ECE Research Initiative
Dan Blumenthal, Optical Communications and Photonics Integration (OCPI) Group | "Silicon Nitride Photonics: From Information to Atoms"
From The ECE Current Newsletter (Fall 2022)
"Our group is consistently pushing the forefront of new technologies that enable the creation and manipulation of light on a chip"
We are in the midst of a new era of photonic integration, one that will impact a broad range of science, engineering, and applications, from data centers and fiber optic communications, to atomic clocks and our most precise ways to measure time (position and gravity), to harnessing the quantum nature of our universe for applications such as computing and sensing. This new photonic integration technology is needed to transform these fields, by moving today’s lab-scale size lasers and optics to the chip-scale to improve reliability, reduce cost, size, and weight, enable portability, and allow experiments to scale to very large numbers of lasers and optics, much like how the transistor and very large scale integration (VLSI) did for computing. In the Blumenthal group at UCSB, we have pioneered "silicon nitride photonics," a new class of enabling integration technology, sitting at the intersection between the way we transmit information with light over optical fibers and the way we use light to interact with and manipulate atoms. Our group then takes these new integration technologies and conducts applications and systems research in areas related to optical communications and atoms. Silicon nitride (Si3N4) is a semiconductor material, used in standard CMOS electronics, that has unique properties related to its a wide bandgap that makes it an ideal optical waveguide with extremely low optical losses operating from the visible (405 nm) to the infrared (2350 nm). The Blumenthal group has demonstrated the lowest optical waveguide losses in the world, less than 0.035 dB/meter at telecommunications 1550 nm wavelengths and less than 0.01 dB/cm at key wavelengths associated with transitions of atoms such as strontium and rubidium (atoms used for atomic clocks and sensors). These waveguides have been used to set world records on other devices including the highest quality factor (Q) resonators in the visible and IR. The Blumenthal group today is made up of a team of highly talented PhD students, whose work is described in this article: Nitesh Chauhan, Debapam Bose, Jiawei Wang, Mark Harrington, Kaikia Liu, and Andrei Isichenko.
Our Si3N4 ultra-low loss waveguides are used to move light around on a chip called a photonic integrated circuit (PIC), and to create other important tools that can be integrated on the same chips, such as: such as: extremely narrow linewidth (spectrally- pure) lasers that can carry a lot of information and connect to atoms without disturbing their state; optical modulators for putting information on the light and allowing control of the light; stabilization cavities for stabilizing laser light and reducing its noise; and high performance diffraction gratings to form free space beams from waveguides to cool and trap atoms. By combining these technologies on the same chip, our group is integrating systems that normally occupy tabletops and racks, to the chip-scale. Since our waveguide technology supports light from the visible to the IR, we are able to address a wide range of applications spanning atoms to optical communications and cross-pollinate techniques between these different areas. For example, applying the stabilization of light in the atomic world to solve energy efficiency problems in the data center, the focus of our ARPA-E sponsored FRESCO (Frequency Stabilized Coherent Optical Links for Energy Efficient Communications) project. Our work is funded by a mix of government programs and industry funding, including the NSF, DARPA, and ARL. Examples of integrated silicon nitride systems-on-chip for communications and atom applications are shown in Fig. 1.
Our group is consistently pushing the forefront of new technologies that enable the creation and manipulation of light on a chip. With light so pure and optical losses so low, experiments which usually require labs filled with tables and racks of lasers, optics, and controls, can be moved to the chip- scale. For example, shown in Fig. 2a, published in Nature Photonics, our silicon nitride integrated Brillouin laser with under 1 Hz fundamental linewidth; such lasers before were built using fibers and discrete components in boxes. Also shown in Fig. 2, our 4-meter long integrated reference cavity, integrated PZT stress-optic modulator, and 200 mm CMOS wafer with 4-meter long reference cavities. Recently our group has reported the world’s first rubidium atom cooling and trapping using photonic integrated beam delivered in a 3-dimensional magneto-optic trap (3D-MOT) for the cooling and repump beams. A 3D-MOT uses a combination of six laser beams and magnetic fields to contain and cool atoms (in this case rubidium) for various applications including optical clocks and sensors. Typically such systems require discrete lenses, mirrors, and other components to deliver laser beams to a vacuum cell that contains the rubidium atoms. In this work we have constructed a rubidium 3D-MOT (shown schematically in Fig. 3a), that uses a silicon nitride integrated circuit (Fig. 3b) to convert laser beams from an optical fiber to waveguide to free-space beams that are input to the vacuum cell. In this experiment we successfully used our chip to cool over five million rubidium atoms to a temperature of 200 microkelvin (uK), as seen in Fig. 3c.
In summary, the potential for integrated systems on-chip using silicon nitride and associated integration technologies to transform applications from information to atoms will impact everything from fiber optical communications to atomic computing, sensing, and quantum systems. We will see a new era where lab-scale experiments move to the chip, enabling new science and portable applications, such as atomic clocks and gravitational measurements using atom interferometery in space, quantum computers and sensors that are able to manipulate and readout a large number of atoms or ions, fundamental particle detection and molecular science, and more, using a technology that is compatible with CMOS wafer-scale foundries and promises to continue to increase in performance and functionality.
The ECE Current Newsletter (Fall 2022) "New Frontiers of Computing with Neurons and Optics" (pages 12-13)
Blumenthal's Optical Communications and Photonic Integration (OCPI) Group