Author was the YouTube user who uploaded this video. This translates to channelID which the video is uploaded in v3.
So in your search you can define channelId.
GET https://www.googleapis.com/youtube/v3/search?part=snippet&channelId= UC1oFl9_Y7LM5tjCcpTpqA1g&q=irem+yagci&key={YOUR_API_KEY}
To get channelId of any legacy username you can use:
GET https://www.googleapis.com/youtube/v3/channels?part=id&forUsername=ewakolik&key={YOUR_API_KEY}