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MID-IR PULSE
GENERATION
USING Cr2+
:ZnSe
CHELSEY CROSSE
DPT. OF CHEMISTRY | COLORADO STATE
UNIVERSITY
ECE 503 | ULTRAFAST OPTICS
MAY 16, 2013
0
OUTLINE
• Applications of mid-IR pulse generation
• Review of optical properties
• Material properties
• Comparison of Ti3+:Al2O3 and Cr2+:ZnSe
• Benefits and considerations
1
APPLICATIONS
• mid-IR frequency combs
• Non-invasive medical diagnosis
• Non-destructive chemical probing
• Free space communication
• Environmental/atmospheric sensing
• Access to important spectroscopic regions through
OPA/OPO
2
DeLoach, Page, Wilke, Payne, Krupke. IEEE J. Quant. Elec. 32, 6 (1996).
Common stretching modes in the IR region.
3
IR SPECTROSCOPIC REGIONS
Hynes et. al. BMC Medical Imaging 5, 2 (2005).
2.5Wavenumber (/cm3)
Wavelength (mm)
3.3 5 10
ULTRAFAST PULSE
GENERATION
• BROAD STIMULATED EMISSION (SE)
BANDWIDTH
• HIGH INTENSITY
• STABILITY
4
Schematic of interference of different wavelengths to produce a pulse.
Gauthier and Boyd. “Fast Light, Slow Light and Optical Precursors: What Does It All Mean?” Photonics Spectra (2007). 5
BROAD SE BANDWIDTH
t
KERR LENS EFFECT
n = n0 + n2 I
• n0 >> n2
• Requires high intensity
6
Bartels. “Fundamentals of Lasers” 7
HIGH INTENSITY
Schematic of pulse selection by Kerr lens mode-locking.
n = n0 + n2 I
Schematic of laser cavity supporting pulse generation.
8
STABILITY
Lambda Photometrics <http://www.lambdaphoto.co.uk/press_releases/200689>.
circulating pulse
supported modes
high reflector partial reflector
output pulse
BROAD SE BANDWIDTH
MATERIAL PROPERTIES
FOR
9
Emission bandwidths of common laser materials.
10
STIMULATED EMISSION BANDWIDTHS
Weber Handbook of laser wavelengths, CRC Press (1999) http://en.wikipedia.org/wiki/File:Commercial_laser_lines.svg.
Emission bandwidths of a variety of laser materials.
11
STIMULATED EMISSION BANDWIDTHS
Schematic of a host crystal with two different active ion dopants.
12
ANATOMY of an ION DOPED CRYSTAL
Yoshida. “Process for producing a heavily nitrogen doped ZnSe crystal.” US Patent 5891243. Feb 11, 1998.
Active Ions
Host Crystal
Schematic of four level laser.
STIMULATED EMISSION
13
??
Absorption
Emission
Non-Radiative
Relaxation
0
1
2
3
Schematic of splitting of electronic energy levels in an electronic field.
14
STARK EFFECT SPLITTING
Ele
ctr
onic
Fie
ld
Courtney, Spellmeyer, Jiao, Kleppner Phys Rev A 51 (1995).
Example of SE energy level splitting of Er3+ crystal
15
CRYSTAL FIELD SPLITTING
??
0
1
2
3
2b
1c
2d
2a
1a
1b
2c
1d
INTENSITY
MATERIAL PROPERTIES
FOR
16
INTENSITY
INCREASED BY: DECREASED BY:
• Thermal relaxation
• Phonon relaxation
• Non-radiative
transitions
17
• Mode locking
• Dopant concentration
• Emission probability
RESULT:
• quantum efficiency
• stimulated emission cross section
Schematic of non-radiative transitions between neighboring ions.
18
NON-RADIATIVE TRANSITIONS
Boulon. Optical Materials. 34 (2012).
Resonant Transitions Cross-Relaxation Up-Conversion
STABILITY
MATERIAL PROPERTIES
FOR
19
• thermal lensing
• system
complexity
Malacarne, Astrath, Baesso. Journal of the Optical Society of America B. 29, 7 (2012). 20
PERTURBATIONS
DAMAGE
• Thermal damage
• high thermal conductance
• mechanically stable at high temperatures
• chemically stable at high temperatures
• Photo-reactivity
• chemically stable under exposure to high intensity light
21
MATERIAL PROPERTIES
FOR
• BROAD SE BANDWIDTH
• HIGH INTENSITY
• quantum efficiency
• stimulated emission cross section
• STABILITY
• Perturbations
• Damage
22
Cr2+
:ZnSe
23
Log scale gain spectrum of Cr2+:ZnSe and Ti3+:Sapphire
24
SE BANDWIDTH
Boulon. Optical Materials. 34 (2012).
SE BANDWIDTH
1Boulon. Optical Materials. 34 (2012).2Wagner, Carrig, Page, Schaffers, Ndap, Ma, Burger. Opt. Lett. 24, 19 (1999).3Cizmeciyan. App. Phys B 106 (2012). 2
5
Cr:ZnSe Ti:Sapphire
Mode-locked
pulse duration
~4ps [1]
92 fs [3]
18 fs [2] theoretical
5 fs
Mode-locked
output power
80-400 mW >1 W
Cr:ZnSe Ti:Sapphire
Peak emission
cross-section 90 39
Peak
absorption
cross-section
87-110 6.5
Optical
quantum
efficiency
63-71% 40%
• quantum
efficiency
• stimulated
emission cross
section
26
HIGH INTENSITY
Boulon. Optical Materials. 34 (2012).
ZnSe Al2O3
Thermal conductivity
(W/m K)18[1]
19[2] 27
dn/dT (10-6/K)[1]
70 12
(1/n)(dn/dT) (10-6/K)[2]
26 6.8
PERTURBATIONS
• THERMAL LENSING
• SYSTEM
COMPLEXITY
DAMAGE
1Boulon. Optical Materials. 34 (2012).2Mirov, Fedorov, Martyshkin, Moskalev, Mirov, Gapontsev. Opt. Mat. Exp. 1, 5 (2011).
27
STABILITY
Optical quantum
efficiency[1] 63-71% 40%
CHALLENGES
• Still under development
• Expensive optics
• Pulses are not near the theoretical limit yet
• Cr2+:ZnSe is difficult to manufacture
28
Using post-growth thermal
diffusion:
• Difficult to predict final
dopant levels
• Dopant is not
homogeneous
• Host crystal sublimation
• Poor repeatability
Recent results:
• “uniform” doping at 7mm
depth
• Scattering loss of 1-
2 %/cm
29
MANUFACTURING
3.3. Thermal diffusion
The diffusion of the TM ions into II-VI semiconductors has been studied for more than 60
years (e.g., [34].). This technique utilizes thermally activated diffusion of transition metal
ions into II-VI crystals. Thermal diffusion is usually realized from the TM metal film
deposited on the crystal surface or from the vapor phase. In the first case, Cr or Fe films are
deposited on the crystal surface, using pulsed laser deposition, thermal deposition, or
magnetron spattering. At the second stage, thermal diffusion is carried out in sealed
vacuumed (~10-5
Torr) ampoules at temperature of 900-1100°C over 7-20 days. In the vapor
phase diffusion method, II-VI samples together with TM (Cr, Fe, Co, Ni) or TM compounds
(CrS, CrSe, FeSe) are placed in the different parts of the ampoules. The ampoules are sealed
at low pressure and annealed. The specific details of the thermal-diffusion process are
reported in [35–37].
Thermal diffusion method in comparison with crystal growth is very cost-effective,
simple, and has been used quite extensively. Its main drawbacks include qualitative nature of
doping (hard to fabricate crystals with a pre-assigned concentration of dopant), non-uniform
doping, large concentration gradients, degradation of optical quality of the crystals due to
sublimation of Zn and Se sub-lattices, and, finally, the procedure has poor repeatability.
Therefore, preparation of the large-scale samples with homogeneous TM ions distribution and
low optical losses requires special technological arrangements. Scientists from the University
of Alabama at Birmingham in collaboration with the IPG Photonics Corporation solved these
issues and developed commercial, quantitative (accuracy of the pre-assigned concentration of
dopant is better than 3%) and fast thermo-diffusion process of TM ions in II-VI polycrystals
with suppressed sublimation in Zn/Cd and Se/S sublattices [15]. In result, the fabricated
crystals are uniformly doped through the thickness of up to 7 mm (see Fig. 2) and feature a
low scattering loss of 1-2% per cm for samples with Cr concentration of 5x1018
cm-3
.
Consistently high optical quality of fabricated thermo-diffusion doped Cr:ZnSe/S and
Fe:ZnSe polycrystals with low depolarization factor enabled the first polycrystalline based
Cr:ZnSe femtosecond oscillator [38], ultra-broad tunability (1973 – 3339 and 1962 – 3195 nm
for CW lasers based on polycrystalline Cr:ZnSe and Cr:ZnS, respectively [39]), and highest
up-to-date output characteristics in CW (13W Cr:ZnSe [40], 10W Cr:ZnS [41]) and gain-
switched regimes of operation (20mJ Cr:ZnSe [42], 4.7 mJ Fe:ZnSe [43]).
Fig. 2. Thermo-diffusion doped Cr:ZnSe and Cr:ZnS crystals. Cr:ZnS crystals with undoped
ends were fabricated by post-growth directed diffusion of active ions in the crystal.
3.4. Hot press TM doped II-VI ceramics
The major advantage of laser ceramics is in advanced ceramic processing enabling affordable
mass production and design flexibility of the laser elements (undoped ends, waveguiding
#147996 - $15.00 USD Received 23 May 2011; revised 19 Jul 2011; accepted 30 Jul 2011; published 11 Aug 2011
(C) 2011 OSA 1 September 2011 / Vol. 1, No. 5 / OPTICAL MATERIALS EXPRESS 905
3.3. Thermal diffusion
The diffusion of the TM ions into II-VI semiconductors has been studied for more than 60
years (e.g., [34].). This technique utilizes thermally activated diffusion of transition metal
ions into II-VI crystals. Thermal diffusion is usually realized from the TM metal film
deposited on the crystal surface or from the vapor phase. In the first case, Cr or Fe films are
deposited on the crystal surface, using pulsed laser deposition, thermal deposition, or
magnetron spattering. At the second stage, thermal diffusion is carried out in sealed
vacuumed (~10-5
Torr) ampoules at temperature of 900-1100°C over 7-20 days. In the vapor
phase diffusion method, II-VI samples together with TM (Cr, Fe, Co, Ni) or TM compounds
(CrS, CrSe, FeSe) are placed in the different parts of the ampoules. The ampoules are sealed
at low pressure and annealed. The specific details of the thermal-diffusion process are
reported in [35–37].
Thermal diffusion method in comparison with crystal growth is very cost-effective,
simple, and has been used quite extensively. Its main drawbacks include qualitative nature of
doping (hard to fabricate crystals with a pre-assigned concentration of dopant), non-uniform
doping, large concentration gradients, degradation of optical quality of the crystals due to
sublimation of Zn and Se sub-lattices, and, finally, the procedure has poor repeatability.
Therefore, preparation of the large-scale samples with homogeneous TM ions distribution and
low optical losses requires special technological arrangements. Scientists from the University
of Alabama at Birmingham in collaboration with the IPG Photonics Corporation solved these
issues and developed commercial, quantitative (accuracy of the pre-assigned concentration of
dopant is better than 3%) and fast thermo-diffusion process of TM ions in II-VI polycrystals
with suppressed sublimation in Zn/Cd and Se/S sublattices [15]. In result, the fabricated
crystals are uniformly doped through the thickness of up to 7 mm (see Fig. 2) and feature a
low scattering loss of 1-2% per cm for samples with Cr concentration of 5x1018
cm-3
.
Consistently high optical quality of fabricated thermo-diffusion doped Cr:ZnSe/S and
Fe:ZnSe polycrystals with low depolarization factor enabled the first polycrystalline based
Cr:ZnSe femtosecond oscillator [38], ultra-broad tunability (1973 – 3339 and 1962 – 3195 nm
for CW lasers based on polycrystalline Cr:ZnSe and Cr:ZnS, respectively [39]), and highest
up-to-date output characteristics in CW (13W Cr:ZnSe [40], 10W Cr:ZnS [41]) and gain-
switched regimes of operation (20mJ Cr:ZnSe [42], 4.7 mJ Fe:ZnSe [43]).
Fig. 2. Thermo-diffusion doped Cr:ZnSe and Cr:ZnS crystals. Cr:ZnS crystals with undoped
ends were fabricated by post-growth directed diffusion of active ions in the crystal.
3.4. Hot press TM doped II-VI ceramics
The major advantage of laser ceramics is in advanced ceramic processing enabling affordable
mass production and design flexibility of the laser elements (undoped ends, waveguiding
#147996 - $15.00 USD Received 23 May 2011; revised 19 Jul 2011; accepted 30 Jul 2011; published 11 Aug 2011
(C) 2011 OSA 1 September 2011 / Vol. 1, No. 5 / OPTICAL MATERIALS EXPRESS 905
Mirov, Fedorov, Martyshkin, Moskalev, Mirov, Gapontsev. Opt. Mat. Exp. 1, 5 (2011).
30