Unless the work you are doing is super secret, any work you do for anyone anywhere—even if they “own” it—can be used by you for self-promotional purposes in a portfolio.
When you say this:
“Hence, I want to ask my employer to permit me to use and publish the works I did for them in my portfolio…”
You are being overly submissive to your employer. Especially when you say this:
“I'm aware that without a specific contract, whatever work is done by a salaried designer, is the company's property.”
Yes, the design work you did is their property. But that is simply a copyright/trademark concern. Unless you plan on mimicking the company you work for online or in print, you are overthinking this.
The only case where you might need explicit permission from an employer is if you signed an NDA (non-disclosure agreement) when you were hired. An NDA basically states you work for XYZ company and the details of your work cannot be disclosed by you for any reason.
That said, NDAs are fairly common to the point of uselessness and overly restrictive for cases like portfolios.
For example, I work as web developer. I have signed NDAs with many employers. But my online portfolio still highlights work I did for them. How can I do that? Simple! I’m not revealing deep code or system specifics which is what the goal of these NDAs are in my case.
In general, I believe that you are really overthinking this for a portfolio. I mean you realize pretty much any work anyone presents in online portfolios is work done for other clients and I am fairly confident 99.9% of these people did not ask their clients for permission? Because it is automatically assumed you will use your work to build your portfolio.
Now yes, there are cases where companies like Apple and Google might not want you to even acknowledge that you work for them, but that is a draconian extreme.