Through a glass not-so-darkly ... infrared to visible up-conversion

I've been familiar with the phenomenon whereby two waveforms can interact such that a third and fourth, the sum and difference of the two wavelengths, are generated. This has been used in radio transmission and reception for decades, known as heterodyning. [See the Wikipedia page.]

I now discover that there is a similar phenomenon at light wavelengths which can result in up-conversion allowing near infrared (AKA short wavelength infrared - SWIR) to become visible. The earliest reference would appear to be in a 1967 paper by JE Midwinter and J Warner [Up‐Conversion of Near Infrared to Visible Radiation in Lithium‐meta‐Niobate Journal of Applied Physics 38, 519].

The abstract is as follows ...

Single‐crystal lithium niobate pumped with pulsed ruby‐laser radiation has been used to convert 1.7‐μ radiation to green light with more than 1% efficiency. A narrow infrared bandwidth of 17 Å, set by the phase‐matching requirement only, allows the up‐converter and photomultiplier to operate in place of a monochromator and infrared detector, and the emission spectrum of a mercury lamp has been thus examined in the region of 1.7 μ. A close agreement between theory and practice has been found in all respects except noise performance. Further studies of this aspect are required.

Moving on to mid-June 2021 and we can see that 'further studies' have indeed been done. (This is not so say that Midwinter et al have not been hard at work; I am coming into this rather late.) The NanoWerk web site has published an article entitled Let there be light! New tech allows people to see in the dark. [Link to the article here.] It outlines how a team at the Australian National University (ANU), working with an international team, have prototyped a device, based on nano-technology and metamaterials, which up-converts near infrared to visible wavelengths.

The lead researcher Dr Rocio Camacho Morales, is quoted in the article saying

We’ve made a very thin film, consisting of nanoscale crystals, hundreds of times thinner than a human hair, that can be directly applied to glasses and acts as a filter, allowing you to see in the darkness of the night.

Fortunately, their paper describing the work is openly available online. The title is Infrared upconversion imaging in nonlinear metasurfaces and describes the technique as follows:

In this approach, the IR image is not directly detected; instead, a parametric nonlinear optical process is employed to convert the image to higher frequencies and detect it using regular cameras in a process known as upconversion IR imaging.

So basically what we have here, albeit in rudimentary form, is a piece of 'glass' that shifts the wavelength of radiation passing through from near infrared to visible light without needing cooling or even imaging technology. There are nanoscale antennas on a gallium arsenide wafer, tuned to the relevant wavelengths and what is described as a pump laser beam to interact with the incoming signal (both near infrared). The frequency of the derived waveform is the sum of the target image frequency and the pump laser beam. This process is also very fast, described as having 'femtosecond temporal resolution' which could enable 'ultrafast imaging of chemical reactions in a conventional microscope device', never mind the opportunities for inexpensive imaging of near infrared. I assume it would work at thermal wavelengths with the right wavelength of pump laser.

Check out the paper and see what you think. I'm fairly excited by the possibilities, even if it will presumably take the team a while to get it to photographic resolutions.

I will finish with their abstract, followed by the citation and link.

Infrared imaging is a crucial technique in a multitude of applications, including night vision, autonomous vehicle navigation, optical tomography, and food quality control. Conventional infrared imaging technologies, however, require the use of materials such as narrow bandgap semiconductors, which are sensitive to thermal noise and often require cryogenic cooling. We demonstrate a compact all-optical alternative to perform infrared imaging in a metasurface composed of GaAs semiconductor nanoantennas, using a nonlinear wave-mixing process. We experimentally show the upconversion of short-wave infrared wavelengths via the coherent parametric process of sum-frequency generation. In this process, an infrared image of a target is mixed inside the metasurface with a strong pump beam, translating the image from the infrared to the visible in a nanoscale ultrathin imaging device. Our results open up new opportunities for the development of compact infrared imaging devices with applications in infrared vision and life sciences.

[Rocio Camacho-Morales, Davide Rocco, Lei Xu, Valerio Flavio Gili, Nikolay Dimitrov, Lyubomir Stoyanov, Zhonghua Ma, Andrei Komar, Mykhaylo Lysevych, Fouad Karouta, Alexander A. Dreischuh, Hark Hoe H. Tan, Giuseppe Leo, Costantino De Angelis, Chennupati Jagadish, Andrey E. Miroshnichenko, Mohsen Rahmani, Dragomir N. Neshev, "Infrared upconversion imaging in nonlinear metasurfaces," Adv. Photon. 3(3) 036002 (14 June 2021) ]

doi.org/10.1117/1.AP.3.3.036002.

Frogs and leaf growth

Back in January I noted research by into infrared reflectance of insects carried out by Michael Mielewczik and others. Michael has contacted me again about two more papers on similar subjects.

The first is Non-Invasive Measurement of Frog Skin Reflectivity in High Spatial Resolution Using a Dual Hyperspectral Approach [1] (on PLOS ONE here with a PDF here).



As before, the team used a camera with filtering that split near-infrared (specifically the red-edge between 675-775 nm) and blue to explore the 'colour' of frog skin. They also used a two further hyperspectral cameras sensitive to visible and near-infrared between 400 and 1000 nm and to SWIR (short wave infrared) between 1000 and 2500 nm. This image is of agalychnis callidryas using the red-edge camera.

I've come across hyperspectral cameras before and they're quite fascinating devices. They produce a multi-dimensional image where each of the pixels in the x and y plane have a complete spectrum recorded in the z axis ... so z records intensity at a range of wavelengths. This means that you can choose which wavelength (or wavelengths) to view the scene after the fact. This multiplies the amount of data dramatically of course.

The second paper uses infrared imaging to help a study of leaf growth. The paper is Diel leaf growth of soybean: a novel method to analyze two-dimensional leaf expansion in high temporal resolution based on a marker tracking approach (Martrack Leaf) [2], available on the Plant Methods web site. This study used dark beads attached to the margins of a leaf and a camera fitted with a 940nm narrow bandpass filter. At this wavelength the leaf is brighter than the beads which makes image analysis easier.

[1] Pinto F, Mielewczik M, Liebisch F, Walter A, Greven H, et al. (2013) PLoS ONE 8(9): e73234. doi:10.1371/journal.pone.0073234
[2] Mielewczik M, Friedli M, Kirchgessner N, Walter A. Plant Methods 2013, 9:30 doi:10.1186/1746-4811-9-30

[Note: corrected information about the hyperspectral camera added 31 October]

New IR camera is user-configurable

New near-infrared cameras are few and far between and there is an increasing choice of thermal imagers, even if they tend (with a notable exception) to be very expensive. However, cameras working in the gap between the two are much rarer beasts.

Episensors of Bolingbrook, Illinois have announced a new camera working in the short wave infrared (which lies just beyond photographic/near infrared) and which is intriguing, not just because they describe it as 'low cost' (not sure just how low) but also because of its versatility. Here's a paragraph from their press-release.

The infrared camera company Episensors, Inc. recently launched a new type of portable infrared camera called the Night SWEEP-1 (“NS-1”). Infrared cameras can see light that is invisible to the human eye and provide imaging at night and through obscurants like smoke and fog. What sets NS-1 apart is its portability and customizability, which allows scientists, researchers, and others to utilize the camera in the field, without sacrificing the capability of swapping between short-wave, mid-wave and long-wave infrared focal plane arrays, lenses and other components. Based on a patent-pending design, this infrared camera system is fully customizable. The camera can be configured with a pour fill Dewar or a closed cycle Integrated Dewar Cooler Assembly (IDCA) depending on the customer’s preference.

An excellent technical note on their web site explains the wavelength domain this camera covers. It's notable not just because the user can change the imaged band but that the extended SWIR (short-wave-ir) band, between 1 and 3 µm (1000 and 3000 nm) not only has some haze and smoke penetration ability but also contains a sweet spot where there is some smoke penetration but also the radiation goes through glass. Output resolution is 320 by 256 with plans for 640 by 512. The digital resolution is 14 bit and I assume having a supercooled sensor (that Dewar referred to in the note is a thermos flask of something like liquid nitrogen) will give a low noise floor.

So this camera is a kind of infrared SLR and operates between photographic infrared (which ends around 1500 nm) and the thermal bands and operates using reflected radiation (from the sun for example) while thermal imagers show radiation from the objects themselves. I believe this mid-infrared imaging is sometimes referred to as reflectography and has applications in art restoration amongst other things.

Check out the videos on the web site. They look like photographic infrared rather than thermal but you will notice some smoke penetration. It'll come down to particle size and by configuring the camera the user will be able to balance haze and smoke penetration against things like glass transparency. The nearest I've seen to this in other devices is where a single unit combines two different cameras.

Whether we will see the NS1 on this side of the Atlantic is currently debatable as some of the technology is export-restricted.