Semiconductor Lasers Aashwinder Lubana Brian Urbanczyk Harpaul Singh Kumar Kunal Chopra

Introduction Light Amplification by Stimulated Emission of Radiation. Laser light is monochromatic, coherent, and moves in the same direction. A semiconductor laser is a laser in which a semiconductor serves as a photon source. The most common semiconductor material that has been used in lasers is gallium arsenide. Einstein’s Photoelectric theory states that light should be understood as discrete lumps of energy (photons) and it takes only a single photon with high enough energy to knock an electron loose from the atom it's bound to. Stimulated, organized photon emission occurs when two electrons with the same energy and phase meet. The two photons leave with the same frequency and direction. In 1916 Einstein devised an improved fundamental statistical theory of heat, embracing the quantum of energy. His theory predicted that as light passed through a substance it could stimulate the emission of more light. This effect is at the heart of the modern laser. How Stuff Works

P- and N-type Semiconductors In the compound GaAs, each gallium atom has three electrons in its outermost shell of electrons and each arsenic atom has five. When a trace of an impurity element with two outer electrons, such as zinc, is added to the crystal. The result is the shortage of one electron from one of the pairs, causing an imbalance in which there is a “hole” for an electron but there is no electron available. This forms a p-type semiconductor. When a trace of an impurity element with six outer electrons, such as selenium, is added to a crystal of GaAs, it provides on additional electron which is not needed for the bonding. This electron can be free to move through the crystal. Thus, it provides a mechanism for electrical conductivity. This type is called an n-type semiconductor.

Under forward bias (the p-type side is made positive) the majority carriers, electrons in the n-side, holes in the p-side, are injected across the depletion region in both directions to create a population inversion in a narrow active region. The light produced by radioactive recombination across the band gap is confined in this active region Pictorial View

Early Lasers The first laser diodes were developed in the early 1960s The device shown is an early example. It would require very high current flow to maintain a population inversion, and due to the heat generated by the steady-state current, the device would be destroyed quickly. Laser Focus World

Different types of Lasers are discussed

Vertical Cavity Surface-Emitting Lasers The VCSEL emits its coherent energy perpendicular to the boundaries between the layers. The vertical in VCSEL arises from the fact that laser diodes are typically diagrammed showing the boundaries as horizontal planes. The divergence of a laser beam is inversely proportional to the beam size at the source— the smaller the source, the larger the divergence. The cavity is along the vertical direction, with a very short length, typically 1-3 wavelengths of the emitted light. The reflectivity required for low threshold currents is greater than 99.9%, Distributed Bragg Reflectors (DBRs) are needed for this reflectivity. DBRs are formed by laying down alternating layers of semiconductor or dielectric materials with a difference in refractive index. The DBR layers also carry the current in the device, therefore, more layers increase the resistance of the device. As a result, dissipation of heat and growth may become a problem if the device is poorly designed. Materials used include gallium arsenide (GaAs), aluminum gallium arsenide (AlGaAs), and indium gallium arsenide nitride (InGaAsN).

Examples of VCSELs : Metallic Reflector VCSELEtched Well VCSEL Air Post VCSELBuried Regrowth VCSEL VCSELs have been constructed that emit energy at 850 and 1300 nanometers, which is in the near infrared portion of the electromagnetic spectrum.

Advantages of VCSEL vs. Edge Emitting Diode Lasers The VCSEL is cheaper to manufacture in quantity Easier to test on wafer More efficient The VCSEL requires less electrical current to produce a given coherent energy output. The VCSEL emits a narrow, more nearly circular beam than traditional edge emitters (used in optical fiber) Wavelength is “tunable” Efficiency and speed of data transfer is improved for fiber optic communications

Quantum Cascade Lasers When an electric current flows through a quantum-cascade laser, electrons cascade down an energy staircase emitting a photon at each step. It is composed of a sliver of semiconductor material. Inside, electrons are constrained within layers of gallium and aluminum compounds, called quantum wells, which are a few nanometers thick. The electrons jump from one energy level to another, and tunnel from one layer to the next going through energy barriers separating the wells. When the electrons jump, they emit photons of light. When the lower-energy electron leaves the first well, it enters a region of material where it is collected and sent to the next well.

The invisible beam from a high- power quantum cascade laser lights a match. It emits an optical power in excess of 200 mW from each facet at a wavelength of 8.0 µm. Pictorial View

Benefits of QC Lasers Typically 25 to 75 active wells are arranged in a QC laser, each at a slightly lower energy level than the one before -- thus producing the cascade effect, and allowing 25 to 75 photons to be created per electron journey. By simply changing the thickness of the semiconductor layers, the laser's wavelength can be changed as well. The QCL can be regarded as an ‘’electronic waterfall’’. When a proper bias is applied and an electric current flows through the laser structure, electrons cascade down an energy staircase, and every time they fall down a step they emit a photon

Quantum Dot Lasers Self-organized quantum dot lasers are grown by metal-organic vapor phase epitaxy (MOVPE), molecular beam epitaxy (MBE), and Stranski-Krastanow method Three dimensionally quantum-confined structures, quantum dots, provide atomic-like energy levels and a delta function density of states. Significant milestones in the development of the quantum dot lasers include demonstration of: low threshold at room temperature large differential gain high output power wide spectral tunability better temperature insensitivity of the threshold current than quantum well lasers.

Quantum Dot Lasers Used in fields such as fiber-optic communications and pump sources The discrete energy levels in quantum dots provide for unique laser applications: the lasing in self assembled quantum dot devices has been shown to exist for ground and excited state transitions, which allows for controlled wavelength switching.

Application of Lasers In telecommunications they send signals for thousands of kilometers along optical fibers. In consumer electronics, semiconductor lasers are used to read the data on compact disks and CD-ROMs. The power and tuning range properties of QC lasers makes it ideal for detection of gases and vapors in a smokestack. VCSEL has been proved to be an efficient emitter for fiber data communication in the speed range of 100Mbps to 1Gbps. Medical lasers are used because of their ability to produce thermal, physical, mechanical and welding effects when exposed to tissues. Some of the applications of lasers include stone removal (laser lithotripsy), activation of specific drugs or molecules and denaturizing of tissues and cells in body. Lasers are also used by law enforcement agencies to determine the speed and distance of the vehicles. Lasers are used for guidance purposes in missiles, aircrafts and satellites and make up for a potential replacement of ballistic missiles.

Problems of Nanostructured Lasers Good laser production above room temperature is a problem