Photonic lifeform

Photonic lifeforms (also called photonics) are any form of sentient life based on particles of light and energy. While some photonic lifeforms are naturally occurring, most encounters have been with artificially-created holograms.

Photonic life encountered by Starfleet
In 2365, the USS Enterprise (NCC-1701-D)'s computer created a sentient holographic being, Professor James Moriarty, in response to a command to create an adversary worthy of defeating Lieutenant Commander Data. The simulated character was able to gain control of the starship twice in his quest to be freed from the confines of the holodeck. 

Photonic beings lived in a photonic lattice and inside a protostar in the Delta Quadrant. While studying the star, the USS Voyager crew did not realize that these energy signatures were indeed life, and one being sought refuge in the holodeck recreation of Beowulf, known by the holographic characters as "Grendel". 

Another naturally-occurring photonic species existed in a trans-dimensional realm and was unaware of the existence of biochemical lifeforms. They believed that photonics were living, while carbon-based lifeforms were artificial. They were an exploratory race. In 2375, they encountered the USS Voyager's holodeck, which was running the Captain Proton simulation. These beings became involved in a war against the character of Chaotica, and were capable of being injured or killed by the holographic photonic-based weapons. Voyager's crewmembers were able to negotiate a peace, through the holographic Doctor, and succeeded in defeating Chaotica, ending the war. 

Voyager was once infested by photonic fleas. Their primary source of nourishment was plasma particles. The larvae of the fleas were brought on board in a jar of amber spice from a Kartelan freighter. 

While investigating a strange particle field in the Gamma Quadrant, Odo and Jadzia Dax discovered a colony of holographic Yaderans created by a refugee, Rurigan, from Yadera Prime, after his home was destroyed by the Dominion. These holographic Yaderans had definite personalities, could experience pain and pleasure, were capable of reproducing, and became aware of their holographic origins when Odo and Dax told them that they were being maintained by a hologenerator which was breaking down, emitting omicron radiation and causing them to disappear. Odo and Dax repaired the generator and convinced Rurigan to let them restore the Yaderan colony by making him realize that even though the holographic Yaderans may not have been real in the same way he was, he still cared about them as though they were real. 

In 2374, Julian Bashir's friend Felix completed his work on Vic Fontaine, a holographic 1962 Las Vegas lounge singer who was programmed to have some amount of free will and knowledge of his true nature. Vic, who referred to himself as a "light bulb," had great insight into people and their relationships, and had the ability to turn his program off and on, as well as to transfer himself to other holo-simulations. 

The works of Dr. Lewis Zimmerman
Perhaps the greatest example of the Federation's artificial photonic life is the work of Dr. Lewis Zimmerman. Working at Jupiter Station, he was responsible for the development of the Emergency Medical Holographic Program. Programmed with medical knowledge from thousands of sources, it was self-aware and its matrix could grow and adapt. Though most of the EMH Mark-I's were quickly retired in favor of future models, the Doctor aboard the USS Voyager remained almost continuously online for seven years. In this time, he grew beyond his original purpose, formed a number of interpersonal relationships, and developed a unique personality. Expanded capabilities included skills such as singing, dancing, golf, romance, sexual routines, holoimaging & editing own holographic visual presentation.

Zimmerman also created several other photonic lifeforms, including Haley, a Human woman and aide to the doctor, Leonard, an iguana of few words, and Roy, a fly designed for micro-surveillance. 

Photonic life in Delta Quadrant cultures
A variety of cultures in the Delta Quadrant used holograms, or their version of holograms, as laborers, and treated them as second-class citizens with no rights. Sometimes, these artificial photonics would rise up against their creators and demand freedom and equality, though it occasionally degenerated into a fight for vengeance.

Dejaren, an unstable isomorphic projection used by the Serosians for hazardous tasks, experienced malfunctions in his personality subroutines and killed his crew in 2374. He later attacked two Voyager crewmembers and was forcibly taken offline. Dejaren kept a holographic fish, Spectrum, as a pet, though it is unlikely the animal was sentient. 

The Lokirrim also used self-aware holograms as servants or caretakers for their children. However, by the 2370s, a brutal civil war between the Lokirrim and the photonic insurgents had broken out, costing many lives and often dividing friends such as Jaryn and Emmik. 

The Hirogen used holographic technology from Voyager to create simulated prey, which they could hunt again and again. These holograms were reprogrammed to experience pain, wounds and death, as well as to learn and adapt, becoming more challenging targets. Eventually, the leader of these holograms, Iden, slew his hunter and began liberating other holograms from the Hirogen and other sources. He initially wanted to settle his people on Ha'Dara and form a unique culture that rejected the influence of organics, but soon sought retribution on the Hirogen. Iden was ultimately destroyed, and the other holograms were reprogrammed by Donik and Kejal. 

Background
The photonic lifeform in "Heroes and Demons" was a CGI effect accidentally created by Digital Magic when they were experimenting with a new software package. Ronald B. Moore affectionately referred to the effect as "fettuccini monster". (Star Trek: Communicator, Issue 105, p.57)