# Femtosecond Optical Nonlinearity of Nanodiamond Suspensions
## 2 Materials and Methods  
HP-HT nanodiamonds were obtained from [PERSON], Geneva, Switzerland. The HP-HT nanodiamond particles are monocrystals with a substitutional nitrogen content of ~100 ppm. The purification of nanodiamonds from carbon was carried out by processing in the air at a temperature of 500 \({}^{\circ}\)C for two hours, followed by treatment in hydrochloric acid at a temperature of 70 \({}^{\circ}\)C for two hours. The nanoparticles were dispersed in deionized water at 1.65 wt% concentration and the suspensions were stored in plastic vessels. Detonation nanodiamonds were synthesized by an explosion of an oxygen-deficient explosive mixture of trinitrotoluene and hexogen at a weight ratio of 1:1 in a closed steel chamber. The resulting detonation soot contains the nanodiamond particles (more than 30%), the other allotropic states of carbon, and various metallic impurities. Purification of the diamond soot in the mixture at the high temperature reduces the amount of impurities down to 1 wt%. These detonation nanodiamonds received from a vendor were additionally purified at Adamas Nanotechnologies with HCl, reducing the metal content to 0.4 wt%. Detonation synthesis leads to the formation of nanodiamonds with a primary particle size of about 5 nm. The natural agglomeration of nanodiamonds leads to the formation of solid particles of much larger sizes. To obtain nanoparticles with a given average size the purified nanodiamonds were suspended in deionized water by sonication and processed in a planetary mill for 4 h using zirconia beads. Additional sp\({}^{2}\)-carbon from milling was removed by treatment at 400 \({}^{\circ}\)C in the air for 3 h. After this high-temperature treatment, the resulting product was resuspended in deionized water at 1% \(w/v\) by sonication. Centrifugation at 25,000\(\times g\) forces was used to extract nanodiamond particles with an average diameter of ~10 nm. The nitrogen content in the detonation nanodiamonds is ~10,000 ppm. The major differences in HP-HT and detonation nanodiamonds are in the size of the primary particles, the presence of aggregates consisting of few primary particles in detonation nanodiamonds and nitrogen content. While treatments to reduce sp\({}^{2}\)-carbon content were performed, residual sp\({}^{2}\)-carbon is still present in both types of nanodiamond particles, with a higher content expected in detonation nanodiamonds due to the smaller size of primary particles. For surface and particle size analysis, small droplets of fabricated suspensions of HP-HT and detonation nanodiamonds were placed on a flat glass surface and dried to form a uniform film. The topography images of the nanodiamond films were acquired with an NTEGRA Probe Nanolaboratory atomic-force microscope (AFM) in semicontact mode. The particle size distribution was attained by processing the atomic-force microscope images with Image Analysis 3.5.0 software (NT-MDT Spectrum Instruments, Zelenograd, Russia) based on the combination of the correlation analysis and the method of sections at a certain relative height. The AFM images and the particle size distribution of the nanodiamond films are shown in Figure 1. The performed measurements reveal that more than 75% of the HP-HT and detonation nanodiamonds have sizes in the ranges of 9-20 and 6-13 nm, respectively. Figure 1: Atomic force microscopy images of (**a**) HP-HT and (**c**) detonation nanodiamond films. The image processing reveals the HP-HT and detonation nanodiamond particle size distributions shown in figures (**b**) and (**d**), correspondingly. Raman spectra of the attained nanodiamond films were recorded using Horiba LabRam HR800 Raman spectrometer (Lille, France) with a 632.8 nm laser. A 100\(\times\) objective with the attenuation of the incident radiation (neutral filter D1) was used to provide an intensity (less than about 5 kW/cm\({}^{2}\)) below the graphitization of nanodiamond material at 632.8 nm [29; 52].