4.2 Optical and electromagnetic sensing mechanisms

Optical and electromagnetic sensing mechanisms are crucial in MEMS/NEMS devices. These techniques use light and magnetic fields to detect tiny changes, enabling precise measurements in various applications.

From photodetectors to SQUIDs, these sensing methods offer unique advantages. They allow for miniaturization, improved sensitivity, and integration with other systems, making them essential in modern technology and scientific research.

Optical Sensing

Photodetectors and Photovoltaic Effect

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• Photodetectors convert light into electrical signals by exploiting the photovoltaic effect
• Photovoltaic effect occurs when light strikes a semiconductor material, causing electrons to be excited and generate a current or voltage
• Photodetectors are used in various applications such as image sensors (digital cameras), optical communication systems, and light detection and ranging (LiDAR)
• Common types of photodetectors include photodiodes, phototransistors, and photoresistors (light-dependent resistors)

Interferometry Techniques

• Interferometry is a technique that uses the interference of light waves to make precise measurements of small displacements or changes in optical path length
• Fabry-Perot cavity consists of two parallel, highly reflective mirrors separated by a small gap, creating an optical resonator sensitive to changes in the gap distance
• Fabry-Perot interferometers are used in various applications such as wavelength filters, optical switches, and pressure sensors
• Bragg gratings are periodic structures etched into an optical fiber or waveguide that selectively reflect specific wavelengths of light based on the grating period and refractive index
• Bragg gratings are used in fiber optic sensors for measuring strain, temperature, and pressure by monitoring shifts in the reflected wavelength

Optical MEMS and Microbolometers

• Optical MEMS (Micro-Electro-Mechanical Systems) integrate mechanical and optical components on a microscale to create devices such as micromirrors, optical switches, and tunable filters
• Micromirrors are used in digital light processing (DLP) displays, optical cross-connects, and adaptive optics systems for wavefront correction
• Microbolometers are thermal infrared detectors that measure the temperature change caused by absorbed infrared radiation
• Microbolometers are used in uncooled infrared cameras for thermal imaging applications, such as night vision, surveillance, and thermography
• Optical MEMS and microbolometers leverage the advantages of miniaturization, such as reduced size, lower power consumption, and improved performance compared to traditional optical systems

Electromagnetic Sensing

Hall Effect and Magnetoresistance

• Hall effect is the generation of a voltage difference across an electrical conductor when a magnetic field is applied perpendicular to the current flow
• Hall effect sensors are used to measure magnetic fields, current, and position in applications such as automotive (wheel speed sensors), industrial (proximity sensors), and consumer electronics (smartphone compasses)
• Magnetoresistance is the change in electrical resistance of a material in the presence of a magnetic field
• Giant magnetoresistance (GMR) and tunnel magnetoresistance (TMR) are phenomena used in sensitive magnetic field sensors, such as read heads in hard disk drives and magnetic random-access memory (MRAM)

Superconducting Quantum Interference Devices (SQUIDs)

• SQUIDs are highly sensitive magnetometers that use the quantum interference of superconducting currents to detect extremely weak magnetic fields
• SQUIDs consist of a superconducting loop with one or two Josephson junctions, which are thin insulating barriers between superconductors that allow quantum tunneling of electrons
• SQUIDs are used in various applications requiring ultra-sensitive magnetic field measurements, such as medical imaging (magnetoencephalography), geophysical exploration, and fundamental physics research (detection of gravitational waves)
• The high sensitivity of SQUIDs is achieved by operating them at cryogenic temperatures (typically liquid helium) to maintain the superconducting state and minimize noise