A dichroic mirror, also known as a dual-band mirror, dual-wavelength mirror, or dichroic reflector, is an optical mirror with different optical characteristics at specific wavelengths. It exhibits reflectivity below a certain cut-on wavelength while allowing transmission at longer wavelengths. 

The "cut-on" wavelength is the point below which a dichroic mirror starts to reflect light, and the "cut-off" wavelength is the point above which the mirror stops allowing light to pass through and instead reflects it.

These mirrors are designed to effectively separate light based on its wavelength and serve as an important component in various optical devices. It typically consists of a carefully engineered coating on a glass or substrate surface. The coating is designed to have varying reflectivity and transmissivity properties, enabling the mirror to effectively separate and manipulate light.

Working of Dichroic Mirrors

Dichroic mirrors exhibit their unique behavior due to the selective interference of light waves at different wavelengths. The key component of a dichroic mirror is its optical coating, which is carefully engineered to have varying thicknesses and refractive indices across its surface.

When light strikes a dichroic mirror, it interacts with the optical coating. The coating is designed to create a thin film interference effect, where different wavelengths experience constructive or destructive interference depending on their phase relationships. 

The light that approaches the dichroic mirror contains a range of wavelengths. The mirror's reflective coating reflects certain wavelengths, causing constructive interference for those colors, while the areas between the coatings are transparent for specific wavelengths, leading to destructive interference. This selective behavior allows the dichroic mirror to efficiently separate different colors or wavelengths without significant losses of light intensity.

In the figure above, the first dichroic mirror transmits red and reflects green. The second dichroic mirror transmits red & green and reflects blue.

In the second figure, the first dichroic mirror transmits green and reflects blue & red. The second dichroic mirror transmits red and reflects blue.

Types of Dichroic Mirrors

Dichroic mirrors exhibit different transmission properties, and they can be categorized based on their behavior. There are several types of dichroic mirrors, including longpass dichroic mirrors, shortpass dichroic mirrors, and multi-band dichroic mirrors. Additionally, hot and cold mirrors, which are used to manage thermal sensitivity, also fall under the category of dichroic mirrors.

• Longpass dichroic mirrors possess a cut-on wavelength that separates a band with high reflectivity and transmissivity. Light with longer wavelengths, above the cut-on wavelength, can freely pass through the mirror, while light with shorter wavelengths is reflected back.
• Shortpass dichroic mirror has a cut-off wavelength. Radiation above this cut-off wavelength experiences high reflectance, while radiation below it is transmitted.
• Multi-band dichroic mirrors have both a cut-off and cut-on wavelength, and they exhibit two transmission bands and one reflective band. The wavelength range between the cut-off and cut-on wavelengths reflects light strongly, while wavelengths above the cut-on and below the cut-off are transmitted with high efficiency.

Applications of Dichroic Mirrors

Dichroic mirrors are extensively used in spectroscopic instruments to split light into different wavelengths. They allow the separation of specific wavelengths for analysis, enabling researchers to study the composition and properties of materials.

In microscopy, dichroic mirrors are employed to separate excitation and emission wavelengths in fluorescence microscopy. They reflect the excitation light towards the sample while transmitting the emitted fluorescence for detection, resulting in high-resolution images with minimal background noise.

Dichroic mirrors play an important role in laser systems by combining or separating laser beams of different wavelengths. They enable the precise manipulation and control of laser light, allowing for multi-wavelength operations, beam combining, and beam shaping.

In photography and videography, dichroic mirrors are utilized in beam splitters, allowing light to be simultaneously directed towards the viewfinder or electronic sensor and the lens. This facilitates the creation of optical systems that can capture images or videos while providing real-time feedback to the photographer or videographer.

They are utilized in lighting applications, such as stage lighting and architectural lighting, to create colored lighting effects. These mirrors selectively reflect certain wavelengths while transmitting others, allowing for the creation of vibrant and dynamic lighting displays.

Dichroic mirrors are utilized in solar energy applications to separate and concentrate sunlight into different wavelengths. They can be used to split the incoming sunlight into visible and infrared components, allowing for more efficient solar energy conversion and utilization.

In optical communication systems, dichroic mirrors are used to separate or combine different wavelengths of light in fiber-optic networks. They facilitate the routing and manipulation of optical signals, enabling the transmission of multiple channels of information simultaneously.

These mirrors also find applications in astronomical instruments, such as telescopes and cameras. They allow for the separation of different wavelengths of light, enabling astronomers to observe specific regions of the electromagnetic spectrum and study various astronomical phenomena.