Femto-second Measurements of Semiconductor Laser Diodes David Baxter

1. Femto-second Measurements of Semiconductor Laser DiodesDavid Baxter • Summary • What are diode and ultrafast lasers? • How do we measure with fs resolution? • Why measure in the fs time scale?

2. InP (p) Metal InP (n) Trench InP (p) Active region InP (n) What Are Diode Lasers? To make a laser you need a gain medium (semiconductor) with feedback (facet mirrors). • Light • Amplification by the • Stimulated • Emission of • Radiation Typical dimensions of the active region are 0.2 x 1.5 x 350 m. Light output typically in mW at either 1.3m or 1.5m. Used in telecom, domestic, medical and research applications.

3. What is an Ultrafast Laser? Intensity 100fs Time 12.5ns Intensity To measure with fs resolution, fs events are required. Time

4. Non-linear Optics Optical polarization Induced polarizations within a crystal become non-linear at high E-field intensities due to asymmetric electron potentials. Amplitude Fundamental polarization Linear response Amplitude Amplitude Second-harmonic polarization Non-linear response Amplitude Steady dc polarization Amplitude Amplitude

5. Non-linear Optics Cont… 1,k1 • As a result of non-linear crystals two photons can be combined to form a new photon. This is call frequency Up conversion • The beam is produced as shown to conserve momentum. • Second harmonic beam is ONLY present when incident beams overlap in time and space. 21,k3 Non-linear crystal 1,k2

6. Auto-correlation Set-up By moving retro-reflector the relative path difference is also changed. Beam splitter Lens SHG Non-linear crystal

7. Auto-correlation Cont… • One pulse moves in time with respect to the other pulse. • Signal proportional to the overlap of the two pulses. • Resultant trace is a convolution of the pulse with itself. • Can be de-convoluted to obtain original pulse. • Only gives intensity information, no phase information is gained. Intensity E-field envelopes Time (ps) Overlap generates signal

8. Cross-correlation Set-up Sample SFM

9. Pump Beam • Pump pulse excites a response from the sample under study, for example, a semiconductor laser diode. • Pump pulse does not necessarily have to be same wavelength as probe pulse. Intensity Response Time (ps)

10. Probe Beam • Probe pulse much shorter in time than the response. • Using the delay stage to ‘scan in time’ along the response. • 100 ps in time requires a path change of 15 mm. • 100 fs resolution requires a path change of 15 μm. • Obtain the response intensity as a function of time. Probe Intensity Response Time (ps)

11. Frequency Resolved Optical Gating (FROG) Can extend the previous technique by replacing the detector with a spectrometer. Can therefore measure spectrum as a function of time. This is similar to a music score which dictates the notes (or frequencies) to be played as a function of time. PG FROG* without chirp PG FROG* with chirp Frequency Frequency Requires a FROG algorithm to return intensity AND phase information. Time Time * FROG traces generated using Femtosoft Technologies Java Applet at http://www.femtosoft.biz/frogapp.shtml

12. What Can We Measure With Fs Pulses? • Pulse trains from a laser – direct measurement of the round trip gain/loss • Fast electron decay mechanisms • Chirp • Examine responses from new materials e.g. nitride and quantum dot materials Chirp Intensity Intensity Round-trip loss Time Time

13. Current Status I am currently characterising the fs pulse from the ultrafast laser using my auto-correlation set-up. Acknowledgements • Professor Jeremy Allam • Dr Konstantin Litvinenko