INVITED PAPER Spectral control of nanodiamond using dressed photon–phonon etching R. Nagumo 1 • F. Brandenburg 1 • A. Ermakova 2 • F. Jelezko 2 • T. Yatsui 1 Received: 27 June 2015 / Accepted: 28 July 2015 Ó Springer-Verlag Berlin Heidelberg 2015 Abstract The luminescence of a nitrogen-vacancy (NV) center in a nanodiamond (ND) is of great interest because of its features, especially in the field of nanophotonics. When an NV center in an ND is located in the vicinity of the surface, the emission is often disturbed by any surface defects, resulting in non-radiative recombination. In this work, we performed dressed photon–phonon (DPP) etching of the NDs, and found that the size of the NDs decreased, while the cathodoluminescence (CL) intensity increased. We assume that this increase in the CL intensity originates from the removal of the surface protrusions and/or defects by DPP etching. 1 Introduction The nitrogen-vacancy (NV) center of diamond is of great interest because of its features, especially in the field of nanophotonics. The NV center is an impurity defect con- sisting of a substitutional nitrogen (N) atom next to a vacancy in the diamond lattice. Within the diamond bandgap, an NV center creates an optical active defect with zero phonon line of emission at 637 nm, accompanied by broad phonon sidebands [1–3]. Bright and photostable photoluminescence of NV centers was previously used in multiple applications. For example, it could act as an electrically driven single-photon source at room tempera- ture [4], while other sources need lasers for photonic excitation. The NV center is also attracting attention as a material for application to quantum cryptography. The NV center has excellent spin coherence properties associated with its ground triplet state. Static and AC magnetic fields can be used to control the ground-state spin. In addition, the spin state can be polarized and readout via optical excitation. Optical access to the spin states enables the realization of atomic magnetometers based on NV defects [5]. In addition, the NV centers of nanodiamonds (NDs) have been applied as biomarkers in live cells [6]. As mentioned above, the NV center has many applica- tions, all of which rely on the NV emission. Therefore, it is very important to improve the emission properties. Mechanical chemical polishing (MCP) has been used to flatten the surface of a diamond substrate; however, it is difficult to achieve a surface roughness (R a value) of less than 1 nm. To overcome this difficulty, we realized non- contact and atomic-scale surface flattening using dressed photon–phonon (DPP) etching [7]. In addition to plane surfaces, DPP etching has previously been applied to three- dimensional structures, including curved lenses and the sidewalls of diamond mesa structures [8]. It is expected that DPP etching could be applied to the surface flattening of a nanodiamond (ND). A nanoparticle has a very large surface-to-volume ratio; therefore, it is assumed that the surface will have a large effect on the luminescence [9]. For example, the emission intensity of porous silicon decreases due to the adhesion of bromine or alkoxide to the surface, which acts as a non-radiative recombination center [10]. The dislocation of an Si atom on the surface of the porous silicon produces new energy levels in the bandgap and thus affects the luminescence [11]. As mentioned above, it should be possible to evaluate & T. Yatsui yatsui@ee.t.u-tokyo.ac.jp 1 School of Engineering, University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan 2 Institute for Quantum Optics and Center for Integrated Quantum Science and Technology (IQST), Ulm University, Albert-Einstein-Allee 11, 89081 Ulm, Germany 123 Appl. Phys. A DOI 10.1007/s00339-015-9400-0