Semiconductor Nano Lasers

Ahlam H. J. Al-Musawi

Theoretical Nanoscience Laser and Optoelectronic Devices,

Department of Physics, College of Education for pure Science

(Ibn Al-Haitham), University of Baghdad

Objectives

Main concepts

• Population inversion

• Optical Gain

• Threshold current

Laser Diode

Semiconductor laser from pn-junction to

nano laser

• “LASER” • Light

• Amplification by the

• Stimulated

• Emission of

• Radiation

Electron/Photon Interactions

• Absorption

• Spontaneous

Emission

• Stimulated Emission

Therefore in a laser….

Three key elements in a laser

•Pumping process prepares amplifying medium in suitable state

•Optical power increases on each pass through amplifying medium

•If gain exceeds loss, device will oscillate, generating a coherentoutput

Main requirements for Lasing

• Initial Photons

• Population Inversion

• Threshold Current

Population Inversion in Diode

Laser

Electrons in CB E

Fn

EFp

CB

VB

Eg

Holes in VB

eV

EFn-EfP = eV

eV > Eg

eV = forward bias voltage

Fwd Diode current pumping

injection pumping

More electrons in

the conduction

band near EC

Than electrons in

the valance band

near EV

There is therefore a population inversion between

energies near EC and near EV around the junction.

This only achieved when degenerately doped p-n

junction is forward bias with energy > Egap

Concepts: Population Inversion and

Optical Gain

Semiconductor laser from pn-

junction to nano laser

• Need to build up

stimulated

emissions by a

optical resonator

• Provided by

cleaved and

polished ends of

the crystal

Laser diode structure 1- Homojunction

a. No optical and carrier confinement.

b. The room temperature threshold current density is very high.

c. The dimension of the laser beam is significantly larger than the

active region thickness.

d. Low efficiency.

2- Heterojunction I. Double Heterojunction

Single quantum well

ll. Quantum Well

Multi-quantum well

a. Electrons and holes are confined to the active region

b. Low threshold current density at room temperature

c. Optical confinement

d. High efficiency

• Constant 2D density of states means a large concentration of electron can

easily occur at E1 (and holes at the minimum valence band energy)

• Population inversion occurs quickly without the need for a large current to bring a large number of electrons

• Benefits: Threshold current reduced, linewidth is narrower

Intersubband Lasing Concept

Quantum Cascade Laser

Laser Diode

Structure

Heterojunction

DH QW

SQW MQW

Laser Diode Parameters

Confinement factor

Threshold gain

Threshold current density

Homojunction

Output power density

21

1ln

2

1

RRLg th

thJJ

RRL

RRgEinP

21

1ln2

21

1ln

21

1ln

2

111

0

RRLogin

tJthJ

22t2D ,

22

2

cnrnD

D