TerahertzQuantumCascade Lasers

Dr. Christopher BairdSubmillimeter-Wave Technology Lab

UMass LowellApril 2008

What is a QCL?

A Quantum Cascade Laser (QCL) is a next-generation device that uses electron transitions between man-made quantum well levels, as opposed to traditional lasers that use transitions between atomic levels. A QCL is built by stacking up alternating layers of semiconductors with thicknesses of only a few atoms.

How does a QCL work?

The alternating layers of materials create a set of energy wells that trap electrons into quantum states. By properly adjusting the layer thicknesses, the quantum states are fine-tuned until lasing occurs. The electrons cascade down through the quantum states like a waterfall, emitting laser radiation at each drop.

2.9 THz Barbieri Structure

Why terahertz?

Terahertz (THz) radiation is used at UMass Lowell's STL and in the military for radar imaging of scaled targets. Expensive, room-sized gas lasers must be currently used to generate THz radiation. QCL's are much more compact and potentially cheaper at providing THz radiation.

Why terahertz?

Terahertz (THz) radiation is used at UMass Lowell's STL to image vehicles and to pursue EM scattering, chemical sensor, and medical research. Expensive, room-sized gas lasers must be currently used to generate THz radiation. QCL's are much more compact and potentially cheaper at providing THz radiation.

Our Team's Efforts

Our QCL team includes groups at STL, the Photonics Center, and Spire Corp. Together we:

● Theoretically model, predict, and design QCL performance

● Grow QCL's using MBE● Process the QCL's● Test and characterize the

QCL's● Use results to improve future

QCL'sMolecular Beam Epitaxy (MBE) system used at

UMass to build QCL's layer by layer

Experimental Success

Our team successfully built a 2.4 THz quantum cascade laser based on the 2.9 THz Barbieri structure shown previously.

Student Involvement

● High School Students through the SOS program

● Undergraduate Students working part-time

● Graduate Students working as research assistants

● Graduate Students working on dissertation research

Our QCL team relies heavily on student involvement, providing them with hands-on experience in high-tech research. We are continually looking for additional students to join our team. Current student involvement includes:

QCL Code Prediction

Our QCL code was designed from the first principles of quantum theory in order to accurately predict QCL performance.

PoissonEquation

SchroedingerEquation

Charge DensityEquation

Steady StateEquation

Charge DensityEquation

Built in Voltage

Wavefunctions

Fermi Levels

Charge Distribution

Charge Distribution

repeat until converges

repeat until converges

repeat until converges

Build QCLStructure

CalculateNumber of

Free Electrons& Ionized Donors

Calculate InitialFermi Levels

Load Inputs &Material Params

Find ScatteringTimes

Find Populations

Find Gain,Intensity,Current, etc.

QCL Code Theory

The heart of the QCL code is a numerical algorithm that solvesthe one-electron Schroedinger equation to find the possible electron wave-function states. The other equations add-in the effects of electron-electron interactions

SchrödingerEquation

PoissonEquation

ChargeDensity

Equation

Steady-StateEquation

ddz [ 1

m* z ddz ] z =− 2

ℏ2 E−V z z

−ddz z d z

dz = z

z =−em* z k BT

ℏ2 ∑n∣n z ∣

2 ln 1e−E nE Fz / k BT

ddz [ z z ddz EF z ]=0

QCL Code Success

Our QCL code theoretical predictions matched the experimentalresults better than the author's own theoretical predictions

Barbieri Code Predictions:2.660 THz

Our Code Predictions:2.906 THz

Barbieri Structure Experimental Results: 2.90 THz