In this work we present a comprehensive review of recent work carried out by our group in the field of
optical refrigeration of Nd-doped solids. Several infrared thermography measurements in Nd-doped
KPb2Cl5 crystals and micro-powders both above and below the barycentre of the 4F3/2 are presented.
These include some of our most recent ones obtained by employing a novel technique that allows one to
perform differential temperature measurements. The role of both the direct anti-Stokes absorption
processes and those assisted by either excited state absorption or energy transfer upconversion in the
cooling process is discussed.
In the present work, we report on infrared thermography measurements in Nd-doped KPb2Cl5 crystal and
powder above and below the barycentre of the 4F3/2 level that were performed in order to assess the
relative weights of both the direct anti-Stokes absorption processes and those assisted by either excited
state absorption or energy transfer upconversion when cooling takes place in the material. As the laser
induced temperature changes are usually small, we used a special configuration of the samples that
allowed us to obtain differential measurements where an undoped sample acted as a temperature baseline.
This method allows us to ascertain whether the recorded temperature changes are optically induced or
they are due to some other effect.
The first experimental demonstration of local internal and bulk optical cooling in samples of Nd-doped
KPb2Cl5 crystals and Nd-doped KPb2Cl5 nanocrystalline powders upon laser excitation between the 4F3/2
and 4F5/2 manifolds is reported. The possibility of controlling the dynamics of the bulk optical cooling
process by adequately tuning the excitation wavelength is also demonstrated.
Optical cryocoolers made of luminescent solids are very promising for many applications in the fields of optical telecommunications, aerospace industry, bioimaging, and phototherapy. To the present day, researchers have employed a number of crystal and glass host materials doped with rare-earth ions (Yb3+, Tm3+, and Er3+) to yield anti-Stokes optical refrigeration. In these host materials, the attainable minimum temperature is limited by the average phonon energy of the lattice and the impurity concentration. However, recently Ruan and Kaviany have theoretically demonstrated that the cooling efficiency can be dramatically enhanced when the host material doped with rare-earth ions is ground into a powder made of sub-micron size grains. This is due to two facts: firstly, the phonon spectrum is modified due to finite size of the grains and, secondly, light localization effects increase the photon density, leading to an enhanced absorptivity.
In the present work, we propose that using a photonic crystal doped with rare earth ions offers many advantages with regards to getting a larger cooling efficiency at room temperature when compared to standard bulk materials or nano-powders. Indeed, apart to analogous phenomena to the ones predicted in nano-crystalline powders, there is the possibility of directly controlling the spontaneous emission rate of the ions embedded in the structure and, also, the absorption rate in the Stokes side of the absorption band by adequately tuning the density of photonic states, thus obtaining a large improvement in the cooling efficiency.
Here we report the first experimental evidences of anti-Stokes laser-induced cooling in two different low phonon
erbium-doped matrices: a KPb2Cl5 crystal and a fluorochloride glass. The local cooling was detected by using a
photothermal deflection technique whereas the bulk cooling was detected by means of a calibrated thermal sensitive
camera. The Er3+ ion was excited in the 4I9/2 manifold. It is worthwhile to mention that the cooling was observed in the
spectral region where some upconversion processes that initiate at the pumped level occur. Together with the
spectroscopic characterization, a short discussion on the experimental and theoretical background of the cooling
process including the possible influence of upconversion processes is presented.
The recent finding of new low phonon materials (both glasses and crystals) as rare-earth (RE) hosts that may
significantly decrease the nonradiative emissions from excited state levels has renewed the interest in investigating new
RE anti-Stokes emission channels. In this work we present, together with recent findings on cooling processes in Yb3+-
doped low phonon materials, the first experimental demonstration of anti-Stokes laser-induced cooling in the same
matrices doped with Er3+: a low phonon KPb2Cl5 crystal and a fluorochloride glass. In order to assess the presence of
internal cooling in these systems we used the photothermal deflection and conventional excitation spectroscopy
techniques whereas the bulk cooling in the Er3+ -doped samples was detected by means of a calibrated thermal sensitive
camera. The experimental results are in good agreement.
Laser induced internal cooling has been investigated in a new fluorochloride glass (CNBZn) and a fluoride glass (BIG) doped with 2.1x1020 Yb3+ ions/cm3 and in a Kpb2Cl5 crystal doped with 5x1019 Yb3+ by using collinear photothermal deflection and conventional laser excitation spectroscopies under high photon irradiances. The cooling efficiency for CNBZn glass which is approximately 2% relative to the absorbed laser power at 1010 nm and 300 K falls about 20% at 77 K. The cooling efficiency for BIG glass was only approximately 6% at room temperature. For the Yb3+ doped Kpb2Cl5 crystal we have shown internal laser cooling with a cooling efficiency of about 0.2% at room temperature. This is the third ytterbium-doped crystal, after Kgd(WO4) (Ref.10) and YAG (Ref.11), in which anti-Stokes laser-induced internal cooling has been demonstrated. The observed temperature dependence of the cooling process can be explained by a simple model accounting for the photon-ion- photon interaction.