Self-Limiting Mechanism of Anti-Stokes Optical Cooling in Diamond NV Centers

TL;DR

This study investigates anti-Stokes optical cooling in diamond NV centers, revealing a self-limiting mechanism caused by charge-state conversion that constrains cooling efficiency.

Contribution

It combines experimental spectroscopy and numerical modeling to identify charge-state conversion as a key factor limiting optical cooling in NV centers.

Findings

Pronounced phonon-assisted anti-Stokes emission observed below ZPL.

Charge-state conversion suppresses NV- mediated cooling.

Simulations predict a self-limiting cooling behavior.

Abstract

Anti-Stokes optical cooling in diamond nitrogen-vacancy (NV) centers is experimentally and numerically investigated. Photoluminescence-excitation spectroscopy reveals pronounced phonon-assisted anti-Stokes emission under excitation below the zero-phonon line (ZPL). However, the below-ZPL excitation drives photoinduced charge-state conversion between negatively-charged NV- and neutral NV0, thereby suppressing the NV- mediated cooling channel. Time-resolved photoluminescence (PL) measurements reveal an increase in the effective PL lifetime with excitation density, reflecting an increasing NV0 contribution. By fitting nanosecond and millisecond PL dynamics with a minimal rate-equation model, we extract effective optical pumping and charge-conversion rates, which enables us to quantitatively simulate the cooling performance. The simulations predict a self-limiting behavior of anti-Stokes cooling and clarify the excitation conditions under which net cooling can be sustained within this effective model. The estimated cooling power per NV center is comparable, on a microscopic basis, to values discussed for semiconductor quantum dots and rare-earth optical coolers. These results identify charge-state conversion as a key bottleneck for defect-based optical refrigeration.