Laser Cooling in Semiconductors Chengao Wang Optical Science and Engineering, Department of Physics & Astronomy, University of New Mexico

Historical Review  1929: The concept of laser cooling was established.  1960: Laser was invented.  1995: Laser cooling was first observed in ytterbium-doped glass.  ????: Laser cooling in semiconductors is achieved.

Significance  Heating is a major problem in semiconductor devices.  Optical refrigerator using laser cooling will be free of vibration, mechanically robust and compact.  It has far reaching implications in the area of optical detection systems and optoelectronic devices.

Purpose  invent a practical all-solid-state optical refrigerator to cool semiconductors using laser cooling.  We hypothesize that laser cooling in semiconductors can achieve temperatures ~10K and below

Overview of Methodology 1. Develop a comprehensive theoretical model of laser cooling in semiconductors. 2. Grow new semiconductor materials optimized for laser cooling using MOCVD 3. Demonstrate experimentally the theory of laser cooling in semiconductor devices. 4. Build prototype optical refrigerator in semiconductors.

The concept of laser cooling Cooling Cycle phonons excited state ground state 3 Laser

The concept of laser cooling in semiconductors Pump Semiconductor heterostructure h h f Valence band Conduction band EgEg

Step 1: Develop a comprehensive theoretical model of laser cooling in semiconductors.  Two key issues, luminescence trapping and red-shifting, have not been addressed in the current theory and these issues will frustrate our attempts to achieve semiconductor net cooling

Luminescence Trapping  Total internal reflection Pump semiconductor

Luminescence red-shift

Step1: Deliverable  understanding of which materials are optimum for laser cooling in semiconductors.  predict the possible designs of the future optical refrigerators.

Step 2: Use MOCVD to grow new semiconductor materials  InGaP/GaAs Heterostructures have been predicted to be good candidates for laser cooling in semiconductors.  perform microscopic analysis of each material in order to optimize the materials for laser cooling.

Step2: Deliverable  optimal materials that have a good chance of achieving net cooling.

Step 3: Demonstrate experimentally the theory of laser cooling in semiconductor devices  Do experiment to prove laser cooling in semiconductors

Step3: Deliverable  proof of net cooling in semiconductors  reevaluating our theory about laser cooling in semiconductors and further optimizing the materials.

Step 4: Build prototype optical refrigerator in semiconductors.  In order to build a practical devise, we should first solve some engineering issues.  After making the preliminary optical refrigerator, we may try to make it more compact and efficient.

Step4: Deliverable  an infant machine for practical applications.

The Goal: An All-Solid-State Cryocooler  The research involve all fundamental physics and engineering issues of laser cooling in solids that will pave the way for the development of a practical all-sold-state optical cryocooler. Refrigerant solid fluorescence heat sink laser cold finger element All-solid-state (rugged, compact) No vibrations (no moving parts or fluids) Efficient For space-based sensors

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