[2022] [paper notes] terahertz quantum well——

Preface

type
$Terahertz+Quantum\; well$
Periodical
$Journal\; of\; infrared\; and\; millimeter\; wave$
author
$Zhang\; Zhenzhen,Fu\; Zhanglong,Wang\; Chang,Cao\; Juncheng$
Time
$2022$

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THz Emission source and detector

THz The emission sources are

• High Power THz Quantum cascade lasers QCL
• Ultra wideband photoconductive antenna PA
• Single carrier diode UTC-PD

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Infrared detector
be based on mct CdTe Large scale focal plane imaging detectors of materials have long been commercialized , Double sided metal microcavity The infrared quantum well detector has achieved room temperature operation , New type GeSn Mid infrared photodetectors are expected to achieve CMOS compatible

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THz Band detector

Constant temperature ultra wideband detection PA Can be used for THz Time domain spectral system TDS Realize the detection of the characteristic spectrum of drugs and explosives

be based on Principles of Electronics The Schottky diodes SBD The probe and the UTC-PD Combination , It can make quasi optical modules , The future 6G The foundation of high-speed wireless communication link

be based on Niobium nitride Material super conduction electron thermal radiometer SHEB Can realize the right THz Direct and heterodyne detection of waves

be based on graphene Room temperature field effect transistor of material GFET be used for THz Heterodyne detection can be achieved 1GHz If bandwidth above P wait

be based on Principles of photonics Of THz Quantum well detector QWP Small size 、 Easy integration 、 Fast response speed and high sensitivity , yes THz Ideal device for large-scale focal plane imaging

THz QWP（ Quantum well detector ） With high power THz QCL（ Cascade laser ） Combination , Can achieve THz Fast imaging and high-speed communication of waves

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THz-QWP principle

THz-QWP Is based on Principle of inter subband transition Single stage device

Inter subband transition ：
The electric field is required to have non-zero polarization separation in the growth direction of the quantum well

So usually THz-QWP There is no response to normal incident light

The substrate is generally used 45° Inclined plane incident mode to couple ？

THz-QWP The active region is composed of dozens of quantum well superlattices with hundreds of cycles , Usually GaN/AlGaAs Material system

In the absence of light , Electrons are bound in quantum wells , The device is in High resistance state

stay THz When the optical coupling enters the active region of the device , The bound electrons in the ground state subband in the quantum well absorb photon energy and are excited to the first excited state and close to the edge of the barrier Quasi continuous state

These photogenerated carriers form photocurrent under the action of applied bias , By measuring the change of photocurrent signal, we can realize THz Detection of light

THz-QWP Coupling mode

THz-QWP The thickness of the active region is in the order of subwavelength , The doping concentration in the well is low , Light absorption rate is small
（ Compared with infrared QWIP Quantum well infrared detector ）

• So efficient Coupling mode Very important ！！！

Multi quantum well detector coupling mode ：

• Incident with Brewster angle （ before ）
• 45° Oblique back incidence （ Now? ）

in addition , Subwavelength metal microcavity and surface plasma coupling , These two coupling methods can also Increase the effective light absorption area of quantum well detector , Improve absorption efficiency , Suppressor device dark current , Raise the working temperature

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QWP progress

A peak detection frequency is 5THz Of QWP, use Patch antenna microcavity array coupling structure , Make the peak response ratio 45° Table structure QWP Improved 5 times

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QWIP progress

A plasma lens adds a monolithic integrated infrared quantum cascade detector QWIP. Infrared radiation is incident on （c）(d) Microstructure surface of , Exciting plasma source , Coupling into the active region of the detector

Plasma lens coupler , Expand the optical effective area of the device to , Than the geometric area of the detector table 5 times

The photocurrent response of the detector at room temperature is improved 6 times

Improved absorption efficiency 、 Working performance at room temperature

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THz-QWP Performance indicators

• Characteristics of dark current changing with temperature
• Photocurrent response spectrum （ Measure with Fourier transform spectrometer ）
• Background limiting operating temperature $T_{BLIP}$： The operating temperature of the device when the dark current is equal to the background current
• Response rate
• Detection sensitivity $D^{\ast }$

And QWIP comparison ,THz-QWP The energy gap between the bound state and the continuous state in the quantum well is very small , The dark current is very sensitive to temperature , Affect the working performance of the detector at room temperature

1. In order to suppress dark current ,THz-QWP It needs continuous refrigeration Liquid helium flow Work under the
2. Photocurrent spectrum It is the relationship between light intensity and frequency
3. Background limiting operating temperature $T_{BLIP}$ The higher the , The less affected by dark current
4. By measuring the radiation response rate of standard blackbody , And photocurrent response spectrum , To calibrate the peak Response rate
5. Detection sensitivity $D\ast =\sqrt{Table\; area}/Noise\; equivalent\; power$

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THz-QWP Of Peak response frequency It is determined by the energy difference between the first and second subbands in the active region quantum well

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Conclusion

Terahertz - Quantum well infrared detector
THz QWP advantage ： Small volume 、 Convenient design 、 Simple workmanship 、 Fast response

be applied to ：

• Focal plane imaging
• High speed wireless communication in free space
• Fast imaging
• High speed modulation

In quantum wells Low inter subband absorption efficiency It's restriction THz QWP Important factors of application

In order to improve absorption efficiency , It can be calculated theoretically 、 Optical coupling mode and process preparation to optimize the device , So as to further improve the photoelectric conversion efficiency and working temperature of the device
in addition , THz QWP It's a kind of Narrowband The detector , To adapt to wide-spectrum detection , Different detection frequencies can be used QWP Stack active areas , Prepare multicolor detectors

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problem

About the coupling mode , Can a super surface be used on the detector surface ？

Does quasi optical module refer to small optical module ？

What is the relationship between quantum wells and electromagnetic fields ？

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