今 means now, 今后, from now on. 此 means 'this, this moment, etc' 此后 , since this moment The referential point of 今后 is now, while the referential point of 此后 is more likely some time in the past.

我认为两个词的意思基本差不多，“今后”是指从今以后，从现在起；”此后“也有今后的意思，但也可以从某一时间算起，从过去某一段时间来说，”此后“表达的范围更广，既可以指从过去的某段时间以后，也可以指从现在以后。

此后： 此后，王子和公主过上了幸福的生活。 今后=从今以后=从今天起： 从今天起，我会好好学习。 一般此后用在某件事发生以后；而今后则更侧重时间。

今后 means "from now on", while 此后 means "from then on" or "ever since". So 此后 can only be used after you have just talked about another event that happened earlier. 此 means "this" or "that", which refers to the event you talked about previously. e.g. 他前年去了趟英国, 此后再也没去过. 我今后要好好学习, 做一名合格的共产主义接班人.

此后 is more common in writing