The science of upconversion laser development can lay claim to many critical achievements on the path to a practical device. Among these are broad spectral coverage, room-temperature operation, single-wavelength pumping, and all-solid-state design. Arguable remaining milestones to overtake are high power, low cost, and easy manufacture. In this chapter we present a discussion of the origins and development of this subject with an eye toward conferring on researchers, who are new to the field, a comprehensive grasp of the published literature on upconversion laser experiments. After an account of the early history of upconversion laser research, we divide the topic into parts: the first comprises work involving bulk laser gain media – exclusively rare-earth-doped crystals – whereas the second comprises work on optical fiber gain media – primarily doped fluorozirconate glasses. Following a similar organization, nearly all the publications discussed in this chapter are reprinted under one cover in the anthology by Gosnell (2000). The chapter concludes with a brief discussion of potential directions for future research.

HISTORICAL INTRODUCTION

The notion of multi-photon upconversion in the solid state was first discussed by Bloembergen (1959) in the context of microwave and infrared quantum counters. The idea, depicted in Figure 8.1, is to exploit the photon counting capabilities of photomultiplier tubes sensitive only in the visible and near-infrared spectral range to detect upconversion emission. As seen in the figure, a single low-energy “signal” photon is converted to a single high-energy output photon when an upconverting pump photon supplied by an external source is simultaneously absorbed by the system. Transparent host materials doped with rare-earth and transition-metal ions were specifically named as potential detectors.