Optogenetics denotes “optical stimulation plus genetic engineering”, which integrates optics and genetic engineering to manipulate cells and their information processes. Optogenetics has been broadly applied to study neural circuit structure,activity, function as well as cell physiology of none-neuronal systems.

The concepts of different actuators for activation and inhibition of cells. The stimulus is indicated on the left-hand side (light). The cellular actions, shown on the right, could be changes either in membrane potential. (Werend Boesmans, Nature Reveiews, 2017)

Optogenetic actuators are usually microbial opsins which conduct the flow of ions into or out of cells by illumination at a specific wavelength. A variety of natural microbial opsins have been identified and series of engineering have been conducted to improve precision of spatiotemporal control and to expand absorption spectrum. 

Below is a brief overview of the commonly used microbial opsins. Also, the list is not exhaustive.
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Channelrhodopsins are cation channels discovered in the green algae Chlamydomonas reinhardtii and open when excited by blue light, which in turn depolarizes cells and often evokes action potentials in neurons. Naturally occurred channelrhodopsins include Channelrhodopsin-1 (ChR1) and Channelrhodopsin-2 (ChR2). ChR2 is more commonly used than ChR1 since ChR2 has a higher conductance at physiological pH and is efficiently located to the membrane.

Halorhodopsins are chloride pumps isolated from Halobacteria and transport chloride into cells when illuminated by yellow light which results in cell hyperpolarization. Its variants include NpHR, eNpHR2.0, eNpHR3.0 and Jaws, among which Jaws have a red-shifted absorbance peak and are often used to silent neural activities.

Archaerhodopsin (Arch) from Halorubrum sodomense is also commonly used to inhibit neural activities which is triggered by green-yellow light. ArchT, eArch3.0, and eArchT3.0 are identified Arch variants.

Optogenetic stimulation consists of several steps. (E. A. Claudia Pama , Lorenza S. Colzato ,and Bernhard Hommel, Frontiers in psychology, 2013)