Nanotechnological lasers.pdf

8/14/2019 Nanotechnological lasers.pdf

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Double heterostructureThe first major advance in laserdiode design was the double heterostructuregeometrywhich confines the carriersand the light to the sameregion.

The threshold current density is approximatelyJthdecreases linearlywith d until the guide layer istoo thin to confine the light, atwhich point the overlapdecreases and the thresholdincreases.

Wide-gapsemiconductorNarrow-gap semiconductor

Wide-gap semiconductor

zEg2

Eg2

Eg1

Eg2 Eg1

z

Valence band edge

Conduction band edge

AlGaAs/GaAs/AlGaAs Laser


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Evolution of the threshold current of the semiconductor lasers


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Single-Quantum Well Laser (SQWL)

Double

Heterostructure:

GFpFn EEE >)(1)( VVVC EfhEf >+ or, alternatively,

Basic Laser condition:

nm

h

V> 0

P p N

EV

EC

EFpEFn

Eel

Ehole


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For lasing at the bandgap energy, population inversion must be achieved

requirement: fCe(Ec) > fVe(EV)

(larger probability of electrons near the CB edge than at the VB edge)

Note: electron probability = probability of not having a hole:

Probability of having electron levels populated at the band edges

The condition fCe(Ec) > fVe(EV) can thus be rewritten as FC-FV> EC- EVThe spacing between the quasi Fermi levels must be larger than the bandgap


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Multiple-Quantum Well Laser (MQWL)

P p P

EV

EC

MQW using isotypeSQW:

mini bands

P p P p P p P p P

h h h h

MQW DFB

MQW DFB


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Separate Confinement Heterostructure (SCH)

h

EV

x

P p N

EC

InP

InGaAsP InGaAsP

InP

InGaAsP

InGaAs

MQW regionSCH region SCH regioncladding cladding

5 nm 10 nm 50 nm


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EC

EG( InP )

Graded-Index SCH Laser (GRINSCH L)

EG( InGaAsP )

EG( InGaAs )

EV

GRIN regionGRIN region MQW region

n

cladding cladding

x


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QDL Predicted Advantages

Wavelength of light determined by the energy levels not by bandgapenergy:

improved performance & increased flexibility to adjust the wavelength

Maximum material gain and differential gain

Small volume: low power high frequency operation

large modulation bandwidth

small dynamic chirp

small linewidth enhancement factor low threshold current

Superior temperature stability of I threshold

I threshold (T) = I threshold(Tref).exp ((T-(Tref))/ (T0))

High T0decoupling electron-phonon interaction by increasing the intersubbandseparation.

Undiminished room-temperature performance without external thermal stabilization

Suppressed diffusion of non-equilibrium carriersReduced leakage


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QDL Application Requirements

Same energy level

Size, shape and alloy composition of QDs close to identical

Inhomogeneous broadening eliminatedreal concentration ofenergy states obtained

High density of interacting QDs

Macroscopic physical parameterlight output

Reduction of non-radiative centers Nanostructures made by high-energy beam patterning cannot

be used since damage is incurred

Electrical control

Electric field applied can change physical properties of QDs

Carriers can be injected to create light emission


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QDL Basic characterist ics

An ideal QDL consists of a 3D-array of dots with equal size and shape

Surrounded by a higher band-gap material

confines the injected carriers.

Embedded in an optical waveguide Consists lower and upper cladding layers (n-doped and p-doped shields)


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Structures for enhanced carrier collection.

(a) Dot in a well structure.

(b) Tunnel injection structure.


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Quantum Dot Lasers (QD L)

b) tunneling-injection QD laser:a) schematic:

The electron-injecting QW is wider

than the hole-injecting QW and both

QWs are narrower than the QD toaccomplish resonant alignment of

the majority-carrier subbands with

the QD energy levels. The tunnel

barrier on the electron-injecting side

is made thicker to suppress hole

leakage from the QD.


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a) Prevention of parasitic b) Limit caserecombination in the OCL

n-cladding

p-cladding

OCL

OCL

QD

electrons

holes

QDs are clad by heterostructure barrier layers that block only the minority carrier

transport


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Bottlenecks First, the lack of uniformity.

Second, Quantum Dots density is insufficient.

Third, the lack of good coupling between QD and QD.

Breakthroughs

Fujitsu

Temperature Independent QD laser

2004

Temperature dependence of light-current characteristics


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Q. Dot Laser vs. Q. Well Laser

In order for QD lasers compete with QW lasers:

A large array of QDs since their active volume is small

An array with a narrow size distribution has to be produced toreduce inhomogeneous broadening

Array has to be without defects

may degrade the optical emission by providing alternatenonradiative defect channels

The phonon bottleneck created by confinement limits the numberof states that are efficiently coupled by phonons due to energyconservation

Limits the relaxation of excited carriers into lasing states Causes degradation of stimulated emission

Other mechanisms can be used to suppress that bottleneckeffect (e.g. Auger interactions)


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1. Edge emitting (in plane laser)

Single QW

Double heterostructure semiconductor laser

Multiple QW

Cavity = cleaved crystal surfaces

Injection of electrons in the active region

Narrow gain spectrum

Small line width

High modulation speed

Low output power 100 mW

In arrays up to 50W


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2. Surface emit ting laser (SEL) : vertical laser output

Vertical Cavity SEL

Easy to integrate to fibers

Heating effects in the multiple layer structure


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Advantages of VCSELs

The structure can be integrated in two-dimensional array configuration.

Low threshold currents enable high-density arrays.

Surface-normal emission and nearly identical to the photo detector geometry give

easy alignment and packaging.

Circular and low divergence output beams eliminate the need for corrective optics.

Passive versus active fiber alignment, combined with high fiber-coupling efficiency.

Low-cost potential because the devices are completed and tested at the wafer

level.

Lower temperature-sensitivity compared to edge-emitting laser diodes.High transmission speed with low power consumption.

VCSELs have been constructed that emit energy at 850 nm and 1300 nm.

Common se/c VCSELs: GaAs, AlGaAs, GaInNAs

The main challenge facing engineers today is the development of a high-power

VCSEL device with an emission wavelength of 1550 nm.


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Distributed Bragg Reflector structure

VCSEL advantages :

- short cavity: optical quality, temperature

independent- Small size: low threshold, efficiency, ...

- Surface emission: integrability, density, ...


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Bandgap dipends on- composition

- structure (q-wells, q-dots)

Using available semiconductor compound materials one can engineer emitters

throughout the range ~300-1600nm

For some applications (e.g. detection of organic species, imaging through

scattering media such as rain, clothes) one would like to use longer wavelengths


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Nanotechnological laser: quantum cascade (QC)

Objectives:

- mid-infrared laser with ad hoc emission wavelength (e.g. for trace analysis)

- Extremely high efficiency (low threshold current, high power)

Band-gap engineering through film thickness

Cascade emission of photons

Only electrons involved (unipolar mechanism)

An electron is injected into level 3 of the first

active zone, transition to level 2 produces a

photon (E depends on thickness).The lifetime of the 3 2 transition has to be

longer than the lifetime of level 2 to obtain

population inversion. Then tunneling

through a thin barrier towards active region 2.

The emission process is repeated in a

cascade fashion (many photons from one

injected electron)


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Faist et al., Science 264, 553 (1994)

Lasing based on transitions between levels within the conduction band

device requires electrons, NOT electron hole pairs unipolar

device

The carrier (e) is not lost the process can be repeated


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Faist et al., Science 264, 553 (1994)

This type of device relies on

regions with engineered

conduction band levels

Based on quantum confined

light emitting active areas

(multilayer structure) separated

by multilayer regions

optimized for injecting electrons

The total materials stack can

become rather complicated

The structure on the

right was the system usedfor the first demonstration of

QCL


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Quantum Cascade Laser (QC L)-PrincipleInterband transition :

Intersub-bandtransition :

Eappl

Tunneling rate >>3= 1 ps

e 2= 0.3 ps


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coupled layers exhibit minibands several closely spaced

allowed levels separated by a minigap to the next minibandExcitation

- Injection region designed to

optimize electron energy for

inserting e-into excited state of

active region (3) (lowest energy

level in injector region aligned withexcited state 3)

Relaxation

After lasing transition: quick

relaxation from level 2 to 1 is

required to prevent stimulated

absorption.

Emission

- Lasing transition: transition

between engineered conduction

band levels 3 and 2 (between

subbands). Intraband transition

Device based on tunneling

To minimize thermal escape of the excited electron present in the active region, the next injectorregion is designed such that a minigap lines up with level 3

.

Levels 1 and 2 line up with levels in the lowest miniband of the next injector region, allowing for fast

tunneling out of the active region

Obtained by engineered level spacing to assist phonon

emission matching the transition energy 21 with theenergy of an optical phonon in the structure. This resultsin a non-radiative relaxation time of 21 0.3 ps, significantly faster than 32 2.6 ps, allowing for

population inversion


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QC Laser -Tailoring


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QC Laser Data

Applications:

Military and Security

Commercial, Medical

Free-Space Optical Communication Systems and Astronomy

Gas detection based on laser spectroscopy with CW or pulsed QC DFB

lasers (chemical sensors)

L

[m]Pout

[mW]

Jth[A/cm2] /

Eth[kV/cm]

operation

mode

T first

demo

[year]

3.4 80 200 300

(CW) up to

1000 (PM)

250 290 /

7.5 48PM or CW

on cooler

350 1994

AT&T

Bell Labs

Material systems:GaAs based, InP based, Si / SiGe on GaSb, InAs / AlSb on

GaSb

CW = continuous wave; PM = pulse mode


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Fabrication of QC-laser

Combination of MBE(thickness control)

and lithography(lateral resolution)

Zona attiva

Iniettore


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