Photonic crystals

Photonic crystals guide certain wavelengths of electromagnetic radiation that have twice the wavelength as the size of repetitive parts with 2 different refractive indexes in crystal. For example ~650 nm red light can be captured and transmitted in crystals with ~320 nm repeating regions and ~400 nm blue light can be created with ~200 nm structures. These can also be used for larger wavelengths like radio waves or microwaves. Earlier photonic crystals transmitted microwaves as they have wavelength of ~1 mm to 1 meter making it easy to manually stack suitably thick layers. These crystals are basically waveguides like the metallic tunnels that guide microwaves to intended direction and they too transmit mainly certain wavelength that is twice the diameter of tunnel. Photonic crystals are made from 2 different nonconductors or from 1 metal and 1 nonconductor.   

Rainbow colored animals (including birds and insects) usually have structures (made of keratin in vertebrates or chitin in insects/mushrooms) with similar sizes but if light goes through so small openings they tend to diffract in almost every direction and interference combines them into colors that depend at which angle something is viewed.
 
If gold particles were added to photonic crystal they could increase its absorption of infrared and visible light by 62% and light to electricity conversion increased by 41%. 

 Above illustration shows photonic crystal usable as laser light source. Light is created in region surrounded by white dotted line. By skipping air holes in that region they create "quantum well" (QW above) which can capture photon energy and require certain energy to leave the "well". 

Skipping holes in photonic crystals can create waveguides like seen in a and c part. Dark spots show magnetic fields. Apparently having lattice constant (size of repetitive crystal parts)/wavelength=0,26 caused behavior seen in picture a but having that ratio at 0,24 caused Y shaped spreading seen in image c.

Other defects that can capture energy from photons can vary from having larger hole or no hole in area intended to capture photon energy.

Photonic crystal waveguides can slow light and by slowing it becomes more intense (shown with height on illustration) due to more compact size but same energy content.

This also seems to turn initial infrared laser pulse into visible green light (above) while it is in this compact slowed form. That light compacting is often seen as important part of optical computers as by slowing light it can amplify lights interaction with atoms and electrons. 

Cross section of photonic crystal fiber. Hollow holes have diameter of 4 micrometers. Minimum energy loss seems to be at least 10% (0,37 dB) per kilometer. Manufacturing could start with piece more than cm wide, then after heating it gets stretched, narrowing main piece along with the holes drilled in it. Resulting fiber could be over kilometer long and still preserve parallel holes. Light can travel at least in those air channels.

Photonic crystals in quantum cascade (QC) type lasers can be used for infrared lasers but as limitation it creates light that tries to travel within material parallel to surface and also researchers had to cool it to 10 K. Emitted light left this main photon cascade due to diffraction in holes. Material around holes is chosen so that contrast between refractive indexes would be as large as possible to help with reflecting. Holes could be added with narrow electron or ion beams. As last part titanium and gold were added on this structure so plasmons in them could carry light energy out of hole due to photons already pushing electrons perpendicular to surface with electrons in layer of metal transporting this energy to surface.

Infrared frequencies between 1-10 THz have practical use in detecting chemicals as molecules have resonance frequencies in that region. Authors used quantum cascade type laser shown in previous illustration with 5 K working temperature. This detectors could use change in frequency or change in refractive indexes to figure out substance (both change frequency). Gaseous molecules could reach inside of these air channels and get stimulated by light but different substances in these holes affect which frequencies they resonate. Due to low temperature they form solid layer within opening that affects contrast between refractive indexes and in turn causes lower emitted frequency. Afterwards they emit some wavelength specific to them and that can be measured by seeing how it travels through prism. Because this frequency has wavelength of 30-300 micrometers it is relatively easy to produce structures with that scale compared to 22 nm scale in some computer parts. In best case it seems to be able to detect single molecule.