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.
Stefan Holmlid, with 2673 citations according to Google Scholar
Satu Miettinen, with 2663 citations according to Google Scholar
Esko Kurvinen, with 1853 citations according to Google Scholar
Stefan Moritz, with 1074 citations according to Google Scholar
Milene Gonçalves, with 621 citations according to Google Scholar
LPA Simons, with 466 citations according to Google Scholar
Simo Säde, with 293 citations according to Google Scholar
Pariya Kashfi PhD., with 215 citations according to Google Scholar
Markus Warg, with 182 citations according to Google Scholar
Helena Sustar, with 177 citations according to Google Scholar
Joe J. Marquez, with 164 citations according to Google Scholar
Yeoreum Lee, with 141 citations according to Google Scholar