The problem with glued-up slabs is that over time they tend to warp. Often this is combated by adding battens on the back side. Typically these are horizontal braces of wood. They may solve the problem, or they may not, but they definitely do not look modern.
The ideal solution is engineered materials. After all, a modern style should exploit modern materials and techniques, right?
The simplest approach is to cut slabs out of plywood and use glued on edge banding tape to cover the exposed layers on the four sides. This is fast, cheap and look pretty good, but falls short of the high quality my clients expect. It also is limited to the species and veneer pattern of plywood available. A step up is to use solid wood edge banding, which significantly ups the quality level for just a little more time and materials.
But if you want to use exotic species not available as plywood, veneer is the only way to go. A veneered slab door has a number of advantages: First the variety of woods available is dozens of times wider than simple plywood, and yet far, far less expensive than solid wood. Second, it isn't prone to warping, no battens required. Third, various core materials can be used to reduce the weight if that is a concern (often the case for large doors).
To make a veneered door, start with the core. The core will always be completely hidden from view, so it doesn't matter what it looks like. MDF, plywood (finish grade) or resin-impregnated paper honeycomb are all flat, strong, stable core materials. Each has its strengths and weaknesses so pick according to your needs. The core will be surrounded on four sides by solid wood edges, so cut it undersized accordingly. Make sure it is squared before continuing.
Next apply the edges. A butted glue joint is strong enough, since the joint will be reinforced by the veneers, but if you want to use splines, biscuits or other more complex joints, knock yourself out. Make the edge strips slightly wider than needed to allow trimming after the veneers are applied, which make final clean up MUCH easier. Don't go overboard though if you want to match up graining from one door to the next.
Once the edges are applied sand the front of and back of the panel to ensure a smooth transition between core and edge (don't sand if your using honeycomb, obviously) Don't finish sand, you want the core roughed up, especially if its MDF, to ensure good bonding with the veneer.
Now cut your veneers. I always try to cut doors for the same cabinet out of the same sheet of veneer so the grain lines up. The veneers can be marked with chalk to help keep them organized. On this project I used paper backed veneer sheets, but any veneer will work, so don't limit yourself (I don't recommend paper backed if the edges will be profiled)
Be sure you have a backer veneer. A panel with veneer on only one side will have a tendency to curl toward the veneer sided, by veneering both sides, the veneers fight each other and keep the panel flat and stable. The backer doesn't have to match the front, but I prefer it on doors.
Now apply the glue, position the veneers and clamp them up to cure. This is where a vacuum press is worth its weight in gold! It provides truly uniform pressure across the entire surface without numerous clamps and cauls, and fits any size door.
After the panel is cured (follow the glue manufacturer's recommendations), I trim over hanging veneer with a router, and joint on long edge of the panel. Then trim the panel to with on a table saw. Then trim the short edges with either a circular saw and straight edge, a crosscut sled on the table saw or a radial arm saw.
Contemporary doors are usually very simple, with minimal edge profiles if at all. If your project calls for a profile, now is the time to route it. In this case, no profile was used, I just eased the edges during sanding.
Now it's time to sand and finish the door as you normally would. I like to bore for hinges before final sanding, to reduce the chance of scratching the finish.
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