We report on the growth and optical properties of dense arrays of single GaAs/AlGaAs quantum dot (QD) heterostructures with pitches as small as 300 nm. The samples were grown by organometallic chemical vapor deposition in dense inverted pyramids on {1 1 1}B GaAs substrate pre-patterned using electron beam lithography and wet chemical etching. The growth conditions such as deoxidation and growth temperatures, growth rates, and V/III ratio, had to be chosen quite differently from those employed with micron-size pyramids. Low-temperature micro-photoluminescence and cathodoluminescence spectra of the samples show distinct luminescence from the QDs with a linewidth of less than 1 meV and uniform emission energy for an ensemble of 900 QDs. The possibility of incorporating such QD arrays inside optical microcavity structures is also discussed.