Optical cavities – two mirrors arranged to reflect light multiple times between them – form the basis of lasers and certain optical filters. Since any angle between the two mirrors results in light being scattered away, parallel alignment is essential, yet difficult to maintain. Nevertheless, [Timothy Giles] managed to 3D-print and align such an optical cavity, and used it to detect minute shifts in space and wavelength.

The cavity has two semi-transparent mirrors facing towards each other. One mirror is held in a 3D-printed mount, and the other is attached to the diaphragm of a speaker with a hole drilled through the center. The hole avoids the speaker coil, and allows light exiting the optical cavity through the semi-transparent mirror to appear on a paper target, which is monitored by a webcam. On the other side of the optical cavity, a laser diode coupled to a single-mode fiber shines in through the other mirror. Alignment is challenging, but the webcam makes it easier; as the mirrors tilt relative to each other, the pattern seen on the paper target changes, providing feedback for more precise adjustment.

Light reflected within the cavity can interfere constructively or destructively with incoming light, changing the brightness of the emitted beam. [Tim] used the speaker as a linear actuator to vary the cavity’s length, which, by counting the peaks in brightness, allowed him to measure the diaphragm’s displacement. This also demonstrated a laser diode’s wavelength instability: when the cavity was set to a constant length and the laser started up, the output brightness would cycle a few times. As the diode was warming up, its output wavelength was shifting, creating the same changing interference pattern.

To get a Gaussian beam distribution, [Tim] used a fiber-coupled laser; if you’d like to build one, we’ve seen a coupling mechanism built before. Most lasers are built around an internal optical cavity, but some instead use an external cavity.

Silicon-based CMOS camera sensors are cheap and plentiful, but they’re rarely used to their full potential: they can detect a greater range of wavelengths in the infrared spectrum than they can in the visible spectrum, but in most cameras this is blocked by an IR-cut filter. [Project 326]’s infrared camera system reverses this: it records infrared images in color while blocking out visible light.

The system uses three USB webcams, each with its IR cut filter removed and replaced with a different dichroic IR band-pass filter. One filter is centered at 750 nm, one at 850 nm, and one at 940 nm. There is no band overlap; in testing, each camera only detected an infrared flashlight tuned to its own filter wavelength. The original cameras didn’t hold the sensors in a consistent position, so [Project 326] designed new housings. Using three lenses, each with distinct aberrations, introduced some difficulties in alignment. [Project 326] originally intended to use a pair of beam-splitting prisms with only one lens, but this proved too difficult to align using 3D-printed frames.

A Raspberry Pi records a separate monochromatic stream from each camera, which can then be processed into a composite color video. The first frames need to be manually aligned, but afterwards a script can apply the alignment to the rest of the video. Finally, the channels are mapped to colors, with the precise mapping being freely changeable. There were some few unexpected issues: each camera has its own, not terribly precise, local oscillator, and they drifted apart by about one or two frames per minute. Parallax error, on the other hand, was less severe than might be expected: at close range it’s noticeable, but by a distance of 35 meters, it represents less than one pixel of distortion.

The resulting images look great, and it’s easy to forget that they’re being captured without the use of any visible light. We’ve seen a similar technique (though extending into the visible range) used to recreate the surreal effect of Aerochrome film.

Unlike most cutting lasers, femtosecond lasers don’t vaporize materials; rather, they produce such short, intense bursts of light that the affected region is ablated without having the chance to heat its surroundings. This makes them good at cutting away material without damaging the surroundings, something [Ben Krasnow] exploited to cut cross-sections of samples while still in a scanning-electron microscope.

In this case, the samples were crickets, and before imaging they had to be prepared. First, the bodies were soaked in glutaraldehyde to cross-link the proteins and stabilize the structure. Next, a series of solvent exchanges replaced the water in the bodies with a low-surface-tension solvent; this meant that during the next step, drying, surface tension wouldn’t distort the crickets’ internal structure. Finally, the insect bodies were charred under argon, which made the bodies conductive and more absorptive to laser light.

The laser itself and the scanning galvo are mounted outside the microscope, and shine in through a transparent window. To protect the detector and electron optics from a spray of ablated carbon, a servo motor swings an aluminium shutter between these and the sample while the laser is active. This caused some mysterious problems during testing: after the first ablation run, the electron microscope’s image would contain so much noise as to be unusable, but it would improve over time. As it turned out, the shutter was painted, and the other side of the paint was getting coated with charged carbon particles. This created a small capacitor which disrupted the electron optics as it discharged. Eventually, after solving this and a few other strange problems, [Ben] was able to take several time-lapse videos of the laser gradually ablating a cricket, 30 microns at a time, revealing its inner structure.

Although scanning-electron microscopes are unfortunately shard to come by, it’s still possible to restore a secondhand microscope or, as [Ben] did, build your own. Femtosecond lasers are yet more inaccessible, though they can be used to replicate themselves.

In 1675, while transporting a barometer by night, the astronomer Jean Picard noticed a glow inside its glass tube, just above the mercury. As the mercury sloshed and splashed across the surface of the glass, a static electric charge had built up, which was discharging by ionizing the residual gas molecules inside the evacuated tube. [Styropyro] recreated this effect, and found that the dim glow could be made much stronger by adding some noble gas to the tube.

It starts with a simple recreation: he took a volumetric flask, attached a narrow glass stem to the mouth, added some mercury to the flask, evacuated it with a vacuum pump, and sealed off the glass stem. This produced a faint glow when shaken, but it was only really visible under very low light. When [Styropyro] brought it near a Tesla coil, however, it did glow much more brightly.

Backfilling an identical flask with neon to about 40 millitorr produced a much more spectacular result (a low pressure in the tube is necessary, but moderate pressure variations don’t significantly alter the effect). When shaken even slightly, this neon-containing flask produced a bright orange-red glow just above the surface of the mercury. Points of obstruction, such as those in a zig-zag tube, produced a brighter glow. A krypton-containing tube glowed blue, but less brightly than the neon tube.

Since this is, essentially, a triboelectric effect, other materials besides mercury should work; [Styropyro] tested several materials, and found that pieces of Teflon produced a faint glow, and copper beads a somewhat brighter glow. Unfortunately, Galinstan, the obvious replacement for mercury, wets and coats glass, preventing a charge buildup.

Without an added noble gas, the standard glow of barometric light comes from the excitation of mercury vapors, a glow which can also be seen in mercury rectifiers, and which excites the phosphors of fluorescent light bulbs.

Thanks to [Vik Olliver] for the tip!

Preservationists usually take great care to prevent fungi from appearing the world of art, but in the case of [Kexin Wang]’s Funguy project, the fungus itself is the art. It uses a laser diode to repeatedly trace an outline onto a dish of agar gel in which fungus is growing, and the photophobic fungus grows only up to the edge of the laser-traced figure, potentially creating complex designs.

This project evolved out of a research project in which they developed a computer model for fungal growth, then used its predictions and a laser to control a fungus’s growth pattern. The model has two parts: a temporal convolutional neural network which learns fungi growth patterns from a series of images, and a cellular automaton to simulate these growth patterns under different starting conditions. The cellular automaton’s rules aren’t fixed; each cell runs a small neural network which learns the rules under supervision from the convolutional network. By training these networks on images of the growth stages of three different fungi, it was able to realistically predict the different growth patterns of the different species.

To actually control the growth pattern, the researchers tried a series of different wavelengths and laser powers; shorter wavelengths tended to work better, with a 405 nm laser working best. The growth model complemented the laser setup by predicting in which areas the growth medium had run out of nutrients. Since fungus would no longer spread in these regions, the laser no longer needed to trace these sections. The Funguy kit’s laser system itself is similar to a laser engraver, with an XY-kinematic system seemingly built from a DVD drive frame. It uses fungi from the Mucor genus, though it can print with other photophobic microorganisms, such as slime molds.

This project seems aimed at artistic and educational uses, but considering the various electronic parts that have been made of fungi, more functional applications should be possible.

Ceramic 3D printers, despite using the same fundamental mechanism as standard FDM printers, are much harder to find. Part of this comes down to the material properties of fired ceramics versus thermoplastics, but they’re also significantly harder to build; for example, in his ceramic printer build, [Joshua Bird] had to deal with severe material shrinkage, collapsing bridges, and the surprisingly abrasive effects of clay.

The centerpiece of the printer is the clay extruder: an air compressor pushes clay along a tube into the extruder, which uses an auger to squeeze the clay through the nozzle, while a gap at the top lets trapped air escape. The extruder has enough control for successful retractions, but rheology remained a challenge: the clay needed to be soft enough to flow through the nozzle, but stiff enough to form bridges without collapsing. [Joshua] thus pressurized the clay as much as possible, making it possible to use stiffer clay mixtures. The extruder’s greatest challenge was longevity: [Joshua] tried many 3D-printed plastic augers, but the clay abraded them all much too quickly, often in under an hour of use; a 3D-printed stainless steel extruder solved this.

Printing in ceramic isn’t a simple process: for each part, [Joshua] had to mix the clay, load it into the tube, clean the extruder, actually print the object, let it dry, fire it, apply glaze, and fire it again. The clay’s shrinkage during drying and firing destroyed many prints, but [Joshua] was nevertheless able to print a double-walled cup, a decorative climbing-themed cup, and even a chain-mail mesh.

The 3D printer’s motion system is a polar design, an adaptation of his earlier non-planar 3D printer, which might eventually make it easier to print overhangs. We’ve previously seen a similar auger-based clay extruder, an approach reminiscent of direct-granule FDM printing.

Unlike almost every other kind of microscope, atomic-force microscopes (AFMs) don’t use any kind of optical beam to image their subjects. Instead, they physically detect the subject’s surface with a tiny probe, repeating this thousands of times to build up a height map of the subject, sometimes with a resolution below a single nanometer. [Ben Krasnow] got to use an AFM in an investigation of one of his projects, and shared some unusual uses of it in his latest video.

For his first demonstration, [Ben] took a video of the probe head in action. Since the probe oscillates at nine kilohertz, this was less straightforward than it sounds, but a stroboscopic welding camera filming near that frequency could visualize its motion. The next project was to image some biological samples, particularly bacteria. First, [Ben] let the bacteria from nattō (fermented soybeans) multiply in a sterile growth medium, then centrifuged and washed them.

He spin-coated a thin layer of gelatine onto part of a silicon wafer, which provided a very flat substrate. The gelatine is electrostatically attracted to the bacteria, adhering them to the slide and letting [Ben] wash away other contaminants. This let the AFM image the bacteria clearly, even revealing how a spin-coating step had oriented them all in the same direction.

[Ben] also imaged a few other samples, including silver nanoprisms and track-etched membranes. Track-etched membranes use high-energy radiation and an etchant to cut very consistent, fine holes into a plastic filtration membrane. Finally, [Ben] used it to image his laser-etched diffraction gratings; to find out how the laser had created these diffraction patterns, he tried to selectively etch away the laser-exposed metal, using the AFM to verify that this metal had been stripped away. Neither an acidic nor a basic etch worked, but electrochemical etching seemed promising.

If after seeing this you want your own atomic force microscope, we’ve seen a few DIY AFMs, including one which can resolve individual atoms.

Thanks to [H Hack] for the tip!

When light reflects off a surface, not all of it reflects off at the same wavelength; some photons impart a portion of their energy to raising the vibrational energy of the surface’s molecules, and are thus scattered away at a lower energy and longer wavelength. This is called Raman scattering, and the precise wavelength shifts are characteristic of the particular molecule being illuminated. It can therefore be used in Raman spectroscopy to identify molecules; these spectrometers are normally elaborate, expensive instruments, but [Allegedly Science] was able to build a simple system with surprising sensitivity.

The system is named the CubeRaman, after the cube-shaped body containing the main optical path. It uses a cheap 532-nm laser module as a monochromatic light source, with a bandpass filter to eliminate stray infrared light. The beam then reflects off a 45-degree dichroic mirror and passes through a microscope objective onto the sample. Raman-shifted light then scatters back through the objective, passes through the dichroic mirror and a long-pass optical filter, and is focused by an achromatic lens onto the slit of a spectrometer. The entire housing is 3D-printed, as are most parts of the kinematic mounts; the kinematic mounts use adjustment screws running through inserts in the mount, with the tips of the screws held in place by magnets.

[Allegedly Science]’s first test was with a raw diamond, which clearly showed the expected Raman shift. When trying to test a chemical inside a glass bottle, it mainly returned the signature of silica, making thin-walled cuvettes essential. Ethanol inside a plastic bottle was similarly interesting; varying the focal distance changed whether it detected the characteristic shift of ethanol or polypropylene. Nevertheless, [Allegedly Science] thinks there’s still room for improvement, particularly by eliminating stray light and using a narrower slit in the spectrometer.

Although we’ve seen an open-source Raman spectrometer before, this design is significantly more accessible. It does still require a separate spectrometer, though, so it might be worth considering some other spectrometer options.