Cooling of matter with narrowband radiation was proposed in 1929 and laser cooling of solids was demonstrated by the team of Richard Epstein at Los Alamos in 1995. Works that emerged from a collaboration between Los Alamos and the University of New Mexico under the guidance of Richard Epstein and Mansoor Sheik-Bahae laid the foundation for the vibrant new field of optical refrigeration. We will review key successes and the pivotal role Mansoor Sheik-Bahae played in these developments. With routine cryogenic cooling of payloads achieved by rare-earth doped cryocoolers, the field is facing new exciting challenges in applications, material science and fundamental limits of light-matter interactions.

Membrane-external-cavity surface-emitting lasers (MECSELs) were originally demonstrated by Prof. Mansoor Sheik-Bahae’s group at the University of New Mexico (UNM) in 2015. MECSELs consist of an epitaxial active region combined with at least one transparent heatspreader, typically SiC or diamond, employing external cavity mirrors for feedback. The standalone gain element allows for significant flexibility in emission wavelength and is amenable to enhanced power scaling via optimized thermal management. In an extremely fruitful collaboration with UNM beginning in 2017, we progressed from serially-produced chip-scale prototypes, to 4” wafer-scale manufacturing of double-bonded (SiC/epi/SiC) devices capable of single-mode frequency-doubled output powers in excess of 10 W. Leveraging this unique architecture, we see a bright future ahead for multi-Watt-output optically-pumped semiconductor laser systems emitting throughout the visible and infrared spectral regions.

60 years of nonlinear optics have been marked a number of significant achievements, but for the most part, all of these breakthroughs occurred in the first 10-15 years, after which the progress has been lamentably slow. In my view, the reason for this slowdown has been the fact that the fundamental limits of nonlinear optics in terms of strength and speed of the effects have already been reached, and the powers required for many nonlinear schemes are plainly too high for them to be practical. In this talk I explain the very fundamental origins of the strength-speed trade-off in nonlinear optics, distinguishing between the instant and slow nonlinearities. Then I explore the resonant enhancement of nonlinearities using intrinsic or extrinsic resonances. Using this framework, I will explore the recently widely promoted nonlinearities in graphene, other 2D materials and epsilon near zero materials, as well as newly minted enhancement schemes like Fano resonances, exceptional points, bound states in continuum and so on. Sober but hopefully not entirely useless, conclusions will be drawn.

Higher order Stokes waves are easily generated in optical fibres through nonlinear stimulated Brillouin scattering. Since the Stokes waves have a linewidth limited by the narrow bandwidth of the Brillouin gain of ~20MHz, higher order Stokes waves are near multiples of the first Stokes shift of ~10GHz, generating a comb of frequencies. When Fourier transformed this comb can potentially lead to ultrashort pulse generation. Our investigations showed that short pulses of order 10ps can indeed be generated with a few Stokes waves, starting with a single CW pump. However, in longer cavities, these Stokes orders de-phase and lead to chaotic behaviour. Taming this source could lead to complex instruments being replaced by a simple optical fibre cavity.

Recent advances in cryogenic optical refrigeration have demonstrated cooling of a payload to below 125 K, enabled by several improvements. The parasitic load from pump and fluorescence scattering on the payload are minimized using a textured-MgF2 thermal link. Amplified Spontaneous Emission and lasing are suppressed by spectrally selective coatings on the multi-pass pump circulator. Power scaling of the cooling lift at cryogenic temperatures is yet to be observed, prompting us to investigate the negative contribution of pump saturation on a material’s cooling efficiency and, thus, Minimal Achievable Temperature. A systematic study of the power-dependent performance of the cryocooler is reported.

Since a few years Optical Cooling is seeing growing interest; recent progress make this technology attractive for space applications. It allows to envision miniature, vibration free and cost effective cryocoolers. Thus how to switch from lab experiment to an industrial product? How to manufacture tens of efficient space compatible optical coolers per year? Wallplug efficiency performance, integration and manufacturing scalability are challenges that are covered in this paper. It reviews the main requirements that crystal, laser source and system shall meet to build a successful product. To conclude the forthcoming activities in France are covered.

It has already been demonstrated that cryogenics environment is beneficial to optical systems such as infrared imaging sensors or for electro-optical interconnections for superconducting devices. A Semiconductor Optical Amplifier (SOA) as a multifunctional device could be a key element for optical amplification, optical switching and optical signal processing. As of today their high noise figure and limited amplification prevent their massive adoption in optical systems. In this paper we present how the cold temperatures dramatically improve the optical gain, noise figure, modulation bandwidth and efficiency of a SOA. The paper covers a theoretical model of SOA including the temperature as a parameter and presents experimental static and dynamic results. It concludes with an outlook on future work.

Cesium lead bromide nanocrystals, in contrast to most other materials, exhibit near-unity photoluminescence quantum yields (PLQY). When excited below the band gap, they absorb the photons and show anti-Stokes photoluminescence (ASPL), emitting higher energy, band-gap photons. Simultaneous existence of near-unity PLQY and ASPL can be used to optically cool these materials. In this talk, I will report near-unity ASPL efficiencies in CsPbBr₃ nanocrystals and attribute it to resonant multiple-phonon absorption by polarons. The theory explains paradoxically large efficiencies for intrinsically disfavored, multiple-phonon-assisted ASPL in nanocrystals.

Lead-halide perovskite nanocrystals are promising candidates for semiconductor laser cooling due to their near-unity photoluminescence quantum yields and efficient photon/phonon up-conversion process. This unexpected, efficient sub-gap energy up-conversion implies an unexpectedly strong electron-phonon interaction in perovskite nanocrystals. However, the underlying mechanism remains mostly unclear. Detailed experiments, along with theory, have now been conducted to elucidate the efficient up-conversion in CsPbBr3 NCs, utilizing a combination of techniques: photothermal absorption spectroscopy, up-conversion detuning spectroscopy, and ultrafast transient differential absorption spectroscopy.

CsPbBr3 perovskite nanocrystals (CsPbBr3 NCs) have emerged as a promising materials system to explore for optical cooling technologies, attributed to their pronounced anti-stokes photoluminescence (ASPL) and near unity photoluminescence quantum yield (PLQY). However, the details of their up-conversion mechanism remain unclear. Our research probed the activation energy of up-conversion (Ea) during temperature-dependent luminescent studies, and its correlation with the up-converted frequency shift (ΔE). The findings highlight the influence of surface polaron states. Notably, an energy of rearrangement (ER) was identified, which is modulated by the ligand coverage on the NCs, as well as several other factors. This work informs advances in material design for optical cooling applications.

Superfluorescence (SF) is a unique optical phenomenon that consists of an ensemble of emitters coupling collectively to produce a short but extremely intense burst of light. Despite our recently published works showing that room temperature anti-Stokes shifted SF were achieved in a few randomly assembled or even single lanthanide-doped upconversion nanoparticle (UCNP), the coupling required to produce and optimize Burnham-Chiao ringing (echoing of pulses) is not understood. Such ringing could be particularly useful to provide timing and multiplexing in potential applications as an alternative light source device. We previously found a lack of Burnham Chiao ringing in single nanocrystals, but strong ringing in a random cluster. The ordered assembly of these crystals will not only create a SF superburst, but also enable understanding of the periodicity of the Burnham Chiao ringing. This work explores SF microrod (MR), with enhanced SF performance and the closely spaced assembly of MR result in a greater active volume, which gives rise to greater reabsorption of the initial emission, which is then re-emitted, leading to greater oscillatory fluorescence or Burnham Chiao ringing. We also correlate the MR dimension and orientation with the corresponding SF spectral properties.

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.

The question whether the efficiency of thermodynamic devices such as heat engines and refrigeration is altered through the breaking of time-reversal symmetry has been a topic of significant debate. We experimentally investigate the cooling of nanomechanical resonators in multimode optomechanical devices, in which time-reversal symmetry for thermal fluctuations can be broken through suitable temporal modulation of the radiation pressure control field. We study the resultant nonreciprocal transport of thermal vibrations, and show that the controlled breaking of time-reversal symmetry through synthetic magnetic fields can enhance the cooling performance, yielding lower phonon occupancies than the conventional limit.

Silica glass is an extremely useful material with excellent optical and mechanical properties, that is easily shaped into a wide variety of forms. Unfortunately, due to the high phonon energy of silica, it has been a challenge to use it for laser cooling applications as it requires ultrahigh purity to avoid parasitic heating adding to the non-radiative pathways, and which act as a severe heat load in any cooling event. Our research over the years has focused on trying to incorporate rare earth materials in silica glass for example, in the form of nanocrystals of fluoride through heat treatment, which provide a low phonon environment, thus shielding it from the high phonon energy silica host. As opposed to the untreated glasses, which showed significant heating, this approach resulted in near zero temperature rise. However, this route as well, is an arduous one as purification of the starting materials is still an issue for the casting technique used for glass manufacture. Recently, our work has focused on inducing phase separation of active rare earth oxides in an environment of a non-active rare earth oxides, such as yttrium oxide to form a unique glass. We have shown that through temperature control, the phase separation of RE: yttria can be either enhanced or reversed, transitioning from clear to turbid to clear glass states using a high purity MCVD process. Our work led to the largest successful cooling reported to date of GAYY-PS oxide glass by with a temperature drop of 4 K from the ambient. This presentation reviews these developments and subsequent progress of laser cooling and other applications in these novel and highly promising glasses.

We were interested in identifying design of nanoparticles that will enhance their refrigeration efficiency. We focused on maximizing their quantum yield while minimizing their non-radiative processes. In particular, we developed nanocrystals with and without an inert shell. We characterized the refrigeration efficiency by monitoring the temperature of the nanocrystals optically trapped in vacuum. Overall, we saw an increase in refrigeration efficiency for the shelled nanoparticle with a minimum temperature of ~150K. We will also show works towards controlling the motional temperature of the levitated nanoparticle paving the way towards absolute cooling in levitated optomechanics.

We present the hydrothermal synthesis of highly porous α-NaYF nanocubes with edge lengths on the order of 150 nm using sodium dodecyl sulfate (SDS) as a ligand. The surface area is increased by a factor of four compared to hypothetical ideal cubes of the same size. Furthermore, the surface-to-volume ratio of these particles is higher than that of 50 nm spherical α-NaYF particles commonly obtained with EDTA under similar conditions, resulting in the highest surface-to-volume particles obtained with hydrothermal synthesis. Laser cooling at ambient pressure and laser heating of the same particles in vacuum is demonstrated. The implications of our results for payload cooling of optically levitated nanoscale sensors are discussed.

Calcium difluoride (CaF2) is an industrially important material for high power optical devices due to its high melting point, stoichiometric composition, low background absorption coefficient, and large optical transparency window. Solid state laser refrigeration has recently been reported with single crystals of cubic CaF2 (fluorite crystal structure, space group Fm-3m) doped with trivalent ytterbium ions. In this presentation we present the recent design, hydrothermal synthesis, and characterization of ytterbium-doped CaF2 microspheres with diameters ranging from 500 nm to 4 micrometers. Raman spectroscopy reveals the presence of point defects that compensate for trivalent ytterbium ions that reside at divalent calcium ion sites in the crystal lattice. Near-infrared optical luminescence spectroscopy is used to demonstrate that CaF2 microspheres undergo solid state laser refrigeration under ambient conditions. These materials are anticipated to have significant applications in experimental single molecule biophysics, radiation-balanced microlasing, and the future development of optomechanical quantum sensors.

We will present an update on the Levitated Sensor Detector (LSD) project for detection of high frequency (10-100kHz) gravitational waves above the region previously probed by LIGO. We discuss experimental trapping results of different geometries of NaYF4 hexagonal plates and our recent milestone of increasing the levitated test mass by an order of magnitude. These materials are excellent candidates for laser refrigeration of the internal temperature, which would enable trapping with larger optical intensities and GW detection at frequencies approaching the MHz range. Finally, we examine the progress of the 1-meter prototype that is under construction at Northwestern University.