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.
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