Here are some of the people who write academic or scientific papers on the topic of Service Design with a simple search on Google Scholar. I've listed below all the people that had 100+ citations.

1. Stefan Holmlid, with 2673 citations according to Google Scholar
2. Satu Miettinen, with 2663 citations according to Google Scholar
3. Esko Kurvinen, with 1853 citations according to Google Scholar
4. Stefan Moritz, with 1074 citations according to Google Scholar
5. Milene Gonçalves, with 621 citations according to Google Scholar
6. LPA Simons, with 466 citations according to Google Scholar
7. Simo Säde, with 293 citations according to Google Scholar
8. Pariya Kashfi PhD., with 215 citations according to Google Scholar
9. Markus Warg, with 182 citations according to Google Scholar
10. Helena Sustar, with 177 citations according to Google Scholar
11. Joe J. Marquez, with 164 citations according to Google Scholar
12. Yeoreum Lee, with 141 citations according to Google Scholar