Lasers are devices which can amplify light by stimulated emission to generate coherent radiation. Actually LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. The light that has been emitted can be in various wavelengths, meaning that it corresponds to specific points in the electromagnetic spectrum. Different kinds of lasers can generate light at wavelengths ranging from millimeters to nanometers. However, we will be examining lasers which generate light in visible region of the electromagnetic spectrum, wavelengths of roughly 400-700 nm.
Figure 1: Electromagnetic spectrum [original work by Özgür Can].
Essential Elements of a Laser:
A typical laser consists of three essential components as shown in Figure 2:
i) a gain medium consisting of active molecules, ions or atoms,
ii) an optical feedback structure which confine light inside by oscillations (it can be called as “resonator” or “cavity”),
iii) a pumping source which excites molecules in the gain medium (it can be called as “excitation source” or “pump”)
Although these main elements can be in a variety of different forms, we are focusing on biological counterparts of gain medium and optical feedback structure to build a biological laser.
Figure 2: Basics of a laser [Original work by Özgür Can].
1. Biological Lasers
Biological laser or biolaser is an emerging concept with huge potential in biological and biomedical research. Biolaser is a new type of laser which has biological materials as part of its gain medium or optical feedback.
Biolasers are very close to fluorescence microscopy where active molecules (fluorescent) emit light upon excitation by an external laser. However, biolasers are different from fluorescence with respect to optical feedback and gain. The light emitted from a biolaser is coherent, meaning it provides a high signal to noise ratio, consists of a narrow spectrum, and can focus on a small spot, whereas fluorescence radiation is dispersive and very weak compared to biolasers .
High Signal to Noise Ratio: The signal to noise ratio can be calculated by dividing the power of signal to the power of noise. If the signal has much power when compared with the power of noise, it is said it has a high signal to noise ratio.
Narrow Spectrum: While the spectrum is around 1 nm in normal lasers, it can generally increase up to 50 nm in fluorescence emission. However, biological lasers we are working on will also have a spectrum of around 1nm which will help us to tag cells individually for single cell tracking.
Focusing on a Small Spot: Most of the time we see fluorescent proteins emitting unfocused radiation. However, the model we have put forward overcomes this focus problem.
In the past decade, different biological materials are harnessed as gain mediums or resonators in biological lasers. A research group at Harvard University demonstrated first cell lasers on E. coli  and mammalian cells HEK293  in 2011 (see Figure 3). After these initial demonstrations, different research groups utilized flavin mononucleotide derived from vitamin B2 , fluorescent protein crystals  and synthetic fluorescent dyes [5, 6] as gain medium, whereas synthetic biopolymers such as polylactic acid , synthetic oil droplets , natural lipid droplets (see Figure 5) , solid microspheres made of inorganic materials  (see Figure 4 and Figure 5) and starch molecules  are used as resonators.
Figure 3: Illustration of a single-cell laser.
A live cell expressing GFP is placed inside a resonator consisting of two mirrors.
Figure 4: Illustration of an intracellular laser.
Synthetic dye containing WGM resonator was introduced into a mammalian cell.
Figure 5: Left: Illustration of an intracellular laser. A WGM resonator was introduced into a living cell.
Right: Illustration of an adipocyte cellwhich contains a natural resonator structure made of lipids.
3. Applications of Biolasers
Biolasers initially harness biological materials passively, and these biolasers were able to image and detect changes inside the laser cavity so they can be used as markers for various changes in biological cells, such as detecting cancerous cells [10, 11, 12]. In following years different active materials were used as gain medium such as quantum dots, rare-earth materials, fluorescent proteins etc.
Biological lasers can be employed in a variety of applications such as:
Detecting various activities at the molecular, cellular, and tissue levels
Various molecules 
Physical forces on membranes 
Physical parameters (temperature) 
Biophysical characterization of cells
Single cell tracking
Single cell tracking in cell cultures 
Multiplexed labelling of individual cells inside complex environments such as tissues or organs 
Probing cellular details with microlasers 
cornea, skin, and blood .
Although there are several compatible laser materials that have been used in the past, only fluorescent proteins and organic mediums are biologically producible, biocompatible, and bioabsorbable. These features make them specifically favorable in generating stimulated emission and laser light from, as well as within the living organism. We anticipate that, in vivo optical amplification will be a key factor when overcoming the limited penetration of light in biological tissue, which is formerly a very vital limitation of optical microscopy modalities.
4. Focus of Our Project
We are developing a biological laser concept in which no artificial cavities will be used to confine light as opposed to conventional biolasers. Cavities we will be using are composed of natural biological materials which cover the membrane of the cell. So our biolaser concept is fully composed of biological materials. We achieve this goal by genetically modifying cells to express silicatein and reflectin proteins as well as fluorescent proteins. Expressed silicatein and reflectins will be transported toward the membrane of the cell, and these proteins will be covering the inside and/or outside of the cell.
We envision a future model organism which is genetically engineered to express our cavity proteins and gain medium proteins. Researchers will easily be able to image developmental processes in such a model organism. By this model, researchers will be able to fully characterize the cells and tissues in an organism.
The most important property of our biological lasers is that enhanced light penetration depth by using laser light sources within the living organism.
We plan to use these biological lasers for biosensor applications too. We can obtain meaningful information of the state of the cell by turning them into cell lasers because lasing modes are very dependent on the physical state of the resonator of the laser. Even today, some research groups are harnessing this feature of cell lasers to obtain some information about the cell/tissue state. A research group recently published a paper where they are introducing resonator balls into cardiac cells, and harness the changes on the refractive index of the cell while some geometric changes happening in cell shape for monitoring contractility in cardiac tissue .
We are developing these biological lasers for two main applications: single cell tracking and biosensor for various diseases. Cell tracking is a very important concept for systems biology approaches because we need to know the relative position, interactions, and biophysical state of every single cell in humans to develop rigorous models of health and disease states.
We see this biological laser concept as a new level of imaging technology for especially dynamic imaging. For systems biology perspective, we need a large amount of imaging data for all domains of an organism; genetic material data, proteomics data, biophysical states of cells and tissues and all other physical and chemical information. Our proposed method will make systems biology efforts easier to realize.
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