No, unless they have a warrant for the smartphone. The Fourth Amendment to the U.S. Constitution protects you from unreasonable government searches and seizures, and this protection extends to smartphones. In Riley v. California, a landmark United States Supreme Court case, the Court unanimously held that the warrantless search and seizure of digital contents of a cell phone during an arrest was unconstitutional.
However, law enforcement officers remain free to examine the physical aspects of a phone to ensure that it will not be used as a weapon -- say, to determine whether there is a razor blade hidden between the phone and its case. Once an officer has secured a phone and eliminated any potential physical threats, however, data on the phone can endanger no one, and therefore officers cannot access the smartphone.
Despite the strong privacy protections established in the court's Riley decision, police also have the right to search your phone without a warrant in “exigent circumstances.” What are exigent circumstances? These can be as simple as the police saying that they need to search your phone because of an emergency.
Lastly, if you give your consent to the police officer, they can access your smartphone. You can't argue that your phone was illegally searched if you give them the consent to search it!
If you have any questions about a sexual investigation stemming from a phone seizure or if you have been arrested for a criminal offense where a phone was used, you need to contact an experience criminal defense law firm.