Hello,

We noticed you're browsing in private or incognito mode.

To continue reading this article, please exit incognito mode or log in.

Not a subscriber? Subscribe now for unlimited access to online articles.

Lasers on Chips

First germanium laser could pave the way to laser-powered computing

Using light to move data would make computers much more efficient than they are today, but that requires a practical way to add optical components to silicon chips. MIT researchers have taken a step toward that goal by demonstrating the first germanium laser that can emit light at wavelengths suitable for digital communication. Unlike the materials used in standard lasers, germanium is easy to incorporate into the chip-making process: most manufacturers are already adding it to their silicon chips because it makes them faster.

Chip Shot New germanium lasers can be built directly into silicon chips like these.

The new device is the first germanium laser to operate at room temperature; previous examples, which emitted radiation in the terahertz frequencies, had to be cooled to near absolute zero. It also demonstrates that a class of materials called indirect-band-gap semiconductors can yield practical lasers.

This story is part of the May/June 2010 Issue of the MIT News magazine
See the rest of the issue
Subscribe

In a semiconductor crystal, adding energy to an electron will knock it out of its usual state and into the so-called conduction band, where it can move freely around the crystal. Such an electron can be in one of two states, which determine what happens to what’s left of its extra energy when it eventually falls out of the conduction band. If it’s in the first state, it releases that energy as a photon. In the second state, it releases it as heat.

In direct-band-gap materials, the first state is a lower energy state than the second; in indirect-band-gap materials like germanium, it’s the other way around. An excited electron will naturally occupy the lowest energy state it can find. So in direct-band-gap materials, those electrons tend to go into the photon-emitting state, and in indirect-band-gap materials, they don’t.

By adding phosphorus atoms to germanium, a team of researchers in the lab of materials science professor Lionel Kimerling ‘65, PhD ‘69, led by principal research associate Jurgen Michel and including postdoc Jifeng Liu, PhD ‘07, coaxed excited germanium electrons into the photon-emitting state. Whereas a phosphorus atom has five outer electrons, Kimerling explains, “germanium has only four outer electrons, so each phosphorus gives us an extra electron.” The extra electron fills up the lower energy state in the conduction band, effectively causing excited germanium electrons to spill over into the higher energy state. Previously, according to Michel, other scientists had thought “that indirect-band-gap semiconductors will never lase”–that is, produce a coherent beam of light.

The researchers’ theoretical work suggests that phosphorus doping works best at 1020 atoms per cubic centimeter of germanium, Kimerling says. So far, they have developed a technique that can add 1019 phosphorous atoms to each cubic centimeter of germanium, “and we already begin to see lasing,” he says.

Want to go ad free? No ad blockers needed.

Become an Insider
Already an Insider? Log in.
Next in MIT News
Want more award-winning journalism? Subscribe to MIT Technology Review.
  • Print + All Access Digital {! insider.prices.print_digital !}* Best Value

    {! insider.display.menuOptionsLabel !}

    The best of MIT Technology Review in print and online, plus unlimited access to our online archive, an ad-free web experience, discounts to MIT Technology Review events, and The Download delivered to your email in-box each weekday.

    See details+

    12-month subscription

    Unlimited access to all our daily online news and feature stories

    6 bi-monthly issues of print + digital magazine

    10% discount to MIT Technology Review events

    Access to entire PDF magazine archive dating back to 1899

    Ad-free website experience

    The Download: newsletter delivered daily

  • All Access Digital {! insider.prices.digital !}*

    {! insider.display.menuOptionsLabel !}

    The digital magazine, plus unlimited site access, our online archive, and The Download delivered to your email in-box each weekday.

    See details+

    12-month subscription

    Unlimited access to all our daily online news and feature stories

    Digital magazine (6 bi-monthly issues)

    Access to entire PDF magazine archive dating back to 1899

    The Download: newsletter delivered daily

  • Print Subscription {! insider.prices.print_only !}*

    {! insider.display.menuOptionsLabel !}

    Six print issues per year plus The Download delivered to your email in-box each weekday.

    See details+

    12-month subscription

    Print magazine (6 bi-monthly issues)

    The Download: newsletter delivered daily

/3
You've read of three free articles this month. for unlimited online access. You've read of three free articles this month. for unlimited online access. This is your last free article this month. for unlimited online access. You've read all your free articles this month. for unlimited online access. You've read of three free articles this month. for more, or for unlimited online access. for two more free articles, or for unlimited online access.

MIT News = for alumni only.

Are you an MIT alum?
Sign in now to read all MIT alumni news and class notes— or to manage your magazine subscription.

Sign in and read on