A finger points at a diagram of a battery with two green bars. Above it is another battery with four smaller green bars with a similar area to the first battery's two. The bottom batter is next to a blue box with a blue wave emanating from it and the top battery has a red box with a red wave emanating from it. Below the red wave is written "2x wavelength" and below the top battery is "1/2 energy in a photon."

What Are Photons, Anyway?

August 1, 2024 by Navarre Bartz 33 Comments

Photons are particles of light, or waves, or something like that, right? [Mithuna Yoganathan] explains this conundrum in more detail than you probably got in your high school physics class.

While quantum physics has been around for over a century, it can still be a bit tricky to wrap one’s head around since some of the behaviors of energy and matter at such a small scale aren’t what we’d expect based on our day-to-day experiences. In classical optics, for instance, a brighter light has more energy, and a greater amplitude of its electromagnetic wave. But, when it comes to ejecting an electron from a material via the photoelectric effect, if your wavelength of light is above a certain threshold (bigger wavelengths are less energetic), then nothing happens no matter how bright the light is.

Scientists pondered this for some time until the early 20th Century when Max Planck and Albert Einstein theorized that electromagnetic waves could only release energy in packets of energy, or photons. These quanta can be approximated as particles, but as [Yoganathan] explains, that’s not exactly what’s happening. Despite taking a few classes in quantum mechanics, I still learned something from this video myself. I definitely appreciate her including a failed experiment as anyone who has worked in a lab knows happens all the time. Science is never as tidy as it’s portrayed on TV.

If you want to do some quantum mechanics experiments at home (hopefully with more luck than [Yoganathan]), then how about trying to measure Planck’s Constant with a multimeter or LEGO? If you’re wondering how you might better explain electromagnetism to others, maybe this museum exhibit will be inspiring.

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Stack of Si3N4-LiNbO3 forming the integrated laser and integrated into test setup (d). (Credit: Snigirev et al., 2023)

Fast Adjustable Lasers Using Lithium Niobate Integrated Photonics

August 4, 2023 by Maya Posch 14 Comments

Making lasers smaller and more capable of rapidly alternating between frequencies, while remaining within a narrow band, is an essential part of bringing down the cost of technologies such as LiDAR and optical communication. Much of the challenge here lies understandably in finding the right materials that enable a laser which incorporates all of these properties.

A heterogeneous Si3N4–LiNbO3 chip as used in the study. (Credit: Snigirev et al., 2023)

Here a recent study by [Viacheslav Snigirev] and colleagues (press release) demonstrates how combining the properties of lithium niobate (LiNbO₃) with those of silicon nitride (Si₃N₄) into a hybrid (Si₃N₄)–LiNbO₃ wafer stack allows for an InP-based laser source to be modulated in the etched photonic circuitry to achieve the desired output properties.

Much of the modulation stability is achieved through laser self-injection locking via the microresonator structures on the hybrid chip. These provide optical back reflection that forces the laser diode to resonate at a specific frequency, providing the frequency lock. What enables the fast frequency tuning is that this is determined by the applied voltage on the microresonator structure via the formed electrodes.

With a LiDAR demonstration in the paper that uses one of these hybrid circuits it is demonstrated that the direct wafer bonding approach works well, and a number of optimization suggestions are provided. As with all of these studies, they build upon years of previous research as problems are found and solutions suggested and tested. It would seem that thin-film LiNbO₃ structures are now finding some very useful applications in photonics.

(Heading image: Stack of Si₃N₄-LiNbO₃ forming the integrated laser and integrated into test setup (d). (Credit: Snigirev et al., 2023) )

Creating Lithography-Free Photonic Reprogrammable Circuits

June 20, 2023 by Maya Posch 2 Comments

The field of photonics has seen significant advances during the past decades, to the point where it is now an integral part of high-speed, international communications. For general processing photonics is currently less common, but is the subject of significant research. Unlike most photonic circuits which are formed using patterns etched into semiconductor mask using lithography, purely light-based circuits are a tantalizing possibility. This is the focus of a recent paper (press release, ResearchGate) in Nature Photonics by [Tianwei Wu] and colleagues at the University of Pennsylvania.

What is somewhat puzzling is that despite the lofty claims of this being ‘the first time’ that such an FPGA-like device has been created for photonics, this is far from the case, as evidenced by e.g. a 2017 paper by [Kaichen Dong] and colleagues (full article PDF) in Advanced Materials. Here the researchers used a slab of vanadium dioxide (VO₂) with a laser to heat sections to above 68 °C where the material transitions from an insulating to a metallic phase and remains that way until the temperature is lowered again. The μm-sized features that can be created in this manner allow for a wide range of photonic devices to be created.

A rewritable metacanvas. a) Schematic of laser writing different photonic operator patterns on a metacanvas. b) Temperature-dependentresistance of a VO2 film. c) Optical images from writing and erasing process on the metacanvas. . d) Diagram showing the mathematical matrix (F) is compiled onto a metacanvas in the form of a photonic operator for manipulation of light waveform (I ). e) Schematic of a metacanvas programmed as a beam steerer with a steering angle ϕ. (Credit: Dong et al., 2018) — A rewritable metacanvas. a) Schematic of laser writing different photonic operator patterns on a metacanvas. b) Temperature-dependent resistance of a VO2 film. c) Optical images from writing and erasing process on the metacanvas. . d) Diagram showing the mathematical matrix (F) is compiled onto a metacanvas in the form of a photonic operator for manipulation of light waveform. e) Schematic of a metacanvas programmed as a beam steerer with a steering angle ϕ. (Credit: Dong et al., 2018)

What does appear to be different with the photonic system presented by [Wu] et al. is that it uses a more traditional 2D approach, with a slab of InGaAsP on which the laser pattern is projected. Whether it is more versatile than other approaches remains to be seen, with the use of fully photonic processors in our computers still a long while off, never mind photonics-accelerated machine learning applications.

Generating Entangled Qubits And Qudits With Fully On-Chip Photonic Quantum Source

April 23, 2023 by Maya Posch 8 Comments

As the world of computing and communication draws ever closer to a quantum future, researchers are faced with many of the similar challenges encountered with classical computing and the associated semiconductor hurdles. For the use of entangled photon pairs, for example, it was already possible to perform the entanglement using miniaturized photonic structures, but these still required a bulky external laser source. In a recently demonstrated first, a team of researchers have created a fully on-chip integrated laser source with photonic circuitry that can perform all of these tasks without external modules.

In their paper published in Nature Photonics, Hatam Mahmudlu and colleagues cover the process in detail. Key to this achievement was finding a way to integrate the laser and photonics side into a single, hybric chip while overcoming the (refractive) mismatch between the InP optical amplifier and Si₃N₄ waveguide feedback circuit. The appeal of photon-based quantum entanglement should be obvious when one considers the relatively stable nature of these pairs and their compatibility with existing optical (fiber) infrastructure. What was missing previously was an economical and compact way to create these pairs outside of a laboratory setup. Assuming that the described approach can be scaled up for mass-production, it may just make quantum communications a realistic option outside of government organizations.

Neural Network In Glass Requires No Power, Recognizes Numbers

July 16, 2019 by Al Williams 59 Comments

We’ve all come to terms with a neural network doing jobs such as handwriting recognition. The basics have been in place for years and the recent increase in computing power and parallel processing has made it a very practical technology. However, at the core level it is still a digital computer moving bits around just like any other program. That isn’t the case with a new neural network fielded by researchers from the University of Wisconsin, MIT, and Columbia. This panel of special glass requires no electrical power, and is able to recognize gray-scale handwritten numbers.

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