mid-IR Lasing Research Articles

Mid-IR lasing in HgCdTe multiple quantum well edge-emitting ridges.

Thin HgCdTe/CdHgTe quantum wells (QWs) grown on alternative GaAs (013) substrates have been recently proposed as a material for coherent emitters in the mid-IR region. In this work, we develop a technological process for the fabrication of ridge microresonators in waveguide heterostructures with multiple HgCdTe QWs via photolithography and ion etching. We process two samples with different ridge heights and analyze their emission spectra measured under optical excitation. The width of the emission spectra dropped by an order of magnitude compared to the nonprocessed as-grown structure, allowing one to conclude that lasing at 9.2-7.1µm takes place within the 8-120K temperature range. However, both samples demonstrated faster temperature quenching of lasing than that of the single-pass stimulated emission from the nonprocessed structure, as well as a drop in the carrier lifetimes. These figures of merit are likely to be compromised not by the Q factor of the cavities, but due to defects induced during the etching process. Finally, the implications for HgCdTe-based lasers for the 3-5µm transparency window and longer wavelengths (beyond 20µm) are discussed.

35 W highly effective Ytterbium-Erbium-Thulium tandem all-fiber 1.947 µm laser system at 975 nm diode pumping

We report on progress towards effective mid-IR lasing using a tandem Ytterbium-Erbium/Thulium all-fiber laser system at near-IR diode pumping. ∼35 W 1.947 µm continuous-wave output power at ∼120 W 975 nm pump power (29% overall efficiency) is reached.

Optical, electrical, and EPR studies of polycrystalline Al:Cr:ZnSe gain elements

Transition metal-doped II-VI (TM:II-VI) chalcogenides are well-known laser materials for optically pumped middle-infrared lasers. Cr:ZnSe is a key representative of this class of transition metal doped II-VI gain media and is arguably considered the material of choice for optically pumped middle-infrared lasers. In addition to effective mid-IR lasing under optical excitation, these crystals, being wide-band semiconductors, hold the potential for direct electrical excitation. One way to form n-type conductivity in ZnSe crystals is by annealing them in a melt of Zn-Al alloy. However, this annealing of Cr:ZnSe crystals results in their purification and transfer of chromium to the melt of Zn-Al alloy. In this article, we report on optimizing the doping technique for providing n-type conductivity in Al:Cr:ZnSe crystals while preserving the chromium concentration. Al:Cr:ZnSe samples with resistivities ranging from 10.8 to 992 Ω-cm were fabricated. While the 2 + valence state of Cr is typically dominant in Cr:ZnSe, both Cr2+ and Cr+ were detected in Al:Cr:ZnSe samples. The maximum level of Cr+ concentration was measured to be 4 × 1018 cm-3.

Atomic Layer Deposition (ALD) of Ultra-Thin Diffusion Barriers on ZnSe Microparticles for Phase Stability in Chalcogenide Glasses for Mid-Infrared Optics

ZnSe embedded chalcogenide glass (ChG) composites are optically active in the mid-infrared (IR) and are ideal for niche applications in medical diagnosis, military missile heat-sensing, and chemical identification. The embedded ZnSe crystals play a critical role in providing lasing activity while the ChG - As2S3-xSex matrix, acts as a host that can be drawn into fiber and provides the necessary mid-IR transparency. However, during synthesis, the ZnSe particles are prone to dissolution in the As2S3-xSex matrix due to a high temperature (650 oC) melting and homogenization process. Here we demonstrate that a thin amorphous barrier layer of Al2O3 deposited using atomic layer deposition (ALD) on ZnSe microparticles 5-10 mm in average diameter is successful to prevent ZnSe dissolution in the As2S3-xSex matrix even after extended melting and homogenization at 650 oC for 8 hours. This talk will highlight some of the key process innovations required to overcome challenges of ALD on powdered components for functionalizing and enhancing performance of multiphasic glasses for optical applications.Amorphous aluminum oxide (Al2O3) was deposited using alternate pulses of trimethyl aluminum and H2O on 1 gm batch size ZnSe using a customized rotary barrel reactor inside a viscous-flow ALD furnace attached to a quadrupole mass spectrometer (QMS). The temperature of deposition was kept at 180 oC and the number of cycles were varied from 100 to 300 cycles. The ZnSe particles were sieved to an average size of 5-10 mm and a maximum size ≤ 20 mm. The coated particles were characterized by x-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The crystallinity of the ZnSe-As2S3-xSex glasses were evaluated using x-ray diffraction (XRD) and their composition stability spatially mapped across individual ZnSe particles embedded in As2S3-xSex matrix using Raman microspectroscopy.Whereas 100 ALD cycles provide partial protection to dissolution for the ZnSe in As2S3-xSex, a 300 cycle ALD Al2O3 on ZnSe particles is sufficient to provide phase and composition stability to the ZnSe embedded As2S3-xSex glass. Through our investigation, we exemplify the efficacy and potential of ALD of ZnSe powder in improving the stability of optical materials while still achieving optimal mid-IR lasing and transparency.

Mid-infrared tunable pulsed laser based on Cr2+-doped II–VI chalcogenide

Cr2+-doped II–VI chalcogenides are attractive laser media for direct access to the mid-infrared (IR) region from 2 to 3 μm. We have developed a mid-IR tunable laser using Cr:ZnSe and Cr:CdSe as laser media. We have used a Q-switched Tm:YAG laser as a pump source. For the Cr:ZnSe laser, the tuning range of 2.18–2.83 μm and the maximum pulse energy of 4.98 mJ at 2.38 μm were obtained with a pump energy of 22 mJ. For the Cr:CdSe laser, the tuning range of 2.22–3.04 μm and the maximum pulse energy of 2.42 mJ at 2.54 μm were achieved with a pump energy of 22 mJ. In addition, we have demonstrated a method of extending the mid-IR lasing region by a combined active medium technique with Cr:ZnSe and Cr:CdSe. We observed the broad tuning range from 2.15 to 3.04 μm and the maximum pulse energy of 5.96 mJ at 2.44 μm with a pump energy of 22 mJ.

Femtosecond filament-driven mid-IR lasing under plasma–chemical reaction in high-pressure mixture SF6 -H2 –Ar

A scheme of plasma–chemical mid-IR hydrogen fluoride (HF) lasing is proposed, in which electrons of femtosecond filament are produced in argon and give rise to a chain of plasma–chemical reactions occurring with SF6 and H2 added to the argon bath and resulting in the formation of vibrationally excited HF molecules. The conditions for the occurrence of amplified spontaneous emission and lasing in the range of 2.7–3 μm in a selected gas mixture at a total pressure of several atmospheres can be realized under the action of a femtosecond laser on Cr-forsterite with a wavelength of 1.24 μm. In numerical experiments, it was demonstrated that HF lasing energy at the level of 1.5 μJ are achievable for typical monofilament plasma parameters.

2.92-µm high-efficiency continuous-wave laser operation of diode-pumped Er:YAP crystal at room temperature.

Mid-infrared lasers have attracted attention for application to the fields of medicine and industry. In this study, we demonstrate continuous-wave laser operation of a diode-pumped 5 at % Er-doped YAlO3 (YAP) single-crystal lasing at 2.92 µm with near-quantum-defect slope efficiency at room temperature. A high slope efficiency of 31% is achieved with a maximum output power of 0.674 W for a cavity length of 18 mm and an output coupler transmittance of 2.5%. This efficiency is 94% of the theoretical quantum-defect efficiency. Our results indicate that Er:YAP lasers can potentially be utilized to realize high-power mid-IR lasing.

Ground state depletion – A step towards mid-IR lasing of doped silver halides

We show for the first time ground state absorption saturation in a doped silver halide crystal (AgClxBr1−x), specifically with cobalt. Spectroscopic studies showed absorption bands in the 1.4–2.5μm region and emission bands in the 3.8–5.0μm region, with a 1.5ms lifetime at low temperatures. Absorption saturation indicates a good low and room temperature lasing feasibility at 4.1μm. In addition, a comparison of cobalt, nickel and iron as dopants is presented. These doped silver halide crystals can be extruded to form optical fibers, possibly introducing a new family of fiber lasers for the middle infrared.

Mid-IR photoluminescence of Fe(2+) and Cr(2+) ions in ZnSe crystal under excitation in charge transfer bands.

Spectroscopic characterization of Fe:ZnSe(Cr:ZnSe) crystals under visible excitation into the charge transfer bands of Transition Metal ions were studied. The excitation efficiencies of mid-IR photoluminescence between (5)T(2)((5)E) and (5)E((5)T(2)) states via direct relaxation to the upper laser levels and via metastable upper (3)T(1) were investigated. It was demonstrated that the latter route is the dominant process for Cr(2+) ions and could provide sufficient pump rate for mid-IR lasing. The pump efficiencies via direct relaxation to the upper laser levels were estimated to be <2% for both ions under 532 nm excitation wavelength.

Progress in Mid-IR Lasers Based on Cr and Fe-Doped II–VI Chalcogenides

Transition metal (TM) doped II-VI chalcogenide laser materials offer a unique blend of physical, spectroscopic, and technological parameters that make them the gain media of choice for cost effective broadly tunable lasing in the Mid-IR. The II-VI semiconductor hosts provide a low phonon cut-off, broad IR transparency, and high thermal conductivity. When doped with transition metal ions, these materials feature ultrabroadband gain, low saturation intensities, and large pump absorption coefficients. This combined with the low-cost mass production technology of crystal fabrication by postgrowth thermal diffusion, as well as broad availability of convenient pump sources, make these materials ideal candidates for broadly tunable mid-IR lasing in CW, gain-switched, free running, and mode-locked regimes of operation. This review summarizes experimental results on optically pumped lasers based on Cr and Fe doped II-VI wide band semiconductors providing access to the 1.9-6 μm spectral range with a high (exceeding 60%) efficiency, multi-Watt-level (18 W in gain switch and 30 W in pure CW) output powers, tunability in excess of 1000 nm, short-pulse (<;50 fs) multi-watt oscillation, multi-Joule long-pulse output energy, and narrow spectral linewidth (<;100 kHz).

Design of Mid-IR $\hbox{Er}^{3+}$-Doped Microsphere Laser

A mid-IR lasing system based on a tapered fiber coupled to an Er3+-doped microsphere has been modeled and numerically investigated. In order to design and optimize the device performance, a dedicated 3-D numerical code exploiting the coupled mode theory and the rate equations model has been developed. The main energy level transitions among the Er3+ ions, the most relevant secondary ion-ion interactions, the amplified spontaneous emission, and the fiber-microsphere coupling phenomena have been taken into account. In order to optimize the lasing performance, several parametric simulations have been carried out. The obtained numerical results show that a laser threshold of about 55 mW and an output power of about -17.8 dBm can be obtained by using small microspheres.

Electrical, spectroscopic, and laser characterization of γ-irradiated transition metal doped II-VI semiconductors

We report a comprehensive study of γ-irradiation on optical, electrical, and laser characteristics of pure and transition-metal doped single and polycrystalline ZnS and ZnSe. Polished pure, Cr-doped, and Ag, Au, Cu, Al, In, and Mn co-doped ZnS and ZnSe crystals after absorption and electro-conductivity characterization were γ-irradiated at doses of 1.28x108 rad at −3°C. Dynamic room temperature absorption studies, electro-conductivity measurements, and mid-IR lasing were performed after different exposition times of crystals at room temperature. Cr:ZnSe and Cr:ZnS lasers based on identical γ-irradiated and non-irradiated crystals featured very similar pump thresholds, slope efficiencies, and output powers. New fluorescence band spanning over 1.3-2.1 μm in the γ-irradiated Au:Cr:ZnS was attributed to 3A2→3T2(F) transition of Cr4+.

Mid-IR lasing of iron–cobalt co-doped ZnS(Se) crystals via Co–Fe energy transfer

We report a spectroscopic characterization of iron–cobalt co-doped ZnS and ZnSe laser crystals over 14–300K temperature range. Mid-IR Fe2+ lasing via Co–Fe energy transfer was demonstrated under cobalt excitation at 4A2–4T1 transition by 1.56μm radiation.

Progress in mid-IR Cr^2+ and Fe^2+ doped II-VI materials and lasers [Invited

Recent progress in fabrication and mid-IR lasing of transition metal doped II-VI single crystal and thermo-diffusion doped polycrystalline and hot-pressed ceramic gain media as well as nano and micro-crystalline laser active powders, powders in the liquid suspension, polymer-film, thin film waveguides and chalcogenides glass composites is reported.

Crystalline Cr^2+:ZnSe/chalcogenide glass composites as active mid-IR materials

We propose new transition metal (TM)-doped ZnSe/As₂S₃:As₂Se₃ composite materials for mid-IR fiber lasers. The composites are the suspension of crystalline micro- and nanosized TM²⁺:ZnSe or TM²⁺:ZnS powders in chalcogenide glasses with the refraction index matching. Mid-IR room-temperature lasing of Cr²⁺:ZnSe/As₂S₃:As₂Se₃ microcomposite material is demonstrated at the 2.4 μm wavelength.

Infrared Emission Properties of Ho Doped KPb2Cl5

AbstractWe report on the optical properties of Ho doped KPb2Cl5 (Ho: KPC) for potential applications as an infrared (IR) solid-state gain medium. The investigated crystal was synthesized from commercial starting materials of PbCl2, KCl, and HoCl3 followed by several purification steps including directional freezing, zone-refinement, and chlorination. The Ho: KPC crystal was subsequently grown by Bridgman technique. Following optical excitation at 885 nm, several IR emission bands were observed at room-temperature with average wavelengths at 1.07, 1.18, 1.35, 1.65, 2.00, 2.94, and 3.96 μm. The emission at 3.96 μm originated from the 5I5 -&gt; 5I6 transitions of Ho3+ and was further evaluated for possible applications in mid-IR lasers. The decay time of the 5I5 excited state was measured to be 5.0 ms at room-temperature. The long 5I5 lifetime is favorable for laser applications and indicates that non-radiative multi-phonon relaxations are small in Ho: KPC. Based on a Judd-Ofelt analysis, the emission quantum efficiency was determined to be near unity resulting in a peak emission cross-section of 0.62×10-20 cm2 at 3.96 μm. A drawback for laser applications is the long decay time of the lower 5I6 state with a value of 4.8 ms . Since the 3.96 μm transition terminates in the 5I6 level, its long lifetime will lead to population bottlenecking, which limits possible mid-IR lasing to pulsed and quasi-cw operation.

En route to electrically pumped broadly tunable middle infrared lasers based on transition metal doped II–VI semiconductors

In this work we report the study of Cr 2+:ZnSe photo-luminescence under UV and visible excitation as well as middle-infrared electroluminescence of n-type, Cr doped bulk ZnSe crystals. Photo-conductance studies were performed to verify (2+)→(1+)→(2+)* ionization transitions responsible for Cr 2+ excitation. We report the first to our knowledge, Cr 2+:ZnSe lasing using 532 nm excitation. The first ever room temperature electroluminescence was also achieved in bulk n-type Cr:Al:ZnSe. This electroluminescence over 1800–2800 nm spectral range is in a good agreement with the mid-IR Cr 2+ fluorescence under intra-shell optical excitation. Other spectral bands of electroluminescence were also observed at 600 nm and 8 μm. The visible electroluminescence observed is attributed to V zn–Al complexes in conductive crystals. The nature of the 8 μm electroluminescence requires additional studies. Photo-ionization results are essential for optical pumping of Cr:ZnSe by easily available visible lasers, and, most importantly, both these and the electroluminescence results open a pathway for Cr:ZnSe broadband mid-IR lasing under direct injection of free carriers. Future directions for electrical excitation of low dimensional II–VI structures, where quantum confinement of the atomic impurity is used, could result in a much more efficient transfer of energy from the host to the localized impurity, are discussed.

The time-resolved spectroscopy of InGaAs/AlGaAs heterostructures with asymmetric funnel-shape quantum wells for near- and mid-IR lasing

The dynamics of interband transitions in InGaAs/AlGaAs heterostructures with funnel-shaped quantum wells, when a narrow and deep well is positioned asymmetrically in wide and shallow ones, has been studied experimentally. These structures have been proposed as a dual-colour laser in mid- and near-IR range simultaneously. The lifetime of electrons in the ground and excited states in the quantum wells has been determined by the time of the photoluminescence intensity decline. The obtained lifetime values allow precise identification of peaks in the photoluminescence spectra and a deep insight into the process of high electron states populating. The experimentally found electron lifetime for the interband transitions corresponds on the order of magnitude with the interband transitions lifetime in GaAs. Analysis of the photoluminescence spectra as well as the time dependence of the photoluminescence intensity decline indicates a possibility of population inversion achievement in these structures under high excitation.