capacitance and silicon-based. We are designing and fabricating photodetectors in a silicon-on-sapphire material system. Using pump-probe techniques, we were able to measure picosecond-range signals with good temporal resolution. We have studied how different designs affect photodetector performance.

Due to the recent discovery of the Quantum-Confined Stark Effect in SiGe/Ge quantum wells, we have been designing a slew of new optical devices. Characterization of the underlying material properties of the SiGe material system is crucial to understanding how these devices operate. This is in part because the strain inherent to epitaxially grown SiGe QWs alters the material bandgap in a unique and potentially useful manner.

We are trying to make use of the optical properties of metals to design sub-wavelength optoelectronic devices. Our aim is to come up with simple models that can describe the radiation and wave guiding properties of various nanometallic structures.

Low-voltage surface-normal electroabsorption modulators are attractive transmitter devices for optical interconnections between CMOS electronic chips. Our quasi-waveguide angled-facet electroabsorption modulator (QWAFEM) architecture offers high contrast ratio over a wide wavelength range for a low, CMOS-compatible voltage in addition to misalignment tolerance of the input optical beam relative to the device.