1

Laser Physics II

PH482/582-3A (Mirov)

Tunable Solid State Lasers

Lecture 4-5

Fall 2006C. Davis, “Lasers and Electro-optics”

2

Broadband media overview• Natural definition of bandwidth is Δλ/λ0• Number of optical cycles per pulse scales as (Δλ/λ0)−1• Natural definition of bandwidth is Δλ/λ0• Number of optical cycles per pulse scales as (Δλ/λ0)−1

3

Alexandrite Lasers

4

5

6

7

8

Ti-Sapphire Lasers

9

10

11

12

Chromium doped LiSAF and LiCAF Lasers

13

14

Room Temperature Middle Infrared lasers based on Transition Metal

(TM: Cr2+, Fe2+, Co2+, Ni2+ doped II-VI (ZnS, ZnSe, ZnTe, CdS, CdSe)

semiconductor crystals

15

OUTLINEI. Motivation

Why TM2+: II-VI?Why Cr2+ and Fe2+?

II. Overview of Cr and Fe-doped II-VI lasersState-of-the-art Cr: ZnS and ZnSe

Sample preparation

III. New regimes of excitation and lasingMicrochip cw and gain-switched lasingMulti-line or ultrabroadband lasing in spatially dispersive cavitiesLasing via photoionization transitionsMid-IR Cr:Al:ZnSe electroluminescence

IV. Fe2+:ZnSe spectroscopic characterization

V. RT Fe2+:ZnSe gain switched lasing over 3.9-4.8μm spectral range

VI. Conclusions and Future Outlook

16

Why broadband laser media?Ultra-short pulses

– time-resolved measurements – chemical reaction control – XUV generation – frequency standards

Ultra-broad spectra– optical coherence tomography – spectral slicing– intracavity absorption

Broadly tunable narrowband sources– optical coherence tomography– spectroscopy– cavity ring-down measurements– photoacoustic measurements

17

Interest for infrared wavelengthsFor femto-chemistry, molecular time-resolved measurements, molecular spectroscopy, tracegas analysis, biomedical applications, etc. one should directly reach molecular fingerprint 2-20 μm region.

Mid-IR tunable, cw-fs sources are required

Requirements:• Sufficient bandwidth• Low cost, compact, directly diode-pumped low threshold• High brightness, i.e. good spatial coherence (TEM00).

Solutions:• OPO (bulk ZGP, PPLN, orientation-patterned GaAs): almost ideal solutions, but rather

complex and costly• QCL: nice solution for λ > 3.4 µm, not as broadband• Semiconductor InGaAsSb/GaSb lasers: narrow tuning, no fs, gap around 2.7-3 µm• Crystalline vibronic lasers: ultrabroadband up to 45 % λ, cw-fs, room-temperature

0.2 0.5 1 2 5 10 20 µm

Athmospherictransmission

Ti:S/Cr:LiSAF

Molecular frequenciesCr:YAG

Cr:Zn/CdSe

Tm:laser

Fe:ZnSeCo:MgF2

18

MOTIVATION for TM:II-VI

Crystalline vibronic lasers – viable solutionActive interest in TM doped II-VI compounds is explained by the fact that these media are close mid-IR analogues of the titanium-doped sapphire (Ti-S) in terms of spectroscopic and laser characteristics and it isanticipated that TM2+ doped chalcogenides will lase in the mid-IR with a great variety of possible regimes of oscillation, similar to the Ti-S laser.

Significant advantages:TM2+ doped II-VI materials can be directly pumpable with radiation of fiber lasers and/or InGaAsP/InP diodes or diode arrays. In comparison with Ti-S they feature higher maximum permissible dopant concentration, higher cross sections and in result microchip lasing arrangements are feasible.Due to effective thermo-diffusion of TM in II-VI, diffusion doping of starting material is feasible. Have potential for direct electrical excitation (doped QD or QW lasers).

For molecular spectroscopy, trace gas analysis, biomedical applications, etc. one should directly reach molecular fingerprint 2-20 μm region. Mid-IR tunable, cw-fs sources are required.

19

Why II-VI are so special for Mid-IR lasing?

Host Phonon cut-off

ZnTe 210 cm-1

ZnSe 250 cm-1

ZnS 350 cm-1

YAG 850 cm-1

YLF 560 cm-1

The heavy anions of II-VI crystals ensure that the optical phonon cutoff occurs at very low energy, thus maximizing the prospectsfor radiative decay of mid-IR luminescence in these crystals.

20

Why Cr2+ & Fe2+? Calculated Multiplet Structure for 3d impurities in ZnSe (after A Fazzio, et al., Phys. Rev. B, 30, 3430 (1984)

First excited levels lie at the right energy to generate 2-3 (Cr) & 3.5-5 μm (Fe) mid-IR emission.The ground and and first excited levels have the same spin, and therefore will have a relatively high cross-section of emission.Higher lying levels have spins that are lower than the ground and first excited levels, greatly mitigating the potential for significant excited state absorption at the pump or laser transition wavelengths.The orbital characteristics of the ground and first excited levels are different, and will experience a significant Franck-Condon shift between absorption and emission, resulting in broadband “dye-like” absorption and emission characteristics, suitable for a broadly tunable laser.

21

The top two: Cr:ZnSe and Cr:ZnS

1400 1600 1800 2000 2200 2400 2600 2800 30000

5

10

15

Tm:Y

ALO

Tm:Y

AG

σ (1

0-19 c

m2 )Er

:fibe

rIn

GaA

sP d

iode

s

Co:MgF2

Gain

Absorption

Wavelength, nm

Cr:ZnS

1400 1600 1800 2000 2200 2400 2600 2800 30000

5

10

15

Tm:Y

ALO

Tm:Y

AG

σ (1

0-19 c

m2 )Er

:fibe

rIn

GaA

sP d

iode

s

Co:MgF2

Gain

Absorption

Wavelength, nm

Cr:ZnSe

I.T. Sorokina, E. Sorokin, S. Mirov, V. Fedorov, V. Badikov, V. Panyutin, K. Schaffers, Opt. Lett., 27, 1042 (2002).

The only two types so far to allow tunable CW (also diode-pumped!) operation *).ZnS ZnSe

Lattice constant 5.4 Å 5.67 Å

Crystal structure mixed-polytype cubic

Bandgap 3.8 eV 2.8 eV

Lifetime (300 K) 5 µs 4 µs

Slope efficiency 53 % 71 %

dn/dT (10-6 K-1) 46 70

Transparency 0.4–14 µm 0.5–20 µm

Hardness (Knoop) 160 120

Thermal conductivity

27 W/mK(cubic)

17 W/mK (hex)18 W/mK

Refractive index 2.27 2.45

Emission peak 2350 nm 2450 nm

Absorption peak 1680 nm 1780 nm

22

State of the art in Cr:ZnSe and Cr:ZnS

Cr:ZnSe and ZnS

CW output power, polarized TEM00 1.8 W I.Sorokina MICS 2005

Output power, pulsed 18 W Carrig, Photonics Europe 2004

Tuning range 2000-3100 nm I. T. Sorokina et al, CLEO/Europe, 2001

CW and Gain-switched microchips 0.5 W and 1 mJ S.B. Mirov, V. Fedorov et al. CLEO, 2002

Lasing of hot-pressed ceramic 2 mJ A. Gallian, V. Fedorov, S.Mirov.. CLEO, 2005

Lasing via photoionization excitation 532 nm pump A. Gallian, V. Fedorov, S.Mirov.. ASSP, 2005

Multiwavelength lasing Up to 60 lines I. S. Moskalev, V. Fedorov, S.Mirov.. . Optics Express 12, 2004

Acousto-optic active Mode Loking 4.4 ps, 82 mW G. J.Wagner et al, Opt. Lett. 24, 19 (1999).

Active-passive Mode Locking 4 ps, 400 mW I. T. Sorokina et al, ASSP, 50,157(2001)

Electroluminescence in n-Cr:ZnSe 2000-3000 nm A. Gallian, L. Luke, V. V. Fedorov, S.MirovWB21 ASSP 2006

Zn-chalcogenide lasers, ( Livermore Group L. DeLoach, R. Page et al 1996)Cd-chalcogenide lasers, ( K. Schepler et al (Cr:CdSe), and S. B. Trivedi et al (Cr:CdTe and compounds) 1997)

23

1800 2000 2200 2400 2600 2800 3000 32000

300

600

900

1200

1500

Out

put p

ower

(mW

)

Wavelength (nm)

0

5

10 GainAbsorption

Fluorescence

Infrared set (2800 nm)Broadband set (2350 nm)

Cro

ss-s

ectio

n (x

10-1

9 cm

2 )State of the art in Cr:ZnSe and Cr:ZnS

Cr:ZnSe CW output power, polarized TEM00 1.8 W TU Vienna-Univ. Innsbruck BEC experiment 2005

Oscillation linewidth < 600 MHz E. Sorokin et al, Appl. Phys. Lett. 80, 3289 (2002).

Cr:ZnS CW output power, polarized TEM00 0.7 W I. Sorokina et al, Opt. Lett., 27, 1040 (2002)

Output power, pulsed 18 W T. Carrig, Photonics Europe 2004Tuning range 2000-3100 nm I. T. Sorokina et al, CLEO/Europe 2001.

Tuning range 2170-2840 nm ibid

0 1 2 30.0

0.2

0.4

0.6

0.8

1.0

OC = 10%λ = 2350 nm

CVT, slope 40% PVT, slope 34%PVT, 50% duty cycle

Out

put p

ower

, W

Absorbed pump power, W

Cr:ZnSe

Mode-locking(Cr:ZnSe)

Acousto-optic active ML 4.4 ps 82 mW G. J.Wagner et al, Opt. Lett. 24, 19 (1999).

4 ps

20 ps11 psInAs MQW SESAM 400 mW C. R. Pollock et al, TOPS, ASSP,98 (2005).

Active-passive ML (KLM?) 400 mW I. T. Sorokina et al, TOPS,ASSP, 50,157(2001).

Active ML, ceramic Cr:ZnSe 150 mW E. Sorokin et al, TOPS,ASSP,83,227(2003).

24

Crystal-field engineering enables new Cr2+lasers

Abs

orpt

ion E

mis

sion5E

5E

Energy

Q

Energy 5T2

10

20

00 1 Dq/B

5D

3H

5T2

5E

3T2

3T2

1T1

1A1

~5000 cm-1

E/B

2000 2500 30000.0

0.5

1.0

1.5

Cr:CdSe

Cr:ZnSe

Cr:ZnS

Em

issi

on (x

10-1

8 cm

2 )

Wavelength (nm)1400 1600 1800 2000 2200

0.0

0.5

1.0

1.5

Cr:CdSe

Cr:ZnSe

Cr:ZnS

Abs

orpt

ion

(x10

-18 c

m2 )

Wavelength (nm)

The central frequency of the vibronic transition is defined by the crystal-field splitting .

In case of Cr2+, the splitting is proportional to the crystal field parameter Dq/B, which inversely scales with the unit cell size:

Example:ZnS: a0 = 5.4 ÅZnSe: a0 = 5.67 ÅCdSe: a0 = 6.05 Å

01

00 )/(/ aBDqc ∝∝= −νλ

25

Crystal field engineering limits

V. I. Sokolov, Fiz. Tverd. Tela 29, 1848 (1987)

Caveats in crystal-field engineering:

Changing the unit cell by crystal composition, will also change:

1. Bandgap (Eg decreases drastically with growing unit cell)

2. Position of the ion levels inside the bandgap.

Result: Not every ion-host combination can be used.

Possible solution: use quantum confinement effects to indepentendtly change the bandgap (Eg grows with decreasing particle size in the 10-nm range)

26

Ternary hosts are also possibleDemonstrated Cr2+-doped II-VI lasers:

Binary systems AIIBVI: Cr:ZnSe, Cr:ZnS, Cr:CdTe, Cr:CdSe

Ternary systems (AIIBII)CVI :Cr:CdMnTe (Trivedi et al, 1997)Cr:CdZnTe (Cerny et al, ASSP 2005)

Ternary systems AII(BVICVI) :Cr:ZnSSe (Sorokina, ASSP 2005)

Important difference: since Cr2+ substitutes an AII

ion, mixing of the BVICVI ions occurs in theimmediate neighbourhood of chromium ion.The crystal field should be strongly affected bymixing and we expect large inhomogeneousbroadening.

27

Bulk Crystal Preparation

Crystals were synthesized by Crystals were synthesized by CVT using iodine as transport CVT using iodine as transport agent ( C(Iagent ( C(I22)=2)=2--5 mg/cm5 mg/cm3 3 ))Ampoule size: Ampoule size: ∅∅20mm x 20mm x 200mm200mmTemperature range 1200Temperature range 1200ooC C --11001100ooCC

P=10P=10--55 torrtorrT= 1000 C T= 1000 C t=7t=7--20 days20 days

Pulsed laser Pulsed laser depositiondeposition

Thermal annealingThermal annealing

The polished samples of The polished samples of 11--2 mm thickness and up 2 mm thickness and up to 5 mm in aperture were to 5 mm in aperture were used for spectroscopic used for spectroscopic and laser measurementsand laser measurements

Chromium thin Chromium thin film deposition film deposition on the crystals on the crystals wafer by means wafer by means of pulsed laser of pulsed laser deposition deposition methodmethod

Chemical Vapor Chemical Vapor Transport (CVT)Transport (CVT)

Crystal

PowderCr

Crystal

Crystalλ=532 nm

E=500 mJZnS+I2↔ ZnI2+1/2S2

28

Why Hot-Pressed Ceramic ?The technology of crystal growth of chromium doped II –VI• Bridgman technique• Chemical vapour transport (CVT)• Physical vapour transport methods (PVT)• The post-growth thermally diffusion doping by chromium

But these growth methods have problems with control of dopantconcentration.

29

Hot-pressed Ceramic Preparation

ZnSe+ CrSe(0.1-0.01%)

ZnSe

P=60MPa

ZnSe+ CrSe(1%) ∅15 mm

P=30-350MPa

30

Pulsed Laser Deposition Growth of Thin Films

• Target: ZnS/Cr • Substrate: Si at 550oC

or 650oC• Target-substrate

separation: 5 ~ 10cm• Pressure vacuum: 2.6

× 10-6 torr• Laser: 248nm KrF, 30

Hz rate, energy density 2J/cm2

• Post annealing is applied

31

X-ray diffraction (XRD) of pure ZnS thin films deposited at different substrate temperatures

25 30 35 40 45 50 55 60 65

2 theta (deg)

inte

nsity

a.u

.ZnS bulk powder

Room temperature

270°C

370°C

470°C

570°C

670°C

32

Surface & cross section SEM (Cr:ZnS results)

Cross section 2500x Surface 5000x

1.5 hours deposition + 1 hour post annealing, at 550oC substrate temperature

33

MID-IR TRANSITION METAL DOPED II-VI MICROCHIP LASERS

Coating mirror and cutting

Chromium deposition and thermal annealing

II-VI WaferTransition Metal doped

semiconductorsMicrochip Lasers Telecom Laser diode

Diode Laser- Microchip Packaging

High Power laser source for 2-3 μm spectral range

The unique combination of technological (low-cost ceramic material) and spectroscopic characteristics (ultrabroadband gain bandwidth, high στ product and high absorption coefficients) make these materials ideal candidates for “non-traditional” regimes of operation such as microchip

34

Ceramic Cr:ZnSe laser

0 200 400 600 800 1000 12000

100

200

300

Single crystal Cr:ZnSe slope efficiency 31%

Polycrystalline Cr:ZnSe slope efficiency 29%

Absorbed pump power, mW

Out

put p

ower

, mW

2200 2400 2600 2800 3000 32000

100

200

300

400

500

600

Mirror set 2750 nmMirror set 2450 nm

Single crystal

Polycrystalline

Out

put p

ower

, mW

Wavelength, nm

70

80

90

100

Tra

nsm

issi

on, %

Air

Polycrystalline sample of Cr:ZnSe has been diffusion-doped to Cr2+ concentration of 2.1019 cm-3 and compared to the single-crystal sample at equal conditions.

Polycrystalline Cr:ZnSe Single-crystal Cr:ZnSe

Slope efficiency 29% 31%Passive losses 8 %/cm 4 %/cmMaximum output power 340 mW 660 mW

Diode-pumped and mode-locked operation have been realized

35

State of the art in Cr:ZnS and ZnSe microchips

nm2550 2600

Inte

nsity

, arb

.un.

Cr2+:ZnS

nm2300 2350

Inte

nsity

, arb

. un.

Cr2+:ZnSeA

B

A

B

2400 2600

C

Pabs,W0 1 2 3 4

Pout

,W

0 .0

0 .1

0.2

0.3

0.4

0.5

0.6

Ge Filter

Output coupler3.5% transmission over 2300-2500 nm

Coupling optics

Er fiber or diode

laser

ZnS:Cr2+ (d=1 mm) or ZnSe:Cr2+ (d=2.5 mm)

CrystalInput mirror

1.55-1.9 μm

2280-2360 μm (ZnS) 2480-2590μm (ZnSe)

Cr:ZnSe microchip

mm-10 -8 -6 -4 -2 0 2 4 6 8 10 12 14

Inte

nsity

, arb

. un.

0.0

0.2

0.4

0.6

0.8

1.0

36

Cr2+: ZnSe gain switched microchip laser. No mirrors deposited on the crystal facets!

Ge Filter

Fresnel reflection 18%

Raman shifted Q-switched

Nd:YAG laser pumping

ZnSe or ZnS(3 mm) Substrate

Fresnel reflection 18%

1.56 μm, 5 ns, D=1mm

2480-2590μm (ZnSe) 2280-2360 μm (ZnS)

ZnS:Cr2+ (d=0.1 mm) or ZnSe:Cr2+ (d=0.2 mm)

active layers

B

Pump energy, mJ5 10 15 20 25 30 35

Out

put E

nerg

y, m

J

0.0

0.2

0.4

0.6

0.8

1.0

η=4%

Output-input energies for pulsed ZnSe microchip laser (t andhrepresent different spots on crystal).

37

Multiline and ultrabroadband lasersU.S. Patent No. 5,471,493, U.S. Patent No 6,236,666 “Application of laser beam shaping for spectral control of “spatially dispersive” lasers” Chapter 7, pp.241-267, in Laser Beam Shaping Applications, Dickey, Holswade, Shealy - Eds., Taylor & Francis, ISBN 0-8247-5941-9, 2005.

Wavelengh, μm1.10 1.15 1.20 1.25

I, a.

e.

0

1

640 645 650 655 660 665 670 675 680

Inte

nsity

, rel

ativ

e un

its

0.0

0.2

0.4

0.6

0.8

1.0luminescencemultiline lasing

LiF CCL Multiline 1& 2ω Multiline diode laser Multiline Cr2+:ZnSe

I. Moskalev, et. al. Optics Express 12, 2004

I. Moskalev et al. Opt. Comm. 220, 161

2003T. Basiev et al., Appl. Optics 36, 2515 1997

(b)

Wavelength, nm

1550 1560 1570 1580 1590In

tens

ity, r

elat

ive

units

0.0

0.2

0.4

0.6

0.8

1.0luminescencemultiline lasing

38

Spatially dispersive Cr:ZnSe lasers

I. Moskalev, et. al. Optics Express 12, 2004

39

Cr:ZnSe Multiline regime of operationA

wavelength, nm

2200 2400 2600 2800

Lase

r Int

ensi

ty. a

.u.

B

wavelength, nm

2200 2400 2600 2800

A

wavelength, nm

2400 2500 2600

Lase

r Int

ensi

ty. a

.u.

B

wavelength, nm

2200 2400 2600 2800

40

Electrically pumpable Cr and Fe doped Quantum Dot and Quantum well broadly tunable mid-IR lasers

Cr and Fe doped II-VI lasers combine the versatility of the ion-doped solid-state lasers with the engineering capabilities of semiconductor lasers, paving the route to the future electrically pumped ultrabroad-band solid-state lasers.

Why QD and QW structures are so critical for electrical excitation?

• the phenomena of quantum confinement of the atomic impurity in quantum well and quantum dot active layers results in much more efficient transfer of energy from the host to the localized impurity accompanied by a significant increase in the oscillator strength of impurity transitions.

41

Electrically pumpable Cr and Fe doped Quantum Dot and Quantum well broadly tunable mid-IR lasers

Scheme of the Mid-IR laser based on QW Cr:ZnSe

heterostructure.

GaAs Substrate

n-ZnSe Chromium doped CdSe

QW

p-ZnSe

Molecular beam epitaxy

2 µm 3 µmwavelength

Inte

nsity

~100 μm

2-3 mm

P - Contact

p - Clad Layer

Nanocomposite Active Region

n - Clad Layer

n+ - Substrate

N- Contact

Scheme of the Mid-IR laser based on nanocomposite

QW/QD Cr:ZnSe - conductive polymer structure prepared by

layer by layer method

n-GaAs substrate

n+-ZnSe

n-ZnxMg1-xSySe1-y

p-ZnxMg1-xSySe1-y

In electrode

Au electrode

p-ZnS1-xSex

n-ZnS1-xSex

2-5 µm Cladding

2-5 µm

1-2 µm Guiding

42

Inter-band excitation of Cr2+ and Fe2+ in bulk II-VI semiconductors

e

h

(Cr2+)* (Cr2+)*

D

A

Cr2+

Cr+ (Cr2+)*

h

e a)

D

A

e

h

b)

e

h

h

Cr2+

Cr+ (Cr2+)*

h

c) d)

e) direct excitation of the Cr2+ centers in ZnSe by the impact of hot carriers, or through impact ionization

43

Lasing of Cr2+:ZnSe via Ionization Transitions (ASSP’05)

μs0 2 4 6 8

mV

0

5

10

15

20

25

0 2 4

Pump Pulse

Lasing from 532nm pumping

Lasing from 1560nm pumping

μs

Build up time

Lasing from 532nm pumping

Pump Energy, mJ5 10 15

Lasi

ng In

tens

ity, a

b.un

.

0

5

10

15

20

25

wavelength, nm1800 2000 2200 2400 2600 2800 30000

1

Inte

nsity

, ab.

un.

Lasing from 532nm pumping

Luminescence from 532nm pumping

Lasing and Luminescence spectrum under 532nm excitation

Lasing output

intensity

versus pump

energy

ZnSe CB

VB

Eac

Ed

Cr2+/Cr+

Time+ +

hνpump

hνos

Cr2+*

5E

5T2

Cr2+*

44

Mid-IR Electroluminescence of n-Cr:ZnSe (ASSP’06 WB21)Absorption Spectra Al:Cr:ZnSe

K (cm

-1)

0

2

4

6

8

Wavelength (nm)1200 1600 1800 2000

I-V Curves for Cr-Al:ZnSe Samples # 1 & 2

Voltage(V)-80 -60 -40 -20 0 20 40 60 80

Cur

rent

(mA

)

-10-8-6-4-2

246810

Sample#2

Sample#10

~30KΩ

~7.5KΩ

Time (µs)-200 0

Vol

ts

-2

mid-IR optical signal

Electrical pulse

200

-4

Visible Luminescence ofVZn-Al complex

Inte

nsity

, a.u

.

mid-IR Cr2+ electroluminescence

Wavelength (nm)

Inte

nsity

, a.u

.

Wavelength (nm)400 600 800 1800 2200 2600

Cr2+ optical pumping

45

Fe2+:ZnSe problemIt was believed that because of the small energy gap , thermally activated

multiphonon quenching in Fe:ZnSe mandates cryogenic laser operation.

PL lifetime vs temperature (Adams, OL24, 1720 (1999)

Possible solution – gain switched regime of operation with pump pulse duration shorter than Fe2+ lifetime at 300K

46

Absorption spectra of Absorption spectra of Fe:ZnSeFe:ZnSe at RT (i) and T=20K (iii), at RT (i) and T=20K (iii), Fe:Cr:ZnSeFe:Cr:ZnSe (ii) at RT(ii) at RT

wavelength, nm

2000 3000 4000 5000

abso

rptio

n cm

-1

0

2

4

6

8

10

12

14

(ii)

(i)

(iii)

Doping of ZnSe samples was achieved by inserting ZnSe crystals together with FeSe or CrSe powder into quartz ampoules that were evacuated and sealed under 10-4 Torrpressure. The sealed ampoules were then placed in a furnace and annealed at 820-1120°C for 5-14 days. Once removed from the furnace and cooled, doped crystals were extracted from the ampoules and polished.

47

Room temperature emission of Room temperature emission of Fe:ZnSeFe:ZnSe and and Fe:Cr:ZnSeFe:Cr:ZnSe crystalscrystals

A

wavelength, nm

3500 4000 4500 5000

Inte

nsity

( W

xnm

-1 x

m-2

x sr

ad-1

)

CO2 absorption

(i)

(ii)

Room temperature emission of Fe:ZnSe crystal under 2.92 μm excitation ((i) Epump= 2 mJ, (ii) Epump=5 mJ

wavelength, nm

2000 3000 4000 5000

x35

iviii B

Room temperature emission of Fe:Cr:ZnSe (iii) and Cr:ZnSe (iv) crystal under 1.56 μm excitation.

48

Excitation of FeExcitation of Fe2+2+ over Ionization Transitionsover Ionization Transitions

P.B. Klein,J.E.Furneaux, R.L.Henry, “Laser Oscillation at 3.53μm from Fe2+ in n:InP:Fe”App.Phys.Let. 42, 638 (1983)

107K 532nm pump

100000

150000

200000

250000

300000

350000

400000

3500 4000 4500 5000

Wavelength (nm)

Counts

ZnSe CB

VB

Fe2+/Fe3+

hνos

5T2 hνos

-

Fe2+*532 nm

5E

49

Room temperature absorption and emission crossRoom temperature absorption and emission cross--sections of sections of Fe:ZnSeFe:ZnSe crystalcrystal

Room Temperature Absorption (i) and Emission cross-sections (ii) and (iii). Emission cross-section calculated using reciprocity method for Zu/Zl=1.5 (ii) and Fuchtbauer-Landenburg equation (iii)

wavelength, nm2000 3000 4000 5000

Cro

ssse

ctio

n x1

018 c

m2 ,

0

1

2

3

(i)(ii)

(iii)

Emission cross sections at room temperature are determined using either the Fuchtbauer-Landenburg(FL) equation

( ) ( )( )∫

=λλλτπ

λλλσdIcn

I

radem 2

5

8

( )∫ −=

λνσλπτ

dcngg

abl

urad 42

18

1

( ) ( ) [ ]( )kThEexpZZ

zlu

labem ννσνσ −=

the reciprocity method (RM)

( )∑ −=j

jju kTEexpgZ

( )∑ −=i

iil kTEexpgZ

50

Luminescence lifetime versus temperature for Luminescence lifetime versus temperature for Fe:ZnSeFe:ZnSe crystalscrystals

*J. Adams, C. Bibeau, R. H. Page, D. M. Krol, L. H. Furu, and S. A. Payne, “4.0-4.5μm lasing of Fe:ZnSe below 180 K, a new mid-infrared laser material,” Opt. Lett. 24, 1720 (1999).

Luminescence lifetime versus temperature for Fe:ZnSecrystals. (◊)- experimental data according to [*]; temperature quenching stage was fitted by strong (ii) and weak (i) electron-phonon coupling.

μS

)exp( kTEWW aa Δ=

strong electron-phonon coupling

τ (RT)= 0.3-0.7 μs

The best fit was obtained with parameters ΔEa=1100-1200 cm-1 and 1/Wa=5ns.

Temperature, K

0 100 200 300

Life

time,

μs

0.1

1

10

100

51

Experimental setExperimental set--up for up for Fe:ZnSeFe:ZnSe RT laser experimentsRT laser experiments..

The 2nd Harmonic , 1st Stokes, 2nd Stokes outputs of the Nd3+:YAG laser were used as a pump source for laser experiments. The pump radiation at 2.92 μm with a beam diameter of 1.5mm excited the Fe2+:ZnSe crystal at the Brewster angle of incidence. The crystal temperature remained 300K during all laser measurements.

τ= 5ns

CaF2

Detector

Spectrometer Fe2+:ZnSe

G

A

M2

P

M1 3.8-4.9 μm θBR

λ1=0.53 μm

λ2=1.56 μm λ3=2.92 μm

M3

52

RT RT Fe:ZnSeFe:ZnSe Lasing in Nonselective cavityLasing in Nonselective cavity

B

Pump Energy, mJ/cm2

50 100 150Si

gnal

, a.u

.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4Ai - 40mJ/cm2

ii -110 mJ/cm2

iii-170 mJ/cm2

wavelength, nm

3750 4000 4250 4500 4750 5000

Sign

al, a

.u.

i

ii

iii

Emission spectra of Fe:ZnSe crystal versus pump density; B- Output of RT gain-switched Fe:ZnSe lasing in nonselective cavity versus pump

density

53

RT RT Fe:ZnSeFe:ZnSe Lasing in Lasing in LittrowLittrow Cavity Cavity

wavelength, nm

3800 4000 4200 4400 4600 4800

Lase

r out

put ,

a.u

.

0.0

0.2

0.4

0.6

0.8

1.0

i

ii

Tuning curve of RT gain-switched Fe:ZnSe laser (i) and example of oscillation spectrum at 4490 nm (ii)

54

Potential Commercial Applications• Non-invasive medical diagnostics • Optical coherence tomography• Multi-analyte eye-safe laser radar and remote

sensing of atmospheric constituents• Multi-analyte species-specific gas monitoring in

production facilities• High density free space optical communication• Information coding • Counter-terrorism applications• Military applications, e.g., target designation,

obstacle avoidance, and infrared countermeasures

55

ConclusionsRecent progress in transition metal doped II-VI semiconductor materials (mainly Cr2+:ZnSe and ZnS) make them the laser sources of choice when one needs a compact system with continuous tunability over 2-3.1 μm, output powers up to 2W, and high (up to 70%) conversion efficiency. The unique combination of technological (low-cost ceramic material) and spectroscopic characteristics (ultrabroadband gain bandwidth, high στ product and high absorption coefficients) make these materials ideal candidates for “non-traditional” regimes of operation such as microchip and multi-line lasing. Emerging Fe2+:ZnSe lasers having potential to operate at room temperature over the spectral range extended to 3.7-5.1 μm are described.Cr and Fe doped II-VI lasers combine the versatility of the ion-doped solid-state lasers with the engineering capabilities of semiconductor lasers, paving the route to the future electrically pumped ultrabroad-band solid-state lasers. This work shows the initial steps towards achieving this goal by studying Cr2+ ion excitation into the upper laser state 5E via photoionization transitions as well as via direct electrical excitation. .

56

Future OutlookFurther improvements are possible with Fe:ZnSe crystal optimization. Cr2+ lasers have come of age and are ready for real-world applications. Now it is Fe2+ lasers turn to reach the level of performance necessary for practical applications.

There are two different fiber-bulk hybrid systems. First utilizes approach of direct fiber laser pumping of the 4I13/2 manifold of Er or 5I7 of Ho featuring long fluorescence lifetime of ~10 ms and provides access to the high energy storage capability of Er and Ho and effectively converts CW radiation of fiber laser into high energy Q-switched radiation. Second type of fiber-bulk hybrid system (Er or Tm fiber-TM:II-VI bulk) provides effective conversion of monochromatic fiber laser radiation into mid-IR broadly tunable narrowline radiation.

Future combination of two type of fiber-bulk hybrid systems one as pump and another as single frequency seed of the optical parametric generator wil result in a high-power, ultra-broadly and rapidly tunable mid-IR source capable of analyzing a complex gas mixtures with a full chemicalspecificity, with ultimate sensitivity, and with highest (real time) speed.