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Related Concept Videos

Semiconductors01:22

Semiconductors

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There is variation in the electrical conductivity of materials - metals, semiconductors, and insulators that are showcased with the help of the energy band diagrams.
Metals such as copper (Cu), zinc (Zn), or lead (Pb) have low resistivity and feature conduction bands that are either not fully occupied or overlap with the valence band, making a bandgap non-existent. This allows electrons in the highest energy levels of the valence band to easily transition to the conduction band upon gaining...
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Types of Semiconductors01:20

Types of Semiconductors

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Intrinsic semiconductors are highly pure materials with no impurities. At absolute zero, these semiconductors behave as perfect insulators because all the valence electrons are bound, and the conduction band is empty, disallowing electrical conduction. The Fermi level is a concept used to describe the probability of occupancy of energy levels by electrons at thermal equilibrium. In intrinsic semiconductors, the Fermi level is positioned at the midpoint of the energy gap at absolute zero. When...
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Metal-Semiconductor Junctions01:24

Metal-Semiconductor Junctions

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The contact of metal and semiconductor can lead to the formation of a junction with either Schottky or Ohmic behavior.
Schottky Barriers
Schottky barriers arise when a metal with a work function (Φm) contacts a semiconductor with a different work function (Φs). Initially, electrons transfer until the Fermi levels of the metal and semiconductor align at equilibrium. For instance, if Φm > Φs, the semiconductor Fermi level is higher than the metal's before contact. The...
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Biasing of Metal-Semiconductor Junctions01:27

Biasing of Metal-Semiconductor Junctions

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Biasing metal-semiconductor junctions involves applying a voltage across the junction. Specifically, the metal is connected to a voltage source, while the semiconductor is grounded. This technique is essential for controlling the direction and magnitude of current flow in electronic devices, including diodes, transistors, and photovoltaic cells.
In Schottky junctions, where the semiconductor is n-type, applying a positive voltage to the metal relative to the semiconductor reduces its Fermi...
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Frequency-dependent Selection01:21

Frequency-dependent Selection

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When the fitness of a trait is influenced by how common it is (i.e., its frequency) relative to different traits within a population, this is referred to as frequency-dependent selection. Frequency-dependent selection may occur between species or within a single species. This type of selection can either be positive—with more common phenotypes having higher fitness—or negative, with rarer phenotypes conferring increased fitness.
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Temperature Dependence on Reaction Rate02:55

Temperature Dependence on Reaction Rate

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The Collision Theory
Atoms, molecules, or ions must collide before they can react with each other. Atoms must be close together to form chemical bonds. This premise is the basis for a theory that explains many observations regarding chemical kinetics, including factors affecting reaction rates.
The collision theory is based on the postulates that (i) the reaction rate is proportional to the rate of reactant collisions, (ii) the reacting species collide in an orientation allowing contact between...
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Related Experiment Video

Updated: Jan 23, 2026

Generation and Coherent Control of Pulsed Quantum Frequency Combs
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Generation and Coherent Control of Pulsed Quantum Frequency Combs

Published on: June 8, 2018

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Room temperature terahertz semiconductor frequency comb.

Quanyong Lu1, Feihu Wang1, Donghai Wu1

  • 1Center for Quantum Devices, Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, IL, 60208, USA.

Nature Communications
|June 5, 2019
PubMed
Summary
This summary is machine-generated.

A room temperature terahertz (THz) harmonic frequency comb was developed using a mid-infrared quantum cascade laser. This advancement enables high-resolution THz spectroscopy at room temperature.

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Area of Science:

  • Optics and Photonics
  • Quantum Cascade Lasers
  • Terahertz Technology

Background:

  • Terahertz (THz) frequency combs are crucial for high-resolution spectroscopy, metrology, and sensing.
  • Previous cryogenic-cooled THz quantum cascade laser (QCL) combs offered high power and broad spectra but required low temperatures.

Purpose of the Study:

  • To report a room temperature THz harmonic frequency comb.
  • To enable chip-based THz spectroscopy applications without cryogenic cooling.

Main Methods:

  • Difference-frequency generation from a mid-infrared QCL to create a THz harmonic frequency comb (2.2–3.3 THz).
  • Intracavity generation by down-converting a mid-infrared comb with an integrated mid-infrared single mode.
  • Utilizing a distributed-feedback grating with a detuned Bragg wavelength to suppress dispersion and operate in the high gain spectral range.
  • Employing multiheterodyne spectroscopy with a reference Fabry-Pérot comb to confirm operation.

Main Results:

  • Successfully generated a THz harmonic frequency comb operating at room temperature.
  • Demonstrated comb operation in the 2.2 to 3.3 THz range.
  • Confirmed the THz comb operation through multiheterodyne spectroscopy.

Conclusions:

  • The developed room temperature THz harmonic frequency comb is a significant advancement for THz technology.
  • This technology paves the way for compact, chip-based THz spectroscopic systems operating without cryogenic cooling.
  • Potential applications in room temperature chip-based THz spectroscopy are highlighted.