Mid-IR Pulse Generation Using Cr2+:ZnSe

Editor's Notes

• #2: Thank you for having me speak on the properties of chromium doped zinc selenide that make fast pulse generation possible.I hope to illuminate some of the potential benefits of the material as well as important differences from other ultrafast source materials.
• #3: I will first explain why ultrafast mid-IR sources are of interest,Then I will give a brief review of the propertiesof Kerr lens mode-locked ultrafast laser sources.Then I’ll move on to describe the spectroscopic, chemical and physical material mechanisms that make ultrafast pulse generation possible.It will then be useful to show how these processes combine in titanium doped sapphire to create ultrafast pulses;and how spectroscopic differences manifest as pulse differences in other materials, for example chromium doped zinc selenide.
• #4: Identify and quantify molecules absorbing in this regionNon-intrusive medical diagnosticsnon-destructiveIndustrial process controlEnvironmental monitoring/atmospheric sensingWavelength longer than 3mm needed, many organic compounds have vibronic transitions in this region. Cr:CdSe goes to 3.6Free space communicationOil prospectingInfrared countermeasures (DOD)Monitoring of munitions disposal (DOD)Stand-off detection of explosion hazards (DOD)[DeLoach et al, IEEE J. Quantum Electron. 32, 6 (1996).]Direct & overtone spectroscopyMolecular fingerprint region<<WHY ARE SHORT PULSES NECESSARY? HIGHER RESOLUTION?>><<ANY APPLICATIONS THAT ARE NOT CHEMICAL?>>
• #5: Cr:CdSe (2.26-3.61)Fe:ZnS (3.49-4.65)
• #6: There are three key optical requirements to generate short pulses using Kerr lens mode locking:a broad optical bandwidthhigh pulse intensityand laser cavity stability.
• #7: We have seen this image a few times in this class, but here it is again.Broad bandwidth is required to localize the pulse by interference.Constructive interference must be present between frequencies at one time to create a pulseDestructive interference must occur to minimize noise outside of the pulse.To create maximum constructive interference, each frequency must be in phase at one time—this is called “mode locking”.(add x axis)
• #8: These short length, high intensity pulses can be preferentially selected using the Kerr lens effect, described by this equation.The Kerr lens effect describes the different refractive index of a material experienced by high intensity light:Because the zeroth-order term is significantly larger than the second-order termThe refractive index is dependent on optical intensity,More intense light will be focused into the laser cavityLess intense light will be rejected.n2 = 1.2ee-18 m2/W [Cizmeciyan et al, apphys b 106 (2012).
• #9: This figure shows the use of the Kerr lens effect in selecting pulses.The Kerr lens effect can be used to preferentially select high intensity pulses and exclude continuous, random phase light.This process is called “Kerr lens mode-locking.”To use Kerr lens mode-locking, the laser source materials must withstand these high intensity pulses, so they must be very stable
• #10: This schematic shows a laser cavity supporting several frequencies as is necessary to generate ultrafast pulses.Because the cavity must be properly aligned to stimulate emission of pulses and mode lock the different frequencies, the lasing material must be stable under conditions of operation. This is especially challenging because the circulating pulse has very high intensity, making damage prevention important.
• #11: With bandwidth, intensity and cavity stability in mind, we will now move into the material properties that make pulse generation possible.
• #12: Looking at laser sources in this figure, there are a lot to choose from.Line sources like Ruby would not be able to generate short pulses because they do not emit at a wide enough range of frequencies.
• #13: While the industry standard, Ti:sapph, emits at a range of 460 nm. What mechanism allows Ti:sapph to support such a broad bandwidth?Looking at all of the broad band sources, about 30% of the materials are ion doped crystals. These crystals are good candidates for ultrafast pulse generation because they canparticipate in Kerr lens mode-locking and withstand high intensity pulses better than their dye and gas counterparts.
• #14: This figure shows the structure of an ion doped crystal. An ion doped crystal consists of:an emitting active ion—like chromium, titanium, or iron—suspended in the host material—anda transparent host crystal—like sapphire, borosilicate, zinc selenide—corresponding to the emission wavelength.
• #15: This figure shows a standard, four level laser.The electronic orbitals that are available within the active ion determine at which frequencies the material will absorb and emit radiation. To emit a wide bandwidth of different frequencies, a range of different energy states must be present at the first and second energy levels.
• #16: This figure shows the D orbitals of hydrogen. In the active ion, just like in hydrogen, the first and second laser levels also have multiple orbitals of the same energy. These are called degenerate orbitals. Like in hydrogen, these orbitals all have different shapes.If those orbitals are subjected to an electric field, they will experience different changes in potential energy.This will cause the energy degenerate orbitals to split into orbitals of different energy.----- Meeting Notes (5/14/13 13:45) -----The stark effect can be used to increase these energy levels by splitting degenerate energy levels.
• #17: Combining all of these effects in our doped crystal, the crystal host acts as an effective electric field.The first and second levels of the active ionare split into multiple energy levelswhich will have many possible emission energiesleading to a broad distribution of frequencies of emitted light and a broad bandwidth
• #18: <<second order non linearity for ZnSe (pV/m) = 30>>
• #19: Quantum efficiency is effected by these processes and is a good indicator of the total<<STIUMLATION/DIPOLE TRANSITION STATE COUPLING?>>
• #20: Low maximum phonon frequency: decreases non-radiative decay rate and increases quantum yield  what is the mechanism for that?Energy migration due to (a) resonant transitions (b) self-quenching by cross-relaxation and (c) self-quenching by up-conversion. These processes are nearly absent in Cr2+:ZnSe.“Another important advance is reflected by numerous articles on processes between excited state levels as can be seen in Fig. 6 in which migration due to resonant transitions, self-quenching by cross-relaxation, and self-quenching by up-conversion mechanisms are shown. This recent discovery of the infrared tunability seen in both Fe2+-doped ZnSe … is the consequence of such advance in the infrared range with selenides … .”
• #21: Stability is a combination of minimizing perturbations to the system during use and preventing damage of the components.
• #22: Small changes to the system during use are mostly due to temperature change.Prevent thermal changes to the refractive index of the crystal (dn/dT) that cause thermal lensing and would change optical path length<<Photobleaching?>>
• #24: How does chromium doped zinc selenide compare to industry standard?
• #25: Note that this range in wavelength is not a wide range in frequency: DO THE MATH (~1/10th the frequency bandwidth)Bandgap for ZnSe is 2.8 eVTetrehedral coordination of the active ions gives ~twice smaller crystal field splittingSapphire is octrahedral<<strong electron-phonon coupling  significant broadening?>>
• #26: 1.973-3.339 mm reported by Mirov et al1.880-3.349 mm reported by cizmeciyan
• #27: Vibrational lifetimes are ~100fs (except in the gas phase)Heavy anions provide low energy optical phonon cut-off, non-radiative decay via phonons is less likely and gives high fluorescence yieldMax phonon for ZnSe is 250 cm-1
• #28: 18 is a reasonably high thermal conductivity and can be workable.70 is a relatively high thermal lensing parameter, does need to be given some attention by proper thermal management but can generally be compensated for with low thermal load due to absence of excited state absorption and up-conversion in ZnSe, as illustrated by optical quantum efficiency
• #29: Expensive optics- Dispersion compensation using CaF2 and MgF2, slabs of BK7 and YAG
• #30: DopingDuring growth (physical vapor transport) [Su et al j. cryst. Growth 207, 1-2 (1999).; Rai et al. J applphys 83, 11 (1998); Korostelin et al j alloy comp 371, 25-30 (2004)]Progress in PVT, but never better than post growth diffusionDifficult to achieve homogenous impurity distribution with predictable concentration, especially at high doping conc.After growth by diffusion8.35ee18 ion/mLMetal film deposited on the surface900-1100 deg. C, 7-10 daysCr/Fe or vapor phase annealingSeal metal and substrate in different parts of low pressure ampouleHeat metal to ionizeDiffuse into substrateDrawbacksDifficult to fabricate crystals with exact dopant levelsNon-uniform dopingSublattice sublimationPoor repeatabilityMirov et al opt. mat. Exp 1,5 (2011) reports“Uniform” doping at 7mm depthLow scattering loss 1-2%/cmZnSe crystalstwin-type stacking faults (interlayers of hexagonal phase in the cubic matrix) 0-2% of the crystalNo twins observed in Cr2+ doped crystals at minimum concentration of 3ee19 /mL [Kulakov et al, Neorg. Mater. 12, 10 (1976).Cz growth requires high T & P, not commercially viableBridgman growth—melt & cool with seed crystal—sometimes has uncontrolled contamination & leads to parasitic absorptions.
• #31: Thank you for your time.
• #32: Hosts are chosen to transmit the stimulated emission, not to emit, so they need to have a wide range of transmittance in the region of stimulated emission. The electronic transitions that lead to absorption and emission take place within the active ion.<<Outside of the transmission region, absorption is usually dissipated as phonon vibration in the host rather than being reemited. Some resonant transitions may exist, like the peaks in borosilicate.>>----- Meeting Notes (5/14/13 13:45) -----Pause for slides, introduce graphs.