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