Photonic is defined as the generation, manipulation, transport, detection, and use of light information and energy whose quantum unit is the photon. Photonics is based on the science of optics and electronics. The origins of optical technology (photonics) date back to the remote past. In the coming century, photonics promises to be a vital part of the information revolution.
To enable us to understand and apply photonics, it is necessary to have a basic understanding of the behavior and properties of light.
Photonics Opportunities
There are ten broad areas of employment that are likely to need increasing numbers of photonics technicians:
| Photonics Opportunities | |
|---|---|
| 1.- Medicine - biomedical | 2.- Environmental |
| 3.- Energy | 4.- Transportation |
| 5.- Defense | 6.- Public safety |
| 7.- Aerospace | 8.- Computers |
| 9.- Energy | 10.- Manufacturing with photonics and test and analysis | 11.- Communication and information technology |
What is light? This question has been debated for many centuries. The sun radiates light, electric lights brighten our darkness, and many others uses of light impact our lives daily. The answer, in short, is light is a special kind of electromagnetic energy.
The speed of light, although quite fast, is not infinite. The speed of light in the vacuum is expressed as c = 2.99 x 108 m/s. Light travels in the vacuum at a constant speed, and this pseed is considered a universal contant. It's important to note that speed changes for light traveling trough nonvacuum media such as air (0.03% slower) or glass (30.0% slower)
For most purpose, we may represent light in terms of its magnitud and direction. In a vacuum, light will travel in a straight line at fixed speed, carrying energy from one place to another. Two key properties of light interacting with a medium are:
1.- It can be deflected upon passing from one medium to another (refraction).
2.- It can be bounced off a surface (reflection).
The field of detection and measurement of light energy is called radiometry. It uses a standardized system for characterizing radiant energy. Table 2 defines the standard terms used.
| Term | Definition | Symbol | Units |
|---|---|---|---|
| Quantity | Radiant energy | Q | Joule (J) |
| Flux | Rate of radiant energy | Φ | Watt (W); Joule/second (J/s) |
| Flux density(irradiance) | Flux per unit area | E | Watts per square meter (W/m2) |
| Intensity | Flux per solid angle | I | Watts per steradian (W/sr) |
| Radiance | Flux per unit area per unit solid angle | L | Watts per square meter per steradian (W/m2⋅sr) |
| Spectral radiance | Radiance per unit wavelength | Lλ | Watts per square meter steradian per nanometer W⁄(m2⋅sr⋅Δλ) |
Dual Nature of Light
Scientist build models of physical processes to help them understand and predict behavior. So it is with light energy. It is through seeing the effects of light that the models are developed. Scientists have observed that light energy can behave like a wave as it moves through space, or it can behave like a discrete particle with a discrete amount of energy (quantum) that can be absorbed and emitted. As we study and use light, both models are helpful
Concept of a photon
The particle-like nature of light is modeled with photons. A photon has no mass and no charge. It is carrier of electromagnetic energy and interacts with others discrete particles (e.g., electrons, atoms, and molecules).
A beam of light is modeled as a stream of photons, each carrying a well-defined energy that is dependent upon the wavelength of the light. The energy of a given photon can be calculated by:
Photon energy (E = hc/λ)
Where E is in joules
h = Planck's constant = 6.625 x 10 -34 J⋅s.
c = Speed of light = 2.998 x 10 8/sup> m/s.
λ = Wavelength of the light in meters.
When ultraviolet light shines on some metal surfaces, it causes electrons to be emitted. This effect is shown in figure 1, the photoelectric effect did not produce results that matched the early predictions of wave theory. Two concerns were:
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| Figure 1.- Photoelectric effect |
1.- More intense radiation (larger-amplitude waves) did not cause emitted electrons to have more energy.
2.- The energy of the emitted electron was dependent on the wavelength of the light, not the amplitude of the wave.
In the photoelectric effect experiment shown in figure 1, light strikes as a metal plate. Electrons are immediately released. The flow of electricity in the external circuit can be measured and the number of electrons generated for a given light signal can be determinated
If light were a continuous wave, it might wash over the metal surface and interact with the electrons to give them the needed energy to escape at lower light levels (intensities), but only after long delays. However, faint light at high frequencies (short wavelengths) caused the immediate release of electrons. Thus, light knocked the electrons out of the metal surface as if the light were made of particles — photons.
There is a minimum energy threshold for an electron to escape from the metal. Photons with frequencies below a given threshold eject no electrons, no matter how intense the light. Photons with frequencies above the threshold do eject electrons, no matter how low the intensity. The energy of the released electrons can be calculated from the next equation:
Ee-= hc/λ - p
where:
p = Characteristic escape energy for the metal.
Ee- = The kinetic energy of an escaping electron.
hc/λ = The energy of the photon of wavelength λ.
Wave Model
The particle-like model of light describes large-scale effects such as light passing through lenses or bouncing off mirrors. However, a wavelike model must be used to describe fine-scale effect such as interference and diffraction that occur when light passes through small openings or by sharp edges. The propagation of light or electromagnetic energy through space can be described in terms of a traveling wave motion. The wave moves energy — without moving mass — from one place to another at a speed independent of its intensity or wavelength.
This wave nature of light is the basis of physical optics and describes the interaction of light with media. Many of these processes require calculus and quantum theory to describe them rigorously.
Characteristics of light wavesTo understand light waves, it is important to understand basic wave motion itself. Water waves are sequence of crest (high points) adn troughs (low points) that "move" along the surface of the water. When ocean waves roll in toward the beach, the line of crests and troughs is seen as profiles parallel to the beach. An electromagnetic wave is made of an electric field and a magnetic field that alternately get weaker and stronger. The directions of the fields are at right angles to the direction the wave is moving, just as the motion of the water is up and down while a water wave moves horizontally. Figure 2 is the representation of the electric and magnetic field.
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| Figure 2.- Representation of the electromagnetic wave. |
The maximum value of the wave displacement is called the amplitude (A) of the wave. The cycle starts at zero and repeats after a distance. This distance is called the wavelength (λ). Light can have different wavelengths, such as the blue light and red light shown in figure 2. The inverse of the wavelength (1/λ) is the wave number (v), which is expressed in cm-1. The wave propagates at a wave speed (v). This wave speed in a vacuum is equal to c, and is less than c in a medium. At a stationary point along the wave, the wave passes by in a repeating cycle. The time to complete one cycle is called the cycle time or period (τ) and can be calculated using the next equation:
Another important measure of a wave is its frequency (f). It is measured as the number of waves that pass a given in one second. The unit for frequency is cycles per second, also called hertz (Hz). As can you see, the frequency and the period are reciprocals of one another. If the wave speed and wavelength are known, the frequency can be calculated with the next equation:
It is possible for a wave to have other than sinusoidal shapes; however, the important concept to remember is that light waves are transverse electric and magnetic fields changing in space and time and propagating at the speed of light in a given medium, as we show below.
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| Figure 3.- A wire-grid polarizer converts an unpolarized beam into one with a single linear polarization. |
Light waves are complex. They are not one-dimensional waves but rather are composed of mutually perpendicular electric and magnetic fields with wave motions at right angles to both fields, as illustrated in figure 3. The wave carries light energy with it. The amount of energy that flows per second across a unit area perpendicular to the direction of travel is called the irradiance (flux density) of the wave.
Electromagnetic waves share six properties with all forms of wave motion:
- Polarization
- Superposition
- Reflection
- Refraction
- Diffraction
- Interference
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