Thermal lens spectroscopy has been performed with a single-mode Ti:sapphire laser in a fluoride doped with trivalent transition metal ions. In samples with background absorption as high as 10-3 cm-1 an experimental quantum efficiency of 100% was determined at room temperature and the cooling efficiency via anti-Stokes fluorescence on an electric dipole transition has been shown to reach threshold in the near infrared spectral range. Cooling efficiencies as high as 20% are predicted for these fluoride crystals with impurity absorption coefficients reduced to 10-4 cm-1.
Background absorption has been studied for an YLF:10%Yb3+ crystal at different intensities at room temperature. The cooling efficiency was measured by both the DLT (Differential Luminescence Thermometry) and TLS (Thermal Lens Spectroscopy) methods. Results show that background and coolant absorption saturate differently at room temperature. A cw Ti:sapphire pump beam was tuned from 920 nm to 1040 nm with an intensity range from 100 W/cm2 to 20,000 W/cm2. Changes of temperature and thermal strength were measured by DLT and TLS methods, respectively. The cooling efficiencies with these approaches at different wavelengths were then compared based on theoretical fits to the experimental results. The cooling efficiency at 1000 nm was found to be independent of pump intensity. There, the saturation of intensity of background absorption had the same value as that of Ytterbium. The cooling efficiency below 1000 nm dropped at elevated intensity. In this range, Ytterbium absorption saturated easily, reducing the cooling ion absorption, while the absorption of background impurities did not saturate as much as Ytterbium. Consequently the cooling efficiency was lowered. For wavelengths above 1000 nm, increases in the pump intensity led to improved cooling efficiency. In this range, background absorption saturated more easily than absorption of coolant ions and parasitic heating was reduced, leading to higher cooling efficiency. Thus we have devised a method of measuring differential absorption saturation and determined its effect on laser cooling at room temperature. Saturation effects of this kind have important consequences in the heat equation for radiation-balanced lasers.
We present the first observation of nonlinear optical response mediated by ultrafast magneto-electric (ME) rectification. The control of magnetic properties of materials by ultrafast optical field enable novel sensing technology, energy conversion, terahertz emission, and ultrafast data storage. However, the interaction of the magnetic field of light with materials is normally ignored due to low magnetic susceptibilities at high frequencies. Optical nonlinearities driven jointly by electric and magnetic field components of light provide a new route in controlling magnetic properties of bulk media. Several novel physical phenomena arise from curved motion of bound electrons driven jointly by electric and magnetic fields such as longitudinally polarized second harmonic radiation, induced transverse magnetization at the optical frequency, and charge separation along the propagation direction. We investigate an ME charge separation in pentacene semiconductors using a time-resolved second harmonic generation technique. A femtosecond laser beam acted as an optical pump with photon energy well below the bandgap of the material to induce ME charge separation. The DC electric field from the ME charge separation interacted with the optical field from a second laser beam, the probe, in a four-wave-mixing interaction that induced second harmonic (M-EFISH) generation. We also sought evidence of ME charge separation by searching for THz emission. By monitoring time-resolved M-EFISH and THz emission, we were able to study the ME charge separation dynamics for the first time.
A 3-D thermal model applicable to arbitrary sample geometries was developed in COMSOL to analyze laser cooling of 1% Yb3+:KYW crystals. The model includes the effects of thermal conduction, black-body radiation, and background impurities. The simulation results showed that the largest limitation to cooling was the thermal conduction between the crystal and its glass capillary tube supports. Although glass has a low thermal conductivity, it absorbs a significant amount of heat through black body radiation because of its high emissivity (~0.9). The absorbed heat is transferred to the crystal sample through the thermal contact, causing an observable dip in the curve of temperature versus time thereby reducing the net, steady-state cooling power. This limitation was overcome using silicon aerogel, whose conductivity and emissivity are 3 orders and 1 order smaller than glass, respectively. The aerogel maintains the temperature gradient and the heat transported to the crystal is negligible, resulting in a much lower minimum achievable temperature and removing the dip in the temperature evolution curve. By changing the sample support from glass to aerogel, the minimum achievable temperature under ambient conditions was lowered from 0.2 K to 1.5 K in a 1% Yb:KYW crystal with 1W pump at 1023 nm. These results, together with analysis of radiation balance in a 10 mm long crystal of 1% Yb:KYW, were used for a preliminary investigation of self-cooled lasing in this tungstate host.
We explore candidate materials for solid-state laser cooling on electric-dipole allowed transitions that could accelerate refrigeration. We analyze the required external quantum efficiencies and tolerable impurity absorption required to achieve net cooling, and examine limitations imposed by charge transfer and excited state absorption in Yb2+:SrF2, Ce3+:LiCAF and Ti3+:Al2O3.
We report the first observation of laser cooling in 1%Yb3+:KYW and discuss factors that limit the cooling efficiency. Cooling by 10 K from room temperature at atmospheric pressure was achieved in this crystal at a wavelength of 1025 nm using 8 W from a seeded fiber amplifier. The temperature of the sample was measured using a calibrated differential luminescence thermometry method and was verified with a thermal camera. Infrared imagery and 3-D modeling were used to analyze the impact of thermal conduction, thermal convection, black-body radiation, and background impurities. The simulated results agree with experimental measurements confirming that the chief limitation of laser cooling at room temperature and atmospheric pressure is thermal conduction from sample supports. Best results were obtained for samples mounted on silica aerogel. Theoretical improvement of cooling efficiency in vacuum by exploiting impurity absorption saturation is also discussed.
Rare-earth doped single crystal (SC) yttrium aluminum garnet (YAG) fibers have great potential as high-power laser gain media. SC fibers combine the superior material properties of crystals with the advantages of a fiber geometry. Improving processing techniques, growth of low-loss YAG SC fibers have been reported. A low-cost technique that allows for the growth of optical quality Ho:YAG single crystal (SC) fibers with different dopant concentrations have been developed and discussed. This technique is a low-cost sol-gel based method which offers greater flexibility in terms of dopant concentration. Self-segregation of Nd ions in YAG SC fibers have been observed. Such a phenomenon can be utilized to fabricate monolithic SC fibers with graded index.
A new AFOSR MURI program, devoted to the pursuit of cooling solid state lasers internally, is underway and will be described. Comprising research teams from four universities, this program will focus on fiber and disc lasers and the demonstration of optical and/or phonon-based processes capable of maintaining beam quality as power loading of the medium rises. Emphasis will be placed on leveraging novel resonator designs to enhance a targeted optical field-material interaction such that localized cooling occurs within the gain medium. Examples will be given of two systems that are being pursued initially.
Rare-earth doped single-crystal (SC) Yttrium Aluminum Garnet (YAG) fibers are excellent candidates for high power lasers. These SC fiber optics combine the favorable low Stimulated Brillouin Scattering (SBS) gain coefficient and excellent thermal properties to make them an attractive alternative to glass fiber lasers and amplifiers. Various rare-earth doped SC fibers have been grown using the laser heated pedestal growth (LHPG) technique. Several cladding methods, including in-situ and post-growth cladding techniques, are discussed in this paper. A rod-in-tube approach has been used by to grow a fiber with an Erbium doped SC YAG fiber core inserted in a SC YAG tube. The result is a radial gradient in the distribution of rare-earth ions. Post cladding methods include sol-gel deposited polycrystalline.
The ability to image through a scattering or diffusive medium such as tissue or hazy atmosphere is a goal which has
garnered extensive attention from the scientific community. Existing imaging methods in this field make use of phase
conjugation, time of flight, iterative wave-front shaping or statistical averaging approaches, which tend to be either time
consuming or complicated to implement. We introduce a novel and practical way of statistical averaging which makes
use of a rotating ground glass diffuser to nullify the adverse effects caused by speckle introduced by a first static diffuser
/ aberrator. This is a Fourier transform-based, holographic approach which demonstrates the ability to recover detailed
images and shows promise for further remarkable improvement. The present experiments were performed with 2D flat
images, but this method could be easily adapted for recovery of 3D extended object information. The simplicity of the
approach makes it fast, reliable, and potentially scalable as a portable technology. Since imaging through a diffuser has
direct applications in biomedicine and defense technologies this method may augment advanced imaging capabilities in
many fields.
Single crystal (SC) yttrium aluminum garnet (YAG, Y3Al5O12) as a host material has the ability to be doped with high
concentrations of Er3+ ions. We utilize this ability to grow a 50% Er3+ doped YAG SC fiber, which was inserted into
a SC YAG tube. This rod-in-tube was used as a preform in our laser-heated pedestal growth (LHPG) apparatus to
grow a fiber with a radial distribution of Er3+ ions. The work shows that there is a distribution of Er3+ ions from their
fluorescence and two different techniques were used to measure the index of refraction.
High concentrations of the rare-earth elements erbium, holmium and thulium have been successfully doped into single crystal (SC) yttrium aluminum garnet (YAG, Y3Al5O12) fibers by use of the laser heated pedestal growth (LHPG) method. The spontaneous emission spectra and fluorescence were measured in the near-infrared (NIR). The results show progress towards forming a solid state laser able to produce a wavelength in the NIR, for high power applications.
A new method is analyzed for 3-D cooling of solids based on near-resonant, stimulated Raman scattering on a
narrowband transition together with optical pumping on a broadband transition. Estimates of achievable cooling rates
indicate that Raman cooling offers significant improvement over anti-Stkes fluorescence cooling at cryogenic
temperatures and should enable the attainment of sub-Kelvin termperatures, starting from ambient conditions in bulk
samples. Also the method is not restricted to ions with small Stokes shifts, and should therefore lead to diversification of
the material platforms used for optical refrigeration.
In recent years there has been a significant evolution in the development of high purity growth methods for nanoparticles in the 10 nm size range. Concurrently, new processing methods have led to the emergence of laser quality transparent ceramics prepared from rare-earth-doped nanopowders. Output powers and efficiencies of ceramic lasers have been reported to compare very well with those of crystal laser systems, causing interest both in nanoscale and macro-scale optical ceramics. In this paper, we first describe highly scattering oxide powders that generate continuous-wave random laser action, are able to store light, exhibit quantum size effects, and sinter to transparency at exceptionally low temperatures. Quantum size effects and modified dopant interactions in transparent ceramics processed from these powders are then considered, and their potential relevance to problems in laser cooling and the engineering of nanostructured ceramics for solid state lasers and nonlinear optics are evaluated.
We present results on the first application of upconversion dynamics for short pulse generation in visible solid state lasers. We also describe prospects for passive mode-locking mechanisms based on cooperative nonlinearities and instabilities.
Pulsed laser spectroscopy has been applied to determine details of the excited state structure of the N-V center in diamond. Excited state splittings and dephasing behavior are reported.
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