Alexander R. Albrecht

Solid-state optical refrigeration to sub-100 Kelvin regime.

Since the first demonstration of net cooling twenty years ago, optical refrigeration of solids has progressed to outperform all other solid-state cooling processes. It has become the first and only solid-state refrigerator capable of reaching cryogenic temperatures, and now the first solid-state cooling below 100 K. Such substantial progress required a multi-disciplinary approach of pump laser absorption enhancement, material characterization and purification, and thermal management. Here we present the culmination of two decades of progress, the record cooling to ≈ 91 K from room temperature.

Optically pumped DBR-free semiconductor disk lasers

We report high power distributed Bragg reflector (DBR)-free semiconductor disk lasers. With active regions lifted off and bonded to various transparent heatspreaders, the high thermal impedance and narrow bandwidth of DBRs are mitigated. For a strained InGaAs multi-quantum-well sample bonded to a single-crystalline chemical-vapor deposited diamond, a maximum CW output power of 2.5 W and a record 78 nm tuning range centered at λ≈1160 nm was achieved. Laser operation using a total internal reflection geometry is also demonstrated. Furthermore, analysis for power scaling, based on thermal management, is presented.

1220–1280-nm Optically Pumped InAs Quantum Dot-Based Vertical External-Cavity Surface-Emitting Laser

We report an InAs quantum dot-based optically pumped vertical external-cavity surface-emitting laser (VECSEL), with a continuously variable emission wavelength from 1220 to 1280 nm through the use of epitaxial gradient across the wafer. We demonstrate the performance of two designs of this VECSEL. The first design, 4 × 3, makes use of a resonant periodic gain structure where three dot-in-a-well (DWELL) layers are located at the antinode of the E-field standing wave, with a total of four sets being used. The second design, 12 × 1, makes use of a single DWELL layer per antinode which is repeated 12 times for the same total of 12 DWELL layers. We demonstrate the lasing performance of the two structures as a function of distance from the center of the wafer and use known radial composition gradients in the molecular beam epitaxy reactor to achieve 50 nm continuous wavelength variation in the 4 × 3 structure and 60 nm in the 12 × 1 structure. We also demonstrate that the performance of the 12 × 1 structure is significantly improved compared to the 4 × 3 structure for all measured laser parameters, possibly due to increased pump absorption in the former structures thicker barriers.

High power 1.25 μm InAs quantum dot vertical external-cavity surface-emitting laser

The authors demonstrate InAs quantum dot (QD)-based optically pumped vertical external-cavity surface-emitting lasers grown by molecular beam epitaxy. Active region designs utilizing two different resonant periodic gain (RPG) structures are compared. The first RPG structure is a more traditional design consisting of three QD layers per antinode of the E-field standing wave, repeated four times, for a total of 12 QD layers. The second RPG has a single-QD layer per antinode, repeated 12 times. The single-QD layer per antinode structure allows for both superior strain relief as well as more complete pump absorption and thus results in significantly improved device performance over the traditional multi-QD layer per antinode design. The authors were able to demonstrate over 3 W of output power at room temperature using a thermal grade polycrystalline chemical-vapor deposition diamond heat spreader mounted on the backside of a sample thinned by mechanical polishing.

First solid-state cooling below 100K

Abstract : Advances in material purity and laser light absorption offer new possibilities for vibration-free cryogenic cooling. Material properties change as a function of temperature, and cryogenic refrigeration allows us to obtain very useful properties not available at higher temperatures. For instance, in the temperature range 77 150K, superconductivity, long- and mid-wave IR detectors, and ultra-stable laser cavities become usable.1 Currently, these low temperatures are reached using liquid or solid cryogens or mechanical refrigerators. Unfortunately, liquids and solids require regular attention to refill after evaporating away, and mechanical refrigerators introduce vibrational noise and mechanically wear over time. Space-based applications, and ultra-stable laser cavities in particular, cannot tolerate these drawbacks. A solid-state solution is preferable for its inherent vibration-free operation and potentially long lifetime. Optical refrigeration via anti-Stokes fluorescence is currently the only solid-state cooling technology capable of reaching cryogenic temperatures.

DBR-free optically pumped semiconductor disk lasers

Optically pumped semiconductor disk lasers (SDLs) provide high beam quality with high average-power power at designer wavelengths. However, material choices are limited by the need for a distributed Bragg reflector (DBR), usually monolithically integrated with the active region. We demonstrate DBR-free SDL active regions, which have been lifted off and bonded to various transparent substrates. For an InGaAs multi-quantum well sample bonded to a diamond window heat spreader, we achieved CW lasing with an output power of 2 W at 1150 nm with good beam quality.

80 nm tunable DBR-free semiconductor disk laser

We report a widely tunable optically pumped distributed Bragg reflector (DBR)-free semiconductor disk laser with 6 W continuous wave output power near 1055 nm when using a 2% output coupler. Using only high reflecting mirrors, the lasing wavelength is centered at 1034 nm and can be tuned up to a record 80 nm by using a birefringent filter. We attribute such wide tunability to the unique broad effective gain bandwidth of DBR-free semiconductor disk lasers achieved by eliminating the active mirror geometry.

Self-mode-locked vertical external-cavity surface-emitting laser (VECSEL)

Self-mode-locking has been observed in an InGaAs VECSEL at 1030 nm with sub-500 fs pulses at 1 GHz. The mechanism is attributed to negative ultrafast Kerr lensing in the gain structure.

Intracavity laser cooling using a VECSEL

We report on the first observation of intracavity laser cooling inside of a vertical external-cavity surface-emitting laser (VECSEL). A Yb:YLF crystal is placed under Brewster angle inside the cavity of an InGaAs quantum well VECSEL emitting around 1030 nm. With the crystal in air, we observed cooling by about 0.5 degrees. By placing the sample and cavity end mirror inside a vacuum chamber, with the window also at Brewster angle to the laser mode, cooling by 20 degrees has been realized. Furthermore, the development of a compact and efficient integrated cryocooler device is underway.

Broadly tunable DBR-free semiconductor disk laser

We report a DBR-free semiconductor disk lasers centered at 1160 nm with a tuning range of 78 nm, and ongoing effort on our DBR-free SDL centered at 1040 nm. Compared with conventional semiconductor disk lasers, DBR-free SDLs have a broader effective gain bandwidth. In CW operation, 2.5 W output power at 1160 nm and 6 W at 1055 nm were collected from the two lasers without thermal-rollover. Intracavity loss mitigation, currently underway, should improve power scaling and efficiency in these systems.